Energy Storage Publications

Bethel Tarekegne, Rebecca O'Neil, Jeremy Twitchell."Energy Storage as an Equity Asset."Current Sustainable/Renewable Energy Reports 8, 149-155 (September 2021).
Abstract: This review offers a discussion on how energy storage deployment advances equitable outcomes for the power system. It catalogues the four tenets of the energy justice concept—distributive, recognition, procedural, and restorative—and shows how they relate to inequities in energy affordability, availability, due process, sustainability, and responsibility. Energy storage systems have been deployed to support grid reliability and renewable resource integration, but there is additional emerging value in considering the connections between energy storage applications and equity challenges in the power system. Through a thorough review of the energy justice and energy transitions literature, this paper offers the equity dimensions of storage project design and implementations. Emerging energy programs and projects are utilizing energy storage in pursuit of improved equity outcomes. Future research and policy design should integrate energy justice principles to align storage penetration with desired equity outcomes.

Charlie Vartanian, Matt Paiss, Vilayanur Viswanathan, Jaime Kolln, David Reed."Review of Codes and Standards for Energy Storage Systems."Current Sustainable/Renewable Energy 8, 138-148 (September 2021).
Abstract: This article summarizes key codes and standards (C&S) that apply to grid energy storage systems. The article also gives several examples of industry efforts to update or create new standards to remove gaps in energy storage C&S and to accommodate new and emerging energy storage technologies. While modern battery technologies, including lithium ion (Li-ion), increase the technical and economic viability of grid energy storage, they also present new or unknown risks to managing the safety of energy storage systems (ESS). This article focuses on the particular challenges presented by newer battery technologies. Prior publications about energy storage C&S recognize and address the expanding range of technologies and their unique characteristics. However, there remains significant need and opportunity for researchers to add to the knowledge base that informs the development of technical references and standards, and ultimately, the application of published standards for the effective and safe design and use of modern ESS.

Patrick Balducci, Kendall Mongird, Mark Weimar."Understanding the Value of Energy Storage for Power System Reliability and Resilience Applications."Current Sustainable/Renewable Energy Reports 8, 131-137 (September 2021).
Abstract: The need for energy storage in the electrical grid has grown in recent years in response to a reduced reliance on fossil fuel baseload power, added intermittent renewable investment, and expanded adoption of distributed energy resources. While the methods and models for valuing storage use cases have advanced significantly in recent years, the value of enhanced resilience remains an open research question. The findings of the recent research indicate that energy storage provides significant value to the grid, with median benefit values for specific use cases ranging from under $10/kW-year for voltage support to roughly $100/kW-year for capacity and frequency regulation services. While the value of lost load is used widely to estimate the benefits of mitigating short-duration outages, reaching as high as $719/kilowatt-year, there is no consensus when it comes to monetizing the value of improving grid resilience. This paper presents a use case taxonomy for energy storage and uses the taxonomy to conduct a meta-analysis of an extensive set of energy storage valuation studies. It reviews several approaches for monetizing reliability and resiliency services and presents a proposed approach for valuing resiliency for energy storage investments.

Xiang Li, Peiyuan Gao, Yun-Yu Lai, J. David Bazak, Aaron Hollas, Heng-Yi Lin, Vijayakumar Murugesan, Shuyuan Zhang, Chung-Fu Cheng, Wei-Yao Tung, Yueh-Ting Lai, Ruozhu Feng, Jin Wang, Chien-Lung Wang, Wei Wang, Yu Zhu."Symmetry-breaking design of an organic iron complex catholyte for a long cyclability aqueous organic redox flow battery."Nature Energy 6, 873-881 (September 2021).
Abstract: The limited availability of a high-performance catholyte has hindered the development of aqueous organic redox flow batteries (AORFB) for large-scale energy storage. Here we report a symmetry-breaking design of iron complexes with 2,2′-bipyridine-4,4′-dicarboxylic (Dcbpy) acid and cyanide ligands. By introducing two ligands to the metal centre, the complex compounds (M4[Fe^II(Dcbpy)₂(CN)₂], M = Na, K) exhibited up to a 4.2 times higher solubility (1.22 M) than that of M₄[Fe^II(Dcbpy)₃] and a 50% increase in potential compared with that of ferrocyanide. The AORFBs with 0.1 M Na_4M/[Fe^II(Dcbpy)₂(CN)₂] as the catholyte were demonstrated for 6,000 cycles with a capacity fading rate of 0.00158% per cycle (0.217% per day). Even at a concentration near the solubility limit (1 M Na₄[Fe^II(Dcbpy)₂(CN)₂]), the flow battery exhibited a capacity fading rate of 0.008% per cycle (0.25% per day) in the first 400 cycles. The AORFB cell with a nearly 1:1 catholyte:anolyte electron ratio achieved a cell voltage of 1.2 V and an energy density of 12.5 Wh l^–1.

Ismael A. Rodriguez-Perez, Hee-Jung Chang, Matthew Fayette, Bhuvaneswari M. Sivakumar, Daiwon Choi, Xiaolin Li, David Reed."Mechanistic investigation of redox processes in Zn–MnO₂ battery in mild aqueous electrolytes."Journal of Materials Chemistry A 9 (36), 20766-20775 (August 2021).
Abstract: Zinc–MnO₂ based batteries have acquired attention for grid-level applications, due to impressive theoretical performance, cost effectiveness and intrinsic safety. However, there are still many challenges that remain elusive due to the complex and controversial mechanisms of operation that hinder commercialization. In this work, the detailed redox processes that occur at the cathode during Zn–MnO₂ battery operation are elucidated. Using a blend of structural and electrochemical techniques, the redox pairs that occur during operation are mechanistically studied while also showcasing the true impact of the electrolyte additive (0.1 M MnSO₄) in a 1 M ZnSO₄ electrolyte. An electrochemical quartz-crystal microbalance (EQCM) has been leveraged to reveal the effect of zinc hydroxy sulfate salt (Zn₄SO₄(OH)₆·nH₂O) and zinc manganese oxide (Zn_xMn_yO_z) dissolution/deposition, which are believed to be major components during discharge and charge conditions. These results provide insight not currently available, allowing a holistic view of the electrochemical reaction mechanisms during battery operation.

Alasdair J. Crawford, Daiwon Choi, Patrick J. Balducci, Venkat R. Subramanian, Vilayanur V. Viswanathan."Lithium-ion battery physics and statistics-based state of health model."Journal of Power Sources 501, 230032 (July 2021).
Abstract: A pseudo-2d model using COMSOL Multiphysics® software simulates performance degradation of Li-ion batteries when subjected to peak shaving grid service. Multiple degradation pathways are considered, including solid electrolyte interphase (SEI) formation and breakdown, cathode dissolution and its effect on SEI formation. The model is validated by simulating commercial cell performance. We develop a global model simulating performance across all chemistries, along with a model treating chemistries individually. There is good agreement between these two models for various optimization parameters such as SEI equilibrium potential, cathode dissolution exchange current density, solvent diffusivity in the SEI and SEI ionic conductivity. To circumvent time constraints related to the COMSOL model, a 0d global model is developed, which fits data well. Good agreement for various optimization parameters is obtained among the COMSOL global & individual chemistry models and the 0-d model. A top-down, statistics-based model using current, voltage, and anode expansion rate as degradation predictors is developed using insights from the physics-based model. This model predicts degradation for multiple grid services and electric vehicle drive cycles with high accuracy and provides the pathway to develop an efficient battery management system combining machine learning and findings from computationally intensive physics-based algorithms.

Hee-Jung Chang, Ismael A. Rodriguez-Perez, Matthew Fayette, Nathan L. Canfield, Huilin Pan, Daiwon Choi, Xiaolin Li, David Reed."Effects of water-based binders on electrochemical performance of manganese dioxide cathode in mild aqueous zinc batteries."Carbon Energy 3: (3), 473-481 (July 2021).
Abstract: In the majority of rechargeable batteries including lithium-ion batteries, polyvinylidene fluoride (PVdF) binders are the most commonly used binder for both anode and cathode. However, using PVdF binder requires the organic solvent of N-methyl-2-pyrrolidone which is expensive, volatile, combustible, toxic, and has poor recyclability. Therefore, switching to aqueous electrode processing routes with non-toxic binders would provide a great leap forward towards the realization of ideally fully sustainable and environmentally friendly electrochemical energy storage devices. Various water-soluble binders (aqueous binders) were characterized and compared to the performance of conventional PVdF. Our study demonstrates that the electrochemical performance of Zn/MnO₂ aqueous batteries is significantly improved by using sodium carboxymethyl cellulose (CMC) binder. In addition, CMC binders offer desirable adhesion, good wettability, homogeneous material distribution, and strong chemical stability at certain pH levels (3.5–5) without any decomposition for long-cycle life.

Bhuvaneswari M. Sivakumar, Venkateshkumar Prabhakaran, Kaining Duanum, Edwin Thomsen, Brian Berland, Nicholas Gomez, David Reed, Vijayakumar Murugesan."Long-Term Structural and Chemical Stability of Carbon Electrodes in Vanadium Redox Flow Battery."ACS Applied Energy Materials 4: (6), 6074-6081 (June 2021).
Abstract: Predicting the performance decay in carbon electrodes is critical to maximizing the longevity of redox flow battery (RFB) systems. This study investigates the effect of long-term cycling (over 8000 cycles) on the structural and chemical evolution of carbon electrodes. We find that the microstructural aspects such as graphitic stacking order and interlayer spacing along with overall morphological construct remain largely unchanged even after the prolonged cycling process. Conversely, significant changes in surface chemistry such as the evolution of functional groups and point defects are evident from our combined multimodal spectroscopic and computational analysis. The X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analysis reveal chemical absorption of chloride counter anions at point defects within the graphitic surface. Additionally, our results suggest that vanadium cation plays an important role in counter anion–carbon surface interaction and subsequently the surface chemistry evolutions. Our findings provide insights about surface chemical evolution that is critical for predicting electrode performance and longevity of RFB.

Xiaowen Zhan, Minyuan M. Li, J. Mark Weller, Vincent L. Sprenkle, Guosheng Li."Recent Progress in Cathode Materials for Sodium-Metal Halide Batteries."Materials 14: (12), 3260 (June 2021).
Abstract: Transitioning from fossil fuels to renewable energy sources is a critical goal to address greenhouse gas emissions and climate change. Major improvements have made wind and solar power increasingly cost-competitive with fossil fuels. However, the inherent intermittency of renewable power sources motivates pairing these resources with energy storage. Electrochemical energy storage in batteries is widely used in many fields and increasingly for grid-level storage, but current battery technologies still fall short of performance, safety, and cost. This review focuses on sodium metal halide (Na-MH) batteries, such as the well-known Na-NiCl₂ battery, as a promising solution to safe and economical grid-level energy storage. Important features of conventional Na-MH batteries are discussed, and recent literature on the development of intermediate-temperature, low-cost cathodes for Na-MH batteries is highlighted. By employing lower cost metal halides (e.g., FeCl₂, and ZnCl₂, etc.) in the cathode and operating at lower temperatures (e.g., 190 °C vs. 280 °C), new Na-MH batteries have the potential to offer comparable performance at much lower overall costs, providing an exciting alternative technology to enable widespread adoption of renewables-plus-storage for the grid.

Ruozhu Feng, Xin Zhang, Vijayakumar Murugesan, Aaron Hollas, Ying Chen, Yuyan Shao, Eric Walter, Nadeesha P. N. Wellala, Litao Yan, Kevin M. Rosso, Wei Wang."Reversible ketone hydrogenation and dehydrogenation for aqueous organic redox flow batteries." Science 372: (6544), 836-840 (May 2021).
Abstract: Aqueous redox flow batteries with organic active materials offer an environmentally benign, tunable, and safe route to large-scale energy storage. Development has been limited to a small palette of organics that are aqueous soluble and tend to display the necessary redox reversibility within the water stability window. We show how molecular engineering of fluorenone enables the alcohol electro-oxidation needed for reversible ketone hydrogenation and dehydrogenation at room temperature without the use of a catalyst. Flow batteries based on these fluorenone derivative anolytes operate efficiently and exhibit stable long-term cycling at ambient and mildly increased temperatures in a nondemanding environment. These results expand the palette to include reversible ketone to alcohol conversion but also suggest the potential for identifying other atypical organic redox couple candidates.

J. David Bazak, Allison R. Wong, Kaining Duanmu, Kee Sung Han, David Reed, Vijayakumar Murugesan."Concentration-Dependent Solvation Structure and Dynamics of Aqueous Sulfuric Acid Using Multinuclear NMR and DFT."Journal of Physical Chemistry B 125 (19), 5089-5099 (May 2021).
Abstract: Sulfuric acid is a ubiquitous compound for industrial processes, and aqueous sulfate solutions also play a critical role as electrolytes for many prominent battery chemistries. While the thermodynamic literature on it is quite well-developed, comprehensive studies of the solvation structure, particularly molecular-scale dynamical and transport properties, are less available. This study applies a multinuclear nuclear magnetic resonance (NMR) approach to the elucidation of the solvation structure and dynamics over wide temperature (−10 to 50 °C) and concentration (0–18 M) ranges, combining the ¹⁷O shift, line width, and T₁ relaxation measurements, 33S shift and line width measurements, and ¹H pulsed-field gradient NMR measurements of proton self-diffusivity. In conjunction, these results indicate a crossover between two regimes of solvation structure and dynamics, occurring above the concentration associated with the deep eutectic point (∼4.5 ^M), with the high-concentration regime dominated by a strong water–sulfate correlation. This description was borne out in detail by the activation energy trends with increasing concentration derived from the relaxation of both the H₂O/H₃O⁺ and H₂SO₄/HSO₄^-;/SO₄2^2-17O resonances and the 1H self-diffusivity. However, the 17O chemical shift difference between the H₂O/H₃O⁺ and H₂SO₄/HSO₄^2-resonances across the entire temperature range is nevertheless strikingly linear. A computational approach coupling molecular dynamics simulations and density functional theory NMR shift calculations to reproduce this trend is presented, which will be the subject of further development. This combination of multinuclear, dynamical NMR, and computational methods, and the results furnished by this study, will provide a platform for future studies on battery electrolytes where aqueous sulfate chemistry plays a central role in the solution structure.

Junhua Song, Kang Xu, Nian Liu, David Reed, Xiaolin Li."Crossroads in the renaissance of rechargeable aqueous zinc batteries." Materials Today 45: 191-212 (May 2021).
Abstract: Aqueous zinc batteries dominate the primary battery market with alkaline chemistries and recently have been rejuvenated as rechargeable devices to compete for grid-scale energy storage applications. Tremendous effort has been made in the past few years and improved cyclability has been demonstrated in both alkaline, neutral, and mild acidic systems. In this review/perspective, we will elucidate the merits of rechargeable aqueous zinc batteries through side-by-side comparison to Li-ion batteries, examine the challenges and progress made in the pursuit of highly rechargeable alkaline and mild acidic batteries, and finally provide a holistic forward look at the technology. The focus is placed on static closed cell designs, while flow batteries and open systems like zinc-air batteries will not be included due to space constraint.

Nimat Shamim, Edwin C. Thomsen, Vilayanur V. Viswanathan, David Reed, Vincent Sprenkle, Guosheng Li."Evaluating ZEBRA Battery Module under the Peak-Shaving Duty Cycles."Materials 14: (9), 2280 (April 2021).
Abstract: With the recent rapid increase in demand for reliable, long-cycle life, and safe battery technologies for large-scale energy-storage applications, a battery module based on ZEBRA battery chemistry is extensively evaluated for its application in peak shaving duty cycles. First, this module is tested with a full capacity cycle consisting of a charging process (factory default) and a discharging process with a current of 40 A. The battery energy efficiency (discharge vs. charge) is about 90%, and the overall energy efficiency is 80.9%, which includes the auxiliary power used to run the battery management system electronics and self-heating to maintain the module operating temperature (265 °C). Generally, because of the increased self-heating during the holding times that exist for the peak shaving duty cycles, the overall module efficiency decreases slightly for the peak-shaving duty cycles (70.7–71.8%) compared to the full-capacity duty cycle. With a 6 h, peak-shaving duty cycle, the overall energy efficiency increases from 71.8% for 7.5 kWh energy utilization to 74.1% for 8.5 kWh. We conducted long-term cycling tests of the module at a 6 h, peak-shaving duty cycle with 7.5 kWh energy utilization, and the module exhibited a capacity degradation rate of 0.0046%/cycle over 150 cycles (>150 days).

Biwei Xiao, Yichao Wang, Sha Tan, Miao Song, Xiang Li, Yuxin Zhang, Feng Lin, Kee Sung Han, Fredrick Omenya, Khalil Amine, Xiao-Qing Yang, David Reed, Yanyan Hu, Gui-Liang Xu, Enyuan Hu, Xin Li, Xiaolin Li. "Vacancy‐Enabled O3 Phase Stabilization for Manganese‐Rich Layered Sodium Cathodes."Angewandte Chemie International Edition60 (15), 8258-8267 (April 2021).
Abstract: Manganese‐rich layered oxide materials hold great potential as low‐cost and high‐capacity cathodes for Na‐ion batteries. However, they usually form a P2 phase and suffer from fast capacity fade. In this work, an O3 phase sodium cathode has been developed out of a Li and Mn‐rich layered material by leveraging the creation of transition metal (TM) and oxygen vacancies and the electrochemical exchange of Na and Li. The Mn‐rich layered cathode material remains primarily O3 phase during sodiation/desodiation and can have a full sodiation capacity of ca. 220 mAh g⁻¹. It delivers ca. 16 mAh g⁻¹ specific capacity between 2–3.8 V with >86 % retention over 250cycles. The TM and oxygen vacancies pre‐formed in the sodiated material enables a reversible migration of TMs from the TM layer to the tetrahedral sites in the Na layer upon de‐sodiation and sodiation. The migration creates metastable states, leading to increased kinetic barrier that prohibits a complete O3‐P3 phase transition.

Di Wu, Xu Ma."Modeling and Optimization Methods for Controlling and Sizing Grid-Connected Energy Storage: A Review." Current Sustainable/Renewable Energy Reports (March 2021).
Abstract: Energy storage is capable of providing a variety of services and solving a multitude of issues in today’s rapidly evolving electric power grid. This paper reviews recent research on modeling and optimization for optimally controlling and sizing grid-connected battery energy storage systems (BESSs). Open issues and promising research directions are discussed. Recent studies on BESS dispatch, evaluation, and sizing focus on advanced modeling and optimization methods to maximize stacked value streams from multiple services. BESS models have been improved to better represent operational characteristics or capture degradation effects. Different solution methods and optimization techniques have been proposed to improve the benefits and cost-effectiveness of BESSs, using deterministic approaches prevalently but with impressive progress in modeling and addressing uncertainties. Recent progress in BESS scheduling and sizing better supports planning and operational decision-making in different use cases, which is highly important to advance the deployment of BESSs. Additional research is required to properly model the trade-off between short-term benefits and service life with multiple degradation effects explicitly considered in the decision-making process. Advanced methods are to be developed for effectively determining optimal BESS sizes that maximize overall benefits within a varying lifetime considering diversified system conditions, as well as uncertainties at planning and operational stages.

Di Wu, Xu Ma, Patrick Balducci, Dhruv Bhatnagar."An economic assessment of behind-the-meter photovoltaics paired with batteries on the Hawaiian Islands." Applied Energy 286 (March 2021).
Abstract: Due to natural variability and uncertainty, the ever-increasing penetration of solar generation in Hawaii presents challenges to power grid operators to maintain reliable system operation. Demand response (DR) has the potential to be a cost-effective tool for Hawaii to reach its aggressive renewable energy goals while maintaining the reliability of power grids. The Hawaii Public Utilities Commission has approved the Hawaiian Electric Company’s revised portfolio of DR programs. The companies have released a grid services purchase agreement and subscribed an initial tranche of load into their DR programs. This paper presents innovative analytical methods and comprehensive economic assessment for distributed photovoltaics (PV) paired with battery energy storage systems (BESSs) for two new DR programs, including fast frequency response and capacity grid service. Optimal dispatch and sizing methods are proposed for the paired system considering different tariff schedules and PV compensation programs across five islands. It was found that while the best resource configuration and potential economic benefits vary with tariff structure, a BESS paired with PV can be optimally dispatched to generate multiple value streams simultaneously. Compensation from DR programs is an important value stream to help increase the cost-effectiveness of the integrated system.

Vijayakumar Murugesan, Zimin Nie, Xin Zhang, Peiyuan Gao, Zihua Zhu, Qian Huang, Litao Yan, David Reed, Wei Wang."Accelerated design of vanadium redox flow battery electrolytes through tunable solvation chemistry."Cell Reports Physical Science 2 (2), 100323 (February 2021).
Abstract: Operational stability of electrolytes is a persistent impediment in building redox flow battery technology. Stabilizing multiple vanadium oxidation states in aqueous solution is a primary challenge in designing reliable large-scale vanadium redox flow batteries (VRBs). Here we demonstrate that rationally selected ionic additives can stabilize the aqua vanadium solvate structures through preferential bonding and molecular interactions despite their relatively low concentrations (≤0.1 M). The competing cations (NH₄⁺ and Mg²⁺) and bonding anions (SO₄²⁻, PO₄³⁻, and Cl⁻) introduced by bi-additives are used to tune the vanadium solvation chemistry and design an optimal electrolyte for VRB technology. Such molecular engineering of VRB electrolytes results in enhancement of the operational temperature window by 180% and energy density by more than 30% relative to traditional electrolytes. This work demonstrates that tunable solvation chemistry is a promising pathway to engineer an optimal electrolyte for targeted electrochemical systems.

