A Flexible Solid Electrolyte Interphase Layer for Long‐Life Lithium Metal Anodes

Recent advances in research on anodes for safe and efficient lithium-metal batteries

The revival of lithium metal anodes (LMAs) makes it a potent influence on the battery research community in the recent years after the popularity of Li-ion batteries with graphite anodes. The main reason is due to the over ten-fold increase in the capacity of LMAs when compared with that obtained when using graphite, as well as the low redox potential of Li/Li⁺. However, the full potential of LMAs is heavily inhibited by several factors, such as dendrite growth, pulverization, side reactions, and volume changes. These adversities lower the cell's Coulombic efficiency dramatically if operated without massively excessive Li usage. In this review, we first introduce some of the most significant progresses made in the understandings of the charging/discharging processes at the anode. The importance of combining advanced characterization techniques with classical methods is highlighted. In particular, we aim to explore the hidden links between those studies for obtaining deeper insights. Two main categories of solutions to address common problems, namely, lithium-electrolyte interfacial engineering and three-dimensional hosting of Li, are subsequently illustrated, where each subsection takes a different methodological perspective to demonstrate the relevant state-of-the-art studies. Some interesting approaches to stop dendrites and a brief note on the practical aspects of lithium-metal batteries are provided, too. This review concludes with our essential discoveries from the current literature and valuable suggestions for future LMA research.

Properties of Thin Lithium Metal Electrodes in Carbonate Electrolytes with Realistic Parameters

To understand the baseline performance of lithium (Li) anode in liquid electrolytes, the electrochemical and physical properties of the Li anode are studied with realistic parameters, including thin thickness (50 μm), practical areal capacity (1-4 mA h cm^-2), practical areal current (0.5-2 mA cm^-2), and low electrolyte/capacity ratio. Two different Li salts, lithium hexafluorophosphate (LiPF₆) and lithium bis(fluorosulfonyl)imide (LiFSI), are used to probe the effects of the electrolyte chemistry and concentration. The cycling of Li/Li symmetric cells, combined with the scanning electron microscopic investigation, demonstrates that the soft-short of Li/Li cells is induced by the continuous volume expansion of Li electrodes during cycling instead of dendrites. The volume change of a Li electrode is dictated by the depth of deposition and stripping (i.e., areal capacity) and the electrolyte/capacity ratio, with no strong correlation with the type of Li salt and concentration. On the other hand, the average Coulombic efficiency (CE) measurement demonstrates inherent correlation with the type of Li salt and its concentration in the electrolyte. Li electrode surface chemical analysis indicates that the fluoride-rich surface layer formed in the LiPF₆ electrolyte can be detrimental to both CE and Li deposition-stripping overpotential.

Ionic liquid assisted electrochemical coating zinc nanoparticles on carbon cloth as lithium dendrite suppressing host

The application of lithium metal anode with high specific capacity and energy density is limited by the volume expansion and pulverization caused by dendrite growth during cycle process. We propose a composite lithium anode by immersing molten lithium on the flexible three-dimensional (3D) carbon cloth scaffold with the zinc nanoparticles. The lithiophilic zinc nanoparticles layer of framework is synthesized by fast and easy electrochemical deposition from ionic liquid avoiding high temperature, high pressure and toxic reagent. The lithium is infused into the 3D lithiophilic framework, the composite anode is obtained. The steady network structure can confine the lithium and lead to Li dendrite restraining and reducing volume change due to the low interfacial resistance and reduce the effective current density, which induced the homogeneous Li growth. Benefiting from this, the Li infused 3D carbon cloth-Zn symmetric battery exhibits a low stripping/plating overpotential (~30 mV) and can be stable over 900 h at 1 mA cm^-2. The Li//LiFePO₄ battery delivers higher reversible capacity (140 mAh g^-1 at 2 C and 120 mAh g^-1 at 5 C) and stable cycling for 1500 and 2000 cycles than bare Li.

