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.     

Artificial Solid‐Electrolyte Interphase Enabled High‐Capacity and Stable Cycling Potassium Metal Batteries

Secondary batteries based on earth‐abundant potassium metal anodes are attractive for stationary energy storage. However, suppressing the formation of potassium metal dendrites during cycling is pivotal in the development of future potassium metal‐based battery technology. Herein, a promising artificial solid‐electrolyte interphase (ASEI) design, simply covering a carbon nanotube (CNT) film on the surface of a potassium metal anode, is demonstrated. The results show that the spontaneously potassiated CNT framework with a stable self‐formed solid‐electrolyte interphase layer integrates a quasi‐hosting feature with fast interfacial ion transport, which enables dendrite‐free deposition of potassium at an ultrahigh capacity (20 mAh cm^?2). Remarkably, the potassium metal anode exhibits an unprecedented cycle life (over 1000 cycles, over 2000 h) at a high current density of 5 mA cm^?2 and a desirable areal capacity of 4 mAh cm^?2. Dendrite‐free morphology in carbon‐fiber and carbon‐black‐based ASEI for potassium metal anodes, which indicates a broader promise of this approach, is also observed.     

In Situ Measurement of Solid Electrolyte Interphase Evolution on Silicon Anodes Using Atomic Force Microscopy

In situ measurements of the growth of solid electrolyte interphase (SEI) layer on silicon and the lithiation‐induced volume changes in silicon in lithium ion half‐cells are reported. Thin film amorphous silicon electrodes are fabricated in a configuration that allows unambiguous separation of the total thickness change into contribution from SEI thickness and silicon volume change. Electrodes are assembled into a custom‐designed electrochemical cell, which is integrated with an atomic force microscope. The electrodes are subjected to constant potential lithiation/delithiation at a sequence of potential values and the thickness measurements are made at each potential after equilibrium is reached. Experiments are carried out with two electrolytes—1.2 m lithium hexafluoro‐phosphate (LiPF₆) in ethylene carbonate (EC) and 1.2 m LiPF₆ in propylene carbonate (PC)—to investigate the influence of electrolyte composition on SEI evolution. It is observed that SEI formation occurs predominantly during the first lithiation and the maximum SEI thickness is ≈17 and 10 nm respectively for EC and PC electrolytes. This study also presents the measured Si expansion ratio versus equilibrium potential and charge capacity versus equilibrium potential; both relationships display hysteresis, which is explained in terms of the stress–potential coupling in silicon.     

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.     

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.     

Construction of Stable Li2O-Rich Solid Electrolyte Interphase for Practical PEO-Based Li-Metal Batteries

Polyethylene oxide (PEO)-based solid polymer electrolytes (SPE) have garnered recognition as highly promising candidates for advanced lithium-metal batteries. However, the practical application of PEO-based SPE is hindered by its low critical current density (CCD) resulting from undesired dendrite growth. In this study, a PEO-based SPE that exhibits an ultra-high CCD (4 mA cm⁻²) is presented and enhanced lithium ionic conductivity through the incorporation of small amounts of P₂S₅ (PS). The crystalline Li₂O-rich and P/S-containing solid electrolyte interphase (SEI) is revealed by cryo-electron microscope (cryo-EM) and Time of flight secondary ion mass spectrometry (TOF-SIMS), which inhibits dendrite growth and adverse reactions between SPE and reductive lithium, thus offering a spherical growth behavior for dendrite-free lithium metal anode. Consequently, utilizing the PS-integrated SPE, a Li-Li symmetric cell demonstrates reduced resistance during operation, enabling stable cycles exceeding 200 hours at 0.5 mA cm⁻² and 0.5 mAh cm⁻², a stringent test condition for PEO-based electrolytes. Moreover, a Li/SPE/LiFePO₄ (LFP) pouch cell exhibits 80% capacity retention after 100 cycles with 50 µm Li and 30 µm PEO electrolyte, showcasing its potential for practical applications.     