Xiang Liu, Biwei Xiao, Amine Daali, Xinwei Zhou, Zhou Yu, Xiang Li, Yuzi Liu, Liang Yin, Zhenzhen Yang, Chen Zhao, Likun Zhu, Yang Ren, Lei Cheng, Shabbir Ahmed, Zonghai Chen, Xiaolin Li, Gui-Liang Xu, Khalil Amine."Stress- and Interface-Compatible Red Phosphorus Anode for High-Energy and Durable Sodium-Ion Batteries."ACS Energy Letters 6, 547-556 (February 2021).
Abstract: Sodium-ion batteries are promising candidates for energy storage application, but the absence of high-capacity and low-cost anode materials significantly limits their practical specific energy and cost. Red phosphorus (RP) possesses a high theoretical specific capacity but suffers from large volume change, low electronic conductivity, and unstable solid-electrolyte interphase (SEI). Herein, a hierarchical micro/nanostructured antimony-doped RP/carbon anode was developed, which demonstrates extraordinary electrochemical performance with high initial Coulombic efficiency of ∼90%, high areal capacity (∼1.7 mAh cm^–2), and good cycle stability and rate capability. Combined experimental and computational studies consistently revealed that such a unique structural design can dramatically accommodate the mechanical stress and moreover effectively restrain the undesired decomposition of electrolyte solvents regardless of electrolyte formulation, resulting in superior structural integrity and thin and robust SEI formation during cycling. The present finding has offered an alternative strategy for stress management and interface engineering on high-capacity alloying-based anode materials.

Minyuan M. Li, Xiaochuan Lu, Xiaowen Zhan, Mark H. Engelhard, Jeffrey F. Bonnett, Evgueni Polikarpov, Keeyoung Jung, David M. Reed, Vincent Sprenkle, Guosheng Li."High performance sodium-sulfur batteries at low temperature enabled by superior molten Na wettability."Chemical Communications 57 (1) 45-48 (January 2021).
Abstract: Reducing the operating temperature of molten sodium-sulfur batteries (~350 °C) is critical to create safe and cost-effective devices for large-scale energy storage. By raising the surface treatment temperature with lead acetate trihydrate, we can significantly improve sodium wettability on ß"-Al₂O₃ solid electrolyte at a low temperature of 120 °C, previously unattained. In turn, the Na S cell can reach a capacity as high as 520.2mAh/g and stable cycling over 1000 cycles at 120 °C, which is slightly higher than the melting point of sodium (98 °C). Analyzing surfaces treated at different temperatures, the deposited Pb particles show similar morphologies but distinct compositions, inferring a strong correlation between passivation and performance.

Maitri Uppaluri, Akshay Subramaniam, Lubhani Mishra, Vilayanur Viswanathan, Venkat R. Subramanian."Can a Transport Model Predict Inverse Signatures in Lithium Metal Batteries Without Modifying Kinetics?"Journal of Electrochemical Society 167, number 16, article number 160547 (December 2020).
Abstract: In this study, a one-dimensional transport model is developed and analyzed to predict the inverse overpotential signature observed during lithium metal electrodeposition. This simple approach predicts inverse signatures stemming from the competing interplay between moving boundary rates and mass transfer limitations. The numerical scheme used for the present model simulations is presented in detail which has been further used to study the effect of design parameters on the prevalence and strength of inverse signatures. It was found that the proposed model and the analysis is more pertinent to thick lithium symmetric cells, commonly used for in-depth fundamental studies.

Qian Huang, Bin Li, Chaojie Song, Zhengming Jiang, Alison Platt, Khalid Fatih, Christina Bock, Darren Jang, David Reed. "In Situ Reliability Investigation of All-Vanadium Redox Flow Batteries by a Stable Reference Electrode."Journal of Electrochemical Society 165, number 16, article number 160541 (December 2020).
Abstract: Redox flow batteries (RFBs) have been studied over the past several decades as a promising candidate for stationary energy storage applications. It is therefore important to understand the reliability of RFBs and the mechanisms that cause degradation with time. Contributions from individual electrodes are difficult to separate especially for long-term cycle testing due to the lack of a stable reference electrode. In our work, the reliability and degradation mechanisms of an all-vanadium RFB were investigated by a stable reference electrode based on the dynamic hydrogen electrode (DHE). The newly developed DHE reference electrode demonstrated high accuracy and long-term stability that enables in situ monitoring of individual electrode signals over hundreds of cycles in a vanadium RFB. This approach enables the full cell degradation to be separated into contributions from the cathode and anode. The cathode and anode were found to play quite different roles in the increase in overpotential of the vanadium RFB during long-term cycling. The anode reaction limited both the charge and discharge capacity over 100 cycles. The negative side also appeared to be the rate limiting factor throughout cycling as determined by EIS measurement. The cathode contributed to the performance degradation as cycling exceeded 50 cycles.

Patrick Balducci, Kendall Mongird, Di Wu, Dexin Wang, Vanshika Fotedar, Robert Dahowski."An Evaluation of the Economic and Resilience Benefits of a Microgrid in Northampton, Massachusetts."Energies 13 (18), 4802 (September 2020).
Abstract: Recent developments and advances in distributed energy resource (DER) technologies make them valuable assets in microgrids. This paper presents an innovative evaluation framework for microgrid assets to capture economic benefits from various grid and behind-the-meter services in grid-connecting mode and resilience benefits in islanding mode. In particular, a linear programming formulation is used to model different services and DER operational constraints to determine the optimal DER dispatch to maximize economic benefits. For the resiliency analysis, a stochastic evaluation procedure is proposed to explicitly quantify the microgrid survivability against a random outage, considering uncertainties associated with photovoltaic (PV) generation, system load, and distributed generator failures. Optimal coordination strategies are developed to minimize unserved energy and improve system survivability, considering different levels of system connectedness. The proposed framework has been applied to evaluate a proposed microgrid in Northampton, Massachusetts that would link the Northampton Department of Public Works, Cooley Dickenson Hospital, and Smith Vocational Area High School. The findings of this analysis indicate that over a 20-year economic life, a 441 kW/441 kWh battery energy storage system, and 386 kW PV solar array can generate $2.5 million in present value benefits, yielding a 1.16 return on investment ratio. Results of this study also show that forming a microgrid generally improves system survivability, but the resilience performance of individual facilities varies depending on power-sharing strategies.

Xiaowen Zhan, Jeffrey F. Bonnett, Mark H. Engelhard, David M. Reed, Vincent L. Sprenkle, Guosheng Li."A High‐Performance Na–Al Battery Based on Reversible NaAlCl₄ Catholyte."Advanced Energy Materials Article number 2001378 (September 2020).
Abstract: This work demonstrates a high‐capacity and safe Na–Al battery pairing a sodium metal anode and reversible NaAlCl₄ catholyte for grid scale energy storage applications. The energy‐rich Na anode allows the full use of the aluminum cathode, resulting in a full‐cell capacity of 308 mAh g⁻¹ at a discharge voltage of 1.6 V. Benefiting from the use of a β″‐alumina solid electrolyte, molten sodium anode, and reversible Al deposition/stripping from NaAlCl4 catholyte, the battery presents a stable Coulombic efficiency of 100% and energy efficiency of ≈95%. At a rate of C/3 (6.77 mA cm⁻²), the cell maintains 282 mAh g⁻¹ (447 Wh kg⁻¹) after 200 cycles with an excellent capacity retention of 97.6%. Moreover, pathways to build Na‐anode‐free cells from the discharged state under dry air are elucidated, which further extends the feasibility of this battery for stationary storage applications. These findings are expected to provide a new platform for the development of practical aluminum batteries.

Matthew Fayette, Hee-Jung Chang, Ismael A. Rodriguez-Perez, Xiaolin Li, David Reed."Electrodeposited Zinc-Based Films as Anodes for Aqueous Zinc Batteries."ACS Applied Materials & Interfaces 12 (38) 42763-42772 (August 2020).
Abstract: Zinc-based batteries have attracted extensive attention in recent years, due to high safety, high capacities, environmental friendliness, and low cost compared to lithium-ion batteries. However, the zinc anode suffers primarily from dendrite formation as a mode of failure in the mildly acidic system. Herein, we report on electrochemically deposited zinc (ED Zn) and copper–zinc (brass) alloy anodes, which are critically compared with a standard commercial zinc foil. The film electrodes are of commercially relevant thicknesses (21 and 25 μM). The electrodeposited zinc-based anodes exhibit low electrode polarization (∼0.025 V) and stable cycling performance in 50 cycle consecutive experiments from 0.26 to 10 mA cm^–2 compared to commercial Zn foil. Coulombic efficiencies at 1 mA cm^–2 were over 98% for the electrodeposited zinc-based materials and were maintained for over 100 cycles. Furthermore, full cells with an electrodeposited Zn/brass anode, electrolytic manganese dioxide (EMD) cathode, in 1 M ZnSO₄ + 0.1 M MnSO₄ delivered capacities of 96.3 and 163 mAh g^–1, respectively, at 100 mA g^–1 compared to 92.1 mAh g^–1 for commercial Zn. The electrodeposited zinc-based anodes also show better rate capability, delivering full cell capacities of 35.9 and 47.5 mAh g^–1 at a high current of up to 3 A g^–1. Lastly, the electrodeposited zinc-based anodes show enhanced capacity for up to 100 cycles at 100 mA g^–1, making them viable anodes for commercial use.

Ke Lu, Bomin Li, Xiaowen Zhan, Fan Xia, Olusola J. Dahunsi, Siyuan Gao, David M. Reed, Vincent L. Sprenkle, Guosheng Le, Yingwen Cheng."Elastic Na_xMoS₂-Carbon-BASE Triple Interface Direct Robust Solid–Solid Interface for All-Solid-State Na–S Batteries."Nano Letters 20:(9), 6837-6844 (August 2020).
Abstract: The developments of all-solid-state sodium batteries are severely constrained by poor Na-ion transport across incompatible solid–solid interfaces. We demonstrate here a triple Na_xMoS₂-carbon-BASE nanojunction interface strategy to address this challenge using the β″-Al₂O₃ solid electrolyte (BASE). Such an interface was constructed by adhering ternary Na electrodes containing 3 wt % MoS₂ and 3 wt % carbon on BASE and reducing contact angles of molten Na to ∼45°. The ternary Na electrodes exhibited twice improved elasticity for flexible deformation and intimate solid contact, whereas Na_xMoS₂ and carbon synergistically provide durable ionic/electronic diffusion paths, which effectively resist premature interface failure due to loss of contact and improved Na stripping utilization to over 90%. Na metal hosted via triple junctions exhibited much smaller charge-transfer resistance and 200 h of stable cycling. The novel interface architecture enabled 1100 mAh/g cycling of all-solid-state Na–S batteries when using advanced sulfur cathodes with Na-ion conductive PEO_10-NaFSI binder and Na_xMo₆S₈ redox catalytic mediator.

Xiaowen Zhan, Xiaochuan Lu, David M. Reed, Vincent L. Sprenkle, and G. Li."Emerging soluble organic redox materials for next-generation grid energy-storage applications."MRS Communications 10 (2):215-229 (June 2020).
Abstract: Because of their structural versatility, fast redox reactivity, high storage capacity, sustainability, and environmental friendliness, soluble organic redox molecules have emerged as materials that have potential for use in energy-storage systems. Considering these advantages, this paper reviews recent progress in implementing such materials in aqueous soluble organic redox flow batteries and organic alkali metal/air batteries. We identify and discuss major challenges associated with molecular structures, cell configurations, and electrochemical parameters. Hopefully, we provide a general guidance for the future development of soluble organic redox materials for emerging energy-storage devices used in the electricity grid.

Biwei Xiao."Intercalated water in aqueous batteries."Carbon Energy 2 (2):251-264 (May 2020).
Abstract: The unprecedentedly growing demand for energy storage devices in recent years calls for diversified chemistries with unique advantages. When it comes to safety and cost, aqueous battery systems have attracted tremendous attention. Owing to its small size, high polarity, and hydrogen bonding, water in the electrode materials, either in the form of structural water or cointercalated hydrated cations, drastically change the electrochemical behavior through multiple aspects. This review discusses the roles of water in aqueous batteries from how water molecules coordinate with cations to examples of water‐mediated reactions in different types of host materials.

Yulun Zhang, Yuxiao Lin, Lingfeng He, Vijayakumar Murugesan, Gorakh Pawar, Bhuvana M. Sivakumar, Hanping Ding, Dong Ding, Boryann Liaw, Eric J. Dufek, and Bin Li."Dual Functional Ni₃S₂@Ni Core–Shell Nanoparticles Decorating Nanoporous Carbon as Cathode Scaffolds for Lithium–Sulfur Battery with Lean Electrolytes."dACS Applied Energy Materials3 (5):4173-4179 (May 2020).
Abstract: Lithium–sulfur batteries are very promising for next-generation energy storage. However, most studies use flooded electrolytes to achieve a high specific capacity at the expense of lowering the specific energy. Understanding lithium–sulfur battery performance with lean electrolytes is highly desirable. Herein, a modified Pechini method is developed to synthesize a nanoporous carbon host decorated with Ni₃S₂@Ni particles. Such a cathode delivers enhanced specific capacities with extended cycling life in lean electrolytes, due to the dual functions of the Ni₃S₂ shell, which can both facilitate reaction kinetics and promote electrolyte wetting. This work highlights a strategy to rationally design cathodes for high-energy lithium–sulfur batteries.

Di Wu, Xu Ma, Sen Huang, Tao Fu, Patrick Balducci."Stochastic optimal sizing of distributed energy resources for a cost-effective and resilient Microgrid."Energy 198 (May 2020).
Abstract: Recent developments and advances in distributed energy resources (DERs) make them more affordable, accessible, and prevalent in microgrids. Research on designing and operating a microgrid with various DERs has received increasing attention during the past few years. This paper proposes a two-stage stochastic mixed-integer programming method for jointly determining optimal sizes of various DERs, considering both economic benefits and resilience performance. The proposed method explicitly models the interaction between DER sizing at the planning stage and hourly or sub-hourly microgrid dispatch at the operating stage in both grid-connected and island modes, considering stochastic grid disturbances, load, and renewable generation. A formulation method is then proposed to convert the stochastic sizing problem to an equivalent mix-integer linear programming problem, which can be efficiently solved even with a large number of system operating conditions. Using the proposed stochastic sizing method, a resource planning analysis for a military base in the U.S. is presented. It is found that the proposed method can effectively determine the optimal DER sizes to meet a required resilience goal at the maximum net-benefit. Impacts of several key factors including tariff rates, discount rate, and survivability level on optimal DER sizes are analyzed through case studies.

Junhua Song, Kuan Wang, Jianming Zheng, Mark H. Engelhard, Biwei Xiao, Enyuan Hu, Zihua Zhu, Chongmin Wang, Manling Sui, Yuehe Lin, David Reed, Vincent Sprenkle, Pengfei Yan, Xiaolin Li."Controlling Surface Phase Transition and Chemical Reactivity of O3-Layered Metal Oxide Cathodes for High-Performance Na-Ion Batteries."ACS Energy Letter 5 6: 1718-1725 (April 2020).
Abstract: O3-layered metal oxides are promising cathode materials for high-energy Na-ion batteries (SIBs); however, they suffer from fast capacity fade. Here, we develop a high-performance O3-NaNi_0.68Mn_0.22Co_0.10O₂ cathode for SIBs toward practical applications by suppressing the formation of a rock salt layer at the cathode surface with an advanced electrolyte. The cathode can deliver a high specific capacity of ∼196 mAh g^–1 and demonstrates >80% capacity retention over 1000 cycles. NaNi_0.68Mn_0.22Co_0.10O₂ hard carbon full-cells with practical loading (>2.5 mAh cm^–2) and lean electrolyte (∼40 μL) demonstrate ∼82% capacity retention after 450 cycles. A 60 mAh single-layer pouch cell has also been fabricated and demonstrated stable performance. This work represents a significant leap in SIB development and brings new insights to the development of advanced layered metal oxide cathodes for alkaline-ion batteries.

Jan Alam, Patrick Balducci, Kevin Whitener, Steve Cox."Energy Storage Control Capability Expansion: Achieving Better Technoeconomic Benefits at Portland General Electric's Salem Smart Power Center."IEEE Power and Energy Magazine 18 (2) (March 2020).
Abstract: The value proposition for energy storage systems (ESSs) is a key topic for creating and advancing its acceptance within the electric power sector, particularly for electric utilities. Although ESS as a technology is gaining popularity within the electric utility industry, its anticipated value streams are not fully understood, quantified, and demonstrated. The unavailability of suitable demonstration sites/projects, the lack of a deep understanding of available economic opportunities, and the deployment complexities associated with pursuing those opportunities are some of the reasons that complicate its value demonstration. The lessons learned from holistic demonstration projects covering key steps, e.g., economic value stream identification, evaluation, and its subsequent realization via suitable control strategies, could help electric utilities learn to manage ESS adoption challenges better.

Xiaowen Zhan, Mark E. Bowden, Xiaochuan Lu, Jeffery F. Bonnett, Teresa Lemmon, David M. Reed, Vincent L. Sprenkle, and Guosheng L. " A Low‐Cost Durable Na‐FeCl₂ Battery with Ultrahigh Rate Capability " Advanced Energy Materials (March 2020).
Abstract:Na‐based batteries have long been regarded as an inexpensive, sustainable candidate for large‐scale stationary energy storage applications. Unfortunately, the market penetration of conventional Na‐NiCl₂ batteries is approaching its limit for several reasons, including limited rate capability and high Ni cost. Herein, a Na‐FeCl₂ battery operating at 190 °C is reported that allows a capacity output of 116 mAh g^-1 at an extremely high current density of 33.3 mA cm^-2 (≈0.6C). The superior rate performance is rooted in the intrinsically fast kinetics of the Fe/Fe²⁺ redox reaction. Furthermore, it is demonstrated that a small amount of Ni additive (10 mol%) effectively mitigates capacity fading of the Fe/NaCl cathode caused by Fe particle pulverization during long‐term cycling. The modified Fe/Ni cathode exhibits excellent cycling stability, maintaining a discharge energy density of over 295 Wh kg^-1 for 200 cycles at 10 mA cm^-2 (≈C/5).

Zhan X., J.P. Sepulveda, X. Lu, J.F. Bonnett, N.L. Canfield, T.L. Lemmon, and K. Jung.D. Reed, V. Sprenkle, G. Li. " Elucidating the role of anionic chemistry towards high-rate intermediate-temperature Na-metal halide batteries." Energy Storage Materials 24:177-187 (January. 2020).
Abstract:Sodium (Na)-based battery technologies that are economical (because Na is abundant) and have long cycle life are gaining importance for large-scale energy storage applications. Among the widely studied Na-based battery systems, intermediate-temperature (IT) Na-metal halide (Na-MH) batteries have demonstrated several advantages over conventional high-temperature Na batteries, including superior battery safety, lower operating temperature and manufacturing cost, potentially longer cycle life, and easier assembly. However, the rate performance of IT Na-MH batteries is inevitably affected by the lower operating temperature. In pursuit of faster charge-transfer reaction kinetics, we extended our studies of cathode materials beyond the extensively investigated NiCl2 to NiBr2 (NaBr/Ni) and NiI2 (NaI/Ni) compounds. We systematically investigated the synergetic effects of anion chemistry on the electrochemical properties. Surprisingly, among three tested cathodes, the NaBr/Ni cathode showed the highest energy density of 174 Wh/kg at 33.3 mA/cm2 (~0.8C), which is 2.5 and 1.9 times higher than those of NaCl/Ni and NaI/Ni cells. We explored the underlying enhancement mechanism in great detail via multiple structural characterization and electrochemical techniques. The sodium-halide salt dissolution in molten NaAlCl4 was found to be the determining factor in rate improvement. Our findings will greatly advance IT Na-MH battery technologies and pave the way towards fundamental understanding of reaction kinetics for high-temperature batteries in general.

Xiaoqin Zang, Litao Yan, Yang Yang, Huilin Pan, Zimin Nie, Ki Won Jung,Zhiqun Daniel Deng, and Wei Wang."Monitoring the State‐of‐Charge of a Vanadium Redox Flow Battery with the Acoustic Attenuation Coefficient: An In Operando Noninvasive Method."Small Methods (December 2019).
Abstract: Redox flow battery technology has been increasingly recognized as a promising option for large‐scale grid energy storage. Access to high‐fidelity information on the health status of the electrolyte, including the state‐of‐charge (SOC), is vital to maintaining optimal and economical battery operation. In this study, an ultrasonic probing cell that can be used to measure SOC in real time is designed. This unprecedented, new measurement approach overcomes the influence of varying temperatures by measuring the acoustic attenuation coefficient of the redox flow battery electrolyte online and noninvasively. The new approach is used to estimate the SOC of a vanadium redox flow battery in operando from measured acoustic properties. The accuracy of the SOC estimated from the acoustic properties is validated against SOC calculated by the titration method. The results show that the acoustic attenuation coefficient is a robust parameter for SOC monitoring, with a maximum error of 4.8% and extremely low sensitivity to temperature, while sound speed appears to be less accurate in the benchmark‐inference method, with a maximum error of 22.5% and high sensitivity to temperature. The acoustic measurement approach has great potential for inexpensive real‐time SOC monitoring of redox flow battery operations.

Jie Bao, Vijayakumar Murugesan, Carl Justin Kamp, Yuyan Shao, Litao Yan, Wei Wang."Machine Learning Coupled Multi‐Scale Modeling for Redox Flow Batteries."Advanced Theory and Simulations 3 (2):article number 1900167 (November 2019).
Abstract: The framework of a multi‐scale model that couples a deep neural network, a widely used machine learning approach, with a partial differential equation solver and provides understanding of the relationship between the pore‐scale electrode structure reaction and device‐scale electrochemical reaction uniformity within a redox flow battery is introduced. A deep neural network is trained and validated using 128 pore‐scale simulations that provide a quantitative relationship between battery operating conditions and uniformity of the surface reaction for the pore‐scale sample. Using the framework, information about surface reaction uniformity at the pore level to combined uniformity at the device level is upscaled. The information obtained using the framework and deep neural network against the experimental measurements is also validated. Based on the multi‐scale model results, a time‐varying optimization of electrolyte inlet velocity is established, which leads to a significant reduction in pump power consumption for targeted surface reaction uniformity but little reduction in electric power output for discharging. The multi‐scale model coupled with the deep neural network approach establishes the critical link between the micro‐structure of a flow‐battery component and its performance at the device scale, thereby providing rationale for further operational or material optimization.

