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

Kinetic understanding of lithium metal electrodeposition for lithium anodes

Lithium, a representative alkali metal, holds the coveted status of the "holy grail" in the realm of next-generation rechargeable batteries, owing to its remarkable theoretical specific capacity and low electrode potential. However, the inherent reactivity of Li metal inevitably results in the formation of the solid-electrolyte interphase (SEI) on its surface, adding complexity to the Li electrodeposition process compared to conventional metal electrodeposition. Attaining uniform Li deposition is crucial for ensuring stable, long-cycle performance and high Coulombic efficiency in Li metal batteries, which requires a comprehensive understanding of the underlying factors governing the electrodeposition process. This review delves into the intricate kinetics of Li electrodeposition, elucidating the multifaceted factors that influence charge and mass transfer kinetics. The intrinsic relationship between charge transfer kinetics and Li deposition is scrutinized, exploring how parameters such as current density and electrode potential impact Li nucleation and growth, as well as dendrite formation. Additionally, the applicability of classical mass-transfer-controlled electrodeposition models to Li anode systems is evaluated, considering the influence of ionic concentration and solvation structure on Li+ transport, SEI formation, and subsequent deposition kinetics. The pivotal role of SEI compositional structure and physicochemical properties in governing charge and mass transfer processes is underscored, with an emphasis on strategies for regulating Li deposition kinetics from both electrolyte and SEI perspectives. Finally, future directions in Li electrodeposition research are outlined, emphasizing the importance of ongoing exploration from a kinetic standpoint to fully unlock the potential of Li metal batteries.

Unveiling the In Situ Evolution of Li2O-Rich Solid Electrolyte Interface on CoOx Embedded Carbon Fibers as Li Anode Host

Modification of three-dimensional (3D) carbon hosts with metal oxides has been considered as advantageous for the formation of Li2O-rich solid electrolyte interface (SEI), which can show fast Li+ diffusion, and meanwhile alleviate dendrite problems caused by fragility and nonuniformity of native SEIs. However, the lack of convincing experimental evidence has made it difficult to unveil the true origin of oxygen in Li2O-rich SEIs until now. Herein, CoOx embedded carbon nanofibers (CNF-CoOx) are successfully prepared as high-performance Li anode hosts. By employing 18O isotope labeling, the role of CoOx during SEI evolution is elucidated, revealing that CoOx contributes significantly to Li2O formation by delivering oxygen. Benefiting from the rich Li2O content, the as-formed SEIs greatly improve the Li+ migration kinetics, and therefore, the CNF-CoOx@Li anode can exhibit excellent cycling stability in half, symmetrical, and full cells.

In Situ Formed Gel Polymer Electrolytes Enable Stable Solid Electrolyte Interface for High-Performance Lithium Metal Batteries

Carbonate-based electrolytes show distinct advantages in high-voltage cathodes but generate nonuniform and mechanically fragile solid-electrolyte interphase (SEI) in lithium (Li) metal batteries. Herein, we propose a LiF-rich SEI incorporating an in situ polymerized poly(hexamethylene diisocyanate)-based gel polymer electrolyte (GPE) to improve the homogeneity and mechanical stability of SEI. Fluoroethylene carbonate (FEC) as a fluorine-based additive for building LiF-rich SEI on Li metal electrodes. With this strategy, the assembled Li symmetric batteries cycled stably for 700 h, and the formation of byproducts on the Li electrode surface was significantly inhibited. The Li/LiFePO4 battery delivered significant capacity retention (91% retention after 800 cycles) at 1 C. With high-voltage LiNi0.8Co0.1Mn0.1O2 (NCM811) as cathode, the Li/GPE-FEC/NCM811 cell delivered a discharge capacity of 168.9 mAh g-1 with a capacity retention of 82% after 300 cycles at 0.5 C. From the above, the work could assist the rapid development of high-energy-density rechargeable Li metal batteries toward remarkable performance.

High-voltage electrosynthesis of organic-inorganic hybrid with ultrahigh fluorine content toward fast Li-ion transport

Hybrid materials with a rational organic-inorganic configuration can offer multifunctionality and superior properties. This principle is crucial but challenging to be applied in designing the solid electrolyte interphase (SEI) on lithium metal anodes (LMAs), as it substantially affects Li+ transport from the electrolyte to the anode. Here, an artificial SEI with an ultrahigh fluorine content (as high as 70.12 wt %) can be successfully constructed on the LMA using a high-voltage electrosynthesis strategy. This SEI consists of ultrafine lithium fluoride nanocrystals embedded in a fluorinated organic matrix, exhibiting excellent passivation and mechanical strength. Notably, the organic-inorganic interface demonstrates a high dielectric constant that enables fast Li+ transport throughout the SEI. Consequently, LMA coated with this SEI substantially enhances the cyclability of both half-cells and full cells, even under rigorous conditions. This work demonstrates the potential of rationally designed hybrid materials via a unique electrosynthetic approach for advanced electrochemical systems.

