High rate and stable cycling of lithium metal anode

Lithium metal is a promising anode material for lithium metal batteries (LMBs). However, dendrite growth and limited Coulombic efficiency (CE) during cycling have prevented its practical application in rechargeable batteries. Herein, a highly concentrated electrolyte composed of an ether solvent and lithium bis(fluorosulfonyl)imide (LiFSI) salt is introduced, which enables the cycling of a lithium metal anode at a high CE (up to ≈99%) without dendrite growth, even at high current densities. Using 3.85 m LiFSI in tetrahydrofuran (THF) as the electrolyte, a Li||Li symmetric cell can be cycled at 1.0 mA cm-2 for more than 1000 h with stable polarization of ≈0.1 V, and Li||LFP cells can be cycled at 2 C (1 C = 170 mA g-1 ) for more than 1000 cycles with a capacity retention of 94.5%. These excellent performances are observed to be attributed to the increased cation-anion associated complexes, such as contact ion pairs and aggregate in the highly concentrated electrolyte; revealed by Raman spectroscopy and theoretical calculations. These results demonstrate the benefits of a high-concentration LiFSI-THF electrolyte system, generating new possibilities for high-energy-density rechargeable LMBs.

Rechargeable metal batteries, such as Li metal batteries are considered the “holy grail” of energy storage systems. However, dendritic metal growth and limited Coulombic efficiency (CE) during metal deposition/stripping have prevented their practical applications in rechargeable batteries.1-3 During the last a few years, we have developed several approaches to suppress metal dendrite growth and enhance the Coulombic efficiency (CE) of metal deposition/stripping processes.2-7 Several electrolyte additives, including CsPF6, RbPF6, and trace-amount of H2O (25-50 ppm) have been found to be effective for achieving dendrite-free Li metal deposition in LiPF6-based electrolytes. Furthermore, we have developed a highly concentrated electrolytes composed of the lithium bis(fluorosulfonyl)imide (LiFSI) salt and 1,2-dimethoxyethane (DME) solvent which enables high rate cycling of Li metal anode at high CE (up to 99.1 %) without dendrite growth. It is demonstrated that a Li|Li cell can be cycled at high rates (10 mA cm-2) for more than 6,000 cycles with no increase in the cell impedance and no dendritic Li growth. A Li|Cu cell can be cycled at 4 mA cm-2 for more than 1,000 cycles with an average CE of 98.4%. We also demonstrate that the dendrite growth and CE of Li deposition is strongly depends on several other factors, such as substrate treatment, charge and discharge protocols, and cell pressure. By optimizing the electrolyte compositions and various operating parameters, CE of Li deposition/stripping has been further improved without dendrite growth. In addition to the high CE cycling of Li metal electrode, novel electrolyte was also developed which enables cycling of Na metal anode with an excellent CE (> 99.1%) as compared to a very low CE of less than 30% in conventional electrolytes. Further development of these approaches may lead to long term stable operation of Li and Na metal batteries. Acknowledgements This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U. S. Department of Energy, Office of Science, Basic Energy Sciences (BES) and by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, the Advanced Battery Materials Research Programs of the U.S. Department of Energy (DOE). PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RLO1830.

A New General Paradigm for Understanding and Preventing Li Metal Penetration through Solid Electrolytes

The application of lithium (Li) metal anodes in rechargeable batteries is hindered by Li dendrite growth during Li deposition and low Li Coulombic efficiency (CE), where the nonaqueous electrolyte plays a critical role. In this work, the effects of different carbonate solvents and Li salts on Li deposition morphology and CE were systematically investigated. Typically, cyclic carbonates favor the formation of uniform Li films and improve Li CE more than linear carbonates do. Several specific cyclic carbonates that are conventionally used as solid electrolyte interphase (SEI) formation additives in Li-ion batteries can also improve the CE of Li anodes. Furthermore, among the nine electrolyte salts studied, LiAsF6 and lithium bis(oxalato)borate (LiBOB) lead to the highest CE. LiBOB also leads to better uniformity of deposited Li than other salts do. Considering the better safety of LiBOB as compared to LiAsF6, LiBOB is a promising salt for rechargeable Li metal batteries with high CE. By combining the best electrolyte solvent/salt that can lead to high CE with novel electrolyte additives that can prevent dendrite formation, it is possible to find an electrolyte that not only prevents Li dendrite formation but also leads to high CE during Li deposition/stripping processes.