Ke Lu, Siyuan Gao, Guosheng Li, Jacob Kaelin, Zhengcheng Zhang, Yingwen Cheng."Regulating Interfacial Na-Ion Flux via Artificial Layers with Fast Ionic Conductivity for Stable and High-Rate Na Metal Batteries."ACS Materials Letters 1: 303-309 (August 2019).
Abstract: Metallic Na electrodes are promising anodes for low-cost and high-energy density batteries due to their natural abundance and high specific capacity. Unfortunately, they are extremely reactive and spontaneously form unstable solid-electrolyte interphases, which lead to critical challenges including growth of dendritic/mossy Na structures and fast degradation. We report here the design of artificial interphase films that have intrinsic high Na⁺-ion conductivity, which enable protected Na electrodes with simultaneously improved surface stability and redox kinetics. They were prepared from Mo₆S₈ films, which transform to Na_xMo₆S₈ (x ≈ 16) through an in-situ sodiation process when pressed onto Na metal. The protected Na electrodes were stable in dry air for days and exhibited 2.5 times higher exchange current density compared with pristine Na electrodes. They enabled symmetric batteries with stable cycling for 1200 h at 0.5 mA cm^–2 and fast Na metal batteries with substantially improved high-rate performance and robust durability for 1000 cycles.

Twitchell JB."A Review of State-Level Policies on Electrical Energy Storage."Current Sustainable/Renewable Energy Reports 6 (2): 35-41 (June 2019).
Abstract: Since California adopted its energy storage mandate in 2013, 14 other states have developed energy storage policies designed to encourage adoption or reduce barriers. This paper reviews those efforts to identify what types of policies are being developed, the underlying goals and rationale behind different approaches, and the early outcomes of those policies. State activity related to energy storage has accelerated in recent years, and numerous policies have emerged. Generally, those policies take one of two approaches: facilitating operational experience with energy storage by ensuring its presence on the grid, or enabling future deployments by removing or reducing barriers. Through detailed review of state policy actions, this paper explores the drivers, design, and implementation of these five specific types of energy storage policy. A taxonomy of state policies related to energy storage is presented, as well as recent research findings that support the different approaches and specific examples of how, where, and why those policies have been implemented. Finally, early impacts of these policies are considered.

Biwei Xiao, Kuan Wang, Gui-Liang Xu, Junhua Song, Zonghai Chen, Khalil Amine, David Reed, Maling Sui, Vincent Sprenkle, Yang Ren, Pengfei Yan, Xiaolin Li."Revealing the Atomic Origin of Heterogeneous Li‐Ion Diffusion by Probing Na."Advanced Materials 31 (29): 1005889 (May 2019).
Abstract: Tracing the dynamic process of Li‐ion transport at the atomic scale has long been attempted in solid state ionics and is essential for battery material engineering. Approaches via phase change, strain, and valence states of redox species have been developed to circumvent the technical challenge of direct imaging Li; however, all are limited by poor spatial resolution and weak correlation with state‐of‐charge (SOC). An ion‐exchange approach is adopted by sodiating the delithiated cathode and probing Na distribution to trace the Li deintercalation, which enables the visualization of heterogeneous Li‐ion diffusion down to the atomic level. In a model LiNi_1/3Mn_1/3Co_1/3O₂ cathode, dislocation‐mediated ion diffusion is kinetically favorable at low SOC and planar diffusion along (003) layers dominates at high SOC. These processes work synergistically to determine the overall ion‐diffusion dynamics. The heterogeneous nature of ion diffusion in battery materials is unveiled and the role of defect engineering in tailoring ion‐transport kinetics is stressed.

Vijayakumar Murugesan, Jong Soo Cho, Niranjan Govind, Amity Andersen, Matthew J. Olszta, Kee Sung Han, Guosheng Li, Hongkyung Lee, David M. Reed, Vincent L. Sprenkle, Sungjin Cho, Satish K. Nune, Daiwon Choi."Lithium Insertion Mechanism in Iron Fluoride Nanoparticles Prepared by Catalytic Decomposition of Fluoropolymer."ACS Applied Energy Materials 2 (3): 1832-1843 (March 2019).
Abstract: Metal fluorides with high redox potential and capacity from strong metal–fluoride bond and conversion reaction make them promising cathodic materials. However, detailed lithium insertion and extraction mechanisms have not yet been clearly understood and explained. Here we report low-temperature synthesis of electrochemically active FeF₃/FeF₂ nanoparticles by catalytic decomposition of a fluoropolymer [perfluoropolyether (PFPE)] using a hydrated iron oxalate precursor both in air and in inert atmosphere. Freshly synthesized FeF₃ nanoparticle delivered specific capacity above 210 mAh/g with decent cycling performance as a Li-ion battery cathode. Both in situ and ex situ characterization techniques were used to investigate the detailed PFPE decomposition and fluorination mechanisms leading to FeF₃/FeF₂ formation as well as the lithium insertion mechanism in a FeF₃ cathode. Specifically, a detailed understanding was investigated using thermogravimetry–mass spectroscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, nuclear magnetic resonance, transmission electron microscopy, scanning electron microscopy/energy dispersive spectroscopy, and X-ray absorption near-edge structure. The novel synthesis route developed not only offers access to electrochemically active metal fluorides but also offers a catalytic approach for decomposing highly inert fluoropolymers for environmental protection.

Dana Jin, Sangjin Choi, Woosun Jang, Aloysius Soon, Jeongmin Kim, Hongjae Moon, Wooyoung Lee, Younki Lee, Sori Son, Yoon-Cheol Park, Hee-Jung Chang, Guosheng Li, Keeyoung Jung, Wooyoung Shim."Bismuth Islands for Low-Temperature Sodium-Beta Alumina Batteries."ACS Applied Materials & Interfaces 11 (3):2917-2924 (January 2019).
Abstract: Wetting of the liquid metal on the solid electrolyte of a liquid metal battery controls the operating temperature and performance of the battery. Liquid sodium electrodes are particularly attractive because of their low cost, natural abundance, and geological distribution. However, they wet poorly on a solid electrolyte near its melting temperature, limiting their widespread suitability for low-temperature batteries to be used for large-scale energy storage systems. Herein, we develop an isolated metal-island strategy that can improve sodium wetting in sodium-beta alumina batteries that allows operation at lower temperatures. Our results suggest that in situ heat treatment of a solid electrolyte followed by bismuth deposition effectively eliminates oxygen and moisture from the surface of the solid electrolyte, preventing the formation of an oxide layer on the liquid sodium, leading to enhanced wetting. We also show that employing isolated bismuth islands significantly improves cell performance, with cells retaining 94% of their charge after the initial cycle, an improvement over cells without bismuth islands. These results suggest that coating isolated metal islands is a promising and straightforward strategy for the development of low-temperature sodium-β alumina batteries.

Biwei Xiao, Teofilo Rojo, Xiaolin Li."Hard Carbon as Sodium‐Ion Battery Anodes: Progress and Challenges."ChemSusChem 12 (1):133-144 (January 2019).
Abstract: Hard carbon (HC) is the state‐of‐the‐art anode material for sodium‐ion batteries due to its excellent overall performance, wide availability, and relatively low cost. Recently, tremendous effort has been invested to elucidate the sodium storage mechanism in HC, and to explore synthetic approaches that can enhance the performance and lower the cost. However, disagreements remain in the field, particularly on the fundamental questions of ion transfer and storage and the ideal HC structure for high performance. This Minireview aims to provide an analysis and summary of the theoretical limitations of HC, discrepancies in the storage mechanism, and methods to improve the performance. Finally, future research on developing ideal structured HCs, advanced electrolytes, and optimized electrolyte–electrode interphases are proposed on the basis of recent progress.

Junhua Song, Dongdong Xiao, Jaiping Jia, Guomin Zhu, Mark Engelhard, Biwei Xiao, Shuo Feng, Dongsheng Li, David Reed, Vincent Sprenkle, Yuehe Lin, Xiaolin Li."A comparative study of pomegranate Sb@C yolk–shell microspheres as Li and Na-ion battery anodes."Nanoscale 11 (1): 348-355 (January 2019).
Abstract: Alloy-based nanostructure anodes have the privilege of alleviating the challenges of large volume expansion and improving the cycling stability and rate performance for high energy lithium- and sodium-ion batteries (LIBs and SIBs). Yet, they face the dilemma of worsening the parasitic reactions at the electrode–electrolyte interface and low packing density for the fabrication of practical electrodes. Here, pomegranate Sb@C yolk–shell microspheres were developed as a high-performance anode for LIBs and SIBs with controlled interfacial properties and enhanced packing density. Although the same yolk–shell nanostructure (primary particle size, porosity) and three-dimensional architecture alleviated the volume change induced stress and swelling in both batteries, the SIBs show 99% capacity retention over 200 cycles, much better than the 78% capacity retention of the LIBs. The comparative electrochemical study and X-ray photoelectron spectroscopy characterization revealed that the different SEIs, besides the distinct phase transition mechanism, played a critical role in the divergent cycling performance.

Xiaochuan Lu, Hee-Jung Chang, Jeffery F. Bonnett, Nathan L. Canfield, Keeyoung Jung, Vincent L. Sprenkle, Guosheng Li."An Intermediate-Temperature High-Performance Na–ZnCl₂ Battery."ACS Omega 3 (11): 15702-15708 (November 2018).
Abstract: The Na−β-alumina battery (NBB) is one of the most promising energy storage technologies for integrating renewable energy resources into the grid. In the family of NBBs, Na–NiCl₂ battery has been extensively studied during the past decade because it has a lower operating temperature, better safety, and good battery performance. One of the major issues with the Na–NiCl₂ battery is material cost, which is primarily from Ni metal in the battery cathode. As an alternative, Zn is much cheaper than Ni, and replacing Ni with Zn in the cathode can significantly reduce the cost. In this work, we investigate the performance and reaction mechanism for a Na–ZnCl₂ battery at 190 °C. Two-step reversible reactions are identified. During the first step of charging, NaCl reacts with Zn to produce a ribbon-type Na₂ZnCl₄ layer. This layer is formed at the NaCl–Zn interface rather than covering the surface of the Zn particles, which leads to an excellent cell rate capability. During the second step, the produced Na₂ZnCl₄ is gradually consumed to form ZnCl₂ on the surface of Zn particles. The formed ZnCl₂ covers most of the surface area of the Zn particles and shows a limited rate capability compared to that of the first step. We conclude that this limited performance of the second step is due to the passivation of Zn particles by ZnCl₂, which blocks the electron pathway of the NaCl–Zn cathodes.

Pengcheng Xu, Congxin Xie, Chenhui Wang, Qinzhi Lai, Wei Wang, Huamin Zhang, Xianfeng Li."A membrane-free interfacial battery with high energy density."Chemical Communications 82 (54):11262-11629 (October 2018).
Abstract: A new concept of the membrane-free interfacial battery based on a biphasic system was proposed for the first time. An aqueous ZnBr₂ solution was used as a negative electrolyte, while Br₂ in CCl₄ served as a positive electrolyte. This interfacial Zn/Br₂ battery demonstrated a very impressive performance with a CE of 96% and an EE of 81% at a current density of 15 mA cm⁻².

Hee-Jung Chang, Xiaochuan Lu, Jeffery F. Bonnett, Nathan L. Canfield, Keesung Han, Mark H. Engelhard, Keeyoung Jung, Vincent L. Sprenkle, Guosheng Li."Decorating β′′-alumina solid-state electrolytes with micron Pb spherical particles for improving Na wettability at lower temperatures."Journal of Materials Chemistry A 6 (40)19703-19711 (October 2018).
Abstract: Overcoming poor physical contact is one of the most critical hurdles for batteries using solid-state electrolytes. In particular, overpotential from the liquid–solid interface between molten sodium and a β′′-alumina solid-state electrolyte (BASE) in a sodium–metal halide (Na–MH) battery could be enormous at lower operating temperatures (<200 °C) due to intrinsically poor Na wetting on the BASE surface. In this work, we describe how surface modification with lead acetate trihydrate (LAT) at different temperatures affects Na wetting on BASEs. LAT treatment conducted at a temperature of 400 °C (under a nitrogen gas atmosphere) shows significantly better Na wettability and battery performance than treatments at lower temperatures. The formation of a unique morphology—micron-sized Pb spherical particles—is observed on the surface of the BASE LAT treated at 400 °C. We also observed evolution of the Na wetting configuration from a Cassie drop, to a Wenzel drop, and finally to a sunny-side-up drop, which is clearly different from the Young–Dupré relation, with increasing the contact-angle measurement temperature. We conclude that formation of a thin Na penetrating film (sunny-side-up shape) on Pb-decorated BASEs is crucial for achieving good battery performance at lower operating temperatures. The new observations and fundamental understanding of Na wetting reported here will provide excellent guidance for improving cell performance in general and will further promote development of practical Na–MH battery technologies for large-scale energy storage applications.

Wentao Duan, Bin Li, Dongping Lu, Xiaoliang Wei, Zimin Nie, Vijayakumar Murugesan, James P. Kizewski, Aaron Hollas, David Reed, Vincent Sprenkle, Wei Wang."Towards an all-vanadium redox flow battery with higher theoretical volumetric capacities by utilizing the VO²⁺/V³⁺ couple."Journal of Energy Chemistry 27 (5): 1381-1385 (September 2018).
Abstract: An all-vanadium redox flow battery with V(IV) as the sole parent active species is developed by accessing the VO²⁺/V³⁺ redox couple. These batteries, referred to as V4RBs, possess a higher theoretical volumetric capacity than traditional VRBs. Copper ions were identified as an effective additive to boost the battery performance.

Biwei Xiao, Fernando Soto, Meng Gu, Kee Sung Han, Junhua Song, Hui Wang, Mark H. Engelhard, Vijayakumar Murugesan, Karl T. Mueller, David Reed, Vincent Sprenkle, Perla B. Balbuena, Xiaolin Li."Lithium‐Pretreated Hard Carbon as High‐Performance Sodium‐Ion Battery Anodes."Advanced Energy Materials 8 (24) 1801441 (August 2018).
Abstract: Hard carbon (HC) is the state‐of‐the‐art anode material for sodium‐ion batteries (SIBs). However, its performance has been plagued by the limited initial Coulombic efficiency (ICE) and mediocre rate performance. Here, experimental and theoretical studies are combined to demonstrate the application of lithium‐pretreated HC (LPHC) as high‐performance anode materials for SIBs by manipulating the solid electrolyte interphase in tetraglyme (TEGDME)‐based electrolyte. The LPHC in TEGDME can 1) deliver > 92% ICE and ≈220 mAh g⁻¹ specific capacity, twice of the capacity (≈100 mAh g⁻¹) in carbonate electrolyte; 2) achieve > 85% capacity retention over 1000 cycles at 1000 mA g⁻¹ current density (4 C rate, 1 C = 250 mA g⁻¹) with a specific capacity of ≈150 mAh g⁻¹, ≈15 times of the capacity (10 mAh g⁻¹) in carbonate. The full cell of Na₃V₂(PO₄)₃‐LPHC in TEGDME demonstrated close to theoretical specific capacity of ≈98 mAh g⁻¹ based on Na₃V₂(PO₄)₃ cathode, ≈2.5 times of the value (≈40 mAh g⁻¹) with nontreated HC. This work provides new perception on the anode development for SIBs.

Guosheng Li."Turning Cooler."Nature Energy (August 2018).
Abstract: Harsh operating conditions, such as high temperatures, hinder the grid-scale application of liquid metal batteries (LMBs). Now, their operating temperature is shown to be substantially lowered thanks to a lithium-ion-conducting solid-state electrolyte. Stationary energy storage systems consisting of large-scale rechargeable batteries are increasingly considered as one of the most important components to effectively integrate intermittent renewable sources such as wind and solar into reliable and resilient next-generation grids^1,2. Because there is less restriction on the energy footprint for stationary energy storage systems than portable devices, the high-temperature liquid metal battery (LMB)³ is now facing great opportunities after nearly four decades of dormancy^4,5. Traditional LMBs are typically constructed with molten metal electrodes and molten salt electrolytes that separate the two liquid electrodes (Fig. 1a). As LMBs need to maintain their liquid state under operation, the minimum cell operating temperature is generally above 450 °C, which is determined by the melting points of the electrode and electrolyte materials (Fig. 1c)^3,6. Now, writing in Nature Energy, Hui Wu, Yi Cui and co-workers from China and the United States report using a lithium (Li)-ion-conducting solid-state electrolyte (SSE) to enable operating LMBs at a temperature of merely 240 °C.

Keeyoung Jung, Hee-Jung Chang, Jeffery F. Bonnett, Nathan L. Canfield, Vincent L. Sprenkle, Guosheng Li."An advanced Na-NiCl₂ battery using bi-layer (dense/micro-porous) β″-alumina solid-state electrolytes."Journal of Power Sources 396: 297-303 (August 2018).
Abstract: Sodium metal halide (Na-MH) batteries present tremendous opportunities for grid scale energy storage applications. In this work, we describe an advanced Na-MH battery operating at 190 °C using a bi-layer (thin dense/thick porous layers) β″-alumina solid-state electrolyte (BASE). The novel design of the bi-layer BASE promotes high Na-ion transportation by reducing the Na+ ion path length. The excellent battery performances are achieved with a stable capacity retention of 350 W h/kg up to >350 cycles (∼6 months). Moreover, owing to the thin dense layer of BASE, the round trip energy efficiency (or discharging energy density) of the tested battery shows an ∼8% increase compared to that of state of the art Na-MH battery reported in the literature. Results from this work clearly demonstrate that advanced Na-MH batteries using bi-layer BASEs can have significant impacts on improving battery performances at lower operating temperatures, and further stretch its feasibility in stationary energy storage applications.

Damien Saurel, Brahim Orayech, Biwei Xiao, Daniel Carriazo, Xiaolin Li, Teófilo Rojo."From Charge Storage Mechanism to Performance: A Roadmap toward High Specific Energy Sodium‐Ion Batteries through Carbon Anode Optimization."Advanced Energy Materials 8 (17) 1703268 (June 2018).
Abstract: While sodium‐ion batteries (SIBs) represent a low‐cost substitute for Li‐ion batteries (LIBs), there are still several key issues that need to be addressed before SIBs become market‐ready. Among these, one of the most challenging is the negligible sodium uptake into graphite, which is the keystone of the present LIB technology. Although hard carbon has long been established as one of the best substitutes, its performance remains below that of graphite in LIBs and its sodium storage mechanism is still under debate. Many other carbons have been recently studied, some of which have presented capacities far beyond that of graphite. However, these also tend to exhibit larger voltage and high first cycle loss, leading to limited benefits in terms of full cell specific energy. Overcoming this concerning tradeoff necessitates a deep understanding of the charge storage mechanisms and the correlation between structure, microstructure, and performance. This review aims to address this by drawing a roadmap of the emerging routes to optimization of carbon materials for SIB anodes on the basis of a critical survey of the reported electrochemical performances and charge storage mechanisms.

Junhua Song, Biwei Xiao, Yuehe Lin, Kang Xu, Xiaolin Li."Interphases in Sodium‐Ion Batteries."Advanced Energy Materials 8 (17) 1703082 (June 2018).
Abstract: Sodium‐ion batteries (SIBs) as economical, high energy alternatives to lithium‐ion batteries (LIBs) have received significant attention for large‐scale energy storage in the last few years. While the efforts of developing SIBs have benefited from the knowledge learned in LIBs, thanks to the apparent proximity between Na‐ions and Li‐ions, the unique physical and chemical properties of Na‐ions also distinctly differ themselves from Li‐ions. It is expected that SIBs have drastically different electrode material structure, solvation–desolvation behavior, electrode–electrolyte interphase stabilities, ion transfer properties, and hence electrochemical performance of batteries. In this review, the authors comprehensively summarize the current understanding of the anode solid electrolyte interphase and cathode electrolyte interphase in SIBs, with an emphasis on how the tuning of the stability and ion transfer properties of interphases fundamentally determines the reversibility and efficiency of electrochemical reactions. Through these carefully screened references, the authors intend to reveal the intrinsic correlation between the properties/functionalities of the interphases and the electrochemical performance of batteries.

Aaron Hollas, Xiaoliang Wei, Vijayakumar Murugesan, Zimin Nie, Bin Li, David Reed, Jun Liu, Vincent Sprenkle & Wei Wang."A biomimetic high-capacity phenazine-based anolyte for aqueous organic redox flow batteries."Nature Energy 3: 508-514 (June 2018).
Abstract: Aqueous soluble organic (ASO) redox-active materials have recently attracted significant attention as alternatives to traditional transition metal ions in redox flow batteries (RFB). However, reported reversible capacities of ASO are often substantially lower than their theoretical values based on the reported maximum solubilities. Here, we describe a phenazine-based ASO compound with an exceptionally high reversible capacity that exceeds 90% of its theoretical value. By strategically modifying the phenazine molecular structure, we demonstrate an increased solubility from near-zero with pristine phenazine to as much as 1.8 M while also shifting its redox potential by more than 400 mV. An RFB based on a phenazine derivative (7,8-dihydroxyphenazine-2-sulfonic acid) at its near-saturation concentration exhibits an operating voltage of 1.4 V with a reversible anolyte capacity of 67 Ah l⁻¹ and a capacity retention of 99.98% per cycle over 500 cycles.

Aaron Hollas, Xiaoliang Wei, Vijayakumar Murugesan, Zimin Nie, Bin Li, David Reed, Jun Liu, Vincent Sprenkle, Wei Wang."A biomimetic high-capacity phenazine-based anolyte for aqueous organic redox flow batteries."Nature Energy3: 508-514 (June 2018).
Abstract: Aqueous soluble organic (ASO) redox-active materials have recently attracted significant attention as alternatives to traditional transition metal ions in redox flow batteries (RFB). However, reported reversible capacities of ASO are often substantially lower than their theoretical values based on the reported maximum solubilities. Here, we describe a phenazine-based ASO compound with an exceptionally high reversible capacity that exceeds 90% of its theoretical value. By strategically modifying the phenazine molecular structure, we demonstrate an increased solubility from near-zero with pristine phenazine to as much as 1.8 M while also shifting its redox potential by more than 400 mV. An RFB based on a phenazine derivative (7,8-dihydroxyphenazine-2-sulfonic acid) at its near-saturation concentration exhibits an operating voltage of 1.4 V with a reversible anolyte capacity of 67 Ah l⁻¹ and a capacity retention of 99.98% per cycle over 500 cycles.

