Sequenced Interfacial Chemistry for Stabilizing Reactive Lithium Metal Anodes.

Alkali and alkaline earth metal anodes are gaining increasing research attention because they could provide high energy densities and theoretical specific capacities. Among them, lithium metal is the most investigated anode candidate for next generation batteries. However, due to some challenges, the commercialization of lithium metal anodes is still delayed. Reactions between lithium metal and the electrolyte lead to the formation of an inhomogeneous solid electrolyte interphase (SEI). Consequently, electrodissolution/-deposition of lithium is favored where the SEI is less resistive or cracked, resulting in high surface area lithium and dendrite growth. This does not only lower the coulombic efficiency (CE) and cell specific capacity but also causes safety issues due to an increased risk of short-circuits and thermal runaway.1,2 To enable increased safety in high energy batteries with lithium metal anodes an effective SEI is required to limit lithium dendrite growth. There is a variety of approaches to grow an effective SEI, such as the use of electrolyte additives, mechanical methods and chemical modification.3-5 Recently, alkaline earth metal anodes, such as magnesium or calcium, are also of considerable interest for next generation anodes. Their main advantage is the significantly lower susceptibility to dendrite formation, while still offering high theoretical specific capacities. However, their oxide passivation layer does not allow significant mobility for their divalent ions, leading to large overvoltage and slow reaction kinetics which hinders the cycling. Therefore, electrolytes which enable a non-passivating SEI have to be applied and additives are investigated on their ability to remove the oxide layers.6,7 Herein, we present a novel mechanochemical approach to form an effective SEI on the lithium metal surface by combining mechanical and chemical modification utilizing ionic liquids (ILs) prior to cell assembly. This method suppresses dendrite growth even at high current densities of 10 mA cm-2 in lithium metal batteries with liquid electrolytes. To further highlight the importance of surface treatment and SEI formation, we investigate electrolytes for calcium metal batteries as well as mechanical modification of calcium metal anodes. Acknowledgements: The authors would like to acknowledge financial support from the European Union through the Horizon 2020 framework program for research and innovation within the project “VIDICAT” (829145).

Electronic devices have been advanced rapidly over the last few decades with improved performance through high-level integration. Lithium ion batteries (LIB), which is representative rechargeable energy storage system, is increasingly demanding to high-energy-density storage system for the stable operation of high performance devices like mobile, laptop, etc. In addition, a recent quantum leap in electronic vehicles (EVs) require to energy storage system with long cycling stability incomparable to previous electronic devices. To achieve high performances and long lifetime, the use of lithium metal is one of attractive anode for as LIBs, because Li metal has high theoretical specific capacity of 3860 mAh/g, low redox potential of -3.04V (verse the standard hydrogen electrode) and low gravimetric density of 0.59 g/cm3. Nevertheless, since Li metal anode has the lithiation-delithiation mechanism of the ‘hostless’ electrochemical plating/stripping unlike the conventional graphite anode, it is confronted with serious problems occurred from uncontrollable interfacial reaction between surface of lithium metal anode and electrolyte applying to traditional LIB system; that is (1) the uncontrollable formation of solid electrolyte interphase (SEI) layer limitless volume expansion of Li metal anode (2) the dendritic growth and isolation of Li interface derived from inhomogeneous deposition of Li+ ion flux (3) the Li surface fracture induced by heterogeneously plated Li+ ion. Through these process, repetitive formation of undesirable SEI layer at surface of Li anode make fresh electrolyte dissipation and concurrently the anode surface becomes more distorted. These side-phenomena strongly correlate with low coulombic efficiency and decreasing lifetime of lithium metal battery (LMB) system where lithium metal is used as anode. In this works, in a dimensional aspect, to solve such problem mentioned above through stabilizing of surface of lithium metal anode, we introduce Janus-faced protective layer (JPL) as gel polymer electrolyte. JPL was composed with dual layer of Ni (3~7μm) embedded poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) and bare PVdF-HFP through simple casting method. The process of preparing JPL membrane is so simple to instantly bring in existing roll-to-roll process, and it is also possible to make a low cost process considered relatively low nickel price. In terms of mechanical property, it is based on flexible polymer and shows the merit of being foldable enough to able origami. In addition, JPL play a role of protection of Li metal anode. JPL has two functionalities. The metal part of JPL directly contacted with anode, and this layer was frameworks for preventing irregular SEI layer formation through inward plating of Li ion, the polymer part is stable state enough to be used as a gel polymer electrolyte. When lithium metal anode is subjected to cycle-test while keeping in contact with the nickel side of JPL membrane, Scanning electron microscope (SEM) image showed that the surface of lithium metal anode was very clean without any fluctuation compared to the case where only polypropylene (PP) separator was used. (Figure. 1) Moreover, as shown in Figure 2., the LMB system applied JPL show high stability of tolerable overvoltage within 5mV, 15mV even at high-current of 10mA/cm2, 20mA/cm2, respectively, in Li//JPL/JPL//Li symmetric cell. These results are attributable to effective function of each layer as mentioned earlier. JPL consists of two layer, the tough and adhesive nature of PVdF-HFP prevents the polymer part from being damaged or torn by dendritic growth and simultaneously ensure that metal part functions as robust framework for SEI layer formation without escaped particle from the membrane. This frameworks induce the SEI layer, which can act as a resistive element, to grown in the metal part while surface of Li metal anode become get out of main subject from the SEI layer formation. By virtue of that property, JPL has not allow forming resistive elements on the surface of Li metal anode, preventing the voltage rising up due to the formation of resistive elements at surface, so tolerability of overvoltage with clean surface of anode is maintained. In conclusion, the model study of nickel micro-particle/PVdF-HFP composite suggests the straightforward direction of protective layer to apply lithium metal as anode in LIB system with alleviating the problem caused by the ‘hostless’ property of lithium metal anode. Thus, when JPL is introduced to a LIB system, it shows a fairly positive possibility for high performance Lithium metal battery. Figure 1

