Interface Engineering for Lithium Metal Anodes in Liquid Electrolyte

Interfacial chemistry between lithium metal anodes and electrolytes plays a vital role in regulating the Li plating/stripping behavior and improving the cycling performance of Li metal batteries. Constructing a stable solid electrolyte interphase (SEI) on Li metal anodes is now understood to be a requirement for progress in achieving feasible Li‐metal batteries. Recently, the application of novel analytical tools has led to a clearer understanding of composition and the fine structure of the SEI. This further promoted the development of interface engineering for stable Li metal anodes. In this review, the SEI formation mechanism, conceptual models, and the nature of the SEI are briefly summarized. Recent progress in probing the atomic structure of the SEI and elucidating the fundamental effect of interfacial stability on battery performance are emphasized. Multiple factors including current density, mechanical strength, operating temperature, and structure/composition homogeneity that affect the interfacial properties are comprehensively discussed. Moreover, strategies for designing stable Li‐metal/electrolyte interfaces are also reviewed. Finally, new insights and future directions associated with Li‐metal anode interfaces are proposed to inspire more revolutionary solutions toward commercialization of Li metal batteries.     

Enhanced Stability of Li Metal Anodes by Synergetic Control of Nucleation and the Solid Electrolyte Interphase

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 LiNi_1/3Mn_1/3Co_1/3O₂ 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.     

Tailoring Lithium Deposition via an SEI‐Functionalized Membrane Derived from LiF Decorated Layered Carbon Structure

Lithium (Li) metal is a key anode material for constructing next generation high energy density batteries. However, dendritic Li deposition and unstable solid electrolyte interphase (SEI) layers still prevent practical application of Li metal anodes. In this work, it is demonstrated that an uniform Li coating can be achieved in a lithium fluoride (LiF) decorated layered structure of stacked graphene (SG), leading to the formation of an SEI‐functionalized membrane that retards electron transfer by three orders of magnitude to avoid undesirable Li deposition on the top surface, and ameliorates Li⁺ ion migration to enable uniform and dendrite‐free Li deposition beneath such an interlayer. Surface chemistry analysis and density functional theory calculations demonstrate that these beneficial features arise from the formation of C–F_x surface components on the SG sheets during the Li coating process. Based on such an SEI‐functionalized membrane, stable cycling at high current densities up to 3 mA cm^?2 and Li plating capacities up to 4 mAh cm^?2 can be realized in LiPF₆/carbonate electrolytes. This work elucidates the promising strategy of modifying Li plating behavior through the SEI‐functionalized carbon structure, with significantly improved cycling stability of rechargeable Li metal anodes.     

A Material Perspective of Rechargeable Metallic Lithium Anodes

Rechargeable lithium‐based batteries are long considered as the most promising candidates for application in various electronic devices, electric vehicles, and even electrical grids owing to their ultrahigh energy densities. However, to date, metallic lithium‐based batteries are still far from practical applications due to the low Coulombic efficiency and fast capacity decay of lithium anodes. The poor electrochemical performances of metallic lithium anodes are inherently related to random growth of lithium dendrites and infinite volume charge of lithium anodes. In this review, the failure mechanisms of metallic lithium anodes are summarized and ascribed to the unstable and inhomogeneous solid electrolyte interphase, uneven distributions of electric field, and lithium‐ion flux during the lithium plating processes. Correspondingly, efficient strategies for mitigating these problems, including surficial engineering, electric field, and lithium‐ion flux regulation are discussed from the perspective of anode materials. Finally, an outlook is proposed for the design and fabrication of next‐generation rechargeable metallic lithium anodes that aims to address the intrinsic problems of metallic lithium anodes.     

Exploiting Lithium‐Depleted Cathode Materials for Solid‐State Li Metal Batteries

Solid‐state lithium metal batteries (SSLMBs) may become one of the high‐energy density storage devices for the next generation of electric vehicles. High safety and energy density can be achieved by utilizing solid electrolytes and Li metal anodes. Therefore, developing cathode materials which can match with Li metal anode efficiently is indispensable. In SSLMBs, Li metal anodes can afford the majority of active lithium ions, then lithium‐depleted cathode materials can be a competitive candidate to achieve high gravimetric energy density as well as save lithium resources. Li_0.33MnO₂ lithium‐depleted material is chosen, which also has the advantages of low synthesis temperature and low cost (cobalt‐free). Notably, solid‐state electrolyte can greatly alleviate the problem of manganese dissolution in the electrolyte, which is beneficial to improve the cycling stability of the battery. Thus, SSLMBs enable practical applications of lithium‐depleted cathode materials.     