Patrick J. Balducci, M. Jan E. Alam, Trevor D. Hardy, Di Wu."Assigning value to energy storage systems at multiple points in an electrical grid."Energy & Environmental Science (June 2018).
Abstract: The ability to define the potential value that energy storage systems (ESSs) could generate through various applications in electric power systems, and an understanding of how these values change due to variations in ESS performance and parameters, market structure, utility structures, and valuation methodologies is highly important in advancing ESS deployment. This paper presents a taxonomy for assigning benefits to the use cases or services provided by ESSs, defines approaches for monetizing the value associated with these services, assigns values, or more precisely ranges of values, to major ESS applications by region based on a review of an extensive set of literature, and summarizes and evaluates the capabilities of several available tools currently used to estimate value for specific ESS deployments.

Zhaoxin Yu, Shun-Li Shang, Yue Gao, Daiwei Wang, Xiaolin Li, Ki-Kui Liu, Donghai Wang."A quaternary sodium superionic conductor - Na_10.8Sn_1.9PS_11.8."Nano Energy 47: 325-330 (May 2018).
Abstract: Sulfide-based Na-ion conductors are promising candidates as solid-state electrolytes (SSEs) for fabrication of solid-state Na-ion batteries (NIBs) because of their high ionic conductivities and low grain boundary resistance. Currently, most of the sulfide-based Na-ion conductors with high conductivities are focused on Na₃PS₄ phases and its derivatives. It is desirable to develop Na-ion conductors with new composition and crystal structure to achieve superior ionic conductivities. Here we report a new quaternary Na-ion conductor, Na_10.8Sn_1.9PS_11.8, exhibiting a high ionic conductivity of 0.67 mS cm⁻¹ at 25 °C. This high ionic conductivity originates from the presence of a large number of intrinsic Na-vacancies and three-dimensional Na-ion conduction pathways, which has been confirmed by single-crystal X-ray diffraction and first-principles calculations. The Na_10.8Sn_1.9PS_11.8 phase is further evaluated as an electrolyte in a Na-Sn alloy/TiS₂ battery, demonstrating its potential application in all-solid-state NIBs.

Daiwon Choi, Prashanth H. Jampani, J.R.P. Jayakody, Steven G. Greenbaum, Prashant N. Kumta."Synthesis, surface chemistry and pseudocapacitance mechanisms of VN nanocrystals derived by a simple two-step halide approach."Materials Science and Engineering: B 230: 8-19 (April 2018).
Abstract: Chloroamide precursors generated via a simple two-step ammonolysis reaction of transition metal chloride in the liquid phase at room temperature were heat treated in ammonia at moderate temperature to yield nano-sized VN crystallites. Grain growth inhibited by lowering the synthesis temperature (≈400 °C) yielded agglomerated powders of spherical crystallites of cubic phase of VN with particle sizes as small as 6 nm in diameter. X-ray diffraction, FTIR, mass spectroscopy (MS), and nuclear magnetic resonance (NMR) spectroscopy assessed the ammonolysis and nitridation reaction of the VCl4-NH3 system. X-ray Rietveld refinement, the BET technique and high-resolution transmission microscopy (HRTEM), energy dispersive X-ray (EDX) and thermogravimetric analysis (TGA) helped assess the crystallographic and microstructural nature of the VN nanocrystals. The surface chemistry and redox reaction leading to the gravimetric pseudo-capacitance value of (≈855 F/g) measured for the VN nanocrystals was determined and validated using FTIR, XPS and cyclic voltammetry analyses.

Alasdair J. Crawford, Qian Huang, Michael C.W. Kintner-Meyer, Ji-Guang Zhang, David M. Reed, Vincent L. Sprenkle, Vilayanur V. Viswanathan, Daiwon Choi."Lifecycle comparison of selected Li-ion battery chemistries under grid and electric vehicle duty cycle combinations."Journal of Power Sources 380: 185-193 (March 2018).
Abstract: Li-ion batteries are expected to play a vital role in stabilizing the electrical grid as solar and wind generation capacity becomes increasingly integrated into the electric infrastructure. This article describes how two different commercial Li-ion batteries based on LiNi_0.8Co_0.15Al_0.05O₂ (NCA) and LiFePO₄ (LFP) chemistries were tested under grid duty cycles recently developed for two specific grid services: (1) frequency regulation (FR) and (2) peak shaving (PS) with and without being subjected to electric vehicle (EV) drive cycles. The lifecycle comparison derived from the capacity, round-trip efficiency (RTE), resistance, charge/discharge energy, and total used energy of the two battery chemistries are discussed. The LFP chemistry shows better stability for the energy-intensive PS service, while the NCA chemistry is more conducive to the FR service under the operating regimes investigated. The results can be used as a guideline for selection, deployment, operation, and cost analyses of Li-ion batteries used for different applications.

Saurel, D; Orayech, B; Xiao, B; Carriazo, D; Li,X;Rojo T. "'From Charge Storage Mechanism to Performance: A Roadmap toward High Specific Energy Sodium‐Ion Batteries through Carbon Anode Optimization" Advanced Energy Materials.  (March, 2018)
Abstract: While sodium‐ion batteries (SIBs) represent a low‐cost substitute for Li‐ion batteries (LIBs), there are still several key issues that need to be addressed before SIBs become market‐ready. Among these, one of the most challenging is the negligible sodium uptake into graphite, which is the keystone of the present LIB technology. Although hard carbon has long been established as one of the best substitutes, its performance remains below that of graphite in LIBs and its sodium storage mechanism is still under debate. Many other carbons have been recently studied, some of which have presented capacities far beyond that of graphite. However, these also tend to exhibit larger voltage and high first cycle loss, leading to limited benefits in terms of full cell specific energy. Overcoming this concerning tradeoff necessitates a deep understanding of the charge storage mechanisms and the correlation between structure, microstructure, and performance. This review aims to address this by drawing a roadmap of the emerging routes to optimization of carbon materials for SIB anodes on the basis of a critical survey of the reported electrochemical performances and charge storage mechanisms.

Chang HJ ,Lu X ,Bonnett J F,Canfield N L,Son S ,Park YC ,Jung K ,Sprenkle V L,Li G. "'Ni-less' Cathodes for High Energy Density, Intermediate Temperature"Advanced Materials Interfaces. 1701592 (March, 2018)
Abstract: Among various battery technologies being considered for stationary energy storage applications, sodium–metal halide (Na–MH) batteries have become one of the most attractive candidates because of the abundance of raw materials, long cycle life, high energy density, and superior safety. However, one of issues limiting its practical application is the relatively expensive nickel (Ni) used in the cathode. In the present work, the focus is on efforts to develop new Ni‐based cathodes, and it is demonstrated that a much higher specific energy density of 405 Wh kg⁻¹ (16% higher than state‐of‐the‐art Na–MH batteries) can be achieved at an operating temperature of 190 °C. Furthermore, 15% less Ni is used in the new cathode formula than that in conventional Na–NiCl₂ batteries. Long‐term cycling tests also show stable electrochemical performance for over 300 cycles with excellent capacity retention (≈100%). The results in this work indicate that these advances can significantly reduce the raw material cost associated with Ni (a 31% reduction) and promote practical applications of Na–MH battery technologies in stationary energy storage systems.

Shaofang Fu, Junhua Song, Chengzhou Zhu, Gui-Liang Xu, Khalil Amine, Chengjun Sun, Xiaolin Li, Mark H Engelhard, Dan Du, Yuehu Lin."Ultrafine and highly disordered Ni₂Fe₁ nanofoams enabled highly efficient oxygen evolution reaction in alkaline electrolyte."Nano Energy44: 319-326 (February 2018).
Abstract: Nickel iron hydroxides are the most promising non-noble electrocatalysts for oxygen evolution reaction (OER) in alkaline media. By in situ reduction of metal precursors, compositionally controlled three-dimensional Ni_xFe_y nanofoams (NFs) are synthesized with high surface area and uniformly distributed bimetallic networks. The resultant ultrafine and highly disordered amorphous Ni₂Fe₁ NFs exhibit extraordinary electrocatalytic performance toward OER and overall water splitting in alkaline media. At a potential as low as 1.42 V (vs. RHE), Ni₂Fe₁ NFs can deliver a current density of 10 mA/cm² and show negligible activity loss after 12 h stability test. Even at large current flux of 100 mA/cm², an ultralow overpotential of 0.27 V is achieved, which is about 0.18 V more negative than benchmark RuO₂. Both ex-situ Mӧssbauer spectroscopy and X-ray Absorption Spectroscopy reveal a phase separation and transformation for the Ni₂Fe₁ catalyst during OER process. The evolution of oxidation state and disordered structure of Ni₂Fe₁ might be a key to the high catalytic performance for OER.

Pan, H.; Li, B.; Mei, D.; Nie, Z.; Shao, Y.; Li, G.; Li, X. S.; Han, K. S.; Mueller, K. T.; Sprenkle, V.; Liu, J. " Controlling Solid–Liquid Conversion Reactions for a Highly Reversible Aqueous Zinc–Iodine Battery" ACS Energy Letters 2:2674-2680 (Oct. 2017).

Abstract: Aqueous rechargeable batteries are desirable for energy storage because of their low cost and high safety. However, low capacity and short cyclic life are significant obstacles to their practical applications. Here, we demonstrate a highly reversible aqueous zinc–iodine battery using encapsulated iodine in microporous carbon as the cathode material by controlling solid–liquid conversion reactions. We identified the factors influencing solid–liquid conversion reactions, e.g., the pore size, surface chemistry of carbon host, and solvent effect. Rational manipulation of the competition between the adsorption in carbon and solvation in electrolytes for iodine species is responsible for the high reversibility and cyclic stability. The zinc–iodine battery delivers a high capacity of 174.4 mAh g^–1 at 1C, stable cyclic life over 3000 cycles with ∼90% capacity retention, and negligible self-discharge. We believe that the principles for stabilizing the zinc–iodine system could provide new insight for other conversion systems such as lithium–sulfur systems.

Junhua Song, Pengfei Yan, Langli Luo, Xingguo Qi, Xiaohui Rong, Jianming Zheng, Biwei Xiao, Shuo Feng, Chongmin Wang, Yong-Sheng Hu, Yuehe Lin, Vincent L. Sprenkle, Xiaolin Li."Yolk-shell structured Sb@C anodes for high energy Na-ion batteries."Nano Energy 40: 504-511 (October 2017).
Abstract: Despite great advances in sodium-ion battery developments, the search for high energy and stable anode materials remains a challenge. Alloy or conversion-typed anode materials are attractive candidates of high specific capacity and low voltage potential, yet their applications are hampered by the large volume expansion and hence poor electrochemical reversibility and fast capacity fade. Here, we use antimony (Sb) as an example to demonstrate the use of yolk-shell structured anodes for high energy Na-ion batteries. The Sb@C yolk-shell structure prepared by controlled reduction and selective removal of Sb₂O₃ from carbon coated Sb₂O₃ nanoparticles can accommodate the Sb swelling upon sodiation and improve the structural/electrical integrity against pulverization. It delivers a high specific capacity of ~ 554 mAh g⁻¹, good rate capability (315 mhA g⁻¹ at 10 C rate) and long cyclability (92% capacity retention over 200 cycles). Full-cells of O3-Na_0.9[Cu_0.22Fe_0.30Mn_0.48]O₂ cathodes and Sb@C-hard carbon composite anodes demonstrate a high specific energy of ~ 130 Wh kg⁻¹ (based on the total mass of cathode and anode) in the voltage range of 2.0–4.0 V, ~ 1.5 times energy of full-cells with similar design using hard carbon anodes.

Lu, X, HJ Chang, JF Bonnett, NL Canfield, K Jung, VL Sprenkle, G Li. "Effect of cathode thickness on the performance of planar Na-NiCl₂ battery." Journal of Power Sources 365: 456-462 (Oct. 2017).
Abstract:Na-beta alumina batteries (NBBs) are one of the most promising technologies for renewable energy storage and grid applications. Commercial NBBs are typically constructed in tubular designs, primarily because of their ease of sealing. However, planar designs are considered superior to tubular counterparts in terms of power output, cell packing, ease of assembly, and thermal management. In this paper, the performance of planar NBBs has been evaluated at an intermediate temperature. In particular, planar Na-NiCl₂ cells with different cathode loadings and thicknesses have been studied at 190°C. The effects of the cathode thickness, charging current, and discharging power output on the cell capacity and resistance have been investigated. More than 60% of theoretical cell capacity was retained with constant discharging power levels of 200, 175, and 100 mW/cm² for 1x, 2x, and 3x cathode loadings, respectively. The cell resistance with 1x and 2x cathode loadings was dominated by ohmic resistance with discharging currents up to 105 mA/cm², while for 3x cathode loading, it was primarily dominated by ohmic resistance with currents less than 66.67 mA/cm² and by polarization resistance above 66.67 mA/cm².

X. Wei, W. Pan, W. Duan, A. Hollas, Z. Yang, B. Li, Z. Nie, J. Liu, D. Reed, W. Wang, V. Sprenkle. "Materials and Systems for Organic Redox Flow Batteries: Status and Challenges " ACS Energy Letters 2017, 2,(9),2187-2204. (Aug. 2017).
Abstract:Redox flow batteries (RFBs) are propitious stationary energy storage technologies with exceptional scalability and flexibility to improve the stability, efficiency, and sustainability of our power grid. The redox-active materials are the key component for RFBs with which to achieve high energy density and good cyclability. Traditional inorganic-based materials encounter critical technical and economic limitations such as low solubility, inferior electrochemical activity, and high cost. Redox-active organic materials (ROMs) are promising alternative “green” candidates to push the boundaries of energy storage because of the significant advantages of molecular diversity, structural tailorability, and natural abundance. Here, the recent development of a variety of ROMs and associated battery designs in both aqueous and nonaqueous electrolytes are reviewed. The critical challenges and potential research opportunities for developing practically relevant organic flow batteries are discussed.

Wu D, M Kintner-Meyer, T Yang, P Balducci. "Analytical sizing methods for behind-the-meter battery storage." Journal of Energy Storage 12: 297-304 (Aug. 2017).
Abstract:In behind-the-meter application, battery storage system (BSS) is used to reduce a commercial or industrial customer's payment for electricity use, including energy and demand charges. The potential value of BSS in payment reduction and the optimal size can be determined by formulating and solving standard mathematical programming problems. In such mathematical programming methods, users input system information such as load profiles, energy/demand charge rates, and battery characteristics to construct a standard programming problem, which typically involves a large number of constraints and decision variables. The problems are then solved by optimization solvers to obtain numerical solutions. Such kind of methods cannot directly link the obtained optimal battery sizes to input parameters and requires case-by-case analysis. In this paper, we present an objective quantitative analysis of costs and benefits for customer-side BSS, and thereby identify key factors that affect optimal sizing. We then develop simple but effective guidelines for determining the most cost-effective battery size. The proposed analytical sizing methods are innovative, and provide engineering insights on how the optimal battery size varies with system characteristics. We illustrate the proposed methods using practical building load profile and utility rate. The obtained results are compared with the ones using mathematical programming based methods for validation.

Cho SJ, MJ Uddin, PK Alaboina, SS Han, MI Nandasiri, YS Choi, E Hu, KW Nam, AM Schwarz, SK Nune, JS Cho, KH Oh, D Choi. "Exploring Lithium Deficiency in Layered Oxide Cathode for Li-Ion Battery." Advanced Sustainable Systems 1 (7) (June 2017).
Abstract:The ever-growing demand for high capacity cathode materials is on the rise since the futuristic applications are knocking on the door. Conventional approach to developing such cathode relies on the lithium-excess materials to operate the cathode at high voltage and extract more lithium-ion. Yet, they fail to satiate the needs because of their unresolved issues upon cycling such as, for lithium manganese-rich layered oxides-their voltage fading, and for as nickel-based layered oxides-the structural transition. Here, in contrast, lithium-deficient ratio is demonstrated as a new approach to attain high capacity at high voltage for layered oxide cathodes. Rapid and cost effective lithiation of a porous hydroxide precursor with lithium deficient ratio is acted as a driving force to partially convert the layered material to spinel phase yielding in a multiphase structure (MPS) cathode material. Upon cycling, MPS reveals structural stability at high voltage and high temperature and results in fast lithium-ion diffusion by providing a distinctive solid electrolyte interface (SEI) chemistry-MPS displays minimum lithium loss in SEI and forms a thinner SEI. MPS thus offers high energy and high power applications and provides a new perspective compared to the conventional layered cathode materials denying the focus for lithium excess material.

Chang HJ ,Lu X ,Bonnett J F,Canfield N L,Son S ,Park YC ,Jung K ,Sprenkle V L,Li G. "Development of intermediate temperature sodium nickel chloride rechargeable batteries using conventional polymer sealing technologies." Journal of Power Sources 348: 150-157 (April 2017).
Abstract:Developing advanced and reliable electrical energy storage systems is critical to fulfill global energy demands and stimulate the growth of renewable energy resources. Sodium metal halide batteries have been under serious consideration as a low cost alternative energy storage device for stationary energy storage systems. Yet, there are number of challenges to overcome for the successful market penetration, such as high operating temperature and hermetic sealing of batteries that trigger an expensive manufacturing process. Here we demonstrate simple, economical and practical sealing technologies for Na-NiCl₂ batteries operated at an intermediate temperature of 190°C. Conventional polymers are implemented in planar Na-NiCl₂ batteries after a prescreening test, and their excellent compatibilities and durability are demonstrated by a stable performance of Na-NiCl₂ battery for more than 300 cycles. The sealing methods developed in this work will be highly beneficial and feasible for prolonging battery cycle life and reducing manufacturing cost for Na-based batteries at elevated temperatures (<200°C).

Li Y ,An Q ,Cheng Y ,Liang Y ,Ren Y ,Sun CJ ,Dong H ,Tang Z ,Li G ,Yao Y. "A high-voltage rechargeable magnesium-sodium hybrid battery." Nano Energy 34: 188-194 (April 2017).
Abstract:Growing global demand of safe and low-cost energy storage technology triggers strong interests in novel battery concepts beyond state-of-art Li-ion batteries. Here we report a high-voltage rechargeable Mg-Na hybrid battery featuring dendrite-free deposition of Mg anode and Na-intercalation cathode as a low-cost and safe alternative to Li-ion batteries for large-scale energy storage. A prototype device using a Na₃V₂(PO₄)₃ cathode, a Mg anode, and a Mg-Na dual salt electrolyte exhibits the highest voltage (2.60 V vs. Mg) and best rate performance (86% capacity retention at 10C rate) among reported hybrid batteries. Synchrotron radiation-based X-ray absorption near edge structure (XANES), atomic-pair distribution function (PDF), and high-resolution X-ray diffraction (HRXRD) studies reveal the chemical environment and structural change of Na₃V₂(PO₄)₃ cathode during the Na ion insertion/deinsertion process. XANES study shows a clear reversible shift of vanadium K-edge and HRXRD and PDF studies reveal a reversible two-phase transformation and V-O bond length change during cycling. The energy density of the hybrid cell could be further improved by developing electrolytes with a higher salt concentration and wider electrochemical window. This work represents a significant step forward for practical safe and low-cost hybrid batteries.

Chang HJ ,Canfield N L,Jung K ,Sprenkle V L,Li G. "Advanced Na-NiCl₂ Battery Using Nickel-Coated Graphite with Core-Shell Microarchitecture." ACS Applied Materials & Interfaces 9 (13): 11609-11614 (March 2017).
Abstract:Stationary electric energy storage devices (rechargeable batteries) have gained increasing prominence due to great market needs, such as smoothing the fluctuation of renewable energy resources and supporting the reliability of the electric grid. With regard to raw materials availability, sodium-based batteries are better positioned than lithium batteries due to the abundant resource of sodium in Earth's crust. However, the sodium-nickel chloride (Na-NiCl₂) battery, one of the most attractive stationary battery technologies, is hindered from further market penetration by its high material cost (Ni cost) and fast material degradation at its high operating temperature. Here, we demonstrate the design of a core�shell microarchitecture, nickel-coated graphite, with a graphite core to maintain electrochemically active surface area and structural integrity of the electron percolation pathway while using 40% less Ni than conventional Na-NiCl₂ batteries. An initial energy density of 133 Wh/kg (at ~C/4) and energy efficiency of 94% are achieved at an intermediate temperature of 190°C.

Soto F A,Yan P ,Engelhard M H,Marzouk A ,Wang C ,Liu J ,Sprenkle V L,El-Mellouhi F ,Balbuena P B,Li X. "Tuning the Solid Electrolyte Interphase for Selective Li- and Na-Ion Storage in Hard Carbon." Advanced Materials 29 (18): 1606860 (March 2017).
Abstract:Solid-electrolyte interphase (SEI) films with controllable properties are highly desirable for improving battery performance. In this paper, a combined experimental and theoretical approach is used to study SEI films formed on hard carbon in Li- and Na-ion batteries. It is shown that a stable SEI layer can be designed by precycling an electrode in a desired Li- or Na-based electrolyte, and that ionic transport can be kinetically controlled. Selective Li- and Na-based SEI membranes are produced using Li- or Na-based electrolytes, respectively. The Na-based SEI allows easy transport of Li ions, while the Li-based SEI shuts off Na-ion transport. Na-ion storage can be manipulated by tuning the SEI layer with film-forming electrolyte additives, or by preforming an SEI layer on the electrode surface. The Na specific capacity can be controlled to < 25 mAh g^-1;= 1/10 of the normal capacity (250 mAh g^-1). Unusual selective/preferential transport of Li ions is demonstrated by preforming an SEI layer on the electrode surface and corroborated with a mixed electrolyte. This work may provide new guidance for preparing good ion-selective conductors using electrochemical approaches.

Li B, Liu, J. "Progress and directions in low-cost redox-flow batteries for large-scale energy storage." National Science Review 4 (1): 91-105 (Jan. 2017).
Abstract:Compared to lithium-ion batteries, redox-flow batteries have attracted widespread attention for long-duration, large-scale energy-storage applications. This review focuses on current and future directions to address one of the most significant challenges in energy storage: reducing the cost of redox-flow battery systems. A high priority is developing aqueous systems with low-cost materials and high-solubility redox chemistries. Highly water-soluble inorganic redox couples are important for developing technologies that can provide high energy densities and low-cost storage. There is also great potential to rationally design organic redox molecules and fine-tune their properties for both aqueous and non-aqueous systems. While many new concepts begin to blur the boundary between traditional batteries and redox-flow batteries, breakthroughs in identifying/developing membranes and separators and in controlling side reactions on electrode surfaces also are needed.