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.

Deciphering the mechanism of concentrated electrolyte for lithium metal anode via cryogenic electron microscopy

Use of a protective coating on a lithium metal anode (LMA) is an effective approach to enhance its coulombic efficiency and cycling stability. Here, a facile approach to produce uniform silver nanoparticle‐decorated LMA for high‐performance Li metal batteries (LMBs) is reported. This effective treatment can lead to well‐controlled nucleation and the formation of a stable solid electrolyte interphase (SEI). Ag nanoparticles embedded in the surface of Li anodes induce uniform Li plating/stripping morphologies with reduced overpotential. More importantly, cross‐linked lithium fluoride‐rich interphase formed during Ag+ reduction enables a highly stable SEI layer. Based on the Ag‐LiF decorated anodes, LMBs with LiNi1/3Mn1/3Co1/3O2 cathode (≈1.8 mAh cm−2) can retain >80% capacity over 500 cycles. The similar approach can also be used to treat sodium metal anodes. Excellent stability (80% capacity retention in 10 000 cycles) is obtained for a Na||Na3V2(PO4)3 full cell using a Na‐Ag‐NaF/Na anode cycled in carbonate electrolyte. These results clearly indicate that synergetic control of the nucleation and SEI is an efficient approach to stabilize rechargeable metal batteries.

The solid electrolyte interphase (SEI) is a crucial passivation layer that governs the electrochemical stability, cycling lifespan, and coulombic efficiency (CE) of different battery systems. Research has been extensively performed on electrolyte engineering and artificial design for robust SEI formation, while precise control of electrolyte decomposition dynamics is largely overlooked. Recently, interfacial catalysis has emerged as a promising strategy to guide SEI formation via modulating the electrolyte decomposition behaviors. The selective decomposition of anions in the electrolyte can be accelerated via introducing unique catalytic sites at the electrode-electrolyte interface, which enables precise tuning of SEI composition, microstructure reconstruction, and functionality enhancement. This review timely and systematically summarizes recent progress in the interfacial catalysis engineering of SEI layers, with a focus on catalytic mechanisms, catalyst design principles, and representative applications in various anode systems (e.g., lithium metal anode, sodium metal anode, alloy anodes, and carbonaceous anodes).