Electrokinetic Phenomena Enhanced Lithium‐Ion Transport in Leaky Film for Stable Lithium Metal Anodes

The application of lithium (Li) metal anodes in Li metal batteries has been hindered by growth of Li dendrites, which lead to short cycling life. Here a Li‐ion‐affinity leaky film as a protection layer is reported to promote a dendrite‐free Li metal anode. The leaky film induces electrokinetic phenomena to enhance Li‐ion transport, leading to a reduced Li‐ion concentration polarization and homogeneous Li‐ion distribution. As a result, the dendrite‐free Li metal anode during Li plating/stripping is demonstrated even at an extremely high deposition capacity (6 mAh cm^?2) and current density (40 mA cm^?2) with improved Coulombic efficiencies. A full cell battery with the leaky‐film protected Li metal as the anode and high‐areal‐capacity LiNi_0.8Co_0.1Mn_0.1O₂ (NCM‐811) (≈4.2 mAh cm^?2) or LiFePO₄ (≈3.8 mAh cm^?2) as the cathode shows improved cycling stability and capacity retention, even at lean electrolyte conditions.     

Control and Optimization of the Electrochemical and Mechanical Properties of the Solid Electrolyte Interphase on Silicon Electrodes in Lithium Ion Batteries

The formation of the solid electrolyte interphase (SEI) on Si is examined in detail using several in situ techniques. The results show that employing different conditions during the first lithiation cycle produces SEI films with substantially different properties. Longer time at higher potentials produces softer SEI, whereas inorganic phases produced at lower potentials have higher elastic moduli. The SEI thickness stabilizes during the first cycle; however, the SEI resistance decreases during the first 20 cycles (in sharp contrast to typical surface passivation processes, where resistance is expected to increase with time). This behavior is consistent with the slow growth of inorganic constituents at lower potentials, inside of a mesoporous soft SEI that initially forms at higher potentials. This interpretation is based on the premise that these inorganic phases have a lower resistivity than that associated with electrolyte transport through the mesoporous organic phase. Based on this set of observations, the multiphase structure that evolves during initial cycling determines critical electrochemical and mechanical properties of the SEI. A basic model of these tradeoffs is proposed to provide guidelines for creating more stable interfacial films.     

Temperature-Mediated Dynamic Lithium Loss and its Implications for High-Efficiency Lithium Metal Anodes

Lithium (Li) metal has been strongly regarded as the ultimate anode option for next-generation high-energy-density batteries. Nevertheless, the insufficient Coulombic efficiency induced by the extensive active Li loss largely hinders the practical operation of Li metal batteries under wide temperature range. Herein, the temperature-mediated dynamic growth of inactive Li from −20 to 60°C via titration gas chromatograph measurements is quantitatively decoupled. Combined X-ray photoelectronic spectroscopy, cryo-transmission electronic microscopy, and scanning electronic microscopy methods depicted that both solid electrolyte interphase (SEI) characteristics and Li deposition compactness can be profoundly manipulated by working temperature. The elevation of temperature is found to fundamentally aggravate the parasitic reactions and deteriorate the spatial uniformity of SEI, yet promote the lateral growth of Li by kinetic reason. The opposite effects of temperature on SEI properties and Li deposition compactness can properly explain the intricate temperature-dependent growth rates of SEI-Li⁺ and dead Li⁰ capacity loss observed under titration gas chromatograph measurements. Design implications towards more stable Li metal anodes with higher reversibility can thus be yielded.     

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.     

