Energy Storage Publications

Tzu-Ling Chen, Rui Sun, Carl Willis, Bert Krutzer, Brian F. Morgan, Frederick L. Beyer, Kee Sung Han, Vijayakumar Murugesan, Yossef A. Elabd."Impact of ionic liquid on lithium ion battery with a solid poly(ionic liquid) pentablock terpolymer as electrolyte and separator."Polymer 209, 122975 (November 2020).
Abstract: In this study, the physical, transport, mechanical, morphological, and electrochemical properties of a ternary blend solid polymer electrolyte (SPE) (poly(ionic liquid) (PIL) multiblock polymer, lithium salt, and ionic liquid (IL)) were systematically investigated as a function of IL concentration. With increasing IL concentration, the conductive volume increases along with the polymer chain segmental mobility. This facilitates high ionic conductivity, while the mechanical modulus exhibits a percolation threshold. Surprisingly, at higher IL concentrations, there is a reduction in the lithium cation mobility as evidenced by pulsed-field gradient nuclear magnetic resonance, which coincides with an increased overpotential evidenced by lithium metal stripping and plating. Stable lithium ion battery cycling durability (over 100 cycles at room temperature) is demonstrated with the ternary blend SPE as the electrolyte and separator. This work provides valuable insights into the design of new SPEs with both high ionic conductivity and improved battery stability.

Hui Wang, Yuyan Shao, Huilin Pan, Xuefei Feng, Ying Chen, Yi-Sheng Liu, Eric D. Walter, Mark H. Engelhard, Kee Sung Han, Tao Deng, Guoxi Ren, Dongping Lu, Xiaochuan Lu, Wu Xu, Chunsheng Wang, Jun Feng, Karl T. Mueller, Jinghua Guo, Ji-Guang Zhang."A lithium-sulfur battery with a solution-mediated pathway operating under lean electrolyte conditions."Nano Energy 76 (October 2020).
Abstract: Lithium-sulfur (Li–S) battery is one of the most promising candidates for the next generation energy storage systems. However, several barriers, including polysulfide shuttle effect, the slow solid-solid surface reaction pathway in the lower discharge plateau, and corrosion of Li anode still limit its practical applications, especially under the lean electrolyte condition required for high energy density. Here, we propose a solution-mediated sulfur reduction pathway to improve the capacity and reversibility of the sulfur cathode. With this method, a high coulombic efficiency (99%) and stable cycle life over 100 cycles were achieved under application-relevant conditions (S loading: 6.2 mg cm⁻²; electrolyte to sulfur ratio: 3 mL_{E gs}⁻¹; sulfur weight ratio: 72 wt%). This result is enabled by a specially designed Li₂S_4-rich electrolyte, in which Li₂S is formed through a chemical disproportionation reaction instead of electrochemical routes. A single diglyme solvent was used to obtain electrolytes with the optimum range of Li₂S₄ concentration. Operando X-ray absorption spectroscopy confirms the solution pathway in a practical Li–S cell. This solution pathway not only introduces a new electrolyte regime for practical Li–S batteries, but also provides a new perspective for bypassing the inefficient surface pathway for other electrochemical processes.

M.S. Lee, K.S. Han, J. Lee, Y. Shin, T.C. Kaspar, Y. Chen, M.H. Engelhard, K.T. Mueller, V. Murugesan."Defect-induced anisotropic surface reactivity and ion transfer processes of anatase nanoparticles."Materials Today Chemistry 17 (September 2020).
Abstract: Surface reactivity and ion transfer processes of anatase TiO₂ nanocrystals were studied using lithium bis(trifluoromethylsulfone)imide (LiTFSI) as a probing molecule. Analysis of synthesized anatase TiO₂ by electron microscopy reveals aggregated nanoparticles (average size ~8 nm) with significant defects (holes and cracks). With the introduction of LiTFSI salt, the Li⁺-adsorption propensity towards the surface along the anatase (100) step edge plane is evident in both x-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) analysis. Ab initio molecular dynamics (AIMD) analysis corroborates the site-preferential interaction of Li⁺ cations with oxygen vacancies and the thermodynamically favorable transport through the (100) step edge plane. Using ⁷Li nuclear magnetic resonance (NMR) chemical shift and relaxometry measurements, the presence of Li⁺ cations near the interface between TiO₂ and the bulk LiTFSI phase was identified, and subsequent diffusion properties were analyzed. The lower activation energy derived from NMR analysis reveals enhanced mobility of Li⁺ cations along the surface, in good agreement with AIMD calculations. On the other hand, the TFSI^– anion interaction with defect sites leads to CF₃ bond dissociation and subsequent generation of carbonyl fluoride-type species. The multimodal spectroscopic analysis including NMR, electron paramagnetic resonance (EPR), and x-ray photoelectron spectroscopy (XPS) confirms the decomposition of TFSI– anions near the anatase surface. The reaction mechanism and electronic structure of interfacial constituents were simulated using AIMD calculations. Overall, this work demonstrates the role of defects at the anatase nanoparticle surface on charge transfer and interfacial reaction processes.

Ying Chen, Nicholas R. Jaegers, Hui Wang, Kee Sung Han, Jian Zhi Hu, Karl T. Mueller, Vijayakumar Murugesan."Role of Solvent Rearrangement on Mg²⁺ Solvation Structures in Dimethoxyethane Solutions using Multimodal NMR Analysis."Journal of Physical Chemistry Letters 11: 6443-6449 (July 2020).
Abstract: One of the main impediments faced for predicting emergent properties of a multivalent electrolyte (such as conductivity and electrochemical stability) is the lack of quantitative analysis of ion–ion and ion–solvent interactions, which manifest in solvation structures and dynamics. In particular, the role of ion–solvent interactions is still unclear in cases where the strong electric field from multivalent cations can influence intramolecular rotations and conformal structural evolution (i.e., solvent rearrangement process) of low permittivity organic solvent molecules on solvation structure. Using quantitative ¹H, ¹⁹F, and ¹⁷O NMR together with ¹⁹F nuclear spin relaxation and diffusion measurments, we find an unusual correlation between ion concentration and solvation structure of Mg(TFSI)₂ salt in dimethoxyethane (DME) solution. The dominant solvation structure evolves from contact ion pairs (i.e., [Mg(TFSI)(DME)_1–2]⁺) to fully solvated clusters (i.e., [Mg(DME)₃]²⁺) as salt concentration increases or as temperature decreases. This transition is coupled to a phase separation, which we study here between 0.06 and 0.36 M. Subsequent analysis is based on an explanation of the solvent rearrangement process and the competition between solvent molecules and TFSI anions for cation coordination.

Lynn Trahey, Fikile R. Brushett, Nitash P. Balsara, Gerbrand Ceder, Lei Cheng, Yet-Ming Chiang, Nathan T. Hahn, Brian J. Ingram, Shelley D. Minteer, Jeffrey S. Moore, Karl T. Mueller, Linda F. Nazar, Kristin A. Persson, Donald J. Siegel, Kang Xu, Kevin R. Zavadil, Venkat Srinivasan, George W. Crabtree."Energy storage emerging: A perspective from the Joint Center for Energy Storage Research."Proceedings of the National Academy of Sciences 117 (23) 12550-12557 (June 2020).
Abstract: Energy storage is an integral part of modern society. A contemporary example is the lithium (Li)-ion battery, which enabled the launch of the personal electronics revolution in 1991 and the first commercial electric vehicles in 2010. Most recently, Li-ion batteries have expanded into the electricity grid to firm variable renewable generation, increasing the efficiency and effectiveness of transmission and distribution. Important applications continue to emerge including decarbonization of heavy-duty vehicles, rail, maritime shipping, and aviation and the growth of renewable electricity and storage on the grid. This perspective compares energy storage needs and priorities in 2010 with those now and those emerging over the next few decades. The diversity of demands for energy storage requires a diversity of purpose-built batteries designed to meet disparate applications. Advances in the frontier of battery research to achieve transformative performance spanning energy and power density, capacity, charge/discharge times, cost, lifetime, and safety are highlighted, along with strategic research refinements made by the Joint Center for Energy Storage Research (JCESR) and the broader community to accommodate the changing storage needs and priorities. Innovative experimental tools with higher spatial and temporal resolution, in situ and operando characterization, first-principles simulation, high throughput computation, machine learning, and artificial intelligence work collectively to reveal the origins of the electrochemical phenomena that enable new means of energy storage. This knowledge allows a constructionist approach to materials, chemistries, and architectures, where each atom or molecule plays a prescribed role in realizing batteries with unique performance profiles suitable for emergent demands.

Kee Sung Han, Zhou Yu, Hui Wang, Paul C. Redfern, Lin Ma, Lei Cheng, Ying Chen, Jian Zhi Hu, Larry A. Curtiss, Kang Xu, Vijayakumar Murugesan, Karl T. Mueller."Origin of Unusual Acidity and Li⁺ Diffusivity in a Series of Water-in-Salt Electrolytes."Journal of Physical Chemistry B 124 (25): 5284-5291 (June 2020).
Abstract: Superconcentrated aqueous electrolytes (“water-in-salt” electrolytes, or WiSEs) enable various aqueous battery chemistries beyond the voltage limits imposed by the Pourbaix diagram of water. However, their detailed structural and transport properties remain unexplored and could be better understood through added studies. Here, we report on our observations of strong acidity (pH 2.4) induced by lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) at superconcentration (at 20 mol/kg). Multiple nuclear magnetic resonance (NMR) and pulsed-field gradient (PFG) diffusion NMR experiments, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations reveal that such acidity originates from the formation of nanometric ion-rich structures. The experimental and simulation results indicate the separation of water-rich and ion-rich domains at salt concentrations ≥5 m and the acidity arising therefrom is due to deprotonation of water molecules in the ion-rich domains. As such, the ion-rich domain is composed of hydrophobic −CF₃ (of TFSI^–) and hydrophilic hydroxyl (OH^–) groups. At 20 m concentration, the tortuosity and radius of water diffusion channels are estimated to be ∼10 and ∼1 nm, respectively, which are close to values obtained from hydrated Nafion membranes that also have hydrophobic polytetrafluoroethylene (PTFE) backbones and hydrophilic channels consisting of SO₃^– ion cluster networks providing for the transport of ions and water. Thus, we have discovered the structural similarity between WiSE and hydrated Nafion membranes on the nanometer scale.

Ying Chen, Nicholas R. Jaegers, Kee Sung Han, Hui Wang, Robert P. Young, Garvit Agarwal, Andrew S. Lipton, Rajeev S. Assary, Nancy M. Washton, Jian Zhi Hu, Karl T. Mueller, Vijayakumar Murugesan."Probing Conformational Evolution and Associated Dynamics of Mg(N(SO₂CF₃)₂)₂·Dimethoxyethane Adduct Using Solid-State ¹⁹F and 1H NMR."Journal of Physical Chemistry C 124 (9):4999-5008 (March 2020).
Abstract: Bis(trifluoromethanesulfonimide) or TFSI is widely used as a counter anion in electrolyte design due to its structural flexibility and chemical stability. We studied the conformational variations and associated dynamics of TFSI in adduct of Mg(TFSI)₂ with dimethoxyethane (DME), a solvate crystalline material using solid-state ¹H and ¹⁹F NMR. TFSI molecular motion in this solvate structure falls within the timescale of the ¹⁹F NMR experiment, yielding spectroscopic signatures for unique TFSI conformers under the coordination environment of Mg²⁺ cation. Within the temperature range of −5 to 82 °C, we observe nine distinct TFSI sites in both crystalline and disordered regions using ¹⁹F NMR, reflecting complexity of structural and dynamics of TFSI anions within solvate structure. The four distinguishable sites in the disordered region for the two CF₃ groups of the same TFSI molecule are identified using chemical shift analysis. The exchange rate constants from site to site are calculated through variable temperature ¹⁹F NMR and two-dimensional (2D) exchange spectroscopy (EXSY) experiments, along with respective activation enthalpies using Eyring’s formulation. The flip rate of CF₃ around the S–C bond is estimated as ∼15 s^–1 at 8 °C with ΔH≠ ∼ 22 kJ/mol, but the rotation of the entire TFSI is 4.8 s^–1 at 8 °C with a significantly greater ΔH≠ = 98 ± 10 kJ/mol. Furthermore, the slow conversion of trans to cis conformers at a lower temperature (T ≤ 1 °C) in the crystalline region is monitored, with a conversion rate of ∼2 × 10^–5 s^–1 at −5 °C. Density functional theory (DFT)-based calculations were performed to support further the assignment of experimental chemical shifts, and the activation energy E_a = 21.1 kJ/mol obtained for the cis to trans transition is consistent with experimental values. The combined set of ¹⁹F and ¹H under both one-dimensional (1D) and 2D NMR methods demonstrated here can be further used for examining electrode–electrolyte interfaces to probe the motions of various constituents that can enable detailed studies of interfacial processes and dynamics. Ultimately, such studies will aid in the design and discovery of interfacial constructs in which directed defect chemistry, chemical moiety distribution, and nanostructure are employed to drive efficient charge transport.