In Situ Electrochemical Atomic Force Microscopy: From Interfaces to Interphases

The electrochemical interface formed between an electrode and an electrolyte significantly affects the rate and mechanism of the electrode reaction through its structure and properties, which vary across the interface. The scope of the interface has been expanded, along with the development of energy electrochemistry, where a solid-electrolyte interphase may form on the electrode and the active materials change properties near the surface region. Developing a comprehensive understanding of electrochemical interfaces and interphases necessitates three-dimensional spatial resolution characterization. Atomic force microscopy (AFM) offers advantages of imaging and long-range force measurements. Here we assess the capabilities of AFM by comparing the force curves of different regimes and various imaging modes for in situ characterizing of electrochemical interfaces and interphases. Selected examples of progress on work related to the structures and processes of electrode surfaces, electrical double layers, and lithium battery systems are subsequently illustrated. Finally, this review provides perspectives on the future development of electrochemical AFM.

Application of Li6.4La3Zr1.45Ta0.5Mo0.05O12/PEO Composite Solid Electrolyte in High-Performance Lithium Batteries

Replacing the flammable liquid electrolytes with solid ones has been considered to be the most effective way to improve the safety of the lithium batteries. However, the solid electrolytes often suffer from low ionic conductivity and poor rate capability due to their relatively stable molecular/atomic architectures. In this study, we report a composite solid electrolyte, in which polyethylene oxide (PEO) is the matrix and Li6.4La3Zr1.45Ta0.5Mo0.05O12 (LLZTMO) and Li6.4La3Zr1.4Ta0.6O12 (LLZTO) are the fillers. Ta/Mo co-doping can further promote the ion transport capacity in the electrolyte. The synthesized composite electrolytes exhibit high thermal stability (up to 413 °C) and good ionic conductivity (LLZTMO-PEO 2.00 × 10-4 S·cm-1, LLZTO-PEO 1.53 × 10-4 S·cm-1) at 35 °C. Compared with a pure PEO electrolyte, whose ionic conductivity is in the range of 10-7~10-6 S·cm-1, the ionic conductivity of composite solid electrolytes is greatly improved. The full cell assembled with LiFePO4 as the positive electrode exhibits excellent rate performance and good cycling stability, indicating that prepared solid electrolytes have great potential applications in lithium batteries.

Solid Electrolyte Interphase on Lithium Metal Anodes

Lithium metal batteries (LMBs) represent the most promising next-generation high-energy density batteries. The solid electrolyte interphase (SEI) film on the lithium metal anode plays a crucial role in regulating lithium deposition and improving the cycling performance of LMBs. In this review, we comprehensively present the formation process of the SEI film, while elucidating the key properties such as electronic conductivity, ionic conductivity, and mechanical performance. Furthermore, various approaches for constructing the SEI film are discussed from both electrolyte regulation and artificial coating design perspectives. Lastly, future research directions along with development recommendations are also provided. This review aims to provide possible strategies for the further improvement of SEI film in LMBs and highlight their inspiration for future research directions.

Compositionally Sequenced Interfacial Layers for High-Energy Li-Metal Batteries

Electrolyte additives with multiple functions enable the interfacial engineering of Li-metal batteries (LMBs). Owing to their unique reduction behavior, additives exhibit a high potential for electrode surface modification that increases the reversibility of Li-metal anodes by enabling the development of a hierarchical solid electrolyte interphase (SEI). This study confirms that an adequately designed SEI facilitates the homogeneous supply of Li+, nonlocalized Li deposition, and low electrolyte degradation in LMBs while enduring the volume fluctuation of Li-metal anodes on cycling. An in-depth analysis of interfacial engineering mechanisms reveals that multilayered SEI structures comprising mechanically robust LiF-rich species, electron-rich P-O species, and elastic polymeric species enabled the stable charge and discharge of LMBs. The polymeric outer SEI layer in the as-fabricated multilayered SEI could accommodate the volume fluctuation of Li-metal anodes, significantly enhancing the cycling stability Li||LiNi0.8Co0.1Mn0.1O2 full cells with an electrolyte amount of 3.6 g Ah-1 and an areal capacity of 3.2 mAh cm-2. Therefore, this study confirms the ability of interfacial layers formed by electrolyte additives and fluorinated solvents to advance the performance of LMBs and can open new frontiers in the fabrication of high-performance LMBs through electrolyte-formulation engineering.