Fu S ,Zhu C ,Song J ,Engelhard M H,Li X ,Zhang P ,Xia H ,Du D ,Lin Y. "Template-directed synthesis of nitrogen- and sulfur-codoped carbon nanowire aerogels with enhanced electrocatalytic performance for oxygen reduction." Nano Research 10 (6): 1888-1895 (Dec. 2016).
Abstract:Heteroatom doping, precise composition control, and rational morphology design are efficient strategies for producing novel nanocatalysts for the oxygen reduction reaction (ORR) in fuel cells. Herein, a cost-effective approach to synthesize nitrogen- and sulfur-codoped carbon nanowire aerogels using a hard templating method is proposed. The aerogels prepared using a combination of hydrothermal treatment and carbonization exhibit good catalytic activity for the ORR in alkaline solution. At the optimal annealing temperature and mass ratio between the nitrogen and sulfur precursors, the resultant aerogels show comparable electrocatalytic activity to that of a commercial Pt/C catalyst for the ORR. Importantly, the optimized catalyst shows much better long-term stability and satisfactory tolerance for the methanol crossover effect. These codoped aerogels are expected to have potential applications in fuel cells.

Shamie J S,Liu C H,Shaw L L,Sprenkle V L. "New Mechanism for the Reduction of Vanadyl Acetylacetonate to Vanadium Acetylacetonate for Room Temperature Flow Batteries." ChemSusChem 10 (3): 533-540 (Dec. 2016).
Abstract:In this study, a new mechanism for the reduction of vanadyl acetylacetonate, VO(acac)₂, to vanadium acetylacetonate, V(acac)₃, is introduced. V(acac)₃ has been studied for use in redox flow batteries (RFBs) for some time; however, contamination by moisture leads to the formation of VO(acac)₂. In previous work, once this transformation occurs, it is no longer reversible because there is a requirement for extreme low potentials for the reduction to occur. Here, we propose that, in the presence of excess acetylacetone (Hacac) and free protons (H⁺), the reduction can take place between 2.25 and 1.5 V versus Na/Na⁺ via a one-electron-transfer reduction. This reduction can take place in situ during discharge in a novel hybrid Na-based flow battery (HNFB) with a molten Na-Cs alloy as the anode. The in situ recovery of V(acac)₃ during discharge is shown to allow the Coulombic efficiency of the HNFB to be =100% with little or no capacity decay over cycles. In addition, utilizing two-electron-transfer redox reactions (i.e., V³⁺/V⁴⁺ and V²⁺/V³⁺ redox couples) per V ion to increase the energy density of RFBs becomes possible owing to the in situ recovery of V(acac)₃ during discharge. The concept of in situ recovery of material can lead to more advances in maintaining the cycle life of RFBs in the future.

Murugesan, V, Q Luo, R Lloyd, Z Nie, X Wei, B Li, VL Sprenkle, JD Londono, M Unlu, W Wang. "Tuning the Perfluorosulfonic Acid Membrane Morphology for Vanadium Redox-Flow Batteries."ACS Applied Materials and Interfaces 8 (50): 34327-34334 (Nov. 2016).
Abstract: The microstructure of perfluorinated sulfonic acid proton-exchange membranes such as Nafion significantly affects their transport properties and performance in a vanadium redox-flow battery (VRB). In this work, Nafion membranes with various equivalent weights ranging from 1000 to 1500 are prepared and the morphology-property-performance relationship is investigated. NMR and small-angle X-ray scattering studies revealed their composition and morphology variances, which lead to major differences in key transport properties related to proton conduction and vanadium-ion permeation. Their performances are further characterized as VRB membranes. On the basis of this understanding, a new perfluorosulfonic acid membrane is designed with optimal pore geometry and thickness, leading to higher ion selectivity and lower cost compared with the widely used Nafion 115. Excellent VRB single-cell performance (89.3% energy efficiency at 50 mA·cm-2) was achieved along with a stable cyclical capacity over prolonged cycling.

Park, M, J Ryu, W Wang, J Cho. "Material design and engineering of next-generation flow-battery technologies." Nature Review Materials 2 (Nov. 2016).
Abstract: Spatial separation of the electrolyte and electrode is the main characteristic of flow-battery technologies, which liberates them from the constraints of overall energy content and the energy/power ratio. The concept of a flowing electrolyte not only presents a cost-effective approach for large-scale energy storage, but has also recently been used to develop a wide range of new hybrid energy storage and conversion systems. The advent of flow-based lithium-ion, organic redox-active materials, metal-air cells and photoelectrochemical batteries promises new opportunities for advanced electrical energy-storage technologies. In this Review, we present a critical overview of recent progress in conventional aqueous redox-flow batteries and next-generation flow batteries, highlighting the latest innovative alternative materials. We outline their technical feasibility for use in long-term and large-scale electrical energy-storage devices, as well as the limitations that need to be overcome, providing our view of promising future research directions in the field of redox-flow batteries.

Cheng, Y, HJ Chang, H Dong, D Choi, VL Sprenkle, J Liu, Y Yao, G Li. "Rechargeable Mg-Li hybrid batteries: status and challenges."Journal of Materials Research 31 (20) :3125-3141 (Oct. 2016).
Abstract: A magnesium-lithium (Mg-Li) hybrid battery consists of an Mg metal anode, a Li⁺ intercalation cathode, and a dual-salt electrolyte with both Mg²⁺ and Li⁺ ions. The demonstration of this technology has appeared in literature for few years and great advances have been achieved in terms of electrolytes, various Li cathodes, and cell architectures. Despite excellent battery performances including long cycle life, fast charge/discharge rate, and high Coulombic efficiency, the overall research of Mg-Li hybrid battery technology is still in its early stage, and also raised some debates on its practical applications. In this regard, we focus on a comprehensive overview of Mg-Li hybrid battery technologies developed in recent years. Detailed discussion of Mg-Li hybrid operating mechanism based on experimental results from literature helps to identify the current status and technical challenges for further improving the performance of Mg-Li hybrid batteries. Finally, a perspective for Mg-Li hybrid battery technologies is presented to address strategic approaches for existing technical barriers that need to be overcome in future research direction.

Wang Y, P Yan, J Xiao, X Lu, J Zhang, VL Sprenkle. "Effect of Al2O3 on the sintering of garnet-type Li6.5La3Zr1.5Ta0.5O12."Solid State Ionics 294: 108-115 (Oct. 2016).
Abstract:It is widely recognized that Al plays a dual role in the fabrication of garnet-type solid electrolytes, i.e., as a dopant that stabilizes the cubic structure and a sintering aid that facilitates the densification. However, the sintering effect of Al2O3 has not been well understood so far because Al is typically "unintentionally" introduced into the sample from the crucible during the fabrication process. In this study, we have investigated the sintering effect of Al on the phase composition, microstructure, and ionic conductivity of Li_6.5La₃Zr_1.5Ta_0.5O₁₂ by using an Al-free crucible and intentionally adding various amounts of y-Al₂O₃. It was found that the densification of Li_6.5La₃Zr_1.5Ta_0.5O₁₂ occurred via liquid-phase sintering, with evidence of morphology change among different compositions. Among all of the compositions, samples with 0.05mol Al per unit formula of garnet oxide (i.e., 0.3wt% Al₂O₃) exhibited the optimal microstructure and the highest total ionic conductivity of 5x10^-4Scm^-1 at room temperature.

Choi D, C Zhu, S Fu, D Du, MH Engelhard, Y Lin. "Electrochemically Controlled Ion-exchange Property of Carbon Nanotubes/Polypyrrole Nanocomposite in Various Electrolyte Solutions." Electroanalysis 29 (3): 929-936 (Sept. 2016).
Abstract: The electrochemically controlled ion-exchange properties of multi-wall carbon nanotube (MWNT)/electronically conductive polypyrrole (PPy) polymer composite in the various electrolyte solutions have been investigated. The ion-exchange behavior, rate and capacity of the electrochemically deposited polypyrrole with and without carbon nanotube (CNT) were compared and characterized using cyclic voltammetry (CV), chronoamperometry (CA), electrochemical quartz crystal microbalance (EQCM), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). It has been found that the presence of carbon nanotube backbone resulted in improvement in ion-exchange rate, stability of polypyrrole, and higher anion loading capacity per PPy due to higher surface area, electronic conductivity, porous structure of thin film, and thinner film thickness providing shorter diffusion path. Chronoamperometric studies show that electrically switched anion exchange could be completed more than 10 times faster than pure PPy thin film. The anion selectivity of CNT/PPy film is demonstrated using X-ray photoelectron spectroscopy (XPS).

Fu, S, C Zhu, J Song, MH Engelhard, X Li, D Du, Y Lin. "Highly Ordered Mesoporous Bimetallic Phosphides as Efficient Oxygen Evolution Electrocatalysts."ACS Energy Letters 1 (4) :792-796 (Sept. 2016).
Abstract: Oxygen evolution from water using earth-abundant transition-metal-based catalysts is of importance for the commercialization of water electrolyzers. Herein, we report a hard templating method to synthesize transition metal phosphides with uniform shape and size. By virtue of the structural feature, synergistic effects among metals, and the in situ formed active species, the as-prepared phosphides with optimized composition present enhanced electrocatalytic performance toward the oxygen evolution reaction in alkaline solution. In detail, the catalyst with optimized composition reaches a current density of 10 mA/cm² at a potential of 1.511 V vs a reversible hydrogen electrode, which is much lower than that of a commercial RuO₂ catalyst. Our work offers a new strategy to optimize the catalysts for water splitting by controlling the morphology and composition.

Li X, P Yan, MH Engelhard, AJ Crawford, V Viswanathan, C Wang, J Liu, VL Sprenkle. "The importance of solid electrolyte interphase formation for long cycle stability full-cell Na-ion batteries."Nano Energy 27: 664-672 (Sept. 2016).
Abstract: Na-ion battery, as an alternative high-efficiency and low-cost energy storage device to Li-ion battery, has attracted wide interest for electrical grid and vehicle applications. However, demonstration of a full-cell battery with high energy and long cycle life remains a significant challenge. Here, we investigated the role of solid electrolyte interphase (SEI) formation on both cathodes and anodes and revealed a potential way to achieve long-term stability for Na-ion battery full-cells. Pre-cycling of cathodes and anodes leads to preformation of SEI, and hence mitigates the consumption of Na ions in full-cells. The example full-cell of Na0.44MnO2-hard carbon with pre-cycled and capacity-matched electrodes can deliver a specific capacity of ~116 mAh/g based on Na0.44MnO2 at 1 C rate (1 C=120 mA/g). The corresponding specific energy is ~313 Wh/kg based on the cathode. Excellent cycling stability with ~77% capacity retention over 2000 cycles was demonstrated at 2 C rate. Our work represents a leap forward in Na-ion battery development.

Wei X, W Duan, J Huang, L Zhang, B Li, DM Reed, W Xu, VL Sprenkle. "A High-Current, Stable Nonaqueous Organic Redox Flow Battery."ACS Energy Letters 1: 705-711 (Sept. 2016).
Abstract:Nonaqueous redox flow batteries are promising in pursuit of high energy density storage systems owing to the broad voltage windows (>2 V) but currently are facing key challenges such as limited cyclability and rate performance. To address these technical hurdles, here we report the nonaqueous organic flow battery chemistry based on N-methylphthalimide anolyte and 2,5-di-tert-butyl-1-methoxy-4-[2'-methoxyethoxy]benzene catholyte, which harvests a theoretical cell voltage of 2.30 V. The redox flow chemistry exhibits excellent cycling stability under both cyclic voltammetry and flow cell tests upon repeated cycling. A series of Daramic and Celgard porous separators are evaluated in this organic flow battery, which enable the cells to be operated at greatly improved current densities as high as 50 mA·cm^-2 compared to those of other nonaqueous flow systems. The stable cyclability and high-current operations of the organic flow battery system represent significant progress in the development of promising nonaqueous flow batteries.

Cheng Y, L Luo, L Zhong, J Chen, B Li, W Wang, SX Mao, C Wang, VL Sprenkle, G Li, J Liu. "Highly Reversible Zinc-Ion Intercalation into Chevrel Phase Mo6S8 Nanocubes and Applications for Advanced Zinc-Ion Batteries."ACS Applied Materials and Interfaces 8 (22):13673-13677 (June 2016).
Abstract: This work describes the synthesis of Chevrel phase Mo6S8 nanocubes and its application as the anode material for rechargeable Zn-ion batteries. Mo6S8 can host Zn(2+) ions reversibly in both aqueous and nonaqueous electrolytes with specific capacities around 90 mAh/g, and exhibited remarkable intercalation kinetics and cyclic stability. In addition, we assembled full cells by integrating Mo6S8 anodes with zinc-polyiodide (I(-)/I3(-))-based catholytes, and demonstrated that such full cells were also able to deliver outstanding rate performance and cyclic stability. This first demonstration of a zinc-intercalating anode could inspire the design of advanced Zn-ion batteries.

Li B, J Liu, Z Nie, W Wang, DM Reed, J Liu, P McGrail, VL Sprenkle. "Metal-Organic Frameworks as Highly Active Electrocatalysts for High-Energy Density, Aqueous Zinc-Polyiodide Redox Flow Batteries." Nano Letters 16 (7): 4335-4340 (June 2016).
Abstract: The new aqueous zinc-polyiodide redox flow battery (RFB) system with highly soluble active materials as well as ambipolar and bifunctional designs demonstrated significantly enhanced energy density, which shows great potential to reduce RFB cost. However, the poor kinetic reversibility and electrochemical activity of the redox reaction of I₃-/I- couples on graphite felts (GFs) electrode can result in low energy efficiency. Two nanoporous metal-organic frameworks (MOFs), MIL-125-NH₂ and UiO-66-CH₃, that have high surface areas when introduced to GF surfaces accelerated the I₃-/I- redox reaction. The flow cell with MOF-modified GFs serving as a positive electrode showed higher energy efficiency than the pristine GFs; increases of about 6.4% and 2.7% occurred at the current density of 30 mA/cm² for MIL-125-NH₂ and UiO-66-CH₃, respectively. Moreover, UiO-66-CH₃ is more promising due to its excellent chemical stability in the weakly acidic electrolyte. This letter highlights a way for MOFs to be used in the field of RFBs.

Estevez L, DM Reed, Z Nie, AM Schwarz, MI Nandasiri, JP Kizewski, W Wang, EC Thomsen, J Liu, J Zhang, VL Sprenkle, B Li. "Tunable oxygen functional groups as electrocatalysts on graphite felt surfaces for all-vanadium flow batteries."ChemSusChem 9 (12): 1455-1461 (May 2016).
Abstract: A dual oxidative approach using O2 plasma followed by treatment with H2O2 to impart oxygen functional groups onto the surface of a graphite felt electrode. When used as electrodes for an all-vanadium redox flow battery (VRB) system, the energy efficiency of the cell is enhanced by 8.2 % at a current density of 150 mA cm-2 compared with one oxidized by thermal treatment in air. More importantly, by varying the oxidative techniques, the amount and type of oxygen groups was tailored and their effects were elucidated. It was found that O-C=O groups improve the cells performance whereas the C-O and C=O groups degrade it. The reason for the increased performance was found to be a reduction in the cell overpotential after functionalization of the graphite felt electrode. This work reveals a route for functionalizing carbon electrodes to improve the performance of VRB cells. This approach can lower the cost of VRB cells and pave the way for more commercially viable stationary energy storage systems that can be used for intermittent renewable energy storage.

Shen F, W Luo, J Dai, Y Yao, M Zhu, E Hitz, Y Tang, Y Chen, VL Sprenkle, X Li, L Hu. "Ultra-Thick, Low-Tortuosity, and Mesoporous Wood Carbon Anode for High-Performance Sodium-Ion Batteries."Advanced Energy Materials 6 (14) (May 2016).
Abstract: Pyrolysis of earth-abundant wood yields to ultra-thick, low-tortuosity, and mesoporous carbon anodes for sodium-ion batteries. Such a low-tortuosity and porous structure promotes electrolyte diffusion and provides fast transport channels for Na ions, which enables a high areal capacity.

Dong H, Y Li, Y Liang, G Li, CJ Sun, Y Ren, Y Lu, Y Yao. "A magnesium-sodium hybrid battery with high operating voltage."Chemical Communications 52: 8263-8266 (April 2016).
Abstract: We report a high performance magnesium-sodium hybrid battery utilizing a magnesium-sodium dual-salt electrolyte, a magnesium anode, and a Berlin green cathode. The cell delivers an average discharge voltage of 2.2 V and a reversible capacity of 143 mA h g-¹. We also demonstrate the cell with an energy density of 135 W h kg-¹ and a high power density of up to 1.67 kW kg-¹.

Cheng Y, D Choi, KS Han, KT Mueller, J Zhang, VL Sprenkle, J Liu, G Li. "Toward the design of high voltage magnesium-lithium hybrid batteries using dual-salt electrolytes." Chemical Communications 52: 5379-5382 (Feb. 2016).
Abstract: We report a design of high voltage magnesium-lithium (Mg-Li) hybrid batteries through rational control of the electrolyte chemistry, electrode materials and cell architecture. Prototype devices with a structure of Mg-Li/LiFePO₄ (LFP) and Mg-Li/LiMn₂O₄ (LMO) have been investigated. A Mg-Li/LFP cell using a dual-salt electrolyte 0.2 M [Mg₂Cl₂(DME)₄][AlCl₄]₂ and 1.0 M LiTFSI exhibits voltages higher than 2.5 V (vs. Mg) and a high specific energy density of 246 W h kg^-1 under conditions that are amenable for practical applications. The successful demonstrations reported here could be a significant step forward for practical hybrid batteries.

Choi D, X Li, WA Henderson, Q Huang, SK Nune, JP Lemmon, VL Sprenkle. "LiCoPO₄ cathode from a CoHPO₄xH₂O nanoplate precursor for high voltage Li-ion batteries." Heliyon 2 (2) (Feb. 2016).
Abstract: A highly crystalline LiCoPO₄/C cathode material has been synthesized without noticeable impurities via a single step solid-state reaction using CoHPO₄xH₂O nanoplate as a precursor obtained by a simple precipitation route. The LiCoPO₄/C cathode delivered a specific capacity of 125 mAhg-¹ at a charge/discharge rate of C/10. The nanoplate precursor and final LiCoPO₄/C cathode have been characterized using X-ray diffraction, thermogravimetric analysis - differential scanning calorimetry (TGA-DSC), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) and the electrochemical cycling stability has been investigated using different electrolytes, additives and separators.

Li G, X Lu, JY Kim, KD Meinhardt, HJ Chang, NL Canfield, VL Sprenkle. "Advanced intermediate temperature sodium-nickel chloride batteries with ultra-high energy density." Nature Communications 7:10683 (Feb. 2016).
Abstract: Sodium-metal halide batteries have been considered as one of the more attractive technologies for stationary electrical energy storage, however, they are not used for broader applications despite their relatively well known redox system. One of the roadblocks hindering market penetration is the high operating temperature. Here we demonstrate that planar sodium-nickel chloride batteries can be operated at an intermediate temperature of 190°C with ultra-high energy density. A specific energy density of 350 Wh kg ^-1, higher than that of conventional tubular sodium-nickel chloride batteries (280°C), is obtained for planar sodium-nickel chloride batteries operated at 190°C over a long-term cell test (1,000 cycles), and it attributed to the slower particle growth of the cathode materials at the lower operating temperature. Results reported here demonstrate that planar sodium-nickel chloride batteries operated at an intermediate temperature could greatly benefit this traditional energy storage technology by improving battery energy density, cycle life and reducing material costs.

Reed DM, EC Thomsen, B Li, W Wang, Z Nie, BJ Koeppel, VL Sprenkle. "Performance of a low cost interdigitated flow design on a 1 kW class all vanadium mixed acid redox flow battery." Journal of Power Sources 306: 24-31 (Feb. 2016).
Abstract: Three flow designs were operated in a 3-cell 1 kW class all vanadium mixed acid redox flow battery. The influence of electrode surface area and flow rate on the coulombic, voltage, and energy efficiency and the pressure drop in the flow circuit will be discussed and correlated to the flow design. Material cost associated with each flow design will also be discussed.

Wang W, VL Sprenkle. "Energy storage: Redox flow batteries go organic."Nature Chemistry 8 (3): 204-206 (Feb. 2016).
Abstract: Access to sustainable and affordable energy is the foundation for the economic growth and future prosperity of our society. Given the drive to also reduce the carbon footprint associated with traditional fossil-based electricity generation, renewable resources could provide a clean and sustainable energy future. However, while the amount of energy produced from renewable resources such as solar and wind is steadily increasing, and the generation costs continuously falling, it still only represents a small fraction of current energy production. One big issue is the intermittent and fluctuating nature of energy produced from renewables and this will threaten the stability of the grid when the energy share from these resources surpasses 20% of the overall energy capacity¹. Electrical energy storage is a potentially cost-effective approach to solving this problem and would be beneficial for renewable energy integration, balancing the mismatch between supply and demand, as well as improving the overall reliability and efficiency of the grid.

Liu L, X Wei, Z Nie, VL Sprenkle, W Wang. "A Total Organic Aqueous Redox Flow Battery Employing a Low Cost and Sustainable Methyl Viologen Anolyte and 4-HO-TEMPO Catholyte." Advanced Energy Materials 6(3):1-8 (Dec. 2015).
Abstract: Increasing worldwide energy demands and rising CO2 emissions have motivated a search for new technologies to take advantage of renewables such as solar and wind energies. Redox flow batteries (RFBs) with their high power density, high energy efficiency, scalability (up to MW and MWh), and safety features are one suitable option for integrating such energy sources and overcoming their intermittency. However, resource limitation and high system costs of current RFB technologies impede wide implementation. Here, a total organic aqueous redox flow battery (OARFB) is reported, using low-cost and sustainable methyl viologen (MV, anolyte) and 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-HO-TEMPO, catholyte), and benign NaCl supporting electrolyte. The electrochemical properties of the organic redox active materials are studied using cyclic voltammetry and rotating disk electrode voltammetry. The MV/4-HO-TEMPO ARFB has an exceptionally high cell voltage, 1.25 V. Prototypes of the organic ARFB can be operated at high current densities ranging from 20 to 100 mA cm2, and deliver stable capacity for 100 cycles with nearly 100% Coulombic efficiency. The MV/4-HO-TEMPO ARFB displays attractive technical merits and thus represents a major advance in ARFBs.