Air‐Stable and Dendrite‐Free Lithium Metal Anodes Enabled by a Hybrid Interphase of C60 and Mg

Li metal is an ideal anode material for rechargeable high energy density batteries, but its sensitivity to humid air and uncontrolled dendrite growth limit its practical applications. A novel hybrid interphase is fabricated to address these issues. This interphase consists of dense fullerene (C₆₀) and magnesium metal bilayers, which are deposited successively on lithium foil by vacuum evaporation deposition and contribute to moisture resistance and lithium dendrite suppression. Thanks to this dual‐functional feature, the assembled cells with the modified anodes and commercial LiFePO₄ cathodes exhibit long cycle life (>200 cycles) with high capacity retention (>98.5%). Moreover, even the modified anodes that are exposed to humid air (30% relative humidity) for over 12 h; the cells still deliver excellent performance, comparable to those without exposure. Such a unique hybrid interphase provides a new promising method for fabricating air‐stable and dendrite‐free lithium metal batteries.     

The Failure of Solid Electrolyte Interphase on Li Metal Anode: Structural Uniformity or Mechanical Strength?

Practical application of lithium (Li) metal anodes has been hindered by Li dendrite growth, which renders a low Coulombic efficiency and short lifespan of working Li metal batteries. A stable solid electrolyte interphase (SEI) is crucial in suppressing the formation of Li dendrites. Herein the local stress and deformation evolvement status of a SEI layer during Li electrodeposition are investigated through a quantitative electrochemical–mechanical model based on a finite element method. Furthermore, the impacts of structural uniformity and mechanical strength on the stability of the SEI under different working conditions are investigated. Improving the structural uniformity of SEI is the most effective way to enhance the stability of SEI, which regulates ion transportation. In addition, pursuing extremely high mechanical strength is shown to be pointless, and a moderate elastic modulus of 3.0 GPa is suggested. This work affords an insight into the rational design of stable SEI layers and sheds light on a possible pathway toward practical applications of Li metal anodes.     

Thermal Induced Strain Relaxation of 1D Iron Oxide for Solid Electrolyte Interphase Control and Lithium Storage Improvement

High energy lithium ion battery based on multi‐electron redox reaction is often accompanied by inherent large volume expansions, sluggish kinetics, and unstable solid electrolyte interphase layer, leading to capacity failure. Here, thermal induced strain relaxation is proposed to realize the solid electrolyte interphase control. It is demonstrated that through thermal treatment, lattice strain is well released and defect density is well reduced, facilitating the charge transfer, improving the interparticle contacts and the contacts at the interface of electrode to withstand the huge volume expansion/contraction during cycling. In this way, the as‐prepared α‐Fe₂O₃ electrode at 800 °C with no protective shell shows an outstanding reversible capacity of 1200 mA h g^?1 at 100 mA g^?1 and an excellent high‐rate cyclability with a capacity fading of 0.056% per cycle for 1200 cycles at 5 A g^?1. It is expected that such findings facilitate the applications of high capacity anode and cathode material systems that undergo large volume expansion.     

An Interconnected Channel‐Like Framework as Host for Lithium Metal Composite Anodes

Lithium (Li) metal anodes have long been counted on to meet the increasing demand for high energy, high‐power rechargeable battery systems but they have been plagued by uncontrollable plating, unstable solid electrolyte interphase (SEI) formation, and the resulting low Coulombic efficiency. These problems are even aggravated under commercial levels of current density and areal capacity testing conditions. In this work, the channel‐like structure of a carbonized eggplant (EP) as a stable “host” for Li metal melt infusion, is utilized. With further interphase modification of lithium fluoride (LiF), the as‐formed EP–LiF composite anode maintains ≈90% Li metal theoretical capacity and can successfully suppress dendrite growth and volume fluctuation during cycling. EP–LiF offers much improved symmetric cell and full‐cell cycling performance with lower and more stable overpotential under various areal capacity and elevated rate capability. Furthermore, carbonized EP serves as a light‐weight high‐performance current collector, achieving an average Coulombic efficiency ≈99.1% in ether‐based electrolytes with 2.2 mAh cm^?2 cycling areal capacity. The natural structure of carbonized EP will inspire further artificial designs of electrode frameworks for both Li anode and sulfur cathodes, enabling promising candidates for next‐generation high‐energy density batteries.