Jian Zhi Hu, Nicholas R. Jaegers, Ying Chen, Kee Sung Han, Hui Wang, Vijayakumar Murugesan, Karl T. Mueller."Adsorption and Thermal Decomposition of Electrolytes on Nanometer Magnesium Oxide: An in Situ ¹³C MAS NMR Study."ACS Applied Materials & Interfaces 11 (42):38689-38696 (October 2019).
Abstract: Mg batteries have been proposed as potential alternatives to lithium-ion batteries because of their lower cost, higher safety, and enhanced charge density. However, the Mg metal readily oxidizes when exposed to an oxidizer to form a thin MgO passivation surface layer that blocks the transport of Mg²⁺ across the solid electrode–electrolyte interface (SEI). In this work, the adsorption and thermal decomposition of diglyme (G2) and electrolytes containing Mg(TFSI)₂ in G2 on 10 nm-sized MgO particles are evaluated by a combination of in situ ¹³C single-pulse, surface-sensitive ¹H–¹³C cross-polarization (CP) magic-angle spinning (MAS) nuclear magnetic resonance, and quantum chemistry calculations. At 180 °C, neat G2 decomposes on MgO to form surface-adsorbed −OCH₃ groups that are captured as a distinctive peak located at about 50 ppm in the CP/MAS spectrum. At low Mg(TFSI)₂ salt concentration, the main solvation structure in this electrolyte is solvent-separated ion pairs without extensive Mg–TFSI contact ion pairs. G2, likely including a small amount of G2-solvated Mg²⁺, adsorbs onto the MgO surface. At high Mg(TFSI)₂ salt concentrations, contact ion pairs between Mg and TFSI are formed extensively in the solution with the first solvation shell containing one pair of Mg–TFSI and two G2 molecules and the second solvation shell containing up to six G2 molecules, namely, MgTFSI(G2)₂(G2)₆⁺. In the presence of MgO, MgTFSI(G2)₂(G2)₆⁺ adsorbs onto the MgO surface. At 180 °C, the MgO surface stimulates a desolvation process converting MgTFSI(G2)₂(G2)₆⁺ to MgTFSI(G2)₂⁺ and releasing G2 molecules from the second solvation shell of the MgTFSI(G2)₂(G2)₆⁺ cluster into the solution. MgTFSI(G2)₂⁺ and MgTFSI(G2)₂(G2)₆⁺ tightly adsorb onto the MgO surface and are observed by ¹H–¹³C CP/MAS experiments. The results contained herein show that electrolyte composition has a directing role in the species present on the electrode surface, which has implications on the structures and constituents of the solid–electrolyte interface on working electrodes and can be used to better understand its formation and the failure modes of batteries.

Vaithiyalingam Shutthanandan, Manjula Nandasiri, Jianming Zheng, Mark H. Engelhard, Wu Xu, Suntharampillai Thevuthasan, Vijayakumar Murugesan."Applications of XPS in the Characterization of Battery Materials."Journal of Electron Spectroscopy and Related Phenomena 231:2-10 (February 2019).
Abstract: Technological development requires reliable power sources where energy storage devices are emerging as a critical component. Wide range of energy storage devices, Redox-flow batteries (RFB), Lithium ion based batteries (LIB), and Lithium-sulfur (LSB) batteries are being developed for various applications ranging from grid-scale level storage to mobile electronics. Material complexities associated with these energy storage devices with unique electrochemistry are formidable challenge which needs to be address for transformative progress in this field. X-ray photoelectron spectroscopy (XPS) - a powerful surface analysis tool - has been widely used to study these energy storage materials because of its ability to identify, quantify and image the chemical distribution of redox active species. However, accessing the deeply buried solid-electrolyte interfaces (which dictates the performance of energy storage devices) has been a challenge in XPS usage. Herein we report our recent efforts to utilize the XPS to gain deep insight about these interfaces under realistic conditions with varying electrochemistry involving RFB, LIB and LSB.

Amity Andersen, Nav Nidhi Rajput, Kee Sung Han, Huilin Pan, Niranjan Govind, Kristin A. Persson, Karl T. Mueller, Vijayakumar Murugesan."Structure and Dynamics of Polysulfide Clusters in a Nonaqueous Solvent Mixture of 1,3-Dioxolane and 1,2-Dimethoxyethane."Chemistry of Materials 31 (7): 2308-2319 (February 2019).
Abstract: Molecular clustering and associated dynamic processes of lithium polysulfide species were unraveled using classical molecular dynamics and ab initio metadynamics calculations. The spectroscopic signatures of polysulfide clusters were analyzed using a multimodal analysis including experimental and computational nuclear magnetic resonance (NMR) and X-ray absorption spectroscopies. Lithium polysulfide solutes (Li₂S₄, Li₂S₆, and Li₂S₈) and their mixtures in a 1,3-dioxolane and 1,2-dimethoxyethane (DOL/DME) solvent undergo aggregation driven by intramolecular lithium–sulfur (Li–S) interactions, leading to distributions of cluster sizes, which could critically influence the functioning of lithium–sulfur batteries. Representative polysulfide clusters with systematic increases in molecular size were extracted from the classical molecular dynamics trajectories for subsequent structural and spectroscopic property calculations using density functional theory analysis. Structural analysis of these clusters reveals progressively decreasing solvent involvement in Li⁺ coordination varying from Li₂S₄ to Li₂S₈, with more pronounced variation and changes in DME compared with those in DOL. These observations are reflected in the analysis of the experimental and theoretical ⁷Li and ¹⁷O NMR chemical shifts and pulsed field gradient-NMR diffusion measurements. A comparison of experimental and theoretical S K-edge X-ray absorption near edge spectra shows that relatively large lithium sulfide chain clusters are likely to occur in the DOL/DME-solvated lithium sulfide systems. Ab initio metadynamics simulations and NMR analysis indicate that Li⁺ solvated by only the solvent can occur through Li⁺ dissociation from sulfide chains. However, the occurrence of “sulfide-free” Li⁺ is a minor mechanism compared with the dynamic aggregation and shuttling processes of polysulfide solvates in DOL/DME-based electrolytes of Li–S batteries. Overall, atomistic insights gained about clustering and lithium exchange dynamics will be critical for the predictive understanding of the polysulfide shuttling and nucleation process that dictates the Li–S battery performance.

Jian Zhi Hu, Nicholas R. Jaegers, Mary Y Hu, Karl T. Mueller."In situ and ex situ NMR for battery research."Journal of Physics: Condensed Matter 30 (46) (November 2018).
Abstract: A rechargeable battery stores readily convertible chemical energy to operate a variety of devices such as mobile phones, laptop computers, electric automobiles, etc. A battery generally consists of four components: a cathode, an anode, a separator and electrolytes. The properties of these components jointly determine the safety, the lifetime, and the electrochemical performance. They also include, but are not limited to, the power density and the charge as well as the recharge time/rate associated with a battery system. An extensive amount of research is dedicated to understanding the physical and chemical properties associated with each of the four components aimed at developing new generations of battery systems with greatly enhanced safety and electrochemical performance at a significantly reduced cost for large scale applications. Advanced characterization tools are a prerequisite to fundamentally understanding battery materials. Considering that some of the key electrochemical processes can only exist under in situ conditions, which can only be captured under working battery conditions when electric wires are attached and current and voltage are applied, make in situ detection critical. Nuclear magnetic resonance (NMR), a non-invasive and atomic specific tool, is capable of detecting all phases, including crystalline, amorphous, liquid and gaseous phases simultaneously and is ideal for in situ detection on a working battery system. Ex situ NMR on the other hand can provide more detailed molecular or structural information on stable species with better spectral resolution and sensitivity. The combination of in situ and ex situ NMR, thus, offers a powerful tool for investigating the detailed electrochemistry in batteries.

Jinhua Huang, Zheng Yang, Vijayakumar Murugesan, Eric Walter, Aaron Hollas, Baofei Pan, Rajeev S. Assary, Ilya A. Shkrob, Xiaoling Wei, Zhengcheng Zhang."Spatially Constrained Organic Diquat Anolyte for Stable Aqueous Flow Batteries."ACS Energy Letters 3 (10) 2533-2538 (October 2018).
Abstract: Redox-active organic materials (ROMs) are becoming increasingly attractive for use in redox flow batteries as promising alternatives to traditional inorganic counterparts. However, the reported ROMs are often accompanied by challenges, including poor solubility and stability. Herein, we demonstrate that the commonly used diquat herbicides, with solubilities of >2 M in aqueous electrolytes, can be used as stable anolyte materials in organic flow batteries. When coupled with a ferrocene-derived catholyte, the flow cells with the diquat anolyte demonstrate long galvanic cycling with high capacity retention. Notably, the mechanistic underpinnings of this remarkable stability are attributed to the improved π-conjugation that originated from the near-planar molecular conformations of the spatially constrained 2,2′-bipyridyl rings, suggesting a viable structural engineering strategy for designing stable organic materials.

Hui Wang, Brian D. Adams, Huilin Pan, Liang Zhang, Kee Sung Han, Luis Estevez, Dongping Lu, Haiping Jia, Jun Feng, Jinghua Guo, Kevin R. Zavadil, Yuyan Shao, Ji-Guang Zhang."Tailored Reaction Route by Micropore Confinement for Li–S Batteries Operating under Lean Electrolyte Conditions."Advanced Energy Materials 8 (21): 1800590 (May 2018).
Abstract: Lithium‐sulfur (Li–S) batteries are one of the most promising alternative energy storage systems beyond Li‐ion batteries. However, the sluggish kinetics of the nucleation and growth of the solid discharge product of Li₂S/Li₂S₂ in the lower discharge plateau has been recently identified as a critical hurdle for attaining high specific capacity in Li–S batteries with high sulfur loadings under lean electrolyte conditions. Herein, a new strategy of breaking the charge‐transport bottleneck by successful generation of experimentally verified stable Li₂S₂ and a reservoir of quasi‐solid lithium polysulfides within the micropores of activated carbon fiber cloth as a high‐sulfur‐loading host is proposed. The developed Li–S cell is capable of delivering a highly sustainable areal capacity of 6.0 mAh cm⁻² under lower electrolyte to sulfur ratios (<3.0 mL_E g_S⁻¹). Micropore confinement leads to generation of solid Li₂S₂ that enables high utilization of the entire electroactive area by its inherent self‐healing capacity. This strategy opens a new avenue for rational material designs for Li–S batteries under lean electrolyte condition.

Jian Zhi Hu, Nav Nidhi Rajput, Chuan Wan, Yuyan Shao, Xuchu Deng, Nicholas R. Jaegers, Mary Hu, Yingwen Chen, Yongwoo Shin, Joshua Monk, Zhong Chen, Zhaohai Qin, Karl Todd Mueller, Jun Liu, Kristin A. Persson."²⁵Mg NMR and computational modeling studies of the solvation structures and molecular dynamics in magnesium based liquid electrolytes."Nano Energy 46: 436-446 (April 2018).
Abstract: There is increasing evidence that the solvation structure of the active components in a liquid electrolyte solution strongly impacts the performance in electrochemical applications. In this work, the nanoscale solvation structures and dynamics of Mg(BH₄)₂ and Mg(TFSI)₂ dissolved in diglyme (DGM) at various concentrations and ratios of Mg(BH₄)₂/Mg(TFSI)₂ were investigated using a combination of natural abundance ²⁵Mg NMR, quantum chemistry calculations of ²⁵Mg NMR chemical shifts, classical molecular dynamics (MD) calculations, and electrochemical performance tests. By mixing two competing Mg salts, we were able to reduce the strong covalent interactions between Mg²⁺ and BH₄– anions. A small increase is observed in the coordination number of Mg-TFSI and a significant increase in the interaction of Mg²⁺ ions with glymes. Through a combination of NMR, DFT and MD simulations, various stable species around 1 nm in size were detected in the mixed salt solution, which play key roles in the enhanced electrochemical performance of the mixed electrolyte. It is established that for the neat Mg(TFSI)₂ in DGM electrolyte at dilute concentrations the TFSI- is fully dissociated from Mg²⁺. At higher concentrations, Mg²⁺ and TFSI- are only partially dissociated as contact ion pairs are formed. In contrast, at 0.01 M Mg(BH₄)₂ (saturated concentration) in DGM, the first solvation shell of a Mg²⁺ ion contains two BH₄- anions and one DGM molecule, while the second solvation shell consists of five to six DGM molecules. An exchange mechanism between the solvation structures in the combined electrolyte containing both Mg(BH₄)₂ and Mg(TFSI)₂ in DGM was found to result in the observation of a single ²⁵Mg NMR peak. This exchange is responsible for an increase in uncoordinated anions, as well as improved stability and ionic conductivity as compared to single anion solution. Solvent molecule rearrangement and direct Mg-ion exchange between the basic solvation structures are hypothesized as likely reasons for the exchange. We elucidate that the solvent rearrangement is energetically much more favorable than direct Mg-ion hopping and is thus suggested as the dominant exchange mechanism.