The development of energy storage devices that can efficiently operate at temperatures below 0 °C is necessary for both terrestrial and space-based applications. NASA has previously carried out pioneering work that has enabled Li-ion batteries to operate at temperatures down to -30 °C through the development of new carbonate electrolyte formulations. [1-2] However, secondary batteries that utilize lithium metal anodes instead of conventional graphite anodes could significantly boost energy density and specific energy, but the behavior of lithium electrodes at low temperatures are largely unknown. In this work, we investigate the effect of sub-zero temperatures on electrochemical behavior, morphological evolution, and solid-electrolyte interphase (SEI) formation of Li metal anodes in carbonate and ether solvents. Electrolytes made with solvents such as ethylene carbonate and ethyl methyl carbonate with ester co-solvents (e.g., ethyl propionate) exhibit sufficient conductivity and show effective Li deposition/stripping down to -30 °C, but with relatively low average Coulombic efficiencies (CEs) of ~70 % (similar to the room-temperature CE of ~73 %). The introduction of SEI-stabilizing additives such as fluoroethylene carbonate (FEC) boosts the average Coulombic efficiency across a range of temperatures (CE = ~94 %). Ether-based electrolytes, on the other hand, enable Li deposition/stripping down to -70 °C with high CEs (~95 %). Interestingly, investigation of the Li morphology after cycling revealed that distinct Li morphologies form at different temperatures, which may be related to differences in SEI chemistry and structure. These results provide important new information for engineering Li metal anodes for efficient operation under a variety of conditions.

Replacing flammable organic liquid electrolytes by solid-state electrolytes (SSEs) provides a fundamental solution to safety issue for lithium-ion batteries (LIBs), driven by the increasing demand of high energy density, stable cycling performance, low cost, and abuse tolerance for energy storage systems in the widespread applications of electric vehicles (EVs) and smart grids. In addition, SSEs could in principle suppress the lithium dendrite growth and enable the utilization of lithium metal anode that is regarded as a “holy grail” of lithium batteries, in all-solid-state lithium metal batteries (ASSLMBs). However, the problems including sluggish ion transport in SSEs and the interfacial incompatibility of SSEs with electrodes significantly limit the overall performances of ASSLMBs. In this chapter, we first highlight the critical scientific challenges as well as technological progresses for the currently studied SSEs including organic polymers, inorganic electrolytes, and the composites. The fundamental understanding and characterizations on the lithium nucleation, dendrite growth, and formation of solid electrolyte interphase (SEI) during Li electrochemical deposition is discussed systematically. The strategies to address the large interfacial impedances between SSE and Li metal anode attributed to low ionic conductivity and poor instability of SEI, and degraded physical contact are also overviewed. The approaches based on nanostructured materials and nanotechnology advances of great promise to improve the overall performances of ASSLMBs are discussed in detail. Finally, the remaining issues and future opportunities of ASSLMBs for next-generation high-energy storages are prospected.

The construction of a high-quality artificial solid electrolyte interphase (SEI) on lithium metal anodes is of great importance to regulate preferred crystal orientation, thereby realizing interfacial stability and dendrite suppression. Herein, we develop a novel low-temperature gas–liquid composite source (SF6 + silane coupling agent) plasma method for synthesis of dense inorganic–organic hybrid SEI on lithium metal anodes. The designed hybrid SEI layer consists of inorganic LiF and Li3N and organic lithium compounds (e.g., Li–O–Si). In-situ and ex-situ XRD results demonstrate that the modified hybrid SEI can induce preferential growth of the Li(110) crystal plane, thereby promoting lateral and dendrite-free morphology, in contrast to the (200)-oriented vertical dendrites of bare lithium. The preferential growth of the Li(110) plane is due to the synergistic effect of inorganic components (LiF and Li3N) and lithium compounds in SEI, which can enhance interfacial structural stability and inhibit dendrite propagation. Consequently, symmetric cells with modified Li metal anodes (P-SEI@Li) exhibit exceptional stability over 900 h at 1 mA cm–2/1 mAh cm–2 with a low overpotential of ∼11 mV. Full cells with LiNi0.9Co0.05Mn0.05O2 cathodes show improved capacity retention. This work demonstrates the effectiveness of plasma-engineered SEI in regulating crystal plane deposition and provides new insights for developing high-performance lithium metal batteries.