Evaluating Solid-Electrolyte Interphases for Lithium and Lithium-free Anodes from Nanoindentation Features

An effective solid-electrolyte interphase for stable solid-state batteries

Lithium metal is the ideal candidate to replace conventional carbonaceous anodes due to its high theoretical specific capacity of 3860 mAh/g and low negative thermodynamic potential of -3.040 V vs. SHE [1]. Most organic electrolytes are unstable in the presence of Li metal and are reduced to form a solid-electrolyte interphase (SEI). One strategy to mitigate electrolyte decomposition is by upshifting the Li electrode redox potential. Ko et al. report positive shifts of up to 0.6 V in the Li electrode potential can influence coulombic efficiency [2]. In recent years, LiFSI-DME and LiFSI-FEC electrolytes with different concentrations have demonstrated highly reversible Li metal plating and stripping coulombic efficiencies of up to 99.1% [3] and 99.64% [4], respectively. While Qian et al. and Suo et al. briefly commented on the increased amount of inorganic content in the SEI, LiFSI concentration’s role on Li metal redox potential and SEI composition remains unclear. Organic species at the Li anode/electrolyte interphase are measured using a recently developed in-situ FTIR method. Cell level performance is evaluated by measuring coulombic efficiency. Complimentary techniques such as NMR and Raman are used to learn about the solvation structure as well as inorganic and soluble electrolyte decomposition products [5]. Novel insight into the relationship between SEI species, Li redox potential, and coulombic efficiency are presented.References Cheng, Xin-Bing, et al. "Toward safe lithium metal anode in rechargeable batteries: a review." Chemical reviews 117.15 (2017): 10403-10473.Ko, Seongjae, et al. "Electrode potential influences the reversibility of lithium-metal anodes." Nature Energy 7.12 (2022): 1217-1224.Qian, Jiangfeng, et al. "High rate and stable cycling of lithium metal anode." Nature communications 6.1 (2015): 6362.Suo, Liumin, et al. "Fluorine-donating electrolytes enable highly reversible 5-V-class Li metal batteries." Proceedings of the National Academy of Sciences 115.6 (2018): 1156-1161.Hestenes, Julia C., et al. "Transition metal dissolution mechanisms and impacts on electronic conductivity in composite LiNi0. 5Mn1. 5O4 cathode films." ACS Materials Au 3.2 (2022): 88-101.

The lithium metal battery is strongly considered to be one of the most promising candidates for high-energy-density energy storage devices in our modern and technology-based society. However, uncontrollable lithium dendrite growth induces poor cycling efficiency and severe safety concerns, dragging lithium metal batteries out of practical applications. This review presents a comprehensive overview of the lithium metal anode and its dendritic lithium growth. First, the working principles and technical challenges of a lithium metal anode are underscored. Specific attention is paid to the mechanistic understandings and quantitative models for solid electrolyte interphase (SEI) formation, lithium dendrite nucleation, and growth. On the basis of previous theoretical understanding and analysis, recently proposed strategies to suppress dendrite growth of lithium metal anode and some other metal anodes are reviewed. A section dedicated to the potential of full-cell lithium metal batteries for practical applications is included. A general conclusion and a perspective on the current limitations and recommended future research directions of lithium metal batteries are presented. The review concludes with an attempt at summarizing the theoretical and experimental achievements in lithium metal anodes and endeavors to realize the practical applications of lithium metal batteries.