Lu X, G Li, JY Kim, KD Meinhardt, VL Sprenkle. "Enhanced sintering of ß"-Al_2/O₃/YSZ with the sintering aids of TiO₂ and MnO₂." Journal of Power Sources 295:167-174 (Nov. 2015).
Abstract: ß"-Al₂O3 has been the dominated choice for the electrolyte materials of sodium batteries because of its high ionic conductivity, excellent stability with the electrode materials, satisfactory mechanical strength, and low material cost. To achieve adequate electrical and mechanical performance, sintering of ß"-Al₂O₃ is typically carried out at temperatures above 1600°C with deliberate efforts on controlling the phase, composition, and microstructure. Here, we reported a simple method to fabricate ß"-Al₂O₃/YSZ electrolyte at relatively lower temperatures. With the starting material of boehmite, single phase of ß"-Al₂O₃ can be achieved at as low as 1200°C. It was found that TiO₂ was extremely effective as a sintering aid for the densification of ß"-Al₂O₃ and similar behavior was observed with MnO₂ for YSZ. With the addition of 2 mol% TiO₂ and 5 mol% MnO₂, the ß"-Al₂O₃/YSZ composite was able to be densified at as low as 1400°C with a fine microstructure and good electrical/mechanical performance. This study demonstrated a new approach of synthesis and sintering of ß"-Al₂O₃/YSZ composite, which represented a simple and low-cost method for fabrication of high-performance ß"-Al₂O₃/YSZ electrolyte.

Reed DM, EC Thomsen, B Li, W Wang, Z Nie, BJ Koeppel, JP Kizewski, VL Sprenkle. "Stack Developments in a kW Class All Vanadium Mixed Acid Redox Flow Battery at the Pacific Northwest National Laboratory." Journal of the Electrochemical Society 163 (1):A5211-A5219 (Nov. 2015).
Abstract: Over the past several years, efforts have been focused on improving the performance of kW class all vanadium mixed acid redox flow battery stacks with increasing current density. The influence of the Nafion membrane resistance, an interdigitated design to reduce the pressure drop in the electrolyte circuit, the temperature of the electrolyte, and the electrode structure will be discussed and correlated to the electrical performance. Improvements to the stack energy efficiency and how those improvements translate to the overall system efficiency will also be discussed.

Wei X, G Xia, BW Kirby, EC Thomsen, B Li, Z Nie, GG Graff, J Liu, VL Sprenkle, W Wang. "An Aqueous Redox Flow Battery Based on Neutral Alkali Metal Ferri/ferrocyanide and Polysulfide Electrolytes." Journal of The Electrochemical Society 163(1):A5150-A5153 (Nov. 2015).
Abstract: We have demonstrated a new ferri/ferrocyanide - polysulfide (Fe/S) flow battery, which employs less corrosive, relatively environmentally benign neutral alkali metal ferri/ferrocyanide and alkali metal polysulfides as the active redox couples. A cobalt nanoparticle - decorated graphite felt was used at the polysulfide side as the catalyst. Excellent electrochemical performance was successfully acquired in the Fe/S flow cells with high cell efficiencies (99% coulombic efficiency and ~74% energy efficiency) and good cycling stability over extended charge/discharge operations. The positive half-cell appears to be the performance - limiting side in the Fe/S flow battery determined by using a carbon cloth probe. The inexpensive redox materials and possibly cell part materials can lead to reduced capital cost, making the Fe/S flow battery a promising cost-effective energy storage technology.

Crawford A, V Viswanathan, D Stephenson, W Wang, EC Thomsen, DM Reed, B Li, PJ Balducci, M Kinter-Meyer, VL Sprenkle. "Comparative analysis for various redox flow batteries chemistries using a cost performance model." Journal of Power Sources 293: 388-399 (Oct. 2015).
Abstract: The total energy storage system cost is determined by means of a robust performance-based cost model for multiple flow battery chemistries. Systems aspects such as shunt current losses, pumping losses and various flow patterns through electrodes are accounted for. The system cost minimizing objective function determines stack design by optimizing the state of charge operating range, along with current density and current-normalized flow. The model cost estimates are validated using 2-kW stack performance data for the same size electrodes and operating conditions. Using our validated tool, it has been demonstrated that an optimized all-vanadium system has an estimated system cost of < $350 kWh^-1 for 4-h application. With an anticipated decrease in component costs facilitated by economies of scale from larger production volumes, coupled with performance improvements enabled by technology development, the system cost is expected to decrease to 160 kWh^-1 for a 4-h application, and to $100 kWh^-1 for a 10-h application. This tool has been shared with the redox flow battery community to enable cost estimation using their stack data and guide future direction.

Cosimbescu L, X Wei, M Vijayakumar, W Xu, ML Helm, SD Burton, CM Sorensen, J Liu, VL Sprenkle, W Wang. "Anion-Tunable Properties and Electrochemical Performance of Functionalized Ferrocene Compounds." Scientific Reports 5:14117 (Sept. 2015).
Abstract:We report a series of ionically modified ferrocene compounds for hybrid lithium-organic non-aqueous redox flow batteries, based on the ferrocene/ferrocenium redox couple as the active catholyte material. Tetraalkylammonium ionic moieties were incorporated into the ferrocene structure, in order to enhance the solubility of the otherwise relatively insoluble ferrocene. The effect of various counter anions of the tetraalkylammonium ionized species appended to the ferrocene, such as bis(trifluoromethanesulfonyl)imide, hexafluorophosphate, perchlorate, tetrafluoroborate, and dicyanamide on the solubility of the ferrocene was investigated. The solution chemistry of the ferrocene species was studied, in order to understand the mechanism of solubility enhancement. Finally, the electrochemical performance of these ionized ferrocene species was evaluated and shown to have excellent cell efficiency and superior cycling stability.

Lu X, ME Bowden, VL Sprenkle, J Liu. "A Low Cost, High Energy Density, and Long Cycle Life Potassium-Sulfur Battery for Grid-Scale Energy Storage." Advanced Materials 27(39):5915-5922 (Aug. 2015).
Abstract:A potassium-sulfur battery using K⁺-conducting beta-alumina as the electrolyte to separate a molten potassium metal anode and a sulfur cathode is presented. The results indicate that the battery can operate at as low as 150°C with excellent performance. This study demonstrates a new type of high-performance metal-sulfur battery that is ideal for grid-scale energy-storage applications.

Canfield NL, JY Kim, JF Bonnett, RL Pearson III, VL Sprenkle, J Kung. "Effects of fabrication conditions on mechanical properties and microstructure of duplex ß"-Al₂O₃ solid electrolyte." Materials Science and Engineering: B 197: 43-50 (July 2015).
Abstract:Na-beta batteries are an attractive technology as a large-scale electrical energy storage for grid applications. However, additional improvements in performance and cost are needed for wide market penetration. To improve cell performance by minimizing polarizations, reduction of electrolyte thickness was attempted using a duplex structure consisting of a thin dense electrolyte layer and a porous support layer. In this paper, the effects of sintering conditions, dense electrolyte thickness, and cell orientation on the flexural strength of duplex BASEs fabricated using a vapor phase approach were investigated. It is shown that sintering at temperatures between 1500 and 1550°C results in fine grained microstructures and the highest flexural strength after conversion. Increasing thickness of the dense electrolyte has a small impact on flexural strength, while the orientation of load such that the dense electrolyte is in tension instead of compression has major effects on strength for samples with a well-sintered dense electrolyte.

Reed DM, ED Thomsen, W Wang, Z Nie, B Li, X Wei, BJ Koeppel, VL Sprenkle. "Performance of Nafion^® N115, Nafion^® NR-212, and Nafion^® NR-211 in a 1 kW class all vanadium mixed acid redox flow battery."Journal of Power Sources 285:425-430 (July 2015).
Abstract:Three Nafion^® membranes of similar composition but different thicknesses were operated in a 3-cell 1 kW class all vanadium mixed acid redox flow battery. The influence of current density on the charge/discharge characteristics, coulombic and energy efficiency, capacity fade, operating temperature and pressure drop in the flow circuit will be discussed and correlated to the Nafion^® membrane thickness. Material costs associated with the Nafion^® membranes, ease of handling the membranes, and performance impacts will also be discussed.

Wei X, W Xu, J Huang, L Zhang, ED Walter, CW Lawrence, M Vijayakumar, WA Henderson, TL Liu, L Cosimbescu, B Li, VL Sprenkle, W Wang. "Radical Compatibility with Nonaqueous Electrolytes and Its Impact on an All-Organic Redox Flow Battery." Angewandte Chemie 54 (30):8684-8687 (July 2015).
Abstract:Nonaqueous redox flow batteries hold the promise of achieving higher energy density because of the broader voltage window than aqueous systems, but their current performance is limited by low redox material concentration, cell efficiency, cycling stability, and current density. We report a new nonaqueous all-organic flow battery based on high concentrations of redox materials, which shows significant, comprehensive improvement in flow battery performance. A mechanistic electron spin resonance study reveals that the choice of supporting electrolytes greatly affects the chemical stability of the charged radical species especially the negative side radical anion, which dominates the cycling stability of these flow cells. This finding not only increases our fundamental understanding of performance degradation in flow batteries using radical-based redox species, but also offers insights toward rational electrolyte optimization for improving the cycling stability of these flow batteries.

Kim JY, NL Canfield, JF Bonnett, VL Sprenkle, K Jung, I Hong. "A Duplex ß"-Al2O3 Solid Electrolyte Consisting of A Thin Dense Layer and A Porous Substrate."Solid State Ionics 278: 192-197 (June 2015).
Abstract:To improve the performance of Na-beta batteries at intermediate temperatures (≤200°C) where much improved cyclability and reduced degradation can be achieved, there is a need to lower the resistance/polarization coming from BASEs while maintaining good strength. In this paper, the concept of a duplex BASE consisting of a thin dense electrolyte and a porous support was proposed as a solution to achieve low area-specific resistance while maintaining good mechanical strength. The effects of various factors including porosity, composition, and the homogeneity of ingredients on the flexural strength of duplex BASEs were examined. In summary, lower porosity, higher YSZ content in the structure, and the attrition milling of powders resulted in improved strength. The area-specific resistance measurement exhibited that the resistance of duplex BASEs was mainly originated from a dense layer. Overall, the maximum strength of 260 MPa and the ASR value of 0.31 cm2 (at 350°C) was achieved from a duplex BASE consisting of a 50 µm thick dense layer (Al2O3: YSZ = 7:3 in volume) and a 500 µm thick porous support (Al2O3: YSZ = 4:6 in volume with 19% open porosity). The effects of various factors on the properties of duplex BASEs will be explained in details.

Li G, X Lu, JY Kim, V Viswanathan, KD Meinhardt, MH Engelhard, VL Sprenkle. "An Advanced Na-FeCl₂ ZEBRA Battery for Stationary Energy Storage Application." Advanced Energy Materials 5(12) (June 2015).
Abstract:In article number 1500357, Guosheng Li, Jin Y. Kim, and co-workers report a remarkably reliable Na-FeCl₂ ZEBRA battery for stationary energy storage applications. The removal of surface oxide passivation layers on iron particles is critical and it is attributed to polysulfide species generated from sulfur-based additives through polysulfide reactions. The Na-FeCl₂ cells presented can be assembled at the discharge state (NaCl + Fe powder) without handling highly hazardous materials such as anhydrous FeCl₂ and metallic sodium.

Shamie JS, C Liu, LL Shaw, VL Sprenkle. "Room Temperature, Hybrid Sodium-Based Flow Batteries with Multi-Electron Transfer Redox Reactions." Scientific Reports 5, article number 11215 (June 2015).
Abstract:We introduce a new concept of hybrid Na-based flow batteries (HNFBs) with a molten Na alloy anode in conjunction with a flowing catholyte separated by a solid Na-ion exchange membrane for grid-scale energy storage. Such HNFBs can operate at ambient temperature, allow catholytes to have multiple electron transfer redox reactions per active ion, offer wide selection of catholyte chemistries with multiple active ions to couple with the highly negative Na alloy anode, and enable the use of both aqueous and non-aqueous catholytes. Further, the molten Na alloy anode permits the decoupled design of power and energy since a large volume of the molten Na alloy can be used with a limited ion-exchange membrane size. In this proof-of-concept study, the feasibility of multi-electron transfer redox reactions per active ion and multiple active ions for catholytes has been demonstrated. The critical barriers to mature this new HNFBs have also been explored.

Wei X, B Li, W Wang. "Porous Polymeric Composite Separators for Redox Flow Batteries." Polymer Reviews 55(2):247-272 (May 2015).
Abstract:Currently, the most commonly used membranes in redox flow batteries (RFB) are ion-exchange membranes. In particular, in all vanadium flow battery systems (VRB), perfluorinated polymers such as Nafion^® are widely used, owing to their high proton conductivity and chemical stability; however, the extremely high cost of currently available membranes has limited the commercialization of VRB technology. Recently, low-cost porous polymeric composite separators (e.g., polytetrafluoroethylene [PTFE]/silica), as an alternative to traditional ion-exchange membranes, have attracted a great deal of interest because of their significantly lower cost. Porous separators prepared from various polymer materials and inorganic fillers have demonstrated comparable electrochemical performances to that of Nafion^® in flow battery tests with different redox chemistries. This paper provides a review of porous separators for flow battery applications. In addition to discussions of separator material selection and preparation methods, we also emphasize the electrochemical performance of various flow battery systems, especially the capacity fade mechanism that is closely related to ion-transport across porous separator.

Cheng Y, RM Stolley, KS Han, Y Shao, BW Arey, NM Washton, KT Mueller, ML Helm, VL Sprenkle, J Liu, G Li. "Highly Active Electrolytes for Rechargeable Mg Batteries Based on [Mg₂(μ-Cl)2]2+ Cation Complex in Dimethoxyethane. "Physical Chemistry Chemical Physics 17: 13307-13314 (Apr. 2015).
Abstract:A novel [Mg₂(μ-Cl)₂]²⁺ cation complex, which is highly active for reversible Mg electrodeposition, was identified for the first time in this work. This complex was found to be present in electrolytes formulated in dimethoxyethane (DME) through dehalodimerization of non-nucleophilic MgCl₂ by reacting with either Mg salts (such as Mg(TFSI)₂, TFSI = bis(trifluoromethane)sulfonylimide) or Lewis acid salts (such as AlEtCl₂ or AlCl₃). The molecular structure of the cation complex was characterized by single crystal X-ray diffraction, Raman spectroscopy and NMR. The electrolyte synthesis process was studied and rational approaches for formulating highly active electrolytes were proposed. Through control of the anions, electrolytes with an efficiency close to 100%, a wide electrochemical window (up to 3.5 V) and a high ionic conductivity (>6 mS cm^-1) were obtained. The understanding of electrolyte synthesis in DME developed in this work could bring significant opportunities for the rational formulation of electrolytes of the general formula [Mg₂(μ-Cl)₂][anion]_x for practical Mg batteries.

Vijayakumar M, N Govind, B Li, X Wei, Z Nie, S Thevuthasan, VL Sprenkle, W Wang. "Aqua-vanadyl ion interaction with Nafion^® membranes."Frontiers in Energy Research 3, article number 10 (March 2015).
Abstract:Lack of comprehensive understanding about the interactions between Nafion membrane and battery electrolytes prevents the straightforward tailoring of optimal materials for redox flow battery applications. In this work, we analyzed the interaction between aqua-vanadyl cation and sulfonic sites within the pores of Nafion membranes using combined theoretical and experimental X-ray spectroscopic methods. Molecular level interactions, namely, solvent share and contact pair mechanisms are discussed based on vanadium and sulfur K-edge spectroscopic analysis.

Li B, Z Nie, M Vijayakumar, G Li, J Liu, VL Sprenkle, W Wang. "Ambipolar zinc-polyiodide electrolyte for a high-energy density aqueous redox flow battery." Nature Communications 6 article number 6303 (Feb. 2015).
Abstract:Redox flow batteries are receiving wide attention for electrochemical energy storage due to their unique architecture and advantages, but progress has so far been limited by their low energy density (~25Whl^-1). Here we report a high-energy density aqueous zinc-polyiodide flow battery. Using the highly soluble iodide/triiodide redox couple, a discharge energy density of 167Whl^-1 is demonstrated with a near-neutral 5.0 M Znl₂ electrolyte. Nuclear magnetic resonance study and density functional theory-based simulation along with flow test data indicate that the addition of an alcohol (ethanol) induces ligand formation between oxygen on the hydroxyl group and the zinc ions, which expands the stable electrolyte temperature window to from -20 to 50°C, while ameliorating the zinc dendrite. With the high-energy density and its benign nature free from strong acids and corrosive components, zinc-polyiodide flow battery is a promising candidate for various energy storage applications.

Li G, X Lu, JY Kim, MH Engelhard, JP Lemmon, and VL Sprenkle. "The Role of FeS in Initial Activation and Performance Degradation of Na-NiCl2 Batteries." Journal of Power Sources 272:398-403 (Dec 2014).
Abstract: The role of iron sulfide (FeS) in initial cell activation and degradation in the Na-NiCl2 battery was investigated in this work. The research focused on identifying the effects of the FeS level on the electrochemical performance and morphological changes in the cathode. The x-ray photoelectron spectroscopy study along with battery tests revealed that FeS plays a critical role in initial battery activation by removing passivation layers on Ni particles. It was also found that the optimum level of FeS in the cathode resulted in minimum Ni particle growth and improved battery cycling performance. The results of electrochemical characterization indicated that sulfur species generated in situ during initial charging, such as polysulfide and sulfur, are responsible for removing the passivation layer. Consequently, the cells containing elemental sulfur in the cathode exhibited similar electrochemical behavior during initial charging compared to that of the cells containing FeS.

Wei X, W Xu, M Vijayakumar, L Cosimbescu, TL Liu, VL Sprenkle, and W Wang. "TEMPO-based Catholyte for High Energy Density Nonaqueous Redox Flow Batteries." Advanced Materials 26(45):7649-7653 (Dec 2014).
Abstract:A TEMPO-based non-aqueous electrolyte with the TEMPO concentration as high as 2.0 _M is demonstrated as a high-energy-density catholyte for redox flow battery applications. With a hybrid anode, Li|TEMPO flow cells using this electrolyte deliver an energy efficiency of ca. 70% and an impressively high energy density of 126 W h L^-1.

Vijayakumar, M., Nie, Z., Walter, E., Hu, J., Liu, J., Sprenkle, V. and Wang, W. "Understanding Aqueous Electrolyte Stability through Combined Computational and Magnetic Resonance Spectroscopy: A Case Study on Vanadium Redox Flow Battery Electrolytes." ChemPlusChem doi: 10.1002/cplu.201402139 (Sept 2014).
Abstract: Commercial sodium-sulphur or sodium-metal halide batteries typically need an operating temperature of 300-350°C, and one of the reasons is poor wettability of liquid sodium on the surface of beta alumina. Here we report an alloying strategy that can markedly improve the wetting, which allows the batteries to be operated at much lower temperatures. Our combined experimental and computational studies suggest that addition of caesium to sodium can markedly enhance the wettability. Single cells with Na-Cs alloy anodes exhibit great improvement in cycling life over those with pure sodium anodes at 175 and 150°C. The cells show good performance even at as low as 95°C. These results demonstrate that sodium-beta alumina batteries can be operated at much lower temperatures with successfully solving the wetting issue. This work also suggests a strategy to use liquid metals in advanced batteries that can avoid the intrinsic safety issues associated with dendrite formation.

Cheng Y, LR Parent, Y Shao, CM Wang, VL Sprenkle, G Li, and J Liu. "Facile Synthesis of Chevrel Phase Nanocubes and their Applications for Multivalent Energy Storage." Chemistry of Materials 26(17):4904-4907. doi:10.1021/cm502306c (Aug 2014).
Abstract: The Chevrel phases (CPs, MxMo6T8, M=metal, T=S or Se) are capable of rapid and reversible intercalation of multivalent ions and are the most practical cathode materials for rechargeable magnesium batteries. For the first time, we report a facile method for synthesizing Mo6S8 nanoparticles and demonstrate that these nanoparticles have significantly better Mg2+ intercalation kinetics compared with microparticles. The results described in this work could inspire the synthesis of nanoscale CPs, which could substantially impact their application.

Wei X, L Cosimbescu, W Xu, JZ Hu, M Vijayakumar, J Feng, MY Hu, X Deng, J Xiao, J Liu, VL Sprenkle, and W Wang. "Towards High Performance Nonaqueous Redox Flow Electrolyte Via Ionic Modification of Active Species." Advanced Energy Materials (1400678), doi:DOI: 10.1002/aenm.201400678 (Aug 2014).
Abstract: Nonaqueous redox flow batteries are emerging flow-based energy storage technologies that have the potential for higher energy densities than their aqueous counterparts because of their wider voltage windows. However, their performance has lagged far behind their inherent capability due to one major limitation of low solubility of the redox species. Here, a molecular structure engineering strategy towards high performance nonaqueous electrolyte is reported with significantly increased solubility. Its performance outweighs that of the state-of-the-art nonaqueous redox flow batteries. In particular, an ionic-derivatized ferrocene compound is designed and synthesized that has more than 20 times increased solubility in the supporting electrolyte. The solvation chemistry of the modified ferrocene compound. Electrochemical cycling testing in a hybrid lithium-organic redox flow battery using the as-synthesized ionic-derivatized ferrocene as the catholyte active material demonstrates that the incorporation of the ionic-charged pendant significantly improves the system energy density. When coupled with a lithium-graphite hybrid anode, the hybrid flow battery exhibits a cell voltage of 3.49 V, energy density about 50 Wh L-1, and energy efficiency over 75%. These results reveal a generic design route towards high performance nonaqueous electrolyte by rational functionalization of the organic redox species with selective ligand.