Jinhua Huang, Wentao Duan, Jingjing Zhang, Ilya A. Shkrob, Rajeev S. Assary, Baofei Pan, Chen Liao, Zhengcheng Zhang, Xiaoliang Wei, Lu Zhang."Substituted thiadiazoles as energy-rich anolytes for nonaqueous redox flow cells."Journal of Materials Chemistry A 6 (15): 6251-6254 (March 2018).
Abstract: Understanding structure–property relationships is essential for designing energy-rich redox active organic molecules (ROMs) for all-organic redox flow batteries. Herein we examine thiadiazole ROMs for storage of negative charge in the flow cells. These versatile molecules have excellent solubility and low redox potentials, allowing high energy density to be achieved. By systematically incorporating groups with varying electron accepting/withdrawing ability, we have examined substituent effects on their properties of interest, including redox potentials, calendar lives of charged ROMs in electrolyte, and the flow cell cycling performance. While the calendar life of energized fluids can be tuned in a predictable fashion over a wide range, the improvements in the calendar life do not automatically translate into the enhanced cycling performance, indicating that in addition to the slow reactions of charged species in the solvent bulk, there are other parasitic reactions that occur only during the electrochemical cycling of the cell and can dramatically affect the cycling lifetime.

Brian D. Adams, Jianming Zheng, Xiaodi Ren, Wu Xu, Ji-Guang Zhang."Accurate Determination of Coulombic Efficiency for Lithium Metal Anodes and Lithium Metal Batteries."Advanced Energy Materials 8 (7): (March 2018).
Abstract: Lithium (Li) metal is an ideal anode material for high energy density batteries. However, the low Coulombic efficiency (CE) and the formation of dendrites during repeated plating and stripping processes have hindered its applications in rechargeable Li metal batteries. The accurate measurement of Li CE is a critical factor to predict the cycle life of Li metal batteries, but the measurement of Li CE is affected by various factors that often lead to conflicting values reported in the literature. Here, several parameters that affect the measurement of Li CE are investigated and a more accurate method of determining Li CE is proposed. It is also found that the capacity used for cycling greatly affects the stabilization cycles and the average CE. A higher cycling capacity leads to faster stabilization of Li anode and a higher average CE. With a proper operating protocol, the average Li CE can be increased from 99.0% to 99.5% at a high capacity of 6 mA h cm⁻² (which is suitable for practical applications) when a high-concentration ether-based electrolyte is used.

Huilin Pan, Kee Sung Han, Mark H. Engelhard, Ruiguo Cao, Junzheng Chen, Ji-Guang Zhang, Karl T. Mueller, Yuyan Shao, Jun Liu."Addressing Passivation in Lithium–Sulfur Battery Under Lean Electrolyte Condition."Advanced Functional Materials 1707234 (February 2018).
Abstract: Reducing the electrolyte amount is critical for the high specific energy of lithium–sulfur (Li–S) batteries in practice. The reduced electrolyte condition (a so‐called “lean electrolyte”) raises a complex situation for sulfur redox reactions since the reactions rely on the electrolyte mediation. The insulating nature of discharge product Li₂S and its uncontrollable accumulation (passivation) at the cathode interface is one of the major challenges for stable cycling of a Li–S battery under lean electrolyte condition. In this study, it is presented that the NH₄TFSI additive in electrolyte solution greatly alleviates the passivation issue in Li–S batteries under lean electrolyte conditions. The ammonium additive enhances the dissociation of Li₂S and largely reduces the insoluble and large Li₂S particles in the sulfur cathodes, which facilitate the reversible and sustainable redox reactions of sulfur. Therefore, the cycle life of Li–S batteries under lean electrolyte conditions is significantly improved. In addition, it is found that the morphology of Li anodes is dependent on the cathode structures. An ammonium additive enables homogeneous surface of cathode and Li anode and extended cycle life.

J. Zhang, Z. Yang, I. A. Shkrob, R. S. Assary, S. Tung, B. Silcox, W. Duan, J. Zhang, C. Liao, Z. Zhang, W. Wang, L. A. Curtiss, L. Thompson, X. Wei, L. Zhang. " Annulated dialkoxybenzenes as catholyte materials for nonaqueous redox flow batteries: achieving high chemical stability through bicyclic substitution " Advanced Energy Materials. 2017, 7, 1701272. (November 2017).
Abstract: 1,4-Dimethoxybenzene derivatives are materials of choice for use as catholytes in non-aqueous redox flow batteries, as they exhibit high open-circuit potentials and excellent electrochemical reversibility. However, chemical stability of these materials in their oxidized form needs to be improved. Disubstitution in the arene ring is used to suppress parasitic reactions of their radical cations, but this does not fully prevent ring-addition reactions. By incorporating bicyclic substitutions and ether chains into the dialkoxybenzenes, a novel catholyte molecule, 9,10-bis(2-methoxyethoxy)-1,2,3,4,5,6,7,8-octahydro-1, 4:5,8- dimethanenoanthracene (BODMA), is obtained and exhibits greater solubility and superior chemical stability in the charged state. A hybrid flow cell containing BODMA is operated for 150 charge–discharge cycles with a minimal loss of capacity.

Han, K. S.; Chen, J.; Cao, R.; Rajput, N. N.; Murugesan, V.; Shi, L.; Pan, H.; Zhang, J.-G.; Liu, J.; Persson, K. A.; Mueller, K. T. " Effects of Anion Mobility on Electrochemical Behaviors of Lithium–Sulfur Batteries " Chemistry of Materials. 29(21): 9023-9029 (Oct. 2017).
Abstract: The electrolyte is a crucial component of lithium–sulfur (Li–S) batteries, as it controls polysulfide dissolution, charge shuttling processes, and solid-electrolyte interphase (SEI) layer formation. Experimentally, the overall performance of Li–S batteries varies with choice of solvent system and Li-salt used in the electrolyte, and a lack of predictive understanding about the effects of individual electrolyte components inhibits the rational design of electrolytes for Li–S batteries. Here we analyze the role of the counteranions of common Li salts (such as TfO^–, FSI^–, TFSI^–, and TDI^–) when dissolved in DOL/DME (1:1 by vol.) for use in Li–S batteries. The evolution of ion–ion and ion–solvent interactions due to various anions was analyzed using ¹⁷O NMR and pulsed-field gradient (PFG) NMR and then correlated with electrochemical performance in Li–S cells. These data reveal that the formation of the passivation layer on the anode and the loss of active materials from the cathode (evidenced by polysulfide dissolution) are related to anion mobility and affinity with lithium polysulfide, respectively. For future electrolyte design, anions with lower mobility and weaker interactions with lithium polysulfides may be superior candidates for increasing the long-term stability of Li–S batteries.

Brian D. Adams, Emily V. Carino, Justin G. Connell, Kee Sung Han, Ruiguo Cao, Junzheng Chen, Jianming Zheng, Qiuyan Li, Karl T. Mueller, Wesley A. Henderson, Ji-Guang Zhang. " Long Term Stability of Li-S Batteries Using High Concentration Lithium Nitrate Electrolytes" Nano Energy. 2017, 607-617.(Oct. 2017).
Abstract: The lithium-sulfur (Li-S) battery is a very promising candidate for the next generation of energy storage systems required for electrical vehicles and grid energy storage applications due to its very high theoretical specific energy (2500 W h kg−1). However, low Coulombic efficiency (CE) during repeated Li metal plating/stripping has severely limited the practical application of rechargeable Li-S batteries. In this work, a new electrolyte system based on a high concentration of LiNO3 in diglyme (G2) solvent is developed which enables an exceptionally high CE for Li metal plating/stripping and thus high stability of the Li anode in the sulfur-containing electrolyte. The tailoring of electrolyte properties for the Li anode has proven to be a highly successful strategy for improving the capacity retention and cycle life of Li-S batteries. This electrolyte provides a CE of greater than 99% for over 200 cycles of Li plating/stripping. In contrast, the Li anode cycles for less than 35 cycles (with a high CE) in the state-of-the-art 1 M LiTFSI + 0.3 M LiNO3 in 1,3-dioxolane:1,2-dimethoxyethane (DOL:DME) electrolyte under the same conditions. The stable Li anode enabled by the new electrolyte may accelerate the applications of high energy density Li-S batteries in both electrical vehicles and large-scale grid energy storage markets.

Pan, H., Chen, J., Cao, R., Vijay, M., Rajput, N.N., Han, K.S., Persson, K., Estevez, L., Engelhard, M.H., Zhang, J.G., Mueller, K.T., Cui, Y., Shao, Y., Liu, J. " Non-encapsulation Approach for High Performance Li-S Batteries through Controlled Nucleation and Growth" Natrue Energy. 2017, 2,813-820 (Sept. 2017).
Abstract: High-surface-area, nanostructured carbon is widely used for encapsulating sulfur and improving the cyclic stability of Li–S batteries, but the high carbon content and low packing density limit the specific energy that can be achieved. Here we report an approach that does not rely on sulfur encapsulation. We used a low-surface-area, open carbon fibre architecture to control the nucleation and growth of the sulfur species by manipulating the carbon surface chemistry and the solvent properties, such as donor number and Li⁺ diffusivity. Our approach facilitates the formation of large open spheres and prevents the production of an undesired insulating sulfur-containing film on the carbon surface. This mechanism leads to ~100% sulfur utilization, almost no capacity fading, over 99% coulombic efficiency and high energy density (1,835 Wh kg⁻¹ and 2,317 Wh l⁻¹). This finding offers an alternative approach for designing high-energy and low-cost Li–S batteries through controlling sulfur reaction on low-surface-area carbon. 

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.

Manjula I. Nandasir, Luis E. Camacho-Forero, Ashleigh M. Schwarz, Vaithiyalingam Shutthanandan, Suntharampillai Thevuthasan, Perla B. Balbuena, Karl T. Mueller, Vijayakumar Murugesan. "In Situ Chemical Imaging of Solid-Electrolyte Interphase Layer Evolution in Li–S Batteries."Chemistry of Materials 29 (11):4728-4737 (May 2017).
Abstract: Parasitic reactions of electrolyte and polysulfide with the Li-anode in lithium sulfur (Li–S) batteries lead to the formation of solid-electrolyte interphase (SEI) layers, which are the major reason behind severe capacity fading in these systems. Despite numerous studies, the evolution mechanism of the SEI layer and specific roles of polysulfides and other electrolyte components are still unclear. We report an in situ X-ray photoelectron spectroscopy (XPS) and chemical imaging analysis combined with ab initio molecular dynamics (AIMD) computational modeling to gain fundamental understanding regarding the evolution of SEI layers on Li-anodes within Li–S batteries. A multimodal approach involving AIMD modeling and in situ XPS characterization uniquely reveals the chemical identity and distribution of active participants in parasitic reactions as well as the SEI layer evolution mechanism. The SEI layer evolution has three major stages: the formation of a primary composite mixture phase involving stable lithium compounds (Li2S, LiF, Li2O, etc.) and formation of a secondary matrix type phase due to cross interaction between reaction products and electrolyte components, which is followed by a highly dynamic monoanionic polysulfide (i.e., LiS5) fouling process. These new molecular-level insights into the SEI layer evolution on Li-anodes are crucial for delineating effective strategies for the development of Li–S batteries.

Chen, J., Henderson, W.A., Pan, H., Perdue, B.R., Cao, R., Hu, J. Z., Wan, C., Han, K.S., Mueller, K.T., Zhang, J.G., Shao, Y., Liu, J. " Improving Lithium–Sulfur Battery Performance Under Lean Electrolyte Through Nanoscale Confinement in Soft Swellable Gels" Nano Letters. 2017, 17, (5) 3061-3067 (April 2017).
Abstract: Li–S batteries have been extensively studied using rigid carbon as the host for sulfur encapsulation, but improving the properties with a reduced electrolyte amount remains a significant challenge. This is critical for achieving high energy density. Here, we developed a soft PEO₁₀LiTFSI polymer swellable gel as a nanoscale reservoir to trap the polysulfides under lean electrolyte conditions. The PEO₁₀LiTFSI gel immobilizes the electrolyte and confines polysulfides within the ion conducting phase. The Li–S cell with a much lower electrolyte to sulfur ratio (E/S) of 4 g_E/g_S (3.3 mL_E/g_S) could deliver a capacity of 1200 mA h/g, 4.6 mA h/cm², and good cycle life. The accumulation of polysulfide reduction products, such as Li₂S, on the cathode, is identified as the potential mechanism for capacity fading under lean electrolyte conditions.