Owing to its high theoretical capacity (3860 mAh/g, over 10 times that of graphite) and its lowest theoretical electrochemical potential (-3.04 V vs SHE), Li metal is an ideal anode material for improving the capacity of rechargeable Li batteries [1]. However, the practical application of the Li anode is severely hindered by its high reactivity with standard electrolyte compositions, which results in limited Coulombic efficiency (CE) in those electrolytes (for instance, 85-90% with 1 M LiPF6 EC:DEC [2]). Strategies to improve CE have focused on electrolyte engineering, including development of highly concentrated electrolytes (HCE) and more recently, local high-concentration electrolytes (LHCE) [3]. A feature shared by these strategies is the promotion of contact-ion pairing, which enhances anion contributions to the solid electrolyte interphase (SEI). Among the best-performing salts are fluorinated lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI), which are believed to promote stable inorganic-rich SEIs and correlate with higher CE [4]. A second strategy of interest is to pursue development of additives incorporated at relatively low concentrations (e.g., 2-5 wt%) that can favorably alter the SEI, such as carbonates (e.g., vinylene carbonate (VC) [5] and fluoroethylene carbonate (FEC) [6]), sulfates (e.g., ethylene sulfate (DTD) and 1,3-propanesulfonate (PS) [7]), and borates (e.g., lithium difluoro(oxoalate) borate (LiDFOB) [8] and tris (2,2,2-trifluoroethyl) borate (TTFEB) [9]). An advantage of this strategy is the possibility to ultimately lower reliance on high fluorinated salt concentrations, which are costly and can be corrosive towards the Al current collector, and potentially expand the versatility of available electrolyte frameworks having competitive CE. However, compared to the principles developed for salt and solvent selection in HCE and LHCE, the mechanism behind successful additive activation needs elucidation.In this work, we examined the potential of "FSI-like," neutral sulfonyl/sulfamoyl fluorides (R-SO2F and R-R’-NSO2F, Figure 1) as functional additives for Li cycling, motivated by their structural similarity to the high-performance FSI- anion. The examined baseline electrolytes include high-CE systems consisting of LiFSI dissolved in various solvents: fluoroethylene carbonate (FEC, 1 and 4 M salt), 1,2-dimethoxyethane (DME, 4 M), and dimethyl carbonate (DMC, 6 M). The ability for each additive to coordinate with Li+ was first examined by NMR, with trends rationalized in part by supporting microcalorimetry data on the degree of solvent coordination strength. We relate these to CE and cycling outcomes in each electrolyte. Interestingly, we find that additives have negligible effect on CE in FEC-based electrolytes, whereas significant impacts were observed in DME and DMC. We relate these diverse outcomes to the SEI chemical compositions and gases evolved during galvanostatic cycling, as characterized by X-ray photoelectron spectroscopy (XPS) and gas chromatography (GC), which help to rationalize competitive reactions among solvent, anion, and additive. Unfortunately, additives had a negative-to-neutral impact on CE in these systems. Thus, we finally examined a LiPF6 in carbonate electrolyte, 1 M LiPF6 in EC/DMC (LP40), where we hypothesized that competitive reduction of coordinating additives over problematic carbonate solvent would lead to performance gains. Indeed, significant improvements (up to 94%, compared to baseline 89% over the initial 50 cycles) in CE were observed for two additives, with their structural advantages further discussed. Overall, our findings provide insights into the effects of sulfonyl/sulfamoyl fluoride additive structures on Li metal cyclability and the compositions of baseline electrolytes whose electrochemical cycling stability can be effectively modulated by these additives. Xu, W., et al., Lithium metal anodes for rechargeable batteries. Energy & Environmental Science, 2014.7(2):p.513-537.Genovese, M., et al., Combinatorial methods for improving lithium metal cycling efficiency. Journal of The Electrochemical Society, 2018.165(13):p.A3000.Hobold, G.M., et al., Moving beyond 99.9% Coulombic efficiency for lithium anodes in liquid electrolytes. Nature Energy, 2021.6(10):p.951-960.Suo, L., et al., Fluorine-donating electrolytes enable highly reversible 5-V-class Li metal batteries. Proceedings of the National Academy of Sciences, 2018.115(6):p.1156-1161.Aurbach, D., et al., On the use of vinylene carbonate (VC) as an additive to electrolyte solutions for Li-ion batteries. Electrochimica acta, 2002.47(9):p.1423-1439.Markevich, E., et al., Fluoroethylene carbonate as an important component in electrolyte solutions for high-voltage lithium batteries: role of surface chemistry on the cathode. Langmuir, 2014.30(25):p.7414-7424.Han, B., et al., Poor Stability of Li2CO3 in the Solid Electrolyte Interphase of a Lithium ‐Metal Anode Revealed by Cryo ‐Electron Microscopy. Advanced Materials, 2021.33(22):p.2100404.Liu, J., et al., Lithium difluoro (oxalato) borate as a functional additive for lithium-ion batteries. Electrochemistry communications, 2007.9(3):p.475-479.Ma, Y., et al., Enabling reliable lithium metal batteries by a bifunctional anionic electrolyte additive. Energy Storage Materials, 2018.11:p.197-204. Figure 1. (a) Sulfonyl/sulfamoyl fluoride chemical structures and (b) R-S and S-F bond lengths calculated by density functional theory. Figure 1