Progress on Lithium Dendrite Suppression Strategies from the Interior to Exterior by Hierarchical Structure Designs

Lithium (Li) metal is promising for high energy density batteries due to its low electrochemical potential (-3.04 V) and high specific capacity (3860 mAh g^-1 ). However, the safety issues impede the commercialization of Li anode batteries. In this work, research of hierarchical structure designs for Li anodes to suppress Li dendrite growth and alleviate volume expansion from the interior (by the 3D current collector and host matrix) to the exterior (by the artificial solid electrolyte interphase (SEI), protective layer, separator, and solid state electrolyte) is concluded. The basic principles for achieving Li dendrite and volume expansion free Li anode are summarized. Following these principles, 3D porous current collector and host matrix are designed to suppress the Li dendrite growth from the interior. Second, artificial SEI, the protective layer, and separator as well as solid-state electrolyte are constructed to regulate the distribution of current and control the Li nucleation and deposition homogeneously for suppressing the Li dendrite growth from exterior of Li anode. Ultimately, this work puts forward that it is significant to combine the Li dendrite suppression strategies from the interior to exterior by 3D hierarchical structure designs and Li metal modification to achieve excellent cycling and safety performance of Li metal batteries.

Double-Shelled C@MoS2 Structures Preloaded with Sulfur: An Additive Reservoir for Stable Lithium Metal Anodes

The growth of Li dendrites hinders the practical application of lithium metal anodes (LMAs). In this work, a hollow nanostructure, based on hierarchical MoS₂ coated hollow carbon particles preloaded with sulfur (C@MoS₂ /S), was designed to modify the LMA. The C@MoS₂ hollow nanostructures serve as a good scaffold for repeated Li plating/stripping. More importantly, the encapsulated sulfur could gradually release lithium polysulfides during the Li plating/stripping, acting as an effective additive to promote the formation of a mosaic solid electrolyte interphase layer embedded with crystalline hybrid lithium-based components. These two factors together effectively suppress the growth of Li dendrites. The as-modified LMA shows a high Coulombic efficiency of 98 % over 500 cycles at the current density of 1 mA cm^-2 . When matched with a LiFePO₄ cathode, the assembled full cell displays a highly improved cycle life of 300 cycles, implying the feasibility of the proposed LMA.

Stable Lithium Metal Anode Enabled by 3D Soft Host

Lithium (Li) metal is among the most promising anode materials for next-generation rechargeable batteries. However, inevitable Li dendrite growth and huge volume expansion severely restrict its practical application. Here, we propose a melamine sponge@silver nanowires (MS@AgNWs) current collector to achieve highly reversible Li storage. By combining the strength advantages of lithophilic nanoseeds, 3D current rectification structure and stress-releasing soft substrate, the MS@AgNWs host can successfully release the compress stress generated during the Li-plating process and hence give rise to uniform Li deposition. In particular, the MS@AgNWs-Li composite anode shows high Coulomb efficiency of 99.1% over 300 cycles and ultralow overpotentials of 10 mV at 1 mA cm^-2 and 19 mV at 2 mA h cm^-2. Superior long-term cycle stability over 1000 h is attained in symmetric cell under various densities. The assembled full cells with LiFePO₄ cathode deliver excellent cycle performance with capacity retention of 138.2 mAh g^-1 at 1C after 400 cycles and outstanding rate performance (discharge capacity of 119 mAh g^-1 at 10 C). Scalable fabrication of 3D MS@AgNWs flexible host can be easily realized, which is potential for developing practical flexible Li metal based batteries.

Amide-Based Interface Layer with High Toughness In Situ Building on the Li Metal Anode

Lithium metal is considered to be the ultimate anode for lithium-ion batteries (LIBs) because of its ultrahigh capacity and lowest electrochemical potential. However, the high reactivity of the lithium metal triggers continuous electrolyte consumption and dendrite growth, resulting in short cycle lifetime and serious safety issues. Massive efforts have been made to stabilize the surface of the lithium metal anode. Here, we propose an amide-based passivation layer to serve as an electrochemically stable and highly tough SEI on the lithium metal anode by in situ generation. The SEI layer presents a high elasticity modulus of 10 GPa and enables stable cycling in 2500 h. Furthermore, based on our strategy, the Li/LiFePO₄ cell with a cathode loading of ∼19 mg cm^-2 exhibits a long lifespan of 400 cycles. Our approach establishes a meaningful guideline for building a highly strong electrolyte/electrode interface in high-energy density lithium metal batteries.