X. Lu, G. Li, J.Y. Kim, D. Mei, J.P. Lemmon, and V.L. Sprenkle, "Liquid-metal electrode to enable ultra-low temperature sodium–beta alumina batteries for renewable energy storage." Nat. Commun. 5:4578 (Aug 2014).
Abstract: Commercial sodium–sulphur or sodium-metal halide batteries typically need an operating temperature of 300–350°C, and one of the reasons is poor wettability of liquid sodium on the surface of beta alumina. Here we report an alloying strategy that can markedly improve the wetting, which allows the batteries to be operated at much lower temperatures. Our combined experimental and computational studies suggest that addition of caesium to sodium can markedly enhance the wettability. Single cells with Na-Cs alloy anodes exhibit great improvement in cycling life over those with pure sodium anodes at 175 and 15°C. The cells show good performance even at as low as 95°C. These results demonstrate that sodium–beta alumina batteries can be operated at much lower temperatures with successfully solving the wetting issue. This work also suggests a strategy to use liquid metals in advanced batteries that can avoid the intrinsic safety issues associated with dendrite formation.

BR Chalamala, T Soundappan, GR Fisher, MA Anstey, VV Viswanathan, ML Perry. "Redox Flow Batteries: An Engineering Perspective." Proceedings of the IEEE 102 (6): 976 - 999 (June 2014).
Abstract:Redox flow batteries are well suited to provide modular and scalable energy storage systems for a wide range of energy storage applications. In this paper, we review the development of redox-flow-battery technology including recent advances in new redox active materials, cell designs, and systems, all from the perspective of engineers interested in applying this technology. We discuss cost, performance, and reliability metrics that are critical for deployment of large flow-battery systems. The technology, while relatively young, has the potential for significant improvement through reduced materials costs, improved energy efficiency, and significant reduction in the overall system costs.

Yingwen Cheng, Yuyan Shao, Ji-Guang Zhang, Vincent L. Sprenkle, Jun Liu and Guosheng Li, "High performance batteries based on hybrid magnesium and lithium chemistry" Chem. Commun., 2014, 50, 9644-9646 (June 2014).
Abstract: This work studied hybrid batteries assembled with a Mg metal anode, a Li+ ion intercalation cathode and a dual-salt electrolyte containing Mg2+ and Li+ ions. We show that such hybrid batteries were able to combine the advantages of Li and Mg electrochemistry. They delivered outstanding rate performance (83% capacity retention at 15 C) with superior safety and stability (B5% fade for 3000 cycles).

Ran Yi, Jinkui Feng, Dongping Lu, Mikhail Gordin, Shuru Chen, Daiwon Choi, Donghai Wang, "GeO_x/Reduced Graphene Oxide Composite as an Anode for Li-ion Batteries: Enhanced Capacity via Reversible Utilization of Li₂O along with Improved Rate Performance" Advanced Functional Materials, 24, p.1059-1066 (Feb 2014).
Abstract: A self-assembled GeO_x/reduced graphene oxide (GeO_x/RGO) composite, where GeO_x nanoparticles are grown directly on reduced graphene oxide sheets, is synthesized via a facile one-step reduction approach and studied by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, electron energy loss spectroscopy elemental mapping, and other techniques. Electrochemical evaluation indicates that incorporation of reduced graphene oxide enhances both the rate capability and reversible capacity of GeO_x, with the latter being due to the RGO enabling reversible utilization of Li₂O. The composite delivers a high reversible capacity of 1600 mAh g^-1 at a current density of 100 mA g^-1, and still maintains a capacity of 410 mAh g^-1 at a high current density of 20 A g^-1. Owing to the flexible reduced graphene oxide sheets enwrapping the GeO_x particles, the cycling stability of the composite is also improved significantly. To further demonstrate its feasibility in practical applications, the synthesized GeO_x/RGO composite anode is successfully paired with a high voltage LiN_i0.5Mn_1.5O₄ cathode to form a full cell, which shows good cycling and rate performance.

Viswanathan VV, AJ Crawford, DE Stephenson, S Kim, W Wang, B Li, GW Coffey, EC Thomsen, GL Graff, PJ Balducci, MCW Kintner-Meyer, and VL Sprenkle. 2014. "Cost and Performance Model for Redox Flow Batteries." Journal of Power Sources, 247:1040-1051. doi:10.1016/j.jpowsour.2012.12.023 (Feb 2014).
Abstract: A cost model is developed for all vanadium and iron-vanadium redox flow batteries. Electrochemical performance modeling is done to estimate stack performance at various power densities as a function of state of charge and operating conditions. This is supplemented with a shunt current model and a pumping loss model to estimate actual system efficiency. The operating parameters such as power density, flow rates and design parameters such as electrode aspect ratio and flow frame channel dimensions are adjusted to maximize efficiency and minimize capital costs. Detailed cost estimates are obtained from various vendors to calculate cost estimates for present, near-term and optimistic scenarios. The most cost-effective chemistries with optimum operating conditions for power or energy intensive applications are determined, providing a roadmap for battery management systems development for redox flow batteries. The main drivers for cost reduction for various chemistries are identified as a function of the energy to power ratio of the storage system.

G, Li, X. Lu, J.Y. Kim, J.P. Lemmon, and V.L. Sprenkle, "Improved cycling behavior of ZEBRA battery operated at intermediate temperature of 175°C," Journal of Power Sources, 249 (2014) 414-417 (Jan. 2014).
Abstract: Operation of the sodium-nickel chloride battery at temperatures below 200°C reduces cell degradation and improves cyclability. One of the main technical issues with operating this battery at intermediate temperatures such as 175°C is the poor wettability of molten sodium on β"”-alumina solid electrolyte (BASE), which causes reduced active area and limits charging. In order to overcome the poor wettability of molten sodium on BASE at 175°C, a Pt grid was applied on the anode side of the BASE using a screen printing technique. Cells with their active area increased by metallized BASEs exhibited deeper charging and stable cycling behavior.

B Li, M Gu, Z Nie, X Wei, C Wang, V Sprenkle, and W Wang, "Nanorod Niobium Oxide as Powerful Catalysts for an All Vanadium Redox Flow Battery", Nano Letters, 2014, 14, 158-165 (Dec 2013).
Abstract: A powerful low-cost electrocatalyst, nanorod Nb₂O₅, is synthesized using hydrothermal method with monoclinic phases and simultaneously deposited on the surface of graphite felt (GF) electrode in an all vanadium flow battery (VRB). Cyclic voltammetry (CV) study confirmed that Nb₂O₅ has catalytic effects towards redox couples of V(II)/V(III) at the negative side and V(IV)/V(V) at the positive side to facilitate the electrochemical kinetics of the vanadium redox reactions. Because of poor conductivity of Nb₂O₅, the performance of the Nb₂O₅ loaded electrodes is strongly dependent on the nanosize and uniform distribution of catalysts on GFs surfaces. Accordingly, optimal amounts of W-doped Nb₂O₅ nanorods with minimum agglomeration and improved distribution on GFs surfaces are established by adding water-soluble compounds containing tungsten (W) into the precursor solutions. The corresponding energy efficiency is enhanced by ~10.7% at high current density (150 mA.cm^-2) as compared with one without catalysts. Flow battery cyclic performance also demonstrates the excellent stability of the as prepared Nb₂O₅ catalyst enhanced electrode. These results suggest that Nb₂O₅-based nanorods, replacing expensive noble metals, uniformly decorating GFs holds great promise as high-performance electrodes for VRB applications.

G. Li, X. Lu, J.Y. Kim, J.P. Lemmon, and V.L. Sprenkle, "Cell Degradation of a Na-NiCl₂ (ZEBRA) Battery," Journal of Materials Chemistry A, 47 (2013) 14935 - 14942 (Nov 2013).
Abstract: In this work, the parameters influencing the degradation of a Na-NiCl₂ (ZEBRA) battery were investigated. Planar Na-NiCl₂ cells using β"”-alumina solid electrolyte (BASE) were tested with different C-rates, Ni/NaCl ratios, and capacity windows, in order to identify the key parameters for the degradation of Na-NiCl₂ battery. The morphology of NaCl and Ni particles were extensively investigated after 60 cycles under various test conditions using a scanning electron microscope. A strong correlation between the particle size (NaCl and Ni) and battery degradation was observed in this work. Even though the growth of both Ni and NaCl can influence the cell degradation, our results indicate that the growth of NaCl is a dominant factor in cell degradation. The use of excess Ni seems to play a role in tolerating the negative effects of particle growth on degradation since the available active surface area of Ni particles can be still sufficient even after particle growth. For NaCl, a large cycling window was the most significant factor, of which effects were amplified with decrease in Ni/NaCl ratio.

Vijayakumar M, W Wang, Z Nie, VL Sprenkle, and JZ Hu. "Elucidating the Higher Stability of Vanadium (V) Cations in Mixed Acid Based Redox Flow Battery Electrolytes." Journal of Power Sources 241:173-177. doi:10.1016/j.jpowsour.2013.04.072 (Nov 2013).
Abstract:The Vanadium (V) cation structures in mixed acid based electrolyte solution were analysed by density functional theory (DFT) based computational modelling and ⁵¹V and ³⁵Cl Nuclear Magnetic Resonance (NMR) spectroscopy. The Vanadium (V) cation exists as di-nuclear [V₂O₃Cl₂.6H₂O]²⁺ compound at higher vanadium concentrations (=1.75M). In particular, at high temperatures (>295K) this di-nuclear compound undergoes ligand exchange process with nearby solvent chlorine molecule and forms chlorine bonded [V₂O₃Cl₂.6H₂O]²⁺ compound. This chlorine bonded [V₂O₃Cl₂.6H₂O]²⁺ compound might be resistant to the de-protonation reaction which is the initial step in the precipitation reaction in Vanadium based electrolyte solutions. The combined theoretical and experimental approach reveals that formation of chlorine bonded [V₂O₃Cl₂.6H₂O]²⁺ compound might be central to the observed higher thermal stability of mixed acid based Vanadium (V) electrolyte solutions.

B Li, Q Luo, X Wei, Z Nie, E Thomsen, B Chen, V Sprenkle, and W Wang, "Capacity Decay Mechanism of Microporous Separator-Based All-Vanadium Redox Flow Batteries and its Recovery", ChemSusChem, 2014, 7, 577-584 (Oct 2013).
Abstract: The results of the investigation of the capacity decay mechanism of vanadium redox flow batteries with microporous separators as membranes are reported. The investigation focuses on the relationship between the electrochemical performance and electrolyte compositions at both the positive and negative half-cells. Although the concentration of total vanadium ions remains nearly constant at both sides over cycling, the net transfer of solution from one side to the other and thus the asymmetrical valance of vanadium ions caused by the subsequent disproportionate self-discharge reactions at both sides lead to capacity fading. Through in situ monitoring of the hydraulic pressure of the electrolyte during cycling at both sides, the convection was found to arise from differential hydraulic pressures at both sides of the separators and plays a dominant role in capacity decay. A capacity-stabilizing method is developed and was successfully demonstrated through the regulation of gas pressures in both electrolyte tanks.

Soowhan Kim, Edwin Thomsen, Guanguang Xia, Zimin Nie, Jie Bao, Kurtis Recknagle, Wei Wang, Vilayanur Viswanathan, Qingtao Luo, Xiaoliang Wei, Alasdair Crawford, Greg Coffey, Gary Maupin, Vincent Sprenkle. "1 kW/1 kWh advanced vanadium redox flow battery utilizing mixed acid electrolytes". Journal of Power Sources 237 (2013) 300-309 (Sept 2013).
Abstract: This paper reports on the recent demonstration of an advanced vanadium redox flow battery (VRFB) using a newly developed mixed acid (sulfuric and hydrochloric acid) supporting electrolyte at a kW scale. The developed prototype VRFB system is capable of delivering more than 1.1 kW in the operation range of 15~85% state of charge (SOC) at 80 mA cm^-2 with an energy efficiency of 82% and energy content of 1.4 kWh. The system operated stably without any precipitation at electrolyte temperatures >45°C. At similar electrolyte temperatures, tests with a conventional sulfuric acid electrolyte suffered from precipitation after 80 cycles. By operating stably at elevated temperatures (>40°C), the mixed acid system enables significant advantages over the conventional sulfate system, namely; 1) high stack energy efficiency due to better kinetics and lower electrolyte resistance, 2) lower viscosity resulting in reduced pumping losses, 3) lower capital cost by elimination of heat exchanger, 4) higher system efficiency and 5) simplified system design and operation. Demonstration of the prototype stack with the mixed acid electrolyte has been shown to lower the cost of conventional VRFB systems for large-scale energy storage applications.

X. Wei, Q. Luo, B. Li, Z. Nie, E. Miller, J. Chambers, V. Sprenkle, and W. Wang. "Performance Evaluation of Microporous Separator in Fe/V Redox Flow Battery." ECS Transactions 45(26):17-24. (Sept 2013).
Abstract: The newly developed Fe/V redox flow battery has demonstrated attractive cell performance. However, the deliverable energy density is relatively low due to the reduced cell voltage. To compensate this disadvantage and compete with other redox flow battery systems, cost reduction of the Fe/V system is necessary. This paper describes evaluation of hydrocarbon-based Daramic^® microporous separators for use in the Fe/V system. These separators are very inexpensive and have exceptional mechanical properties. Separator B having ion exchange capacity demonstrated excellent capacity retention capability, and exhibited energy efficiency above 65% over a broad temperature range of 5-50°C and at current densities up to 80mA/cm². Therefore, this separator shows great potential to replace the expensive Nafion^® membrane. This will drive down the capital cost and make the Fe/V system a promising low-cost energy storage technology.

W Xu, X Chen, W Wang, D Choi, F Ding, J Zheng, Z Nie, YJ Choi, J Zhang, and Z Yang. "Simply AlF3-treated Li₄Ti₅O₁₂ composite anode materials for stable and ultrahigh power lithium-ion batteries."Journal of Power Sources 236 (2013), 169 -174. (Aug 2013).
Abstract: The commercial Li₄Ti₅O₁₂ (LTO) is successfully modified by AlF₃ via a low temperature process. After being calcined at 400°C for 5 h, AlF₃ reacts with LTO to form a composite material, which mainly consists of Al₃+ and F- co-doped LTO with small amounts of anatase TiO₂. Al³⁺ and F- co-doped LTO demon- strates ultrahigh rate capability comparing to the pristine LTO. Since the amount of the byproduct TiO₂ is relatively small, the modified LTO electrodes retain the main voltage characteristics of LTO with a minor feature similar to those of anatase TiO₂. The doped LTO anodes deliver slightly higher discharge capacity and maintain the excellent long-term cycling stability when compared to the pristine LTO anode. Therefore, Al³⁺ and F- co-doped LTO composite material synthesized at low temperature is an excellent stable and ultra-high power lithium-ion batteries.

Dongyang Chen, Soowhan Kim, Vincent Sprenkle, Michael A. Hickner. "Composite blend polymer membranes with increased proton selectivity and lifetime for vanadium redox flow batteries". Journal of Power Sources 231 (2013) 301-306. (Jun 2013).
Abstract: Composite membranes based on blends of sulfonated fluorinated poly(arylene ether) (SFPAE) and poly(vinylidene fluoride-co-hexafluoropropene) (P(VDF-co-HFP)) were prepared with varying P(VDF-co-HFP) content for vanadium redox flow battery (VRFB) applications. The properties of the SFPAE-P(VDF-co-HFP) blends were characterized by atomic force microscopy, differential scanning calorimetry, and Fourier transform infrared spectroscopy. The water uptake, mechanical properties, thermal properties, proton conductivity, VO²⁺] permeability and VRFB cell performance of the composite membranes were investigated in detail and compared to the pristine SFPAE membrane. It was found that SFPAE had good compatibility with P(VDF-co-HFP) and the incorporation of P(VDF-co-HFP) increased the mechanical properties, thermal properties, and proton selectivity of the materials effectively. An SFPAE composite membrane with 10 wt.% P(VDF-co-HFP) exhibited a 44% increase in VRFB cell lifetime as compared to a cell with a pure SFPAE membrane. Therefore, the P(VDF-co-HFP) blending approach is a facile method for producing low-cost, high-performance VRFB membranes.

Xiaochuan Lu, Guosheng Li, Jin Y Kim, John P. Lemmon, Vincent L Sprenkle, Zhenguo Yang, "A novel low-cost sodium-zinc chloride battery." Energy & Environmental Science 6(6): 1837-1843 (Jun 2013).
Abstract: The sodium-metal halide (ZEBRA) battery has been considered as one of the most attractive energy storage systems for stationary and transportation applications. Even though Na-NiCl2 battery has been widely investigated, there is still a need to develop a more economical system to make this technology more attractive for commercialization. In the present work, a novel low-cost Na-ZnCl2 battery with a planar β"-Al2O3 solid electrolyte (BASE) was proposed, and its electrochemical reactions and battery performance were investigated. Compared to the Na-NiCl2 chemistry, the ZnCl2-based chemistry was more complicated, in which multiple electrochemical reactions including liquid-phase formation occurred at temperatures above 253°C. During the first stage of charge, NaCl reacted with Zn to form Na in the anode and Na2ZnCl4 in the cathode. Once all the residual NaCl was consumed, further charging led to the formation of a NaCl-ZnCl2 liquid phase. At the end of charge, the liquid phase reacted with Zn to produce solid ZnCl2. To identify the effects of liquid-phase formation on electrochemical performance, button cells were assembled and tested at 280°C and 240°C. At 280°C where the liquid phase formed during cycling, cells revealed quite stable cyclability. On the other hand, more rapid increase in polarization was observed at 240°C where only solid-state electrochemical reactions occurred. SEM analysis indicated that the stable performance at 280°C was due to the suppressed growth of Zn and NaCl particles, which were generated from the liquid phase during discharge of each cycle.

Bin Li, Liyu Li, Wei Wang, Zimin Nie, Baowei Chen, Xiaoliang Wei, Qingtao Luo, Zhenguo Yang, and Vincent Sprenkle. "Fe/V Redox Flow Battery Electrolyte Investigation and Optimization". Journal of Power Sources 229 (2013) 1-5. (May 2013).
Abstract: The recently invented iron (Fe)/vanadium (V) redox flow battery (IVB) system has attracted increasing attention because of its long-term cycling stability and low-cost membrane/separator. In this paper, we describe our extensive matrix study of factors such as electrolyte composition; state of charge (SOC), and temperature that influence the stability of electrolytes in both positive and negative half-cells. During the study, an optimized electrolyte that can be operated in a temperature range from -5°C to 50°C without precipitation is identified. Fe/V flow cells using the optimized electrolyte and low-cost separator exhibit satisfactory cycling performance at different temperatures. Efficiencies, capacities, and energy densities of flow batteries at various temperatures are studied.

X. Wei, Z Nie, Q Luo, B Li, V. Sprenkle, and W. Wang, "Polyvinyl Chloride/Silica Nanoporous Composite Separator for All-Vanadium Redox Flow Battery Applications." Journal of the Electrochemical Society, 160(8):A1215 - A1218, 2013. (May 2013).
Abstract: We demonstrate application of a commercial nanoporous polyvinyl chloride (PVC)/silica separator in an all-vanadium redox flow battery (VRB) as a low-cost alternative to expensive Nafion^® membranes. This hydrophilic separator is composed of silica particles enmeshed in a PVC matrix that creates unique porous structures. These nano-scale pores with an average pore size of 45nm and a porosity of 65% serve as ion transport channels that are critically important for flow battery operation. The VRB flow cell using the PVC/silica separator produces excellent electrochemical performance in a mixed-acid VRB system with average energy efficiency (EE) of 79% at the current density of 50mAcm^-2. This separator affords the VRB flow cell with excellent rate capability with its EE higher than that of Nafion^® membrane at current densities above 100mAcm^-2. With this separator, the EE of the VRB flow cell exhibits great tolerance to temperature fluctuations in the typical operational temperature range of the mixed-acid VRB system. More importantly, the flow cell using the separator demonstrates an excellent capacity retention over cycling, which enables the VRB system to operate in the long term with minimal electrolyte maintenance.

D Reed, G Coffey, E Mast, N Canfield, J Mansurov, X Lu, VL Sprenkle. "Wetting of sodium on ß-Al₂O₃/YSZ composites for low temperature planar sodium-metal halide batteries". Journal of Power Sources 227: 94-100 (Apr 2013).
Abstract:Wetting of Na on ß-Al₂O₃/YSZ composites was investigated using the sessile drop technique. The effects of moisture and surface preparation were studied at low temperatures. Electrical conductivity of Na/ß-Al₂O₃/YSZ/Na cells was also investigated at low temperatures and correlated to the wetting behavior. The use of planar ß-Al₂O₃ substrates at low temperature with low cost polymeric seals is realized due to improved wetting at low temperature and conductivity values consistent with the literature.

X. Wei, Z. Nie, Q. Luo, B. Li, B. Chen, K. Simmons, V. Sprenkle, W. Wang, "Nanoporous Polytetrafluoroethylene/Silica Composite Separator as a High-Performance All-Vanadium Redox Flow Battery Membrane". Advanced Energy Materials, 3, 1215-1220, 2013. (Apr 2013).
Abstract:A novel low-cost nanoporous polytetrafluoroethylene (PTFE)/silica composite separator has been prepared and evaluated for its use in an all-vanadium redox flow battery (VRB). The separator consists of silica particles enmeshed in a PTFE fibril matrix. It possesses unique nanoporous structures with an average pore size of 38 nm and a porosity of 48%. These pores function as the ion transport channels during redox flow battery operation. This separator provides excellent electrochemical performance in the mixed-acid VRB system. The VRB using this separator delivers impressive energy efficiency, rate capability, and temperature tolerance. In addition, the flow cell using the novel separator also demonstrates an exceptional capacity retention capability over extended cycling, thus offering excellent stability for long-term operation. The characteristics of low cost, excellent electrochemical performance and proven chemical stability afford the PTFE/silica nanoporous separator great potential as a substitute for the Nafion membrane used in VRB applications.