Duan, W, J Huang, JA Kowalski, IA Shkrob, M Vijayakumar, E Walter, B Pan, Z Yang, JD Milshtein, B Li, C Liao, Z Zhang, W Wang, J Liu, JS Moore, FR Brushett, L Zhang, X Wei. "“Wine-Dark Sea” in an Organic Flow Battery: Storing Negative Charge in 2,1,3-Benzothiadiazole Radicals Leads to Improved Cyclability."ACS Energy Letters 2 (5): 1156-1161 (April 2017).
Abstract: Redox-active organic materials (ROMs) have shown great promise for redox flow battery applications but generally encounter limited cycling efficiency and stability at relevant redox material concentrations in nonaqueous systems. Here we report a new heterocyclic organic anolyte molecule, 2,1,3-benzothiadiazole, that has high solubility, a low redox potential, and fast electrochemical kinetics. Coupling it with a benchmark catholyte ROM, the nonaqueous organic flow battery demonstrated significant improvement in cyclable redox material concentrations and cell efficiencies compared to the state-of-the-art nonaqueous systems. Especially, this system produced exceeding cyclability with relatively stable efficiencies and capacities at high ROM concentrations (>0.5 M), which is ascribed to the highly delocalized charge densities in the radical anions of 2,1,3-benzothiadiazole, leading to good chemical stability. This material development represents significant progress toward promising next-generation energy storage.

Wan C ,Xu S ,Hu M Y,Cao R ,Qian J ,Qin Z ,Liu J ,Mueller K T,Zhang J ,Hu J Z. "Multinuclear NMR Study of the Solid Electrolyte Interface Formed in Lithium Metal Batteries" Applied Materials & Interfaces 9 (17): 14741-14748 (April 2017).
Abstract: The composition of the solid electrolyte interphase (SEI) layers formed in Cu|Li cells using lithium bis(fluorosulfonyi)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,2-dimethoxyethane (DME) electrolytes is determined by a multinuclear solid-state MAS NMR study at high magnetic field. It is found that the “dead” metallic Li is largely reduced in the SEI layers formed in a 4 M LiFSI–DME electrolyte system compared with those formed in a 1 M LiFSI–DME electrolyte system. This finding relates directly to the safety of Li metal batteries, as one of the main safety concerns for these batteries is associated with the “dead” metallic Li formed after long-term cycling. It is also found that a large amount of LiF, which exhibits superior mechanical strength and good Li+ ionic conductivity, is observed in the SEI layer formed in the concentrated 4 M LiFSI–DME and 3 M LiTFSI–DME systems but not in the diluted 1 M LiFSI–DME system. Quantitative 6Li MAS NMR results indicate that the SEI associated with the 4 M LiFSI–DME electrolyte is denser than those formed in the 1 M LiFSI–DME and 3 M LiTFSI–DME systems. These studies reveal the fundamental mechanisms behind the excellent electrochemical performance associated with higher concentration LiFSI–DME electrolyte systems.

Rajput NN ,Murugesan V ,Shin Y ,Han KS ,Lau KC ,Chen J ,Liu J ,Curtiss L A,Mueller K T,Persson K A. "Elucidating the Solvation Structure and Dynamics of Lithium Polysulfides Resulting from Competitive Salt and Solvent Interactions" Chemistry of Materials 29 (8): 3375-3379 (March 2017).
Abstract: Designing optimal electrolytes is key to enhancing the performance of energy storage devices,especially relating to cycle life, efficiency, and stability. Specifically, for future beyond-Li ion energy storage, there is still ample room for electrolyte improvements. Among the candidates for higher gravimetric energy storage, the Li−S battery is considered quite promising, owing to its theoretical specific energy density (2600 Wh/kg) and specific capacity (1675 mAh/g) and significantly lower cost as compared to state-of-art lithium-ion batteries.2−4 However, despite these attractive attributes, successful commercialization of Li−S batteries is currently hindered by poor cycling performance and capacity retention that is primarily caused by the parasitic reactions between the Li metal anode and dissolved polysulfide (PS) species from the cathode during the cycling process.

Vijayakumar, M., Han, K.S., Hu, J., Mueller, K.T. " Molecular Level Structure and Dynamics of Electrolytes Using 17O Nuclear Magnetic Resonance Spectroscopy " eMagRes. 2017, 3, 71-82 (March. 2017).
Abstract: Electrolytes help harness the energy from electrochemical processes by serving as solvents and transport media for redox-active ions. Molecular-level interactions between ionic solutes and solvent molecules – commonly referred to as solvation phenomena – give rise to many functional properties of electrolytes such as ionic conductivity, viscosity, and stability. It is critical to understand the evolution of solvation phenomena as a function of competing counterions and solvent mixtures to predict and design the optimal electrolyte for a target application. Probing oxygen environments is of great interest as oxygens are located at strategic molecular sites in battery solvents and are directly involved in inter- and intramolecular solvation interactions. NMR signals from ¹⁷O nuclei in battery electrolytes offer nondestructive bulk measurements of isotropic shielding, electric field gradient tensors, and transverse and longitudinal relaxation rates, which are excellent means for probing structure, bonding, and dynamics of both solute and solvent molecules. This article describes the use of ¹⁷O NMR spectroscopy in probing the solvation structures of various electrolyte systems ranging from transition metal ions in aqueous solution to lithium cations in organic solvent mixtures.

Duan, W, RS Vemuri, D Hu, Z Yang, X Wei. "A Protocol for Electrochemical Evaluations and State of Charge Diagnostics of a Symmetric Organic Redox Flow Battery." Journal of Visualized Experiments 120 (Feb. 2017).
Abstract: Redox flow batteries have been considered as one of the most promising stationary energy storage solutions for improving the reliability of the power grid and deployment of renewable energy technologies. Among the many flow battery chemistries, non-aqueous flow batteries have the potential to achieve high energy density because of the broad voltage windows of non-aqueous electrolytes. However, significant technical hurdles exist currently limiting non-aqueous flow batteries to demonstrate their full potential, such as low redox concentrations, low operating currents, under-explored battery status monitoring, etc. In an attempt to address these limitations, we recently reported a non-aqueous flow battery based on a highly soluble, redox-active organic nitronyl nitroxide radical compound, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO). This redox material exhibits an ambipolar electrochemical property, and therefore can serve as both anolyte and catholyte redox materials to form a symmetric flow battery chemistry. Moreover, we demonstrated that Fourier transform infrared (FTIR) spectroscopy could measure the PTIO concentrations during the PTIO flow battery cycling and offer reasonably accurate detection of the battery state of charge (SOC), as cross-validated by electron spin resonance (ESR) measurements. Herein we present a video protocol for the electrochemical evaluation and SOC diagnosis of the PTIO symmetric flow battery. With a detailed description, we experimentally demonstrated the route to achieve such purposes. This protocol aims to spark more interests and insights on the safety and reliability in the field of non-aqueous redox flow batteries.

Han KS ,Rajput NN ,Vijayakumar M ,Wei X ,Wang W ,Hu J Z,Persson K A,Mueller K T. "Preferential Solvation of an Asymmetric Redox Molecule" Journal of Physical Chemistry C 120 (49): 27834-27839 (Nov. 2016).
Abstract: The fundamental correlations between solubility and solvation structure for the electrolyte system comprising N-(ferrocenylmethyl)-N,N-dimethyl-N-ethylammonium bistrifluoromethylsulfonimide (Fc1N112-TFSI) dissolved in a ternary carbonate solvent mixture is analyzed using combined NMR relaxation and computational methods. Probing the evolution of the solvent–solvent, ion–solvent and ion–ion interactions with an increase in solute concentration provides a molecular level understanding of the solubility limit of the Fc1N112-TFSI system. An increase in solute concentration leads to pronounced Fc1N112-TFSI contact-ion pair formation by diminishing solvent–solvent and ion–solvent type interactions. At the solubility limit, the precipitation of solute is initiated through agglomeration of contact-ion pairs due to overlapping solvation shells.

Mehdi, BL, A Stevens, J Qian, C Park, W Xu, WA Henderson, J Zhang, KT Mueller, ND Browning. "The Impact of Li Grain Size on Coulombic Efficiency in Li Batteries."Scientific Reports 6: Article number 34267 (Oct. 2016).
Abstract: One of the most promising means to increase the energy density of state-of-the-art lithium Li-ion batteries is to replace the graphite anode with a Li metal anode. While the direct use of Li metal may be highly advantageous, at present its practical application is limited by issues related to dendrite growth and low Coulombic efficiency, CE. Here operando electrochemical scanning transmission electron microscopy (STEM) is used to directly image the deposition/stripping of Li at the anode-electrolyte interface in a Li-based battery. A non-aqueous electrolyte containing small amounts of H₂O as an additive results in remarkably different deposition/stripping properties as compared to the “dry” electrolyte when operated under identical electrochemical conditions. The electrolyte with the additive deposits more Li during the first cycle, with the grain sizes of the Li deposits being significantly larger and more variable. The stripping of the Li upon discharge is also more complete, i.e., there is a higher cycling CE. This suggests that larger grain sizes are indicative of better performance by leading to more uniform Li deposition and an overall decrease in the formation of Li dendrites and side reactions with electrolyte components, thus potentially paving the way for the direct use of Li metal in battery technologies.

Huang, J, B Pan, W Duan, X Wei, RS Assary, L Su, FR Brushett, L Cheng, C Liao, MS Ferrandon, W Wang, Z Zhang, AK Burrell, LA Curtiss, IA Shkrob, JS Moore, L Zhang. "The lightest organic radical cation for charge storage in redox flow batteries." Scientific Reports 6: Article number 32102 (Aug. 2016).
Abstract: In advanced electrical grids of the future, electrochemically rechargeable fluids of high energy density will capture the power generated from intermittent sources like solar and wind. To meet this outstanding technological demand there is a need to understand the fundamental limits and interplay of electrochemical potential, stability, and solubility in low-weight redox-active molecules. By generating a combinatorial set of 1,4-dimethoxybenzene derivatives with different arrangements of substituents, we discovered a minimalistic structure that combines exceptional long-term stability in its oxidized form and a record-breaking intrinsic capacity of 161 mAh/g. The nonaqueous redox flow battery has been demonstrated that uses this molecule as a catholyte material and operated stably for 100 charge/discharge cycles. The observed stability trends are rationalized by mechanistic considerations of the reaction pathways.

Qian, J, BD Adams, J Zheng, W Xu, WA Henderson, J Wang, ME Bowden, S Xu, JZ Hu, J Zhang. "Anode-Free Rechargeable Lithium Metal Batteries." Advanced Functional Materials 26 (39): 7094-7102 (Aug. 2016).
Abstract: Anode-free rechargeable lithium (Li) batteries (AFLBs) are phenomenal energy storage systems due to their significantly increased energy density and reduced cost relative to Li-ion batteries, as well as ease of assembly because of the absence of an active (reactive) anode material. However, significant challenges, including Li dendrite growth and low cycling Coulombic efficiency (CE), have prevented their practical implementation. Here, an anode-free rechargeable lithium battery based on a Cu||LiFePO₄ cell structure with an extremely high CE (>99.8%) is reported for the first time. This results from the utilization of both an exceptionally stable electrolyte and optimized charge/discharge protocols, which minimize the corrosion of the in situly formed Li metal anode.

Cheng, L, LA Curtiss, KR Zavadil, AA Gewirth, Y Shao, KG Gallagher. "Sparingly Solvating Electrolytes for High Energy Density Lithium–Sulfur Batteries." ACS Energy Letters 1 (3): 503-509 (July 2016).
Abstract: Moving to lighter and less expensive battery chemistries compared to contemporary lithium-ion requires the control of energy storage mechanisms based on chemical transformations rather than intercalation. Lithium–sulfur (Li/S) has tremendous theoretical specific energy, but contemporary approaches to control this solution-mediated, precipitation–dissolution chemistry require large excesses of electrolyte to fully solubilize the polysulfide intermediates. Achieving reversible electrochemistry under lean electrolyte operation is the most promising path for Li/S to move beyond niche applications to potentially transformational performance. An emerging Li/S research area is the use of sparingly solvating electrolytes and the creation of design rules for discovering new electrolyte systems that fundamentally decouple electrolyte volume from sulfur and polysulfide reaction mechanism. This Perspective presents an outlook for sparingly solvating electrolytes as a key path forward for long-lived, high energy density Li/S batteries including an overview of this promising new concept and some strategies for accomplishing it.

Pan, H, KS Han, M Vijayakumar, J Xiao, R Cao, J Chen, J Zhang, KT Mueller, Y Shao, J Liu. "Ammonium Additives to Dissolve Lithium Sulfide through Hydrogen Binding for High-Energy Lithium–Sulfur Batteries."Applied Materials & Interfaces 9 (5): 4290-4295 (July 2016).
Abstract: In rechargeable Li–S batteries, the uncontrollable passivation of electrodes by highly insulating Li₂S limits sulfur utilization, increases polarization, and decreases cycling stability. Dissolving Li₂S in organic electrolyte is a facile solution to maintain the active reaction interface between electrolyte and sulfur cathode, and thus address the above issues. Herein, ammonium salts are demonstrated as effective additives to promote the dissolution of Li₂S to 1.25 M in DMSO solvent at room temperature. NMR measurements show that the strong hydrogen binding effect of N–H groups plays a critical role in dissolving Li₂S by forming complex ligands with S2– anions coupled with the solvent’s solvating surrounding. Ammonium additives in electrolyte can also significantly improve the oxidation kinetics of Li₂S, and therefore enable the direct use of Li₂S as cathode material in Li–S battery system in the future. This provides a new approach to manage the solubility of lithium sulfides through cation coordination with sulfide anion.