In Situ Engineering of Inorganic-Rich Solid Electrolyte Interphases via Anion Choice Enables Stable, Lithium Anodes

The discovery of liquid battery electrolytes that facilitate the formation of stable solid electrolyte interphases (SEIs) to mitigate dendrite formation is imperative to enable lithium anodes in next-generation energy-dense batteries. Compared to traditional electrolyte solvents, tetrahydrofuran (THF)-based electrolyte systems have demonstrated great success in enabling high-stability lithium anodes by encouraging the decomposition of anions (instead of organic solvent) and thus generating inorganic-rich SEIs. Herein, by employing a variety of different lithium salts (i.e., LiPF6, LiTFSI, LiFSI, and LiDFOB), it is demonstrated that electrolyte anions modulate the inorganic composition and resulting properties of the SEI. Through novel analytical time-of-flight secondary-ion mass spectrometry methods, such as hierarchical clustering of depth profiles and compositional analysis using integrated yields, the chemical composition and morphology of the SEIs generated from each electrolyte system are examined. Notably, the LiDFOB electrolyte provides an exceptionally stable system to enable lithium anodes, delivering >1500 cycles at a current density of 0.5 mAh g-1 and a capacity of 0.5 mAh g-1 in symmetrical cells. Furthermore, Li//LFP cells using this electrolyte demonstrate high-rate, reversible lithium storage, supplying 139 mAh g(LFP) -1 at C/2 (≈0.991 mAh cm-2 , @ 0.61 mA cm-2 ) with 87.5% capacity retention over 300 cycles (average Coulombic efficiency >99.86%).

Unraveling the Mechanism of Very Initial Dendritic Growth Under Lithium Ion Transport Control in Lithium Metal Anodes

Lithium metal deposition is strongly affected by the intrinsic properties of the solid-electrolyte interphase (SEI) and working electrolyte, but a relevant understanding is far from complete. Here, by employing multiple electrochemical techniques and the design of SEI and electrolyte, we elucidate the electrochemistry of Li deposition under mass transport control. It is discovered that SEIs with a lower Li ion transference number and/or conductivity induce a distinctive current transition even under moderate potentiostatic polarization, which is associated with the control regime transition of Li ion transport from the SEI to the electrolyte. Furthermore, our findings help reveal the creation of a space-charge layer at the electrode/SEI interface due to the involvement of the diffusion process of Li ions through the SEI, which promotes the formation of dendrite embryos that develop and eventually trigger SEI breakage and the control regime transition of Li ion transport. Our insight into the very initial dendritic growth mechanism offers a bridge toward design and control for superior SEIs.

Resolving nanostructure and chemistry of solid-electrolyte interphase on lithium anodes by depth-sensitive plasmon-enhanced Raman spectroscopy

The solid-electrolyte interphase (SEI) plays crucial roles for the reversible operation of lithium metal batteries. However, fundamental understanding of the mechanisms of SEI formation and evolution is still limited. Herein, we develop a depth-sensitive plasmon-enhanced Raman spectroscopy (DS-PERS) method to enable in-situ and nondestructive characterization of the nanostructure and chemistry of SEI, based on synergistic enhancements of localized surface plasmons from nanostructured Cu, shell-isolated Au nanoparticles and Li deposits at different depths. We monitor the sequential formation of SEI in both ether-based and carbonate-based dual-salt electrolytes on a Cu current collector and then on freshly deposited Li, with dramatic chemical reconstruction. The molecular-level insights from the DS-PERS study unravel the profound influences of Li in modifying SEI formation and in turn the roles of SEI in regulating the Li-ion desolvation and the subsequent Li deposition at SEI-coupled interfaces. Last, we develop a cycling protocol that promotes a favorable direct SEI formation route, which significantly enhances the performance of anode-free Li metal batteries.