In Situ Formed LiZn Alloy Skeleton for Stable Lithium Anodes

Lithium metal is the most promising anode for developing high-energy density rechargeable batteries because of its ultrahigh theoretical capacity and extremely low reduction potential. However, the formation of dendritic lithium and the huge volume change of the anode during the charge/discharge process severely hinder the practical application of the lithium anode. Obtaining high-performance, simple methods that can simultaneously modify the interface and restrict the volume change of the Li anode are highly required. Herein, the lithiophilic Zn nanoparticles are introduced into molten lithium directly to obtain a composite anode filled with in situ formed LiZn alloy rods. These micrometer-sized alloy rods can serve as a skeleton to provide a large number of lithium deposition sites as well as volume suppression for lithium deposition. Benefiting from these two aspects, the composite anode exhibits superior electrochemical performance by means of lowering the overpotential and prolonging the cycle life of symmetrical cells. Furthermore, the full cell paired with this composite anode and LiFePO₄ cathode also demonstrates a better capacity retention than its counterpart with raw Li anode.

Organic-Organic Composite Electrolyte Enables Ultralong Cycle Life in Solid-State Lithium Metal Batteries

Ionic conducting polymer electrolytes for solid-state lithium-ion batteries have attracted ever-increasing attention because of their decent ionic conductivity, flexibility, no liquid leakage, and good processability. Poly(vinylidene fluoride) (PVDF)-based polymer electrolytes have recently stood out among the polymer electrolytes due to their high room temperature ionic conductivity. However, the interface between PVDF-based polymer electrolytes and lithium metal decays over time until the batteries break down. Here, we introduce a small amount of poly(acrylic acid) (PAA) into a PVDF-based polymer electrolyte and synthesize an organic-organic composite electrolyte that alleviates the interfacial reaction with lithium metal, which shows great superiority over other modification methods such as coating. The cycle life of lithium symmetric cells is prolonged from 130 to 850 h at 0.44 mA cm^-2 due to the effective suppression of interfacial reaction. The much more stable interface also enables excellent cycle performance in a solid-state LiCoO₂||Li cell at 30 °C with a capacity decay of 0.03% per cycle for 1000 cycles, which is much lower than that of a cell without blending PAA (0.13% per cycle for only 450 cycles). The results would shed light on the applications of PVDF-based polymer electrolytes in solid-state lithium metal batteries.

Solid Electrolyte Interphase Evolution on Lithium Metal in Contact with Glyme-Based Electrolytes

The formation of a stable solid electrolyte interphase (SEI) is a prerogative for functional lithium metal batteries. Herein, the formation and evolution of such SEI in contact with glyme-based electrolytes is investigated under open circuit voltage and several constant current cycles. An important conclusion of the study is that Li_x S_y species are nonbeneficial SEI components, compared to the Li₃ N counterpart. In addition, chemical (X-ray photoelectron spectroscopy, XPS) and electrochemical (impedance spectroscopy) evolution of SEI under galvanostatic conditions are comprehensively tracked.

A Ternary Hybrid-Cation Room-Temperature Liquid Metal Battery and Interfacial Selection Mechanism Study

The dendrite-free sodium-potassium (Na-K) liquid alloy composed of two alkali metals is one of the ideal alternatives for Li metal as an anode material while maintaining large capacity, low potential, and high abundance. However, Na- or K-ion batteries have limited cathode materials that can deliver stably large capacity. Combining advantages of both, a hybrid-cation liquid metal battery is designed for a Li-ion-insertion-based cathode to deliver stable high capacity using a Na-K liquid anode to avoid dendrites. The mechanical property of the Na-K alloy is confirmed by simulation and experimental characterization, which leads to stable cycling performance. The charge carrier selection principle in this ternary hybrid-cation system is investigated, showing consistency with the proposed interfacial layer formation and ion distribution mechanism for the electrochemical process as well as the good stability. With Li ions contributing stable cycling as the cathode charge carrier, the K ion working as charge carrier on the anode, and Na as the medium to liquefy K metal, such a ternary hybrid battery system not only inherits the rich battery chemistry of Li-insertion cathodes but also broadens the understanding of alkali metal alloys and hybrid-ion battery chemistry.