With a theoretical specific energy of 2500 W h kg-1 and energy density of 2800 W h L-1, the Li-S battery system is believed to provide the step-up in energy density necessary for lithium-based battery technologies to expand from portable electronics to transportation and grid-storage applications.1 However, the growth of dendrites during repeated Li plating/stripping and the low coulombic efficiency (CE) of these processes have limited application of rechargeable Li metal batteries.2 For example, a 300% excess amount of lithium often used in these batteries would directly result in halving the theoretical specific energy of the Li/S cells. In this presentation, the design of new electrolyte systems which enable high CE of lithium metal plating/stripping and high stability in the sulfur environment will be discussed. Tailoring of electrolyte properties for the lithium negative electrode has proven to be a successful strategy for improving the capacity retention and cycle life of Li-S full cells. This also enables lower electrolyte/sulfur mass ratios to be used and a lower excess of lithium metal; ultimately increasing the energy density of the system. A new class of electrolytes based on a high concentration of selected lithium salts in pure diethylene glycol dimethyl ether (diglyme) solvent provides a CE for lithium plating/stripping of greater than 99% for over 200 cycles and greater than 95% for over 500 cycles (Figure 1). In contrast, lithium metal cycles for less than 40 cycles at high CE in the standard 1 M LiTFSI + 2wt% LiNO3 in DOL:DME electrolyte. To realize the benefits of sulfur cathodes over intercalation cathodes currently using in Li-ion cells, high loading (high capacity) cathodes need to be used. In Figure 2a, a Cu||Li cell is cycled with the new diglyme-based electrolyte to a capacity of 6 mAh/cm2 at a current density of 0.6 mA/cm2 (C/10). Even at this high capacity, lithium is cycling with >99% CE. Lithium symmetrical cells were also cycled at a current density of 0.5 mA/cm2 to 5 mAh/cm2 (Figure 2b). In this case, the increase in polarization was used a metric to determine the practical cycle life of Li metal in the different electrolytes. No polarization is observed for the diglyme-based electrolyte after 2200 hours in the Li||Li cell. The inexpensive sulfur cathode paired with a low excess of lithium metal and the low-cost salt/solvent system may accelerate the applications of high energy density Li-S batteries in both electrical vehicles and large-scale grid energy storage markets.