Prabhakaran V ,Mehdi B L,Ditto J J,Engelhard M H,Wang B ,Gunaratne KD D,Johnson D C,Browning N D,Johnson G E,Laskin J. "Rational design of efficient electrode–electrolyte interfaces for solid-state energy storage using ion soft landing" Nature Communications 7 (April 2016).
Abstract: The rational design of improved electrode–electrolyte interfaces (EEI) for energy storage is critically dependent on a molecular-level understanding of ionic interactions and nanoscale phenomena. The presence of non-redox active species at EEI has been shown to strongly influence Faradaic efficiency and long-term operational stability during energy storage processes. Herein, we achieve substantially higher performance and long-term stability of EEI prepared with highly dispersed discrete redox-active cluster anions (50 ng of pure ∼0.75 nm size molybdenum polyoxometalate (POM) anions on 25 μg (∼0.2 wt%) carbon nanotube (CNT) electrodes) by complete elimination of strongly coordinating non-redox species through ion soft landing (SL). Electron microscopy provides atomically resolved images of a uniform distribution of individual POM species soft landed directly on complex technologically relevant CNT electrodes. In this context, SL is established as a versatile approach for the controlled design of novel surfaces for both fundamental and applied research in energy storage.

Crawford, A, E Thomsen, D Reed, D Stephenson, V Sprenkle, J Liu, V Viswanathan. "Development and validation of chemistry agnostic flow battery cost performance model and application to nonaqueous electrolyte systems." International Journal of Energy Research 40 (12): 1611-1623 (April 2016).
Abstract: A chemistry agnostic cost performance model is described for a nonaqueous flow battery. The model predicts flow battery performance by estimating the active reaction zone thickness at each electrode as a function of current density, state of charge, and flow rate using measured data for electrode kinetics, electrolyte conductivity, and electrode-specific surface area. Validation of the model is conducted using a 4 kW stack data at various current densities and flow rates. This model is used to estimate the performance of a nonaqueous flow battery with electrode and electrolyte properties used from the literature. The optimized cost for this system is estimated for various power and energy levels using component costs provided by vendors. The model allows optimization of design parameters such as electrode thickness, area, flow path design, and operating parameters such as power density, flow rate, and operating SOC range for various application duty cycles. A parametric analysis is done to identify components and electrode/electrolyte properties with the highest impact on system cost for various application durations. A pathway to 100$ kW h⁻¹ for the storage system is identified. Copyright © 2016 John Wiley & Sons, Ltd.

Cao, R, J Chen, KS Han, W Xu, D Mai, P Bhattacharya, MH Engelhard, KT Mueller, J Liu, J Zhang. "Effect of the Anion Activity on the Stability of Li Metal Anodes in Lithium-Sulfur Batteries." Advanced Functional Materials 26 (18): 3059-3066 (March 2016).
Abstract: With the significant progress made in the development of cathodes in lithium-sulfur (Li-S) batteries, the stability of Li metal anodes becomes a more urgent challenge in these batteries. Here the systematic investigation of the stability of the anode/electrolyte interface in Li-S batteries with concentrated electrolytes containing various lithium salts is reported. It is found that Li-S batteries using LiTFSI-based electrolytes are more stable than those using LiFSI-based electrolytes. The decreased stability is because the N–S bond in the FSI− anion is fairly weak and the scission of this bond leads to the formation of lithium sulfate (LiSO_x) in the presence of polysulfide species. In contrast, in the LiTFSI-based electrolyte, the lithium metal anode tends to react with polysulfide to form lithium sulfide (LiS_x), which is more reversible than LiSO_x formed in the LiFSI-based electrolyte. This fundamental difference in the bond strength of the salt anions in the presence of polysulfide species leads to a large difference in the stability of the anode-electrolyte interface and performance of the Li-S batteries with electrolytes composed of these salts. Therefore, anion selection is one of the key parameters in the search for new electrolytes for stable operation of Li-S batteries.

Chen, J, KS Han, WA Henderson, KC Lau, M Vijayakumar, T Dzwiniel, H Pan, LA Curtiss, J Xiao, KT Mueller, Y Shao, J Liu. "Restricting the Solubility of Polysulfides in Li-S Batteries Via Electrolyte Salt Selection."Advanced Energy Materials 6 (11) (March 2016).
Abstract: Lithium 2-trifluoromethyl-4,5-dicyanoimidazole as a supporting salt in electrolytes achieves a reduced Li₂S₈ solubility by tuning the chemical production to Li₄S₈ dimer. Ab initio molecular dynamics and nuclear magnetic resonance calculation confirms the solvation structure radius increase by 20%. Combined with Li₂S₈ and LiNO₃, the electrolyte can demonstrate a stable 300 cycling battery of practical sulfur loading.

Deng, X, M Hu, X Wei, W Wei, KT Mueller, Z chen, JZ Hu. "Nuclear magnetic resonance studies of the solvation structures of a high-performance nonaqueous redox flow electrolyte." Journal of Power Sources 308: 172-179 (March 2016).
Abstract: Understanding the solvation structures of electrolytes is important for developing nonaqueous redox flow batteries that hold considerable potential for future large scale energy storage systems. The utilization of an emerging ionic-derivatived ferrocene compound, ferrocenylmethyl dimethyl ethyl ammonium bis(trifluoromethanesulfonyl)imide (Fc1N112-TFSI), has recently overcome the issue of solubility in the supporting electrolyte. In this work, ¹³C, ¹H and ¹⁷O NMR investigations were carried out using electrolyte solutions consisting of Fc1N112-TFSI as the solute and the mixed alkyl carbonate as the solvent. It was observed that the spectra of ¹³C experience changes of chemical shifts while those of ¹⁷O undergo linewidth broadening, indicating interactions between solute and solvent molecules. Quantum chemistry calculations of both molecular structures and chemical shifts (¹³C, ¹H and ¹⁷O) are performed for interpreting experimental results and for understanding the detailed solvation structures. The results indicate that Fc1N112-TFSI is dissociated at varying degrees in mixed solvent depending on concentrations. At dilute solute concentrations, most Fc1N112+ and TFSI− are fully disassociated with their own solvation shells formed by solvent molecules. At saturated concentration, Fc1N112+-TFSI- contact ion pairs are formed and the solvent molecules are preferentially interacting with the Fc rings rather than interacting with the ionic pendant arm of Fc1N112-TFSI.

Duan, W, RS Vemuri, JD Milshtein, S Laramie, RD Dmello, J Huang, L Zhang, D Hu, M Vijayakumar, W Wang, J Liu, RM Darling, L Thompson, K Smith, JS Moore, FR Brushett, X Wei. "A symmetric organic-based nonaqueous redox flow battery and its state of charge diagnostics by FTIR." Journal of Materials Chemistry A 4: 5448-5456 (March 2016).
Abstract: Redox flow batteries have shown outstanding promise for grid-scale energy storage to promote utilization of renewable energy and improve grid stability. Nonaqueous battery systems can potentially achieve high energy density because of their broad voltage window. In this paper, we report a new organic redox-active material for use in a nonaqueous redox flow battery, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) that has high solubility (>2.6 M) in organic solvents. PTIO exhibits electrochemically reversible disproportionation reactions and thus can serve as both anolyte and catholyte redox materials in a symmetric flow cell. The PTIO flow battery has a moderate cell voltage of ∼1.7 V and shows good cyclability under both cyclic voltammetry and flow cell conditions. Moreover, we demonstrate that FTIR can offer accurate estimation of the PTIO concentration in electrolytes and determine the state of charge of the PTIO flow cell, suggesting FTIR as a powerful online battery status sensor. This study is expected to inspire more insights in this under-addressed area of state of charge analysis aiming at operational safety and reliability of flow batteries.

Wan, C, MY Hu, O Borodin, J Qian, Z Qin, J Zhang, JZ Hu. "Natural abundance ¹⁷O, ⁶Li NMR and molecular modeling studies of the solvation structures of lithium bis(fluorosulfonyl)imide/1,2-dimethoxyethane liquid electrolytes."Journal of Power Sources 307: 231-243 (March 2016).
Abstract: Natural abundance ¹⁷O and⁶Li NMR experiments, quantum chemistry and molecular dynamics studies were employed to investigate the solvation structures of Li+ at various concentrations of LiFSI in DME electrolytes. It was found that the chemical shifts of both ¹⁷O and ⁶Li changed with the concentration of LiFSI, indicating the changes of solvation structures with concentration. For the quantum chemistry calculations, the coordinated cluster LiFSI(DME)₂ forms at first, and its relative ratio increases with increasing LiFSI concentration to 1 M. Then the solvation structure LiFSI(DME) become the dominant component. As a result, the coordination of forming contact ion pairs between Li+ and FSI− ion increases, but the association between Li+ and DME molecule decreases. Furthermore, at LiFSI concentration of 4 M the solvation structures associated with Li+(FSI−)₂(DME), Li+₂(FSI−)(DME)₄ and (LiFSI)₂(DME)₃ become the dominant components. For the molecular dynamics simulation, with increasing concentration, the association between DME and Li+ decreases, and the coordinated number of FSI− increases, which is in perfect accord with the DFT results.

Kumar G R,Savariraj D A,Karthick S N,Selvam S ,Balamuralitharan B ,Kim HJ ,Viswanathan K K,Vijayakumar M ,Prabakar K. "Phase transition kinetics and surface binding states of methylammonium lead iodide perovskite" Phys. Chem. Chem. Phys. 18: 7284-7297 (Feb. 2016).
Abstract: We have presented a detailed analysis of the phase transition kinetics and binding energy states of solution processed methylammonium lead iodide (MAPbI3) thin films prepared at ambient conditions and annealed at different elevated temperatures. It is the processing temperature and environmental conditions that predominantly control the crystal structure and surface morphology of MAPbI3 thin films. The structural transformation from tetragonal to cubic occurs at 60 °C with a 30 minute annealing time while the 10 minute annealed films posses a tetragonal crystal structure. The transformed phase is greatly intact even at the higher annealing temperature of 150 °C and after a time of 2 hours. The charge transfer interaction between the Pb 4f and I 3d oxidation states is quantified using XPS.

Hu, JZ, Z Zhao, MY Hu, J Feng, X Deng, X Chen, W Xu, J Liu, J Zhang. "In situ⁷Li and ¹³³Cs nuclear magnetic resonance investigations on the role of Cs⁺ additive in lithium-metal deposition process." Journal of Power Sources 304: 51-59 (Feb. 2016).
Abstract: Cesium ion (Cs+) has been reported to be an effective electrolyte additive to suppress Li dendrite growth which prevents the application of lithium (Li) metal as an anode for rechargeable Li batteries. In this work, we investigated the effect of Cs+ additive on Li depositions using quantitative in situ⁷Li and ¹³³Cs nuclear magnetic resonance (NMR) with planar symmetric Li cells. It's found that the addition of Cs+ can significantly enhance both the formation of well aligned Li nanorods and reversibility of the Li electrode. In situ¹³³Cs NMR directly confirms that Cs+ migrates to Li electrode to form a positively charged electrostatic shield during the charging process. Much more electrochemical “active” Li was found in Li films deposited with Cs+ additive, while more electrochemical “dead” and thicker Li rods were identified in Li films deposited without Cs+. Combining the in situ and the previous ex-situ results, a Li deposition model has been proposed to explain these observations.

Savariraj D A,Rajendrakumar G ,Selvam S ,Karthick S N,Balamuralitharan B ,Kim HJ ,Viswanathan K K,Vijayakumar M ,Prabakar K. "Stacked Cu1.8S nanoplatelets as counter electrode for quantum dot-sensitized solar cell" RCS Advances 5: 100560-100567 (Nov. 2015).
Abstract: It is found that the electrocatalytic activity of Cu2−xS thin films used in quantum dot-sensitized solar cells (QDSSCs) as counter electrode (CE) for the reduction of polysulfide electrolyte depends on the surface active sulfide and disulfide species and the deficiency of Cu. The preferential bonding between Cu2+ and S2−, leading to the selective formation of a Cu1.8S stacked platelet-like morphology, is determined by the cetyl trimethyl ammonium bromide surfactant and deposition temperature; the crab-like Cu–S coordination bond formed dictates the surface area to volume ratio of the Cu1.8S thin films and their electrocatalytic activity. The Cu deficiency enhances the conductivity of the Cu1.8S thin films, which exhibit near-infrared localized surface plasmon resonance due to free carriers, and UV-vis absorption spectra show an excitonic effect due to the quantum size effect. When these Cu1.8S thin films were employed as CEs in QDSSCs, a robust photoconversion efficiency of 5.2% was obtained for the film deposited at 60 °C by a single-step chemical bath deposition method.