Regulating the Solvation Structure of Electrolyte via Dual-Salt Combination for Stable Potassium Metal Batteries

Batteries using potassium metal (K-metal) anode are considered a new type of low-cost and high-energy storage device. However, the thermodynamic instability of the K-metal anode in organic electrolyte solutions causes uncontrolled dendritic growth and parasitic reactions, leading to rapid capacity loss and low Coulombic efficiency of K-metal batteries. Herein, an advanced electrolyte comprising 1 M potassium bis(fluorosulfonyl)imide (KFSI) + 0.05 M potassium hexafluorophosphate (KPF₆ ) dissolved in dimethoxyethane (DME) is introduced as a simple and effective strategy of regulated solvation chemistry, showing an enhanced interfacial stability of the K-metal anode. Incorporating 0.05 M KPF₆ into the 1 M KFSI in DME electrolyte solution decreases the number of solvent molecules surrounding the K ion and simultaneously leads to facile K⁺ de-solvation. During the electrodeposition process, these unique features can lower the exchange current density between the electrolyte and K-metal anode, thereby improving the uniformity of K electrodeposition, as well as potentially suppressing dendritic growth. Even under a high current density of 4 mA cm^-2 , the K-metal anode in 0.05 M KPF₆ -containing electrolyte ensures high areal capacity and an unprecedented lifespan with stable Coulombic efficiency in both symmetrical half-cells and full-cells employing a sulfurized polyacrylonitrile cathode.

Morphology-Dictated Mechanism of Efficient Reaction Sites for Li2O2 Decomposition

In the pursuit of a highly reversible lithium-oxygen (Li-O₂) battery, control of reaction sites to maintain stable conversion between O₂ and Li₂O₂ at the cathode side is imperatively desirable. However, the mechanism involving the reaction site during charging remains elusive, which, in turn, imposes challenges in recognition of the origin of overpotential. Herein, via combined investigations by in situ atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS), we propose a universal morphology-dictated mechanism of efficient reaction sites for Li₂O₂ decomposition. It is found that Li₂O₂ deposits with different morphologies share similar localized conductivities, much higher than that reported for bulk Li₂O₂, enabling the reaction site not only at the electrode/Li₂O₂/electrolyte interface but also at the Li₂O₂/electrolyte interface. However, while the mass transport process is more enhanced at the former, the charge-transfer resistance at the latter is sensitively related to the surface structure and thus the reactivity of the Li₂O₂ deposit. Consequently, for compact disk-like deposits, the electrode/Li₂O₂/electrolyte interface serves as the dominant decomposition site, which causes premature departure of Li₂O₂ and loss of reversibility; on the contrary, for porous flower-like and film-like Li₂O₂ deposits bearing a larger surface area and richer surface-active structures, both the interfaces are efficient for decomposition without premature departure of the deposit so that the overpotential arises primarily from the sluggish oxidation kinetics and the decomposition is more reversible. The present work provides instructive insights into the understanding of the mechanism of reaction sites during the charge process, which offers guidance for the design of reversible Li-O₂ batteries.

Probing the Mechanically Stable Solid Electrolyte Interphase and the Implications in Design Strategies

The inevitable volume expansion of secondary battery anodes during cycling imposes forces on the solid electrolyte interphase (SEI). The battery performance is closely related to the capability of SEI to maintain intact under the cyclic loading conditions, which basically boils down to the mechanical properties of SEI. The volatile and complex nature of SEI as well as its nanoscale thickness and environmental sensitivity make the interpretation of its mechanical behavior many roadblocks. Widely varied approaches are adopted to investigate the mechanical properties of SEI, and diverse opinions are generated. The lack of consensus at both technical and theoretical levels has hindered the development of effective design strategies to maximize the mechanical stability of SEIs. Here, the essential and desirable mechanical properties of SEI, the available mechanical characterization methods, and important issues meriting attention for higher test accuracy are outlined. Previous attempts to optimize battery performance by tuning SEI mechanical properties are also scrutinized, inconsistencies in these efforts are elucidated, and the underlying causes are explored. Finally, a set of research protocols is proposed to accelerate the achievement of superior battery cycling performance by improving the mechanical stability of SEI.

Surface engineering toward stable lithium metal anodes

The lithium (Li) metal anode (LMA) is susceptible to failure due to the growth of Li dendrites caused by an unsatisfied solid electrolyte interface (SEI). With this regard, the design of artificial SEIs with improved physicochemical and mechanical properties has been demonstrated to be important to stabilize the LMAs. This review comprehensively summarizes current efficient strategies and key progresses in surface engineering for constructing protective layers to serve as the artificial SEIs, including pretreating the LMAs with the reagents situated in different primary states of matter (solid, liquid, and gas) or using some peculiar pathways (plasma, for example). The fundamental characterization tools for studying the protective layers on the LMAs are also briefly introduced. Last, strategic guidance for the deliberate design of surface engineering is provided, and the current challenges, opportunities, and possible future directions of these strategies for the development of LMAs in practical applications are discussed.