An Outlook on Low-Volume-Change Lithium Metal Anodes for Long-Life Batteries

Rechargeable Li metal batteries are one of the most attractive energy storage systems due to their high energy density. However, the hostless nature of Li, the excessive dendritic growth, and the accumulation of nonactive Li induce severe volume variation of Li anodes. The volume variation can give rise to a fracture of solid electrolyte interphase, continuous consumption of Li and electrolytes, low Coulombic efficiency, fast performance degradation, and finally short cycle life. This Outlook provides a comprehensive understanding of the origin and consequences of Li volume variation. Recent strategies to address this challenge are reviewed from liquid to gel to solid-state electrolyte systems. In the end, guidelines for structural design and fabrication suggestions for future long-life Li composite anodes are presented.

Atom Transfer Radical Polymerization of Acrylic and Methacrylic Acids: Preparation of Acidic Polymers with Various Architectures

The preparation of poly(acrylic acid) (PAA) with tailored architecture and morphology is important for the design of advanced polymer materials. Cu-catalyzed atom transfer radical polymerization (ATRP) of AA is challenging due to the tendency of dormant chains to undergo an intramolecular lactonization reaction with consequent loss of chain-end functionalities, as previously reported for ATRP of methacrylic acid (MAA). In addition, AA can coordinate to the Cu catalyst. Moreover, the lower ATRP reactivity of AA relative to MAA enhances side reactions during polymerizations. These issues were overcome by adjusting the composition of the catalytic system, the polymerization setup, and the initiator nature. AA conversion >70-80% was obtained in 5 h, producing PAA with Đ ≈1.4. Multifunctional water-soluble initiators provided PAA and PMAA with telechelic and star-shaped architectures. Block copolymers of MAA and AA confirmed the retention of chain-end functionalities during ATRPs.

Uniform Li Deposition Sites Provided by Atomic Layer Deposition for the Dendrite-free Lithium Metal Anode

The nonuniform nucleation of lithium (Li) leads to dendritic behavior and formation of dead Li, which seriously hinders the practical application of Li metal batteries. Here, atomic layer deposition (ALD) is used to deposit uniform and conformal ZnO coating (at a low content of 5.96%) on carbon fibers to form a free-standing framework Li host material without uncontrollable dendrites. Compared with the liquid deposition process, the ALD method can achieve homogeneous and conformal ZnO coating and excellent lithiophilicity of the carbon fiber, guiding molten Li infusion into the carbon fiber skeleton to obtain the Li/C composite electrode with a flat surface, thereby minimizing the effective current density. More importantly, the converted LiZn alloy will serve as uniform and numerous nucleation sites for Li and guide synchronous growth of the Li metal along carbon fibers, displaying a dendrite-free morphology after large-current and long-term deposition/dissolution cycling. Therefore, the ALD ZnO-modified carbon fiber/Li exhibits significantly better cycle and rate performances than the liquid deposition ZnO-modified carbon fiber/Li composite anode. The electrodes display an ultralong lifespan up to 400 cycles at 3.0 mA/cm², as well as a high rate performance (with a deposition overpotential of 338 mV at 25.0 mA/cm²) at a high Li deposition areal capacity of 5.0 mA h cm^-2.