Cheng, Y, Y Shao, LR Parent, ML Sushko, G Li, PV Sushko, ND Browning, C Wang, J Liu. "Interface Promoted Reversible Mg Insertion in Nanostructured Tin–Antimony Alloys."Advanced Materials 27 (42): 6598-6605 (Sept. 2015).
Abstract: An interface promoted approach is developed for guiding the design of stable and high capacity materials for Mg batteries using SnSb alloys as model materials. Experimental and theoretical studies reveal that the SnSb alloy has exceptionally high reversible capacity (420 mA h g⁻¹), excellent rate capability, and good cyclic stability for hosting Mg ions due to the stabilization/promotion effects of the interfaces between the multicomponent phases generated during repeated magnesiation–demagnesiation.

Zhu, Z, Y Zhou, P Yan, RS Vemuri, W Xu, R Zhao, X Wang, S Thevuthasan, DR Baer, CM Wang. "In Situ Mass Spectrometric Determination of Molecular Structural Evolution at the Solid Electrolyte Interphase in Lithium-Ion Batteries." Nano Letters 15 (9): 6170-6176 (Aug. 2015).
Abstract: Dynamic structural and chemical evolution at solid–liquid electrolyte interface is always a mystery for a rechargeable battery due to the challenge to directly probe a solid–liquid interface under reaction conditions. We describe the creation and usage of in situ liquid secondary ion mass spectroscopy (SIMS) for the first time to directly observe the molecular structural evolution at the solid–liquid electrolyte interface for a lithium (Li)-ion battery under dynamic operating conditions. We have discovered that the deposition of Li metal on copper electrode leads to the condensation of solvent molecules around the electrode. Chemically, this layer of solvent condensate tends to be depleted of the salt anions and with reduced concentration of Li+ ions, essentially leading to the formation of a lean electrolyte layer adjacent to the electrode and therefore contributing to the overpotential of the cell. This observation provides unprecedented molecular level dynamic information on the initial formation of the solid electrolyte interphase (SEI) layer. The present work also ultimately opens new avenues for implanting the in situ liquid SIMS concept to probe the chemical reaction process that intimately involves solid–liquid interface, such as electrocatalysis, electrodeposition, biofuel conversion, biofilm, and biomineralization.

Deng X, MY Hu, X Wei, W Wang, Z Chen, J Liu, JZ Hu. "Natural abundance ¹⁷O nuclear magnetic resonance and computational modeling studies of lithium based liquid electrolytes." Journal of Power Sources 285: 146-155 (July 2015).
Abstract:Natural abundance ¹⁷O NMR measurements were conducted on electrolyte solutions consisting of Li[CF₃SO₂NSO₂CF₃] (LiTFSI) dissolved in the solvents of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and their mixtures at various concentrations. It was observed that ¹⁷O chemical shifts of solvent molecules change with the concentration of LiTFSI. The chemical shift displacements of carbonyl oxygen are evidently greater than those of ethereal oxygen, strongly indicating that Li⁺ ion is coordinated with carbonyl oxygen rather than ethereal oxygen. To understand the detailed molecular interaction, computational modeling of ¹⁷O chemical shifts was carried out on proposed solvation structures. By comparing the predicted chemical shifts with the experimental values, it is found that a Li⁺ ion is coordinated with four double bond oxygen atoms from EC, PC, EMC and TFSI anion. In the case of excessive amount of solvents of EC, PC and EMC the Li⁺ coordinated solvent molecules are undergoing quick exchange with bulk solvent molecules, resulting in average ¹⁷O chemical shifts. Several kinds of solvation structures are identified, where the proportion of each structure in the liquid electrolytes investigated depends on the concentration of LiTFSI.

Qian, J, W Xu, P Bhattacharya, M Engelhard, WA Henderson, Y Zhang, J Zhang. "Dendrite-free Li deposition using trace-amounts of water as an electrolyte additive." Nano Energy 15: 135-144 (July 2015).
Abstract:Residual water (H₂O) presents in nonaqueous electrolytes has been widely regarded as a detrimental factor for lithium (Li) batteries. This is because H₂O is highly reactive with the commonly used LiPF₆ salt leading to the formation of HF which subsequently corrodes battery materials. In this work, we demonstrate that a controlled trace-amount of H₂O (25–50 ppm) can be an effective electrolyte additive for achieving dendrite-free Li metal deposition in LiPF_6-based electrolytes, while avoid detrimental effects. Detailed analyses revealed that the trace amount of HF derived from the decomposition reaction of LiPF₆ with H₂O is electrochemically reduced during the initial Li deposition process to form a uniform and dense LiF-rich solid electrolyte interphase (SEI) layer on the surface of the substrate. This LiF-rich SEI layer leads to a uniform distribution of the electric field on the substrate surface thereby enabling uniform and dendrite-free Li deposition. Meanwhile, the detrimental effect of HF on the other cell components is diminished due to the consumption of the HF in the LiF formation process. Microscopic analysis reveals that the as-deposited, dendrite-free Li films exhibit a self-aligned and highly-compact Li nanorod structure which is consistent with a vivid blue color due to structural coloration. These findings clearly demonstrate a novel approach to control the nucleation and grow processes of Li metal films using a well-controlled, trace-amount of H₂O, as well as illuminate the effect of H₂O on other electrodeposition processes.

Kerisit, S, M Vijayakumar, KS Han, KT Mueller. "Solvation structure and transport properties of alkali cations in dimethyl sulfoxide under exogenous static electric fields." Journal of Chemical Physics 142 (22) 224502 (June 2015).
Abstract:A combination of molecular dynamics simulations and pulsed field gradient nuclear magnetic resonance spectroscopy is used to investigate the role of exogenous electric fields on the solvation structure and dynamics of alkali ions in dimethyl sulfoxide (DMSO) and as a function of temperature. Good agreement was obtained, for select alkali ions in the absence of an electric field, between calculated and experimentally determined diffusion coefficients normalized to that of pure DMSO. Our results indicate that temperatures of up to 400 K and external electric fields of up to 1 V nm⁻¹ have minimal effects on the solvation structure of the smaller alkali cations (Li⁺ and Na⁺) due to their relatively strong ion-solvent interactions, whereas the solvation structures of the larger alkali cations (K⁺, Rb⁺, and Cs⁺) are significantly affected. In addition, although the DMSO exchange dynamics in the first solvation shell differ markedly for the two groups, the drift velocities and mobilities are not significantly affected by the nature of the alkali ion. Overall, although exogenous electric fields induce a drift displacement, their presence does not significantly affect the random diffusive displacement of the alkali ions in DMSO. System temperature is found to have generally a stronger influence on dynamical properties, such as the DMSO exchange dynamics and the ion mobilities, than the presence of electric fields.

Pan, H, X Wei, WA Henderson, Y Shao, J Chen, P Bhattacharya, J Xiao, J Liu. "On the Way Toward Understanding Solution Chemistry of Lithium Polysulfides for High Energy Li–S Redox Flow Batteries." Advanced Energy Materials 5 (16)(Apr. 2015).
Abstract:Lithium–sulfur (Li–S) redox flow battery (RFB) is a promising candidate for high energy large-scale energy storage application due to good solubility of long-chain polysulfide species and low cost of sulfur. Here, the fundamental understanding and control of lithium polysulfide chemistry are studied to enable the development of liquid phase Li–S redox flow prototype cells. These differ significantly from conventional static Li–S batteries targeting for vehicle electrification. A high solubility of the different lithium polysulfides generated at different depths of discharge and states of charge is required for a flow battery in order to take full advantage of the multiple electron transitions. A new dimethyl sulfoxide based electrolyte is proposed for Li–S RFBs, which not only enables the high solubility of lithium polysulfide species, especially for the short-chain species, but also results in excellent cycling with a high Coulombic efficiency. The challenges and opportunities for the Li–S redox flow concept have also been discussed in depth.

Wei X, W Xu, J Huang, L Zhang, E Walter, C Lawrence, M Vijaykumar, WA Henderson, T Liu, L Cosimbescu, B Li, V Sprenkle, W Wang. "Radical Compatibility with Nonaqueous Electrolytes and Its Impact on an All-Organic Redox Flow Battery."Angewandte Chemie 54 (30): 8684-8687 (Apr. 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.

Cao, R, W Xu, D Lu, J Xiao, J Zhang. "Anodes for Rechargeable Lithium-Sulfur Batteries." Advanced Energy Materials 5 (16)(Apr. 2015).
Abstract:With the significant progress that has been made toward the development of cathode materials and electrolytes in lithium-sulfur (Li-S) batteries in recent years, the stability of the anode in Li-S batteries has become one of the more urgent challenges in order to reach long-term stability of Li-S batteries. In Li-S batteries, a passivation layer is easily formed on the metallic Li anode surface because of the presence of polysulfides and electrolyte additives. Although the passivation layer on the Li metal anode can significantly suppress Li dendrite growth and improve the safety of Li-S batteries, continuous corrosion of the Li metal anode eventually leads to battery failure due to the increased cell impedance and the depletion of electrolyte. Here, the recent developments on the protection of the Li metal anode in Li-S batteries are reviewed. Various strategies used to minimize the corrosion of Li anodes and to reduce its impedance increase are analyzed. Other alternative anodes used in sulfur-based rechargeable batteries are also discussed.

Chen J, D Wu, E Walter, M Engelhard, P Bhattacharya, H Pan, Y Shao, F Gao, J Xiao, J Liu. "Molecular-confinement of polysulfides within mesoscale electrodes for the practical application of lithium sulfur batteries."Nano Energy 13: 267-274 (Apr. 2015).
Abstract:Nitrogen-doped porous carbon (NPC) and multi-wall carbon nanotubes (MWCNT) have been frequently studied to immobilize sulfur in lithium–sulfur (Li–S) batteries. However, neither NPC nor MWCNT itself can effectively confine the soluble polysufides if cathode thickness e.g. sulfur loading is increased. In this work, NPC was combined with MWCNTs to construct an integrated host structure to immobilize sulfur at a relevant scale. The function of doped nitrogen atoms was revisited and found to effectively attract sulfur radicals generated during the electrochemical process. The addition of MWCNT facilitated the uniform coating of sulfur nanocomposites to a practically thickness and homogenized the distribution of sulfur particles in the pristine electrodes, while NPC provided sufficient pore volume to trap the dissolved polysulfides species. More importantly, the difficulty of electrode wetting, a critical challenge for thick sulfur cathodes, is also mitigated after the adoption of MWCNT, leading to a high areal capacity of ca. 2.5 mA h/cm² with a capacity retention of 81.6% over 100 cycles.

Yu, X, H Pan, Y Zhou, P Northrup, J Xiao, S Bak, M Liu, KW Nam, D Qu, J Liu, T Wu, XQ Yang. "Direct Observation of the Redistribution of Sulfur and Polysufides in Li–S Batteries During the First Cycle by In Situ X-Ray Fluorescence Microscopy." Advanced Energy Materials 5 (16) (March 2015).
Abstract:A novel in situ X-ray fluorescence microscopy combined with X-ray absorption spectroscopy technique is reported to investigate the Li–S batteries during electrochemical cycling. The evolution of morphology changes of the electrode is monitored in real time using the X-ray fluorescence images, while the changes of the sulfur chemical state are characterized simultaneously using the X-ray absorption spectroscopy spectra.

Welch D A,Mehdi B L,Hatchell H J,Faller R ,Evans J E,Browning N D. "Using molecular dynamics to quantify the electrical double layer and examine the potential for its direct observation in the in-situ TEM." Advanced Structural and Chemical Imagining 1 (1) (March 2015).
Abstract: Understanding the fundamental processes taking place at the electrode-electrolyte interface in batteries will play a key role in the development of next generation energy storage technologies. One of the most fundamental aspects of the electrode-electrolyte interface is the electrical double layer (EDL). Given the recent development of high spatial resolution in-situ electrochemical fluid cells for scanning transmission electron microscopy (STEM), there now exists the possibility that we can directly observe the formation and dynamics of the EDL. In this paper we predict electrolyte structure within the EDL using classical models and atomistic Molecular Dynamics (MD) simulations. Classical models are found to greatly differ from MD in predicted concentration profiles. It is thus suggested that MD must be used in order to accurately predict STEM images of the electrode-electrolyte interface. Using MD and image simulation together for a high contrast electrolyte (the high atomic number CsCl electrolyte), it is determined that, for a smooth interface, concentration profiles within the EDL should be visible experimentally. When normal experimental parameters such as rough interfaces and low-Z electrolytes (like those used in Li-ion batteries) are considered, observation of the EDL appears to be more difficult.