B. Li, M. Gu, Z. Nie, Y. Shao, Q. Luo, X. Wei, X. Li, J. Xiao, C. Wang, V. Sprenkle, and W. Wang, "Bismuth Nanoparticle Decorating Graphite Felt as a High-Performance Electrode for an All-Vanadium Redox Flow Battery". Nano Letters, 13, 1330-1335, 2013. (Feb 2013).
Abstract: Employing electrolytes containing Bi3+, bismuth nanoparticles are synchronously electrodeposited onto the surface of a graphite-felt electrode during operation of an all-vanadium redox flow battery (VRFB). The influence of the Bi nanoparticles on the electrochemical performance of the VRFB is thoroughly investigated. It is confirmed that Bi is only present at the negative electrode and facilitates the redox reaction between V(II) and V(III). However, the Bi nanoparticles significantly improve the electrochemical performance of VRFB cells by enhancing the kinetics of the sluggish V(II)/V(III) redox reaction, especially under high power operation. The energy efficiency is increased by 11% at high current density (150 mA.cm-2) owing to faster charge transfer as compared with one without Bi. The results suggest that using Bi nanoparticles in place of noble metals offers great promise as high-performance electrodes for VRFB application.

Q Luo, L Li, W Wang, Z Nie, X Wei, B Li, B Chen, Z Yang, and VL Sprenkle. "Capacity Decay and Remediation of Nafion-based All-Vanadium Redox Flow Batteries". ChemSusChem 6(2) 268-274. (Feb 2013).
Abstract: The relationship between the electrochemical performance of vanadium redox flow batteries (VRB) and electrolyte compositions has been investigated, and the reasons for capacity decay over charge-discharge cycling have been analyzed and are discussed in this paper. The results show that the reasons for capacity fading over real charge-discharge cycling include not only the imbalanced vanadium active species, but also the asymmetrical valence of vanadium ions in positive and negative electrolytes. The asymmetrical valence of vanadium ions leads to the SOC range to decrease in positive electrolyte and increase in negative electrolyte, respectively. As a result, the higher SOC range in negative half-cells further aggravate the capacity fading by creating a higher over-potential and possible hydrogen evolution. Based on this finding, we developed two methods for restoring the lost capacity; thereby enabling long-term operation of VRBs to be achieved without the substantial loss of energy resulting from periodic remixing of electrolytes.

W Wang, Q Luo, B Li, X Wei, L Li, and Z Yang. "Recent Progress in Redox Flow Battery Research and Development".Advanced Functional Materials 23 (8), 970-986.(Feb 2013).
Abstract: With the increasing need to seamlessly integrate renewable energy with the current electricity grid, which itself is evolving into a more intelligent, efficient, and capable electrical power system, it is envisioned that energy-storage systems will play a more prominent role in bridging the gap between current technology and a clean sustainable future in grid reliability and utilization. Redox flow battery technology is a leading approach in providing a well-balanced approach for current challenges. In this paper, we review recent progress in the research and development of redox flow battery technology, including cell-level components of electrolytes, electrodes, and membranes. Our review focuses on new redox chemistries for both aqueous and non-aqueous systems.

X. Lu, JP Lemmon, JY Kim, VL Sprenkle, and ZG Yang. "High Energy Density Na-S/NiCl2 Hybrid Battery." Journal of Power Sources 224 (2013) 312- 316. (Feb 2013).
Abstract: High temperature (250-350°C) sodium-beta alumina batteries (NBBs) are attractive energy storage devices for renewable energy integration and other grid related applications. Currently, two technologies are commercially available in NBBs, e.g., sodium-sulfur (Na-S) battery and sodium-metal halide (ZEBRA) batteries. In this study, we investigated the combination of these two chemistries with a mixed cathode. In particular, the cathode of the cell consisted of molten NaAlCl4 as a catholyte and a mixture of Ni, NaCl and Na2S as active materials. During cycling, two reversible plateaus were observed in cell voltage profiles, which matched electrochemical reactions for Na-S and Na-NiCl2 redox couples. An irreversible reaction between sulfur species and Ni was identified during initial charge at 280°C, which caused a decrease in cell capacity. The final products on discharge included Na2Sn with 1< n < 3, which differed from Na2S3 found in traditional Na-S battery. Reduction of sulfur in the mixed cathode led to an increase in overall energy density over ZEBRA batteries. Despite of the initial drop in cell capacity, the mixed cathode demonstrated relatively stable cycling with more than 95% of capacity retained over 60 cycles under 10mA/cm2. Optimization of the cathode may lead to further improvements in battery performance.

Wang W, D Choi, and Z Yang."Li-Ion Battery with LiFePO4 Cathode and Li₄Ti₅O₁₂ Anode for Stationary Energy Storage". Metallurgical and Materials Transactions A, Physical Metallurgy and Materials Science 44A(1 Supplement): 21-25. (Jan 2013).
Abstract: Li-ion batteries based on commercially available LiFePO₄ cathode and Li₄Ti₅O₁₂ anode were investigated for potential stationary energy storage applications. The full cell that operated at flat 1.85 V demonstrated stable cycling up to 200 cycles followed by a rapid fade. A Li-ion full cell with Ketjen black modified LiFePO₄ cathode and an unmodified Li₄Ti₅O₁₂ anode exhibited negligible fade after more than 1200 cycles with a capacity of ~130 mAh/g at C/2. The improved stability, along with its cost-effectiveness, environmental benignity, and safety, make the LiFePO₄/Li₄Ti₅O₁₂ combination Li-ion battery a promising option for storing renewable energy.

X Lu, BW Kirby, W Xu, G Li, JY Kim, JP Lemmon, VL Sprenkle, and ZG Yang. "Advanced Intermediate-Temperature Na-S Battery." Energy & Environmental Science 6(1) (2013) 299 - 306. (Jan 2013).
Abstract: In this study, we reported an intermediate-temperature (~150°C) sodium-sulfur (Na-S) battery. With a reduced operating temperature, this novel battery can potentially reduce the cost and safety issues associated with the conventional high-temperatures (300~350°C) Na-S battery. A dense β"-Al2O3 solid membrane and tetraglyme were utilized as the electrolyte separator and catholyte solvent in this battery. Solubility tests indicated that cathode mixture of Na2S4 and S exhibited extremely high solubility in tetraglyme (e.g., > 4.1 M for Na2S4 + 4 S). CV scans of Na2S4 in tetraglyme revealed two pairs of redox couples with peaks at around 2.22 and 1.75 V, corresponding to the redox reactions of polysulfide species. The discharge/charge profiles of the Na-S battery showed a slope region and a plateau, indicating multiple steps and cell reactions. In-situ Raman spectra during battery operation suggested that polysulfide species were formed in the sequence of Na2S5 + S → Na2S5 + Na2S4 → Na2S4 + Na2S2 during discharge and in a reverse order during charge. This battery showed dramatic improvement in rate capacity and cycling stability over room-temperature Na-S batteries, which makes it extremely attractive for renewable energy integration and other grid related applications.

GS Li, XC Lu, CA Coyle, JY Kim, JP Lemmon, VL Sprenkle, and ZG Yang. "Novel ternary molten salt electrolytes for intermediate-temperature sodium/nickel chloride batteries."Journal of Power Sources 220, 193 -198. (Dec 2012).
Abstract: The sodium-nickel chloride (ZEBRA) battery is typically operated at relatively high temperature (250~350°C) to achieve adequate electrochemical performance. Reducing the operating temperature in the range of 150 to 200°C can lead to enhanced cycle life by suppressing temperature related degradation mechanisms. The operation at these intermediate temperatures also allows for lower cost materials of construction such as elastomeric sealants and gaskets. To achieve adequate electrochemical performance at lower operating temperatures requires an overall reduction in ohmic losses associated with temperature. This includes reduction in the ohmic resistance of β"-alumina solid electrolyte (BASE) and the incorporation of low melting point molten salt as the secondary electrolyte. In present work, planar-type Na/NiCl2 cells with a thin flat BASE (600 μm) and low melting point secondary electrolyte were evaluated at reduced temperatures. Molten salts used as secondary electrolytes were fabricated by the partial replacement of NaCl in the standard secondary electrolyte (NaAlCl4) with other lower melting point alkali metal salts such as NaBr, LiCl, and LiBr. Electrochemical characterization of these ternary molten salts demonstrated improved ionic conductivity and sufficient electrochemical window at reduced temperatures. Furthermore, Na/NiCl2 cells with 50 mol% NaBr-containing secondary electrolyte exhibited reduced polarizations at 175°C compared to the cell with the standard NaAlCl4 catholyte. The cells also exhibited stable cycling performance even at 150°C.

Q Luo, L Li, Z Nie, W Wang, X Wei, B Li, B Chen, Z Yang. "In-situ investigation of vanadium ion transport in redox flow battery." Journal of Power Sources 218 (2012) 15-30 (Nov 2012).
Abstract: Flow batteries with vanadium and iron redox couples as the electroactive species were employed to investigate the transport behavior of vanadium ions in the presence of an electric field. It was shown that the electric field accelerated the positive-to-negative and reduced the negative-to-positive transport of vanadium ions in the charging process and affected the vanadium ion transport in the opposite way during discharge. In addition, a method was designed to differentiate the concentration-gradient-driven vanadium ion diffusion and electric-field-driven vanadium ion migration. A simplified mathematical model was established to simulate the vanadium ion transport in real charge-discharge operation of the flow battery. The concentration gradient diffusion coefficients and electric-migration coefficients of V²⁺, V³⁺, VO²⁺, and VO₂⁺ across a NAFION^® membrane were obtained by fitting the experimental data.

X Wei, L Li, Q Luo, Z Nie, W Wang, B Li, GG Xia, E Millar, J Chambers, Z Yang. "Microporous separators for Fe/V redox flow batteries." (2013) Journal of Power Sources 218 (2012) 39-45 (Nov 2012).
Abstract: The Fe/V redox flow battery has demonstrated promising performance with distinct advantages over other redox flow battery systems. Due to the less oxidative nature of the Fe(III) species, hydrocarbon-based ion exchange membranes or separators can be used. Daramic^® microporous polyethylene separators were tested on Fe/V flow cells using sulfuric/chloric mixed acid-supporting electrolytes. Among them, separator C exhibited good flow cell cycling performance with satisfactory repeatability over a broad temperature range of 5-50°C. Energy efficiency (EE) of C remains around 70% at current densities of 50-80 mA.cm^-2 in temperatures ranging from room temperature to 50°C. The capacity decay problem could be circumvented through hydraulic pressure balancing by means of applying different pump rates to the positive and negative electrolytes. Stable capacity and energy were obtained over 20 cycles at room temperature and 40°C. These results show that extremely low-cost separators ($1-20/m²) are applicable in the Fe/V flow battery system with acceptable energy efficiency. This represents a remarkable breakthrough: a significant reduction of the capital cost of the Fe/V flow battery system, which could further its market penetration in grid stabilization and renewable integration.

D Stephenson, S Kim, F Chen, E Thomsen, V Viswanathan, W Wang, VL Sprenkle. "Electrochemical Model of the Fe/V Redox Flow Battery."Journal of the Electrochemical Society 159 (12): A1993-A2000 (Oct 2012).
Abstract: A zero-dimensional electrochemical model of the Fe/V redox flow battery (RFB) is presented that can model RFB performance at low flow rates (<0.5 mL min^-1 cm^-2) and varied temperatures. The electrochemical model is appropriate for practical RFBs and provides good agreement with experimental data. In addition, a proposed non-ideal electrode model is introduced that accounts for higher voltage losses at low flow rates. Semi-quantitative operation strategies and electrode design guidelines can be obtained from the model. We found that ohmic losses associated with the electrolyte were dominating our electrode losses, which means operating the cell at higher temperature will reduce electrolyte ohmic losses and viscosity, thus leading to higher system efficiency. Thinner electrodes than the 4.5-mm-thick felt used in this study should reduce ohmic losses as well as pumping losses if the same space velocity is maintained. This electrochemical model can be easily incorporated into system-level and cost models, which will help in system optimization, system control, and pump selection and help avoid potential risks of large scale RFB system development.

W Wang, L Li, Z Nie, B Chen, Q Luo, Y Shao, X Wei, F Chen, G Xia, Z Yang. "A new hybrid redox flow battery with multiple redox couples." Journal of Power Sources 216 (2012), 99-103. (Oct 2012).
Abstract: A redox flow battery using V⁴⁺/V⁵⁺ vs. V²⁺/V³⁺ and Fe²⁺/Fe³⁺ vs. V²⁺/V³⁺ redox couples in chloric/sulfuric mixed acid supporting electrolyte was investigated for potential stationary energy storage applications. The Fe/V hybrid redox flow cell using mixed reactant solutions and operated within a voltage window of 0.5~1.7 V demonstrated stable cycling over 100 cycles with energy efficiency ~80% and negligible capacity fading at room temperature. A 66% improvement in the energy density of the Fe/V hybrid cell was achieved compared with the previously reported Fe/V cell using only Fe²⁺/Fe³⁺ vs. V²⁺/V³⁺ redox couples.

X Lu, GS Li, JY Kim, JP Lemmon, VL Sprenkle, and ZG Yang. "The effects of temperature on the electrochemical performance of sodium-nickel chloride batteries." Journal of Power Sources 215 (2012), 288-295. (Oct 2012).
Abstract: The sodium-nickel chloride (ZEBRA) battery is typically operated at relatively high temperatures (≥ 300°C) to achieve adequate electrochemical performance. In the present study, the effects of operating temperature on the electrochemical performance of planar-type sodium-nickel chloride batteries were investigated in order to evaluate the feasibility of the battery operation at low temperatures (≥ 200°C). Electrochemical test results revealed that the battery was able to be cycled at C/3 rate at as low as 175°C despite the higher cell polarization at the reduced temperature. Overall, low operating temperature resulted in a considerable improvement in the stability of cell performance. Cell degradation was negligible at 175°C, while 55% increase in end-of-charge polarization was observed at 280°C after 60 cycles. SEM analysis indicated that the performance degradation at higher temperatures was related to the particle growth of both nickel and sodium chloride in the cathode. The cells tested at lower temperatures (e.g., 175 and 200°C), however, exhibited a sharp drop in cell voltage at the end of discharge due to the diffusion limitation, possibly caused by the limited ionic conductivity of NaAlCl4 melt or the poor wettability of sodium on the β"-Al2O3 solid electrolyte (BASE). Therefore, improvements in the ionic conductivity of a secondary electrolyte and sodium wetting as well as reduction in the ohmic resistance of BASE are required to further enhance the battery performance at low temperatures.

Cao Y, L Xiao, ML Sushko, W Wang, b Schwenzer, J Xiao, Z Nie, LV Saraf, Z Yang, J Liu. "Sodium Ion Insertion in Hollow Carbon Nanowires for Battery Applications." Nano Letters 12 (7): 3783-3787 (June 2012).
Abstract: Hollow carbon nanowires (HCNWs) were prepared through pyrolyzation of a hollow polyaniline nanowire precursor. The HCNWs used as anode material for Na-ion batteries deliver a high reversible capacity of 251 mAh g^-1 and 82.2% capacity retention over 400 charge-discharge cycles between 1.2 and 0.01 V (vs Na⁺/Na) at a constant current of 50 mA g^-1 (0.2 C). Excellent cycling stability is also observed at an even higher charge-discharge rate. A high reversible capacity of 149 mAh g^-1 also can be obtained at a current rate of 500 mA g^-1 (2C). The good Na-ion insertion property is attributed to the short diffusion distance in the HCNWs and the large interlayer distance (0.37 nm) between the graphitic sheets, which agrees with the interlayered distance predicted by theoretical calculations to enable Na-ion insertion in carbon materials.

Wang W, W Xu, L Cosimbescu, D Choi, L Li, and Z Yang. "Anthraquinone with Tailored Structure for Nonaqueous Metal-Organic Redox Flow Battery." Chemical Communications 48(53):6669-6671. (May 2012).
Abstract: A nonaqueous, hybrid metal-organic redox flow battery based on tailored anthraquinone structure is demonstrated to have an energy efficiency of ~82% and a specific discharge energy density similar to these of aqueous redox flow batteries, which is due to the significantly improved solubility of anthraquinone in supporting electrolytes.

Wang W, Z Nie, B Chen, F Chen, Q Luo, X Wei, G Xia, M Skyllas-Kazacos, L Li, and Z Yang. "A New Fe/V Redox Flow Battery Using Sulfuric/Chloric Mixed Acid Supporting Electrolyte." Advanced Energy Materials 2(4): 487-493. (Feb. 2012).
Abstract: A redox flow battery using Fe²⁺/Fe³⁺ and V²⁺/V³⁺ redox couples in chloric/sulfuric mixed-acid supporting electrolyte was investigated for potential stationary energy storage applications. The Fe/V redox flow cell using mixed reactant solutions operated within a voltage window of 0.5~1.35 V with a nearly 100% utilization ratio and demonstrated stable cycling over 100 cycles with energy efficiency > 80% and no capacity fading at room temperature. A 25% improvement in the discharge energy density of the Fe/V cell was achieved compared with the previous reported Fe/V cell using pure chloride-acid supporting electrolyte. Stable performance was achieved in the temperature range between 0°C and 50°C as well as using a microporous separator as the membrane. The improved electrochemical performance makes the Fe/V redox flow battery a promising option as a stationary energy storage device to enable renewable integration and stabilization of the electric grid.

Zhang J, L Li, Z Nie, B Chen, M Vijayakumar, S Kim, W Wang, B Schwenzer, J Liu, and Z Yang. "Effects of additives on the stability of electrolytes for all-vanadium redox flow batteries." Journal of Applied Electrochemistry 41(10 - Special Issue S1):1215-1221. (Oct 2011).
Abstract: The stability of the electrolytes for all-vanadium redox flow battery was investigated with ex-situ heating/cooling treatment and in situ flow-battery testing methods. The effects of inorganic and organic additives have been studied. The additives containing the ions of potassium, phosphate, and polyphosphate are not suitable stabilizing agents because of their reactions with V(V) ions, forming precipitates of KVSO₆ or VOPO₄. Of the chemicals studied, polyacrylic acid and its mixture with CH₃SO₃H are the most promising stabilizing candidates, which can stabilize all the four vanadium ions (V²⁺, V³⁺, VO²⁺, and VO₂⁺) in electrolyte solutions up to 1.8 M. However, further effort is needed to obtain a stable electrolyte solution with >1.8 M V⁵⁺ at temperatures higher than 40°C.

Wang W, S Kim, B Chen, Z Nie, J Zhang, G Xia, L Li, and Z Yang. "A New Redox Flow Battery Using Fe/V Redox Couples in Chloride Supporting Electrolyte." Energy & Environmental Science 4(10):4068-4073. (June 2011).
Abstract: A new redox flow battery using Fe²⁺/Fe³⁺ and V²⁺/V³⁺ redox couples in chloride-supporting electrolyte was proposed and investigated for potential stationary energy storage applications. The Fe/V redox flow cell using mixed reactant solutions operated within a voltage window of 0.5~1.35 V with a nearly 100% utilization ratio and demonstrated stable cycling with energy efficiency around 80% at room temperature. Stable performance was also achieved in the temperature range between 0°C and 50°C. The improved stability and electrochemical activity over a broader temperature range over the current technologies (such as Fe/Cr redox chemistry) potentially eliminate the necessity of external heat management and use of catalysts, making the Fe/V redox flow battery a promising option as a stationary energy storage device to enable renewable integration and stabilization of the electrical grid.

Schwenzer B, S Kim, M Vijayakumar, Z Yang, J Liu. "Correlation of structural differences between Nafion/polyaniline and Nafion/polypyrrole composite membranes and observed transport properties."Journal of Membrane Science 327 (1-2): 11-19 (Apr. 2011).
Abstract:Polyaniline/Nafion and polypyrrole/Nafion composite membranes, prepared by chemical polymerization, are studied by scanning electron microscopy, infrared and nuclear magnetic resonance spectroscopy. Differences in vanadium ion diffusion through the membranes and in the membranes' area specific resistance are linked to analytical observations that polyaniline and polypyrrole interact differently with Nafion. Polypyrrole, a weakly basic polymer, binds less strongly to the sulfonic acid groups of the Nafion membrane. Infrared spectroscopy results suggest that the hydrophobic polymer aggregates in the center of the Nafion channel rather than attaching to the hydrophilic walls containing sulfonic acid groups. This results in a drastically elevated membrane resistance and only slightly decreased vanadium ion diffusion compared to a Nafion membrane. Polyaniline, on the other hand, polymerizes along the sides of the Nafion pores and on the membrane surface, binding tightly to the sulfonic acid groups of Nafion, polyaniline's greater basicity possibly causing the difference in polymerization behavior. This leads to a more effective reduction in vanadium ion transport across the polyaniline/Nafion membranes and the increase in membrane resistance is less severe. The performance of selected polypyrrole/Nafion composite membranes is tested in a static vanadium redox cell. Increased coulombic efficiency, compared to a cell employing a pure Nafion membrane, further confirms the reduced vanadium ion transport through the composite membranes.

Li L, S Kim, W Wang, M Vijayakumar, Z Nie, B Chen, J Zhang, G Xia, JZ Hu, GL Graff, J Liu, and Z Yang. "A Stable Vanadium Redox-Flow Battery with High Energy Density for Large-scale Energy Storage." Advanced Energy Materials 1(3):394-400. (March 2011).
Abstract: The all-vanadium redox flow battery is a promising technology for large-scale renewable and grid energy storage, but is limited by the low energy density and poor stability of the vanadium electrolyte solutions. A new vanadium redox flow battery with a significant improvement over the current technology is reported in this paper. This battery uses sulfate-chloride mixed electrolytes, which are capable of dissolving 2.5 M vanadium, representing about a 70% increase in energy capacity over the current sulfate system. More importantly, the new electrolyte remains stable over a wide temperature range of -5 to 50°C, potentially eliminating the need for electrolyte temperature control in practical applications. This development would lead to a significant reduction in the cost of energy storage, thus accelerating its market penetration.

Yang Z, J Zhang, MCW Kintner-Meyer, X Lu, D Choi, JP Lemmon, and J Liu. "Electrochemical Energy Storage for Green Grid." Chemical Reviews 111(5):3577 -3613. (March 2011).
Abstract: Electrochemical Energy Storage (EES) is an established, valuable approach for improving the reliability and overall use of the entire power system (generation, transmission, and distribution [T&D]). Sited at various T&D stages, EES can be employed for providing many grid services, including a set of ancillary services such as (1) frequency regulation and load following (aggregated term often used is balancing services), (2) cold start services, (3) contingency reserves, and (4) energy services that shift generation from peak to off -peak periods. In addition, it can provide services to solve more localized power quality issues and reactive power support.