Lithium metal is considered as the ultimate anode material for rechargeable batteries because it has the highest theoretical specific capacity (3,860 mAh g−1) and the lowest electrochemical potential (−3.04 V versus standard hydrogen electrode) of all possible candidates. However, dendritic Li growth and low Li Coulombic efficiency (CE) during Li plating/stripping hinder the practical applications of rechargeable lithium-metal batteries. In recent years, highly stable Li plating/stripping cycling has been achieved in the concentrated electrolytes of lithium bis(fluorosulfonyl) imide (LiFSI) in various solvents.1 , 2 Among the reported electrolytes, sulfolane (SL) is a promising solvent for high energy lithium metal batteries, since it has high anodic stability and low flammability. The concentrated electrolyte composed of LiFSI and SL shows high CE of ca. 98% and stable cycling of Li metal anode, however, the effect of LiFSI concentration on the long-term cyclability is unclear. Concentrated LiFSI/SL electrolytes have high viscosity and thus poor wettability toward conventional polyolefin separators, preventing stable Li plating/stripping cycling. We have previously reported that three dimensionally ordered macroporous polyimide (3DOM PI) membrane has high electrolyte wettability owing to its high porosity of ca. 70% and the relatively polar constituent of imide structure.3 Furthermore, uniform distribution of pores in a hexagonal close-packed arrangement of 3DOM PI membrane provides even current distribution during Li plating/stripping processes, preventing the dendritic Li growth. Therefore, 3DOM PI separator is expected to stabilize Li plating–stripping cycling. Here we studied the effect of LiFSI concentration in SL-based electrolyte on long-term Li plating/stripping behavior by using 3DOM PI separator. 3DOM PI separator exhibits good wettability toward concentrated LiFSI/SL electrolyte and highly stable Li plating/stripping than surfactant-coated polypropylene separator, enabling the evaluation of the long-term cyclability. Higher concentrated LiFSI/SL electrolyte realizes superior cyclability with low overpotential and high CEs during cycling. The ratio of F and N is higher, while that of C is much lower in the SEI on the Li metal cycled in the higher concentrated electrolyte. These results suggest that the FSI-derived SEI in the highly concentrated electrolyte suppresses decomposition of SL and subsequent increase in interfacial resistance, leading to long-term stable Li plating–stripping cycling. Acknowledgement This work is supported by The Furukawa Battery Co., Ltd. and 3DOM Inc. References X. Fan, L. Chen, X. Ji, T. Deng, S. Hou, J. Chen, J. Zheng, F. Wang, J. Jiang, K. Xu and C. Wang, Chem, 4, 174 (2018). X. Ren, S. Chen, H. Lee, D. Mei, M. H. Engelhard, S. D. Burton, W. Zhao, J. Zheng, Q. Li, M. S. Ding, M. Schroeder, J. Alvarado, K. Xu, Y. S. Meng, J. Liu, J.-G. Zhang and W. Xu, Chem, 4, 1877 (2018).Y. Maeyoshi, S. Miyamoto, H. Munakata and K. Kanamuraamura, J. Power Sources, 350, 103 (2017).