Xiao J ,Hu J Z,Chen H ,Vijayakumar M ,Zheng J ,Pan H ,Walter E D,Hu M Y,Deng X ,Feng J ,Liaw BY ,Gu M ,Deng Z ,Lu D ,Xu S ,Wang C M,Liu J. "Following the Transient Reactions in Lithium–Sulfur Batteries Using an In Situ Nuclear Magnetic Resonance Technique." Nano Letters 15 (5): 3309-3316 (March 2015).
Abstract:A fundamental understanding of electrochemical reaction pathways is critical to improving the performance of Li–S batteries, but few techniques can be used to directly identify and quantify the reaction species during disharge/charge cycling processes in real time. Here, an in situ ⁷Li NMR technique employing a specially designed cylindrical microbattery was used to probe the transient electrochemical and chemical reactions occurring during the cycling of a Li–S system. In situ NMR provides real time, semiquantitative information related to the temporal evolution of lithium polysulfide allotropes during both discharge/charge processes. This technique uniquely reveals that the polysulfide redox reactions involve charged free radicals as intermediate species that are difficult to detect in ex situ NMR studies. Additionally, it also uncovers vital information about the ⁷Li chemical environments during the electrochemical and parasitic reactions on the Li metal anode. These new molecular-level insights about transient species and the associated anode failure mechanism are crucial to delineating effective strategies to accelerate the development of Li–S battery technologies.

Shao Y ,Rajput NN ,Hu J Z,Hu M Y,Liu T L.,Wei Z ,Gu M ,Deng X ,Xu S ,Han KS ,Wang J ,Nie Z ,Li G ,Zavadil K ,Xiao J ,Wang C M,Henderson W A,Zhang J ,Wang Y ,Mueller K T,Persson K A,Liu J. "Nanocomposite polymer electrolyte for rechargeable magnesium batteries." Nano Energy 12: 750-579 (March 2015).
Abstract:Nanocomposite polymer electrolytes present new opportunities for rechargeable magnesium batteries. However, few polymer electrolytes have demonstrated reversible Mg deposition/dissolution and those that have still contain volatile liquids such as tetrahydrofuran (THF). In this work, we report a nanocomposite polymer electrolyte based on poly(ethylene oxide) (PEO), Mg(BH₄)₂ and MgO nanoparticles for rechargeable Mg batteries. Cells with this electrolyte have a high coulombic efficiency of 98% for Mg plating/stripping and a high cycling stability. Through combined experiment-modeling investigations, a correlation between improved solvation of the salt and solvent chain length, chelation and oxygen denticity is established. Following the same trend, the nanocomposite polymer electrolyte is inferred to enhance the dissociation of the salt Mg(BH₄)₂ and thus improve the electrochemical performance. The insights and design metrics thus obtained may be used in nanocomposite electrolytes for other multivalent systems.

Mehdi, BL, J Qian, E Nasybulin, C Park, DA Welch, R Faller, H Mehta, WA Henderson, W Xu, CM Wang, JE Evans, J Liu, J Zhang, KT Mueller, ND Browning. "Observation and Quantification of Nanoscale Processes in Lithium Batteries by Operando Electrochemical (S)TEM." <Nano Letters 15 (3): 2168-2173 (Feb. 2015).
Abstract:An operando electrochemical stage for the transmission electron microscope has been configured to form a “Li battery” that is used to quantify the electrochemical processes that occur at the anode during charge/discharge cycling. Of particular importance for these observations is the identification of an image contrast reversal that originates from solid Li being less dense than the surrounding liquid electrolyte and electrode surface. This contrast allows Li to be identified from Li-containing compounds that make up the solid-electrolyte interphase (SEI) layer. By correlating images showing the sequence of Li electrodeposition and the evolution of the SEI layer with simultaneously acquired and calibrated cyclic voltammograms, electrodeposition, and electrolyte breakdown processes can be quantified directly on the nanoscale. This approach opens up intriguing new possibilities to rapidly visualize and test the electrochemical performance of a wide range of electrode/electrolyte combinations for next generation battery systems.

Qian J, WA Henderson, W Xu, P Bhattacharya, M Engelhard, O Borodin, J Zhang. "High rate and stable cycling of lithium metal anode."Nature Communications 6: Article number 6362 (Feb. 2015).
Abstract:Lithium metal is an ideal battery anode. However, dendrite growth and limited Coulombic efficiency during cycling have prevented its practical application in rechargeable batteries. Herein, we report that the use of highly concentrated electrolytes composed of ether solvents and the lithium bis(fluorosulfonyl)imide salt enables the high-rate cycling of a lithium metal anode at high Coulombic efficiency (up to 99.1%) without dendrite growth. With 4 M lithium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane as the electrolyte, a lithium|lithium cell can be cycled at 10 mA cm⁻² for more than 6,000 cycles, and a copper|lithium cell can be cycled at 4 mA cm⁻² for more than 1,000 cycles with an average Coulombic efficiency of 98.4%. These excellent performances can be attributed to the increased solvent coordination and increased availability of lithium ion concentration in the electrolyte. Further development of this electrolyte may enable practical applications for lithium metal anode in rechargeable batteries.

Wei X ,Cosimbescu L ,Xu W ,Hu J Z,Vijayakumar M ,Feng J ,Hu M Y,Deng X ,Xiao J ,Liu J ,Sprenkle V L,Wang W. "Towards High-Performance Nonaqueous Redox Flow Electrolyte Via Ionic Modification of Active Species." Advanced Energy Materials 5 (1): 1400678 (Jan. 2015).
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.

Liu, T, JT Cox, D Hu, X Deng, J Hu, MY Hu, J Xiao, Y Shao, K Tang, J Liu. "A fundamental study on the [(μ-Cl)3Mg2(THF)6]+ dimer electrolytes for rechargeable Mg batteries." Chemical Communications 51: 2312-2315 (Jan. 2015).
Abstract:The long sought solvated [MgCl]+ species in the Mg-dimer electrolytes was characterized by soft mass spectrometry. The presented study provides an insightful understanding on the electrolyte chemistry of rechargeable Mg batteries.

Parent LR, Y Cheng, PV Sushko, Y Shao, J Liu, CM Wang, ND Browning. "Realizing the Full Potential of Insertion Anodes for Mg-Ion Batteries Through the Nanostructuring of Sn." Nano Letters 15 (2): 1177-1182 (Dec. 2014).
Abstract:Magnesium is of great interest as a replacement for lithium in next-generation ion-transfer batteries but Mg-metal anodes currently face critical challenges related to the formation of passivating layers during Mg-plating/stripping and anode–electrolyte–cathode incompatibilities.1−6 Alternative anode materials have the potential to greatly extend the spectrum of suitable electrolyte chemistries2,7 but must be systematically tailored for effective Mg²⁺ storage. Using analytical (scanning) transmission electron microscopy ((S)TEM) and ab initio modeling, we have investigated Mg²⁺ insertion and extraction mechanisms and transformation processes in β-SnSb nanoparticles (NPs), a promising Mg-alloying anode material. During the first several charge–discharge cycles (conditioning), the β-SnSb particles irreversibly transform into a porous network of pure-Sn and Sb-rich subparticles, as Mg ions replace Sn atoms in the SnSb lattice. After electrochemical conditioning, small Sn particles/grains (<33 ± 20 nm) exhibit highly reversible Mg-storage, while the Sb-rich domains suffer substantial Mg trapping and contribute little to the system performance. This result strongly indicates that pure Sn can act as a high-capacity Mg-insertion anode as theoretically predicted,8 but that its performance is strongly size-dependent, and stable nanoscale Sn morphologies (<40 nm) are needed for superior, reversible Mg-storage and fast system kinetics.

Zhang Y ,Qian J ,Xu W ,Russell S M,Chen X ,Nasybulin E ,Bhattacharya P ,Engelhard M H,Mei D ,Cao R ,Ding F ,Cresce A V,Xu K ,Zhang J. "Dendrite-Free Lithium Deposition with Self-Aligned Nanorod Structure." Nano Letters 14 (12): 6889-6896 (Nov. 2014).
Abstract:Suppressing lithium (Li) dendrite growth is one of the most critical challenges for the development of Li metal batteries. Here, we report for the first time the growth of dendrite-free lithium films with a self-aligned and highly compacted nanorod structure when the film was deposited in the electrolyte consisting of 1.0 M LiPF₆ in propylene carbonate with 0.05 M CsPF₆ as an additive. Evolution of both the surface and the cross-sectional morphologies of the Li films during repeated Li deposition/stripping processes were systematically investigated. It is found that the formation of the compact Li nanorod structure is preceded by a solid electrolyte interphase (SEI) layer formed on the surface of the substrate. Electrochemical analysis indicates that an initial reduction process occurred at ∼2.05 V vs Li/Li⁺ before Li deposition is responsible for the formation of the initial SEI, while the X-ray photoelectron spectroscopy indicates that the presence of CsPF₆ additive can largely enhance the formation of LiF in this initial SEI. Hence, the smooth Li deposition in Cs⁺- containing electrolyte is the result of a synergistic effect of Cs⁺ additive and preformed SEI layer. A fundamental understanding on the composition, internal structure, and evolution of Li metal films may lead to new approaches to stabilize the long-term cycling stability of Li metal and other metal anodes for energy storage applications.

Adams B D,Black R ,Radtke C ,Williams Z ,Mehdi B L,Browning N D,Nazar L F. "The Importance of Nanometric Passivating Films on Cathodes for Li–Air Batteries" ACS Nano 8 (12): 12483-12493 (Nov. 2014).
Abstract: Recently, there has been a transition from fully carbonaceous positive electrodes for the aprotic lithium oxygen battery to alternative materials and the use of redox mediator additives, in an attempt to lower the large electrochemical overpotentials associated with the charge reaction. However, the stabilizing or catalytic effect of these materials can become complicated due to the presence of major side-reactions observed during dis(charge). Here, we isolate the charge reaction from the discharge by utilizing electrodes prefilled with commercial lithium peroxide with a crystallite size of about 200–800 nm. Using a combination of S/TEM, online mass spectrometry, XPS, and electrochemical methods to probe the nature of surface films on carbon and conductive Ti-based nanoparticles, we show that oxygen evolution from lithium peroxide is strongly dependent on their surface properties. Insulating TiO2 surface layers on TiC and TiN - even as thin as 3 nm–can completely inhibit the charge reaction under these conditions. On the other hand, TiC, which lacks this oxide film, readily facilitates oxidation of the bulk Li2O2 crystallites, at a much lower overpotential relative to carbon. Since oxidation of lithium oxygen battery cathodes is inevitable in these systems, precise control of the surface chemistry at the nanoscale becomes of upmost importance.

Han, KS, NN Rajput, X wei, W Wang, JZ Hu, KA Persson, KT Mueller. "Diffusional motion of redox centers in carbonate electrolytes" Journal of Physical Chemistry 141(10): 104509 (Sept. 2014).
Abstract: Ferrocene (Fc) and N-(ferrocenylmethyl)-N,N-dimethyl-N-ethylammonium bistrifluoromethyl-sulfonimide (Fc1N112-TFSI) were dissolved in carbonate solvents and self-diffusion coefficients (D) of solutes and solvents were measured by (1)H and (19)F pulsed field gradient nuclear magnetic resonance (NMR) spectroscopy. The organic solvents were propylene carbonate (PC), ethyl methyl carbonate (EMC), and a ternary mixture that also includes ethylene carbonate (EC). Results from NMR studies over the temperature range of 0-50 °C and for various concentrations (0.25-1.7 M) of Fc1N112-TFSI are compared to values of D simulated with classical molecular dynamics (MD). The measured self-diffusion coefficients gradually decreased as the Fc1N112-TFSI concentration increased in all solvents. Since TFSI(-) has fluoromethyl groups (CF3), D(TFSI) could be measured separately and the values found are larger than those for D(Fc1N112) in all samples measured. The EC, PC, and EMC have the same D in the neat solvent mixture and when Fc is dissolved in EC/PC/EMC at a concentration of 0.2 M, probably due to the interactions between common carbonyl structures within EC, PC, and EMC. A difference in D (D(PC) < D(EC) < D(EMC)), and both a higher E(a) for translational motion and higher effective viscosity for PC in the mixture containing Fc1N112-TFSI reflect the interaction between PC and Fc1N112(+), which is a relatively stronger interaction than that between Fc1N112(+) and other solvent species. In the EC/PC/EMC solution that is saturated with Fc1N112-TFSI, we find that D(PC) = D(EC) = D(EMC) and Fc1N112(+) and all components of the EC/PC/EMC solution have the same E(a) for translational motion, while the ratio D(EC/PC/EMC)/D(Fc1N112) is approximately 3. These results reflect the lack of available free volume for independent diffusion in the saturated solution. The Fc1N112(+) transference numbers lie around 0.4 and increase slightly as the temperature is increased in the PC and EMC solvents. The trends observed for D from simulations are in good agreement with experimental results and provide molecular level understanding of the solvation structure of Fc1N112-TFSI dissolved in EC/PC/EMC.