Lithium metal anodes are ideal for realizing high-energy-density batteries owing to their advantages, namely high capacity and low reduction potentials. However, the utilization of lithium anodes is restricted by the detrimental lithium dendrite formation, repeated formation and fracturing of the solid electrolyte interphase (SEI), and large volume expansion, resulting in severe "dead lithium" and subsequent short circuiting. Currently, the researches are principally focused on inhibition of dendrite formation toward extending and maintaining battery lifespans. Herein, we summarize the strategies employed in interfacial engineering and current-collector host designs as well as the emerging electrochemical catalytic methods for evolving-accelerating-ameliorating lithium ion/atom diffusion processes. First, strategies based on the fabrication of robust SEIs are reviewed from the aspects of compositional constituents including inorganic, organic, and hybrid SEI layers derived from electrolyte additives or artificial pretreatments. Second, the summary and discussion are presented for metallic and carbon-based three-dimensional current collectors serving as lithium hosts, including their functionality in decreasing local deposition current density and the effect of introducing lithiophilic sites. Third, we assess the recent advances in exploring alloy compounds and atomic metal catalysts to accelerate the lateral lithium ion/atom diffusion kinetics to average the spatial lithium distribution for smooth plating. Finally, the opportunities and challenges of metallic lithium anodes are presented, providing insights into the modulation of diffusion kinetics toward achieving dendrite-free lithium metal batteries.

Lithium metal anodes are promising alternatives to graphite for use in next-generation lithium-ion batteries due to their higher capacity (3,860 mAh/g vs. 372 mAh/g). However, Li anodes still display excessive capacity loss over cycling, leading to underwhelming Coulombic efficiency (CE, <99.9%) compared to graphite (>99.95%).1 These losses can derive from: (1) the formation of electronically isolated Li0 (often referred to as ‘dead’ or inactive’); and (2) the parasitic reactions between Li and the electrolyte that form the solid electrolyte interphase (SEI). Characterization of the Li anode has historically relied on surface characterization techniques such as XPS and FTIR, which cannot quantitatively reveal the total amounts of specific SEI phases. More precise quantification has recently become possible due to advances in chemical titration, which notably revealed that inactive Li0 is the dominant loss mechanism at low-to-moderate CE (<90%).2 At high CE (>90%), SEI losses are increasingly dominant and become the loss mode to minimize. Despite its importance, titration of SEI losses have thus far largely focused on LiH,3-5 with precise quantification of other accompanying phases still lacking.To bridge this gap, here we apply quantitative titration based on GC, ICP-AES and NMR to track an extended array of key SEI phases: semicarbonates (ROCO2Li), lithium carbide (Li2C2), olefins (RLi), LiF, P-containing phases, and total Li loss with, which teach new insights beyond inactive Li0. In 1 M LiPF6 EC/DEC, we demonstrate chemical resolution up to 71% of Li loss and 33% of SEI loss inventory. The analysis was also expanded to additional carbonate electrolytes of varying CEs (from <10% to >96%). Soluble SEI phases were particularly important at ultra-low CE (<10%), and their formation was suppressed higher CEs. Among the quantifiable SEI phases, ROCO2Li was consistently the major phase, but its proportions were invariant with CE. Instead, Li2C2, a minor phase, exhibited clear inverse correlation with CE. These results add further nuance beyond inactive Li0 to the current understanding of capacity loss, and demonstrate that, while minor phases often receive less focus and are harder to characterize, they can play governing roles in SEI function, particularly at high CE when formation of inactive Li0 is minimized. Hobold, G. M.; Lopez, J.; Guo, R.; Minafra, N.; Banerjee, A.; Shirley Meng, Y.; Shao-Horn, Y.; Gallant, B. M., Moving Beyond 99.9% Coulombic Efficiency for Lithium Anodes in Liquid Electrolytes. Nat. Energy 2021, 6 (10), 951-960. Fang, C.; Li, J.; Zhang, M.; Zhang, Y.; Yang, F.; Lee, J. Z.; Lee, M. H.; Alvarado, J.; Schroeder, M. A.; Yang, Y.; Lu, B.; Williams, N.; Ceja, M.; Yang, L.; Cai, M.; Gu, J.; Xu, K.; Wang, X.; Meng, Y. S., Quantifying Inactive Lithium in Lithium Metal Batteries. Nature 2019, 572 (7770), 511-515. Tao, M.; Xiang, Y.; Zhao, D.; Shan, P.; Sun, Y.; Yang, Y., Quantifying the Evolution of Inactive Li/Lithium Hydride and Their Correlations in Rechargeable Anode-Free Li Batteries. Nano Lett. 2022. Xu, G.; Li, J.; Wang, C.; Du, X.; Lu, D.; Xie, B.; Wang, X.; Lu, C.; Liu, H.; Dong, S.; Cui, G.; Chen, L., The Formation/Decomposition Equilibrium of LiH and Its Contribution on Anode Failure in Practical Lithium Metal Batteries. Angew. Chem. Int. Ed. Engl. 2021, 60 (14), 7770-7776. Xiang, Y.; Tao, M.; Zhong, G.; Liang, Z.; Zheng, G.; Huang, X.; Liu, X.; Jin, Y.; Xu, N.; Armand, M.; Zhang, J. G.; Xu, K.; Fu, R.; Yang, Y., Quantitatively Analyzing the Failure Processes of Rechargeable Li Metal Batteries. Sci. Adv. 2021, 7 (46), eabj3423.