Silicon‐Based Nanomaterials for Lithium‐Ion Batteries: A Review

There are growing concerns over the environmental, climate, and health impacts caused by using non‐renewable fossil fuels. The utilization of green energy, including solar and wind power, is believed to be one of the most promising alternatives to support more sustainable economic growth. In this regard, lithium‐ion batteries (LIBs) can play a critically important role. To further increase the energy and power densities of LIBs, silicon anodes have been intensively explored due to their high capacity, low operation potential, environmental friendliness, and high abundance. The main challenges for the practical implementation of silicon anodes, however, are the huge volume variation during lithiation and delithiation processes and the unstable solid‐electrolyte interphase (SEI) films. Recently, significant breakthroughs have been achieved utilizing advanced nanotechnologies in terms of increasing cycle life and enhancing charging rate performance due partially to the excellent mechanical properties of nanomaterials, high surface area, and fast lithium and electron transportation. Here, the most recent advance in the applications of 0D (nanoparticles), 1D (nanowires and nanotubes), and 2D (thin film) silicon nanomaterials in LIBs are summarized. The synthetic routes and electrochemical performance of these Si nanomaterials, and the underlying reaction mechanisms are systematically described.     

Carbon Anodes for Nonaqueous Alkali Metal‐Ion Batteries and Their Thermal Safety Aspects

Since their commercialization by Sony in 1991, graphite anodes in combination with various cathodes have enabled the widespread success of lithium‐ion batteries (LIBs), providing over 10 billion rechargeable batteries to the global population. Next‐generation nonaqueous alkali metal‐ion batteries, namely sodium‐ion batteries (SIBs) and potassium‐ion batteries (PIBs), are projected to utilize intercalation‐based carbon anodes as well, due to their favorable electrochemical properties. While traditionally graphite anodes have dominated the market share of LIBs, other carbon materials have been investigated, including graphene, carbon nanotubes, and disordered carbons. The relationship between carbon material properties, electrochemical performance, and charge storage mechanisms is clarified for these alkali metal‐ion batteries, elucidating possible strategies for obtaining enhanced cycling stability, specific capacity, rate capability, and safety aspects. As a key component in determining cell performance, the solid electrolyte interphase layer is described in detail, particularly for its dependence on the carbon anode. Finally, battery safety at extreme temperatures is discussed, where carbon anodes are susceptible to dendrite formation, accelerated aging, and eventual thermal runaway. As society pushes toward higher energy density LIBs, this review aims to provide guidance toward the development of sustainable next‐generation SIBs and PIBs.     

Uniform High Ionic Conducting Lithium Sulfide Protection Layer for Stable Lithium Metal Anode

Artificial solid‐electrolyte interphase (SEI) is one of the key approaches in addressing the low reversibility and dendritic growth problems of lithium metal anode, yet its current effect is still insufficient due to insufficient stability. Here, a new principle of “simultaneous high ionic conductivity and homogeneity” is proposed for stabilizing SEI and lithium metal anodes. Fabricated by a facile, environmentally friendly, and low‐cost lithium solid‐sulfur vapor reaction at elevated temperature, a designed lithium sulfide protective layer successfully maintains its protection function during cycling, which is confirmed by both simulations and experiments. Stable dendrite‐free cycling of lithium metal anode is realized even at a high areal capacity of 5 mAh cm^?2, and prototype Li–Li₄Ti₅O₁₂ cell with limited lithium also achieves 900 stable cycles. These findings give new insight into the ideal SEI composition and structure and provide new design strategies for stable lithium metal batteries.     

Highly Stable Operation of Lithium Metal Batteries Enabled by the Formation of a Transient High‐Concentration Electrolyte Layer

Lithium (Li) metal has been extensively investigated as an anode for rechargeable battery applications due to its ultrahigh theoretical specific capacity and the lowest redox potential. However, significant challenges including dendrite growth and low Coulombic efficiency are still hindering the practical applications of rechargeable Li metal batteries. It is demonstrated that long‐term cycling of Li metal batteries can be realized by the formation of a transient high‐concentration electrolyte layer near the surface of Li metal anode during high rate discharge process. The highly concentrated Li⁺ ions in this transient layer will immediately be solvated by the available solvent molecules and facilitate the formation of a stable and flexible solid electrolyte interphase (SEI) layer composed of a poly(ethylene carbonate) framework integrated with other organic/inorganic lithium salts. This SEI layer largely suppresses the corrosion of Li metal anode attacked by free organic solvents and enables the long‐term operation of Li metal batteries. The fundamental findings in this work provide a new direction for the development of Li metal batteries that could be operated at high current densities for a wide range of applications.     