Nasybulin E N,Xu W ,Mehdi B L,Thomsen E C,Engelhard M H,Masse R C,Bhattacharya P ,Gu M ,Bennett W D,Nie Z ,Wang C M,Browning N D,Zhang J. "Formation of Interfacial Layer and Long-Term Cyclability of Li-O₂ Batteries." ACS Applied Materials & Interfaces 6 (16): 14141-14151 (July 2014).
Abstract:The long-term operation of Li–O₂ batteries under full discharge/charge conditions is investigated in a glyme-based electrolyte. The formation of stable interfacial layer on the electrode surface during the initial cycling stabilizes reaction products at subsequent cycling stages as demonstrated by quantitative analyses of the discharge products and the gases released during charging. There is a quick switch from the predominant formation of Li₂O₂ to the predominant formation of side products during the first few cycles. However, after the formation of the stable interfacial layer, the yield of Li₂O₂ in the reaction products is stabilized at about 33–40%. Extended cycling under full discharge/charge conditions is achievable upon selection of appropriate electrode materials (carbon source and catalyst) and cycling protocol. Further investigation on the interfacial layer, which in situ forms on air electrode, may increase the long-term yield of Li₂O₂ during the cycling and enable highly reversible Li–O₂ batteries required for practical applications.

Du Y ,Gu M ,Varga T ,Wang C M,Bowden M E,Chambers S A .  "Strain Accommodation by Facile WO6 Octahedral Distortion and Tilting during WO3 Heteroepitaxy on SrTiO3(001)" Applied Materials & Interfaces 6 (16):14253-14258 (July 2014)
Abstract: In this work, we demonstrate that WO6 octahedra in tungsten trioxide (WO3) undergo an unusually large degree of distortion and tilting to accommodate interfacial strain. This motion strongly impacts nucleation, structure, and defect formation during the epitaxial growth of WO3 on SrTiO3(001). A metastable tetragonal phase can be stabilized by heteroepitaxy and a thickness-dependent phase transition (tetragonal to monoclinic) is observed. In contrast to misfit dislocation formation, facile WO6 octahedral deformation gives rise to three types of planar defects. The thicknesses of affected regions can range from several to tens of nanometers with graded lattice parameters, allowing the strain from interfacial lattice mismatch to be relieved gradually. These atomically resolved, unique interfacial defects may significantly alter the electronic, electrochromic, and mechanical properties of WO3 epitaxial films.

Cao R ,Walter E D,Xu W ,Nasybulin E N,Bhattacharya P ,Bowden M E,Engelhard M H,Zhang J. "The Mechanisms of Oxygen Reduction and Evolution Reactions in Nonaqueous Lithium–Oxygen Batteries." ChemSusChem 7 (9):2436-2440 (July 2014).
Abstract:A fundamental understanding of the mechanisms of both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in nonaqueous lithium–oxygen (Li–O₂) batteries is essential for the further development of these batteries. In this work, we systematically investigate the mechanisms of the ORR/OER reactions in nonaqueous Li–O₂ batteries by using electron paramagnetic resonance (EPR) spectroscopy, using 5,5-dimethyl-pyrroline N-oxide as a spin trap. The study provides direct verification of the formation of the superoxide radical anion (O₂^.−) as an intermediate in the ORR during the discharge process, while no O₂^.− was detected in the OER during the charge process. These findings provide insight into, and an understanding of, the fundamental reaction mechanisms involving oxygen and guide the further development of this field.

Mehdi B L,Gu M ,Parent L R,Xu W ,Nasybulin E N,Chen X ,Unocic R R,Xu P ,Welch D A,Abellan P ,Zhang J ,Liu J ,Wang C M,Arslan I ,Evans J E,Browning N D. "In-situ electrochemical transmission electron microscopy for battery research." Microscopy & Microanalysis 20 (2): 484-492 (April 2014).
Abstract:The recent development of in-situ liquid stages for (scanning) transmission electron microscopes now makes it possible for us to study the details of electrochemical processes under operando conditions. As electrochemical processes are complex, care must be taken to calibrate the system before any in-situ/operando observations. In addition, as the electron beam can cause effects that look similar to electrochemical processes at the electrolyte/electrode interface, an understanding of the role of the electron beam in modifying the operando observations must also be understood. In this paper we describe the design, assembly, and operation of an in-situ electrochemical cell, paying particular attention to the method for controlling and quantifying the experimental parameters. The use of this system is then demonstrated for the lithiation/delithiation of silicon nanowires.

Vijayakumar M ,Govind N ,Walter E D,Burton S D,Shukla A K,Devaraj A ,Xiao J ,Liu J ,Wang C M,Karim A M,Thevuthasan S. "Molecular structure and stability of dissolved lithium polysulfide species." Phys. Chem. Chem. Phys. 16: 10923-10932 (March 2014).
Abstract:The ability to predict the solubility and stability of lithium polysulfide is vital in realizing longer lasting lithium–sulfur batteries. Herein we report combined experimental and computational analyses to understand the dissolution mechanism of lithium polysulfide species in an aprotic solvent medium. Multinuclear NMR, variable temperature ESR and sulfur K-edge XAS analyses reveal that the lithium exchange between polysulfide species and solvent molecules constitutes the first step in the dissolution process. Lithium exchange leads to de-lithiated polysulfide ions (S_n²⁻) which subsequently form highly reactive free radicals through dissociation reaction (S_n²⁻ → 2S_n2^˙−). The energy required for the dissociation and possible dimer formation reactions of the polysulfide species is analyzed using density functional theory (DFT) based calculations. Based on these findings, we discuss approaches to optimize the electrolyte in order to control the polysulfide solubility.

Abellan Baeza P ,Mehdi B L,Parent L R,Gu M ,Park C ,Xu W ,Zhang Y ,Arslan I ,Zhang J ,Wang C M,Evans J E,Browning N D. "Probing the Degradation Mechanisms in Electrolyte Solutions for Li-Ion Batteries by in Situ Transmission Electron Microscopy" Nano Letters 14 (3): 1293-1299 (Feb. 2014).
Abstract:Development of novel electrolytes with increased electrochemical stability is critical for the next generation battery technologies. In situ electrochemical fluid cells provide the ability to rapidly and directly characterize electrode/electrolyte interfacial reactions under conditions directly relevant to the operation of practical batteries. In this paper, we have studied the breakdown of a range of inorganic/salt complexes relevant to state-of-the-art Li-ion battery systems by in situ (scanning) transmission electron microscopy ((S)TEM). In these experiments, the electron beam itself caused the localized electrochemical reaction that allowed us to observe electrolyte breakdown in real-time. The results of the in situ (S)TEM experiments matches with previous stability tests performed during battery operation and the breakdown products and mechanisms are also consistent with known mechanisms. This analysis indicates that in situ liquid stage (S)TEM observations could be used to directly test new electrolyte designs and identify a smaller library of candidate solutions deserving of more detailed characterization. A systematic study of electrolyte degradation is also a necessary first step for any future controlled in operando liquid (S)TEM experiments intent on visualizing working batteries at the nanoscale.

Sacci R L,Dudney N J,More K L,Parent L R,Arslan I ,Browning N D. "Direct visualization of initial SEI morphology and growth kinetics during lithium deposition by in situ electrochemical transmission electron microscopy."Chemical Communications 17: 2104-2107 (Dec. 2013).
Abstract: Deposition of Li is a major safety concern existing in Li-ion secondary batteries. Here we perform the first in situ high spatial resolution measurement coupled with real-time quantitative electrochemistry to characterize SEI formation on gold using a standard battery electrolyte. We demonstrate that a dendritic SEI forms prior to Li deposition and that it remains on the surface after Li electrodissolution.

Gu M ,Parent L R,Mehdi B L,Unocic R R,Mcdowell M T,Sacci R L,Xu W ,Connell J G,Xu P ,Abellan Baeza P ,Chen X ,Zhang Y ,Perea D E,Evans J E,Lauhon L ,Zhang J ,Liu J ,Browning N D,Cui Y ,Arslan I ,Wang C M. "Demonstration of an Electrochemical Liquid Cell for Operando Transmission Electron Microscopy Observation of the Lithiation/Delithiation Behavior of Si Nanowire Battery Anodes." Nano Letters 13 (12): 6106-6112 (Nov. 2013).
Abstract:Over the past few years, in situ transmission electron microscopy (TEM) studies of lithium ion batteries using an open-cell configuration have helped us to gain fundamental insights into the structural and chemical evolution of the electrode materials in real time. In the standard open-cell configuration, the electrolyte is either solid lithium oxide or an ionic liquid, which is point-contacted with the electrode. This cell design is inherently different from a real battery, where liquid electrolyte forms conformal contact with electrode materials. The knowledge learnt from open cells can deviate significantly from the real battery, calling for operando TEM technique with conformal liquid electrolyte contact. In this paper, we developed an operando TEM electrochemical liquid cell to meet this need, providing the configuration of a real battery and in a relevant liquid electrolyte. To demonstrate this novel technique, we studied the lithiation/delithiation behavior of single Si nanowires. Some of lithiation/delithation behaviors of Si obtained using the liquid cell are consistent with the results from the open-cell studies. However, we also discovered new insights different from the open cell configuration—the dynamics of the electrolyte and, potentially, a future quantitative characterization of the solid electrolyte interphase layer formation and structural and chemical evolution.

Shao Y ,Liu T L.,Li G ,Gu M ,Nie Z ,Engelhard M H,Xiao J ,Lu D ,Wang C M,Zhang J ,Liu J. "Coordination Chemistry in magnesium battery electrolytes: how ligands affect their performance." Scientific Reports 3 (Nov. 2013).
Abstract:Magnesium battery is potentially a safe, cost-effective, and high energy density technology for large scale energy storage. However, the development of magnesium battery has been hindered by the limited performance and the lack of fundamental understandings of electrolytes. Here, we present a study in understanding coordination chemistry of Mg(BH₄)₂ in ethereal solvents. The O donor denticity, i.e. ligand strength of the ethereal solvents which act as ligands to form solvated Mg complexes, plays a significant role in enhancing coulombic efficiency of the corresponding solvated Mg complex electrolytes. A new electrolyte is developed based on Mg(BH₄)₂, diglyme and LiBH₄. The preliminary electrochemical test results show that the new electrolyte demonstrates a close to 100% coulombic efficiency, no dendrite formation, and stable cycling performance for Mg plating/stripping and Mg insertion/de-insertion in a model cathode material Mo6S8 Chevrel phase.

Xu W ,Wang J ,Ding F ,Chen X ,Nasybulin E N,Zhang Y ,Zhang J. "Lithium metal anodes for rechargeable batteries" Energy & Environmental Science 7: 513-537 (Oct. 2013).
Abstract:Lithium (Li) metal is an ideal anode material for rechargeable batteries due to its extremely high theoretical specific capacity (3860 mA hg^-1), low density (0.59 g cm^-3) and the lowest negative electrochemical potential (−3.040 V vs. the standard hydrogen electrode). Unfortunately, uncontrollable dendritic Li growth and limited Coulombic efficiency during Li deposition/stripping inherent in these batteries have prevented their practical applications over the past 40 years. With the emergence of post-Li-ion batteries, safe and efficient operation of Li metal anodes has become an enabling technology which may determine the fate of several promising candidates for the next generation energy storage systems, including rechargeable Li–air batteries, Li–S batteries, and Li metal batteries which utilize intercalation compounds as cathodes. In this paper, various factors that affect the morphology and Coulombic efficiency of Li metal anodes have been analyzed. Technologies utilized to characterize the morphology of Li deposition and the results obtained by modelling of Li dendrite growth have also been reviewed. Finally, recent development and urgent need in this field are discussed.

Gu M ,Kushima A ,Shao Y ,Zhang J ,Liu J ,Browning N D,Li J ,Wang C M. "Probing the Failure Mechanism of SnO₂ Nanowires for Sodium-Ion Batteries." Nano Letters 13 (11): 5203-5211 (Sept. 2013).
Abstract:Nonlithium metals such as sodium have attracted wide attention as a potential charge carrying ion for rechargeable batteries. Using in situ transmission electron microscopy in combination with density functional theory calculations, we probed the structural and chemical evolution of SnO₂ nanowire anodes in Na-ion batteries and compared them quantitatively with results from Li-ion batteries (Huang, J. Y.; et al. Science 2010, 330, 1515−1520). Upon Na insertion into SnO₂, a displacement reaction occurs, leading to the formation of amorphous NaxSn nanoparticles dispersed in Na2O matrix. With further Na insertion, the Na_xSn crystallized into Na₁₅Sn₄ (x = 3.75). Upon extraction of Na (desodiation), the Na_xSn transforms to Sn nanoparticles. Associated with the dealloying, pores are found to form, leading to a structure of Sn particles confined in a hollow matrix of Na₂O. These pores greatly increase electrical impedance, therefore accounting for the poor cyclability of SnO₂. DFT calculations indicate that Na⁺ diffuses 30 times slower than Li⁺ in SnO₂, in agreement with in situ TEM measurement. Insertion of Na can chemomechanically soften the reaction product to a greater extent than in lithiation. Therefore, in contrast to the lithiation of SnO₂ significantly less dislocation plasticity was seen ahead of the sodiation front. This direct comparison of the results from Na and Li highlights the critical role of ionic size and electronic structure of different ionic species on the charge/discharge rate and failure mechanisms in these batteries.