Tuning Interface Mechanics Via β-Configuration Dominant Amyloid Aggregates for Lithium Metal Batteries

Owing to abundant polar groups and good lithiophilicity, protein materials regain interest for application in lithium metal batteries (LMBs). Current proteins with an α-conformation for modifying lithium (Li) anodes possess typically poor mechanical properties, and there is therefore a significant need for advanced protein materials. Herein, a lysozyme-modified layer is coated onto the poly(vinylidene fluoride) electrospun mat for high mechanical strength and uniform Li-ion flux. The lysozyme membrane can regulate Li⁺ deposition behavior due to complete β-sheet configuration, high lithiophilicity sulfhydryl groups, and columnar nanopores. As a result, the lysozyme-modified Li metal anode exhibits a high stability performance of Li-Li symmetric cells (2800 h) and Li-LiFePO₄ full cell (1450 cycles). Our strategy pushes the protein with β-sheet configuration toward the applications of next-generation LMBs.

Chemomechanics of Rechargeable Batteries: Status, Theories, and Perspectives

Chemomechanics is an old subject, yet its importance has been revived in rechargeable batteries where the mechanical energy and damage associated with redox reactions can significantly affect both the thermodynamics and rates of key electrochemical processes. Thanks to the push for clean energy and advances in characterization capabilities, significant research efforts in the last two decades have brought about a leap forward in understanding the intricate chemomechanical interactions regulating battery performance. Going forward, it is necessary to consolidate scattered ideas in the literature into a structured framework for future efforts across multidisciplinary fields. This review sets out to distill and structure what the authors consider to be significant recent developments on the study of chemomechanics of rechargeable batteries in a concise and accessible format to the audiences of different backgrounds in electrochemistry, materials, and mechanics. Importantly, we review the significance of chemomechanics in the context of battery performance, as well as its mechanistic understanding by combining electrochemical, materials, and mechanical perspectives. We discuss the coupling between the elements of electrochemistry and mechanics, key experimental and modeling tools from the small to large scales, and design considerations. Lastly, we provide our perspective on ongoing challenges and opportunities ranging from quantifying mechanical degradation in batteries to manufacturing battery materials and developing cyclic protocols to improve the mechanical resilience.

In-Situ Electrodeposition of Nanostructured Carbon Strengthened Interface for Stabilizing Lithium Metal Anode

The lithium metal anode (LMA) is regarded as one of the most promising candidates for high-energy Li-ion batteries. However, the naturally formed solid electrolyte interface (SEI) is unsatisfied, which would cause continuous dendrite growth and thus prevent the practical application of the LMA. Herein, a stable electrolytic carbon-based hybrid (ECH) artificial SEI is constructed on the LMA via the in-situ electrodeposition of an electrolyte sovlent at ultrahigh voltage. This nanostructured carbon strengthened SEI exhibits much improved ionic conductivity and mechanical strength, which enables uniform Li⁺ diffusion, stabilizes the interface between the electrolyte and lithium metal, and inhibits Li dendrite breeding and Li pulverization. With the protection of this ECH layer, the symmetrical cells show stable long-term cycling performance over 500 h with an ultrahigh plating capacity of 5 mAh cm^-2 at the current density of 5 mA cm^-2. A full cell assembled with a Li[Ni_0.8Co_0.1Mn_0.1]O₂ or LiFePO₄ cathode exhibits a long-term cycling life and excellent capacity retention.

Modification of Nitrate Ion Enables Stable Solid Electrolyte Interphase in Lithium Metal Batteries

The lifespan of high-energy-density lithium metal batteries (LMBs) is hindered by heterogeneous solid electrolyte interphase (SEI). The rational design of electrolytes is strongly considered to obtain uniform SEI in working batteries. Herein, a modification of nitrate ion (NO₃ ^- ) is proposed and validated to improve the homogeneity of the SEI in practical LMBs. NO₃ ^- is connected to an ether-based moiety to form isosorbide dinitrate (ISDN) to break the resonance structure of NO₃ ^- and improve the reducibility. The decomposition of non-resonant -NO₃ in ISDN enriches SEI with abundant LiN_x O_y and induces uniform lithium deposition. Lithium-sulfur batteries with ISDN additives deliver a capacity retention of 83.7 % for 100 cycles compared with rapid decay with LiNO₃ after 55 cycles. Moreover, lithium-sulfur pouch cells with ISDN additives provide a specific energy of 319 Wh kg^-1 and undergo 20 cycles. This work provides a realistic reference in designing additives to modify the SEI for stabilizing LMBs.