Predicting Calendar Aging in Lithium Metal Secondary Batteries: The Impacts of Solid Electrolyte Interphase Composition and Stability

Calendar aging of lithium metal batteries, in which cells' components degrade internally due to chemical reactions while no current is being applied, is a relatively unstudied field. In this work, a model to predict calendar aging of lithium metal cells is developed using two sets of readily obtainable data: solid electrolyte interphase (SEI) layer composition (measured via X‐ray photoelectron spectroscopy) and SEI stability (measured as a degradation rate using a simple constant current–constant voltage charging protocol). Electrolyte properties such as volume and salt concentration are varied in order to determine their effect on SEI stability and composition, with subsequent impacts to calendar aging. Lower salt concentrations produce a solvent‐based, more soluble SEI, while the highest concentration produces a salt‐based, less soluble SEI. Higher electrolyte volumes promote dissolution of the SEI and thus decrease its stability. The model predicts that lithium metal would be the limiting factor in calendar aging, depleting long before the electrolyte does. Additionally, the relative composition of the electrolyte during aging is modeled and found to eventually converge to the same value independent of initial salt concentration.     

Recent Developments and Understanding of Novel Mixed Transition‐Metal Oxides as Anodes in Lithium Ion Batteries

Mixed transition‐metal oxides (MTMOs), including stannates, ferrites, cobaltates, and nickelates, have attracted increased attention in the application of high performance lithium‐ion batteries. Compared with traditional metal oxides, MTMOs exhibit enormous potential as electrode materials in lithium‐ion batteries originating from higher reversible capacity, better structural stability, and high electronic conductivity. Recent advancements in the rational design of novel MTMO micro/nanostructures for lithium‐ion battery anodes are summarized and their energy storage mechanism is compared to transition‐metal oxide anodes. In particular, the significant effects of the MTMO morphology, micro/nanostructure, and crystallinity on battery performance are highlighted. Furthermore, the future trends and prospects, as well as potential problems, are presented to further develop advanced MTMO anodes for more promising and large‐scale commercial applications of lithium‐ion batteries.     

Fluorinated End‐Groups in Electrolytes Induce Ordered Electrolyte/Anode Interface Even at Open‐Circuit Potential as Revealed by Sum Frequency Generation Vibrational Spectroscopy

Fluorine‐based additives have a tremendously beneficial effect on the performance of lithium‐ion batteries, yet the origin of this phenomenon is unclear. This paper shows that the formation of a solid‐electrolyte interphase (SEI) on the anode surface in the first five charge/discharge cycles is affected by the stereochemistry of the electrolyte molecules on the anode surface starting at open‐circuit potential (OCP). This study shows an anode‐specific model system, the reduction of 1,2‐diethoxy ethane with lithium bis(trifluoromethane)sulfonimide, as a salt on an amorphous silicon anode, and compares the electrochemical response and SEI formation to its fluorinated version, bis(2,2,2‐trifluoroethoxy) ethane (BTFEOE), by sum frequency generation (SFG) vibrational spectroscopy under reaction conditions. The SFG results suggest that the ? CF₃ end‐groups of the linear ether BTFEOE change their adsorption orientation on the a‐Si surface at OCP, leading to a better protective layer. Supporting evidence from ex situ scanning electron microscopy and X‐ray photoelectron spectroscopy depth profiling measurements shows that the fluorinated ether, BTFEOE, yields a smooth SEI on the a‐Si surface and enables lithium ions to intercalate deeper into the a‐Si bulk.     

Long‐Life,High‐Rate Lithium–Sulfur Cells with a Carbon‐Free VN Host as an Efficient Polysulfide Adsorbent and Lithium Dendrite Inhibitor

Lithium–sulfur (Li‐S) batteries are a promising next‐generation energy‐storage system, but the polysulfide shuttle and dendritic Li growth seriously hinder their commercial viability. Most of the previous studies have focused on only one of these two issues at a time. To address both the issues simultaneously, presented here is a highly conductive, noncarbon, 3D vanadium nitride (VN) nanowire array as an efficient host for both sulfur cathodes and lithium‐metal anodes. With fast electron and ion transport and high porosity and surface area, VN traps the soluble polysulfides, promotes the redox kinetics of sulfur cathodes, facilitates uniform nucleation/growth of lithium metal, and inhibits lithium dendrite growth at an unprecedented high current density of 10 mA cm^?2 over 200 h of repeated plating/stripping. As a result, VN‐Li||VN‐S full cells constructed with VN as both an anode and cathode host with a negative to positive electrode capacity ratio of only ≈2 deliver remarkable electrochemical performance with a high Coulombic efficiency of ≈99.6% over 850 cycles at a high 4 C rate and a high areal capacity of 4.6 mA h cm^?2. The strategy presented here offers a viable approach to realize high‐energy‐density, safe Li‐metal‐based batteries.     