Solvent-derived inorganic F and N-rich solid electrolyte interface for stable lithium metal batteries

A reaction-dissolution strategy for designing solid electrolyte interphases with stable energetics for lithium metal anodes

Given concerns about the low earth abundance of lithium (Li) in Li-ion batteries, there is growing interest in developing a beyond-Li materials basis for rechargeable batteries. Divalent batteries based on calcium (Ca) are a compelling alternative due to the ~2000-fold higher concentration of Ca in the earth’s crust, along with attractive theoretical metrics (2073 mAh/cm3, 1337 mAh/g, -2.87 V vs. SHE) if Ca is used as a metallic anode. When most common battery electrolytes come into contact with Ca, however, a passivating solid electrolyte interphase (SEI) forms that prevents Ca2+ transport,[1] and only a handful of electrolytes have been discovered that allow for reversible Ca plating/stripping to any degree. Ca(BH4)2 in tetrahydrofuran (THF) is one of the more widely-tested electrolytes and a current benchmark given its highest-to-date cycling Coulombic efficiency (CE) of ~95%.[2] Yet, there is limited understanding of the reactions forming the SEI and the resulting degree of SEI stability, which impedes further understanding into this electrolyte’s function. To begin to address these knowledge gaps, this study employs gas chromatography (GC) analysis to track gas evolution markers of SEI formation in Ca(BH4)2/THF across a range of cell conditions and timescales.Both H2 and linear hydrocarbon (RH) gases are detected as the major evolved gas species, with the former predominantly evolving from reaction between Ca(BH4)2 and residual water, and RH products evolving from reaction between Ca and THF. RH evolution is found to be dynamic for cells at rest, with distinct timescales attributed to initial and extended SEI formation and spanning several days. When cells are polarized during Ca plating, RH evolution arrests after a certain plated capacity is reached, signaling the formation of a protective yet functional SEI that continues to be permissive to Ca plating to higher capacities. Yet, this SEI is metastable, as further chemical changes are observed when polarization is relaxed. Given the significant role of water in contributing to H2 evolution, we further systematically investigate the impact of water on SEI formation dynamics and composition. Increased water content is observed to mainly shorten SEI formation timescales, but its impact diminishes over extended cycling, and the SEI composition ultimately remains largely independent of initial water content. This work probes the highly-dynamic reactions between Ca, Ca(BH4)2/THF, and residual water, improving understanding into both the fundamentals of this electrolyte system and the unique methodologies required for its study. While H2 evolution presents severe challenges to the practical implementation of any borohydride-based electrolyte, addition of small amounts of Ca(BH4)2 to other Ca electrolyte systems, if compatible, may offer longer term benefits to the chemical stability of a battery, given the salt’s ability to scavenge trace water and accelerate early SEI formation to passivate the anode at a shorter timescale.[1] Aurbach, D., Skaletsky, R. & Gofer, Y. The Electrochemical Behavior of Calcium Electrodes in a Few Organic Electrolytes. Journal of The Electrochemical Society 138, 3536-3545 (1991).[2] Wang, D. et al. Plating and stripping calcium in an organic electrolyte. Nature Materials 17, 16 (2017).

Rechargeable cells with lithium metal anodes have a very large theoretical energy density and are a promising cell system. However, rechargeable lithium metal cells are not yet currently commercially available. One of the biggest problems with the cells is the poor safety aspect resulting from the high chemical reactivity of lithium. We have been studying a cell system consisting of an amorphous (a-)V2O5P2O5 (95:5 in molar ratio) cathode, a lithium (Li) metal anode and an organic electrolyte in fabricating an AA-size prototype. In this paper, we report recent progress on our rechargeable lithium metal cell focusing on its safety.