Lithium-ion batteries are important for applications including portable electronics, electric vehicles, and grid storage. Lithium metal anodes can significantly increase energy density beyond lithium-ion batteries because their theoretical storage capacity is 10x larger than that for state-of-the-art graphite anodes. Development of functional lithium metal anodes is critically important for implementation of next-generation battery concepts like Li-S, Li-O2, and Li-metal batteries paired with traditional insertion cathodes. Unfortunately, lithium metal anodes suffer from parasitic reactions with electrolytes, high aspect ratio morphological evolution, and poor cycling behavior. Many strategies have been proposed to improve lithium metal anode cycling including the development of electrolytes that enable more favorable lithium morphology and minimize side reactions. One of the very promising electrolytes that have been developed is 4 M lithium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane (4 M LiFSI/DME).1 To better understand lithium metal plating and stripping processes in 4 M LiFSI/DME, we plated and stripped lithium metal using in-situ electrochemical scanning transmission electron microscopy (EC-STEM).2 We performed EC-STEM experiments using a customizable, sealed liquid cell STEM chip designed and built at Sandia National Laboratories. The Coulombic efficiency and morphology observed in the in-situ EC-STEM experiments2 differed from that in published coin cell results.1 Through macroscale (not TEM) experiments, we determined that the discrepancy could be explained by the absence of a separator and interfacial pressure at the lithium-electrolyte interface for the in-situ EC-STEM experiments. Thus, we concluded that lithium metal plating and stripping performance depends significantly on interfacial compression. Our data also suggests that compression changes how the solid electrolyte interphase forms, which could in turn affect how lithium metal deposits.Motivated by our observations that lithium plating and stripping behavior depends on interfacial compression, we systematically studied the role of interfacial compression on lithium plating and stripping. We performed electrochemical testing in pouch cells under varied loading and disassembled the cells for subsequent characterization. The electrode stacks were studied by cross sectioning the electrodes with cryo focused ion beam milling. These samples were then imaged using cryo scanning electron microscopy experiments to understand the impact of interfacial compression on morphological evolution. We confirm through these experiments that interfacial compression plays an important role in lithium plating and stripping behavior.This work was supported by the Joint Center for Energy Storage Research and the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525. 1. Qian, J., Henderson, W.A., Xu, W., Bhattacharya, P., Engelhard, M., Borodin, O. and Zhang, J.G., 2015. High rate and stable cycling of lithium metal anode. Nature communications, 6, p. 6362.2. Harrison, K.L., Zavadil, K.R., Hahn, N.T., Meng, X., Elam, J.W., Leenheer, A., Zhang, J.G. and Jungjohann, K.L., 2017. Lithium self-discharge and its prevention: direct visualization through in situ electrochemical scanning transmission electron microscopy. ACS nano, 11(11), pp.11194-11205.

Nowadays, lithium (Li) metal batteries arouse widespread concerns due to its ultrahigh specific capacity (3,860 mAh g−1). However, the growth of Li dendrites has always limited their industrial development. In this paper, the use of concentrated electrolyte with lithium difluoro(oxalate)borate (LiODFB) salt in 1, 2-dimethoxyethane (DME) enables the good cycling of a Li metal anode at high Coulombic efficiency (up to 98.1%) without dendrite growth. Furthermore, a Li/Li cell can be cycled at 1 mA cm−2 for over 3,000 h. Besides, compared to conventional LiPF6-carbonate electrolyte, Li/LiFePO4 cells with 4 M LiODFB-DME exhibit superior electrochemical performances, especially at high temperature (65°C). These outstanding performances can be certified to the increased availability of Li+ concentration and the merits of LiODFB salt. We believe that the concentrated LiODFB electrolyte is help to enable practical applications for Li metal anode in rechargeable batteries.

Lithium Metal Anodes: A Recipe for Protection

In the recent years, all-solid-state batteries have attracted much attention as they are safer in terms of flammability compared to liquid electrolyte systems. There are polymer, oxidic, and sulfidic solid electrolytes with advantages and disadvantages. One major advantage of sulfidic electrolytes is their high ionic conductivity at room temperature which is even comparable with that of liquid electrolytes. Another feature is the possible usability of lithium anodes in this system which leads to very high energy density. One major challenge of the lithium metal anode is the formation and growth of lithium dendrites which results in short circuits. A lot of research is done to prevent dendrite growth but more research is needed to understand the actual dendrite growth mechanism [1]. In our work we analyzed the reasons for increase of overpotentials that is very likely related to lithium dendrite growth in symmetric lithium metal cells with solid electrolyte Li6PS5Cl. In a three-electrode set up, the interface of a lithium symmetric cell is analyzed during stripping and plating with electrochemical impedance spectroscopy. The changes of the resistance contributions are discussed in terms of possible major mechanisms that are involved in lithium dendrite growth. Furthermore, the plating behavior of lithium is investigated which gives insight into possible onset for lithium dendrite growth. Reference s : [1] Xin-Bing Cheng, Rui Zhang, Chen-Zi Zhao, and Qiang Zhang, Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review, Chem. Rev. 2017, 117, 10403−10473.