An Upgraded Lithium Ion Battery Based on a Polymeric Separator Incorporated with Anode Active Materials

Structural/compositional characteristics at the anode/electrolyte interface are of paramount importance for the practical performance of lithium ion batteries, including cyclic stability, rate capacity, and operational safety. The anode‐electrolyte interface with traditional separator technology is featured with inevitable phase discontinuity and fails to support the stable operation of lithium ion batteries based on large‐capacity anodes with structural change in charges/discharges, such as transition metal oxide anodes. In this work, an anode/electrolyte framework based on an oxide anode and an active‐oxide‐incorporated separator is proposed for the first time and investigated for lithium ion batteries. The architecture builds a robust anode‐separator interface in LIBs, shortens Li⁺ diffusion path, accelerates electron transport, and mitigates the volume change of the oxide anode in electrochemical reactions. Remarkably, 4 wt% CuO addition in the separator leads to a 17% enhancement in the overall capacity of a battery with a CuO anode. The battery delivers an unparalleled record reversible capacity of 637.2 mAh g^?1 with a 99% capacity retention after 100 charge/discharge cycles at 0.5 C. The high performance are attributed to the robust anode‐separator interface, which gives rise to enhanced interaction between the oxide anode and the same‐oxide‐incorporated composite in the separator.     

Zipper‐Inspired SEI Film for Remarkably Enhancing the Stability of Li Metal Anode via Nucleation Barriers Controlled Weaving of Lithium Pits

The lithium dendrite, inducing short circuit and breaking solid electrolyte interphase (SEI) films, is deleterious to the stability of Li metal batteries due to the uncontrollable occurrence of miscellaneous stresses. In contrast to conventional suppression routes, herein a strategy is proposed via controlling SEI film broken regions to minimize releasing stress in terms of weaving lithium pits. Inspired by the principle of zippers, zipper‐like SEI films enable offering ordered pattern on the surface of Li anode via mechanical rolling. For the available cells, net‐like sewing/breaking patterns alternatively occur in Li plating/stripping. In the same electrolyte, a stable and dendrite‐free Li homogeneous growth is achieved.     

The Fast Charge Transfer Kinetics of the Lithium Metal Anode on the Garnet‐Type Solid Electrolyte Li6.25Al0.25La3Zr2O12

The charge transfer kinetics between a lithium metal electrode and an inorganic solid electrolyte is of key interest to assess the rate capability of future lithium metal solid state batteries. In an in situ microelectrode study run in a scanning electron microscope, it is demonstrated that—contrary to the prevailing opinion—the intrinsic charge transfer resistance of the Li|Li_6.25Al_0.25La₃Zr₂O₁₂ (LLZO) interface is in the order of 10^?1 Ω cm² and thus negligibly small. The corresponding high exchange current density in combination with the single ion transport mechanism (t⁺ ≈ 1) of the inorganic solid electrolyte enables extremely fast plating kinetics without the occurrence of transport limitations. Local plating rates in the range of several A cm^?2 are demonstrated at defect free and chemically clean Li|LLZO interfaces. Practically achievable current densities are limited by lateral growth of lithium along the surface as well as electro‐chemo‐mechanical‐induced fracture of the solid electrolyte. In combination with the lithium vacancy diffusion limitation during electrodissolution, these morphological instabilities are identified as the key fundamental limitations of the lithium metal electrode for solid‐state batteries with inorganic solid electrolytes.     