Formation sequence of solid electrolyte interphases and impacts on lithium deposition and dissolution on copper: an in situ atomic force microscopic study

Copper is the most widely used substrate for Li deposition and dissolution in lithium metal anodes, which is complicated by the formation of solid electrolyte interphases (SEIs), whose physical and chemical properties can affect Li deposition and dissolution significantly. However, initial Li nucleation and growth on bare Cu creates Li nuclei that only partially cover the Cu surface so that SEI formation could proceed not only on Li nuclei but also on the bare region of the Cu surface with different kinetics, which may affect the follow-up processes distinctively. In this paper, we employ in situ atomic force microscopy (AFM), together with X-ray photoelectron spectroscopy (XPS), to investigate how SEIs formed on a Cu surface, without Li participation, and on the surface of growing Li nuclei, with Li participation, affect the components and structures of the SEIs, and how the formation sequence of the two kinds of SEIs, along with Li deposition, affect subsequent dissolution and re-deposition processes in a pyrrolidinium-based ionic liquid electrolyte containing a small amount of water. Nanoscale in situ AFM observations show that sphere-like Li deposits may have differently conditioned SEI-shells, depending on whether Li nucleation is preceded by the formation of the SEI on Cu. Models of integrated-SEI shells and segmented-SEI shells are proposed to describe SEI shells formed on Li nuclei and SEI shells sequentially formed on Cu and then on Li nuclei, respectively. "Top-dissolution" is observed for both types of shelled Li deposits, but the integrated-SEI shells only show wrinkles, which can be recovered upon Li re-deposition, while the segmented-SEI shells are apparently top-opened due to mechanical stresses introduced at the junctions of the top regions and become "dead" SEIs, which forces subsequent Li nucleation and growth in the interstice of the dead SEIs. Our work provides insights into the impact mechanism of SEIs on the initial stage Li deposition and dissolution on foreign substrates, revealing that SEIs could be more influential on Li dissolution and that the spatial integration of SEI shells on Li deposits is important to improving the reversibility of deposition and dissolution cycling.

Mechanistic Insights into the Structural Modulation of Transition Metal Selenides to Boost Potassium Ion Storage Stability

Atomic-level structure engineering is an effective strategy to reduce mechanical degradation and boost ion transport kinetics for battery anodes. To address the electrode failure induced by large ionic radius of K⁺ ions, herein we synthesized Mn-doped ZnSe with modulated electronic structure for potassium ion batteries (PIBs). State-of-the-art analytical techniques and theoretical calculations were conducted to probe crystalline structure changes, ion/electron migration pathways, and micromechanical stresses evolution mechanisms. We demonstrate that the heterogeneous adjustment of the electronic structure can relieve the potassiumization-induced internal strain and improve the structural stability of battery anodes. Our work highlights the importance of the correlation between doping chemistry and mechanical stability, inspiring a pathway of structural engineering strategy toward a highly stable PIBs.

Mechanical studies of the solid electrolyte interphase on anodes in lithium and lithium ion batteries

A stable solid electrolyte interphase (SEI) layer is key to high performing lithium ion and lithium metal batteries for metrics such as calendar and cycle life. The SEI must be mechanically robust to withstand large volumetric changes in anode materials such as lithium and silicon, so understanding the mechanical properties and behavior of the SEI is essential for the rational design of artificial SEI and anode form factors. The mechanical properties and mechanical failure of the SEI are challenging to study, because the SEI is thin at only ~10-200 nm thick and is air sensitive. Furthermore, the SEI changes as a function of electrode material, electrolyte and additives, temperature, potential, and formation protocols. A variety ofin situandex situtechniques have been used to study the mechanics of the SEI on a variety of lithium ion battery anode candidates; however, there has not been a succinct review of the findings thus far. Because of the difficulty of isolating the true SEI and its mechanical properties, there have been a limited number of studies that can fully de-convolute the SEI from the anode it forms on. A review of past research will be helpful for culminating current knowledge and helping to inspire new innovations to better quantify and understand the mechanical behavior of the SEI. This review will summarize the different experimental and theoretical techniques used to study the mechanics of SEI on common lithium battery anodes and their strengths and weaknesses.