Flexible Ion‐Conducting Composite Membranes for Lithium Batteries

The use of metallic lithium anodes enables higher energy density and higher specific capacity Li‐based batteries. However, it is essential to suppress lithium dendrite growth during electrodeposition. Li‐ion‐conducting ceramics (LICC) can mechanically suppress dendritic growth but are too fragile and also have low Li‐ion conductivity. Here, a simple, versatile, and scalable procedure for fabricating flexible Li‐ion‐conducting composite membranes composed of a single layer of LICC particles firmly embedded in a polymer matrix with their top and bottom surfaces exposed to allow for ionic transport is described. The membranes are thin (<100 μm) and possess high Li‐ion conductance at thicknesses where LICC disks are mechanically unstable. It is demonstrated that these membranes suppress Li dendrite growth even when the shear modulus of the matrix is lower than that of lithium. It is anticipated that these membranes enable the use of metallic lithium anodes in conventional and solid‐state Li‐ion batteries as well as in future Li? S and Li? O₂ batteries.     

Pristine or Highly Defective? Understanding the Role of Graphene Structure for Stable Lithium Metal Plating

The role of graphene host structure/chemistry in plating–stripping in lithium metal anodes employed for lithium metal batteries is first examined in this study. Structural and chemical defects are bad, since highly defective graphene promotes unstable solid electrolyte interphase (SEI) growth. This consumes the fluoroethylene carbonate (FEC) additive in the carbonate electrolyte and is correlated with rapid decay in Coulombic efficiency (CE) and formation of filament‐like Li dendrites. A unique flow‐aided sonication exfoliation method is employed to synthesize “defect‐free” graphene (df‐G), allowing for a direct performance comparison with conventional reduced graphene oxide (r‐GO). At cycle 1, the r‐GO is better electrochemically wetted by Li than df‐G, indicating that initially it is more lithiophilic. With cycling, the nucleation overpotential with r‐GO becomes higher than with df‐G, indicating less facile plating reactions. The df‐G yields state‐of‐the‐art electrochemical performance, with the post cycled metal surface being relatively smooth and dendrite‐free. Conversely, r‐GO templates have CE rapidly degrade from the onset, with extensive dendrites after cycling. Severe SEI growth and associated FEC depletion with r‐GO are further confirmed by electrochemical impedance analysis and surface science methods. A new design rule is provided for Li metal templates: An ideal host must be noncatalytic toward SEI formation.     

Ultrathin Bilayer of Graphite/SiO2 as Solid Interface for Reviving Li Metal Anode

Lithium metal anodes are expected to drive practical applications that require high energy‐density storage. However, the direct use of metallic lithium causes safety concerns, low rate capabilities, and poor cycling performance due to unstable solid electrolyte interphase (SEI) and undesired lithium dendrite growth. To address these issues, a radio frequency sputtered graphite‐SiO₂ ultrathin bilayer on a Li metal chips is demonstrated, for the first time, as an effective SEI layer. This leads to a dendrite free uniform Li deposition to achieve a stable voltage profile and outstanding long hours plating/stripping compared to the bare Li. Compared to a bare Li anode, the graphite‐SiO₂ bilayer modified Li anode coupled with lithium nickel cobalt manganese oxide cathode (NMC111) and lithium titanate shows improved capacity retention, higher capacity at higher rates, longer cycling stability, and lower voltage hysteresis. Graphite acts as an electrical bridge between the plated Li and Li electrode, which lowers the impedance and buffers the volume expansion during Li plating/stripping. Adding an ultrathin SiO₂ layer facilitates Li‐ion diffusion and lithiation/delithiation, provides higher electrolyte affinity, higher chemical stability, and higher Young's modulus to suppress the Li dendrite growth.     

On the Capacity Losses Seen for Optimized Nano‐Si Composite Electrodes in Li‐Metal Half‐Cells

While the use of silicon‐based electrodes can increase the capacity of Li‐ion batteries considerably, their application is associated with significant capacity losses. In this work, the influences of solid electrolyte interphase (SEI) formation, volume expansion, and lithium trapping are evaluated for two different electrochemical cycling schemes using lithium‐metal half‐cells containing silicon nanoparticle–based composite electrodes. Lithium trapping, caused by incomplete delithiation, is demonstrated to be the main reason for the capacity loss while SEI formation and dissolution affect the accumulated capacity loss due to a decreased coulombic efficiency. The capacity losses can be explained by the increasing lithium concentration in the electrode causing a decreasing lithiation potential and the lithiation cut‐off limit being reached faster. A lithium‐to‐silicon atomic ratio of 3.28 is found for a silicon electrode after 650 cycles using 1200 mAhg^?1 capacity limited cycling. The results further show that the lithiation step is the capacity‐limiting step and that the capacity losses can be minimized by increasing the efficiency of the delithiation step via the inclusion of constant voltage delithiation steps. Lithium trapping due to incomplete delithiation consequently constitutes a very important capacity loss phenomenon for silicon composite electrodes.