Flexible, Self-Healing, and Fire-Resistant Polymer Electrolytes Fabricated via Photopolymerization for All-Solid-State Lithium Metal Batteries

The cyclophosphazene-based self-healing polymer electrolytes (CPSHPE) is designed and fabricated via the copolymerization of hexa(4-ethyl acrylate phenoxy) cyclotriphosphazene (HCP), (2-(3-(6-methyl-4-oxo-1,4-dihydropyrimidin-2-yl)ureido)ethyl methacrylate) (UPyMA), and poly(ethylene glycol) methyl ether methacrylate (PEGMA) under UV irradiation. The cross-linking structure formed by HCP could effectively enhance the mechanical strength of the polymer electrolyte, and the cyclotriphosphazene as the core is able to improve the flame-retardant properties. Benefiting from the phenyl groups in HCP and the cross-linking structure, the CPSHPE shows high thermal stability (up to 300 °C). On the other hand, the supramolecular network fabricated by the dynamic ureido-pyrimidinone (UPy) dimers endows the polymer electrolyte with good self-healing capability and is expected to improve the reliability of polymer lithium batteries. Moreover, the cells were fabricated with LiFePO₄ (LFP), CPSHPE, and Li anodes show good reversible specific capacity. The CPSHPE could be a promising candidate as the multifunctional polymer electrolyte to improve the safety performance of lithium metal batteries.

Single-Atom Catalytic Materials for Advanced Battery Systems

Advanced battery systems with high energy density have attracted enormous research enthusiasm with potential for portable electronics, electrical vehicles, and grid-scale systems. To enhance the performance of conversion-type batteries, various catalytic materials are developed, including metals and transition-metal dichalcogenides (TMDs). Metals are highly conductive with catalytic effects, but bulk structures with low surface area result in low atom utilization, and high chemical reactivity induces unfavorable dendrite effects. TMDs present chemical adsorption with active species and catalytic activity promotes conversion processes, suppressing shuttle effect and improving energy density. But they suffer from inferior conductivity compared with metal, and limited sites mainly concentrate on edges and defects. Single-atom materials with atomic sizes, good conductivity, and individual sites are promising candidates for advanced batteries because of a large atom utilization, unsaturated coordination, and unique electronic structure. Single-atom sites with high activity chemically trap intermediates to suppress shuttle effects and facilitate electron transfer and redox reactions for achieving high capacity, rate capability, and conversion efficiency. Herein, single-atom catalytic electrodes design for advanced battery systems is addressed. Major challenges and promising strategies concerning electrochemical reactions, theoretical model, and in situ characterization are discussed to shed light on future research of single-atom material-based energy systems.

Stable Lithium Deposition Enabled by an Acid-Treated g-C3N4 Interface Layer for a Lithium Metal Anode

Li metal has been regarded as one of the most promising anode candidates for high-energy rechargeable lithium batteries. Nevertheless, the practical applications of the Li anode have been hampered because of its low Coulombic efficiency and safety hazards. Here, acid-treated g-C₃N₄ with O- and N-containing groups are coated on Li foil through a facile physical pressing method. The O- and N-containing groups cooperate to rearrange the concentration of Li ions and enhance the Li ion transfer. Hence, the cycle and rate performances of acid-treated g-C₃N₄-coated Li electrodes are greatly improved in symmetric cells, which show cycling stability over 400 h at 1 mA cm^-2 in ester-based electrolytes and over 2100 h in ether-based electrolytes. As for the Li//LiFePO₄ full cells, there is a high capacity retention of 80% over 400 cycles at 1 C. The full cells of Li//S in ether-based electrolytes also exhibit a capacity of 520 mA h g^-1 after 400 cycles at 1 C.

Three-Dimensional Superlithiophilic Interphase for Dendrite-Free Lithium Metal Anodes

Lithium metal is among the most promising anode candidates of high-energy-density batteries. However, the formed dendrites result in low Coulombic efficiency and serious security issues. Designing lithiophilic sites is one of the effective strategies to control Li deposition. Herein, we propose a three-dimensional lithiophilic N-rich carbon nanofiber with the decoration of ZnO granules as a protective layer for a dendrite-free lithium metal anode. Theoretical evaluation indicates the synergistic effects of lithiophilic ZnO and N-containing functional groups enhance lithium adsorption and trigger uniform deposition. With the lithiophilic interlayer, the lithium deposition overpotential is only ∼20, 50, and 74 mV at 1, 3, and 6 mA cm^-2, respectively, which are much lower than those without the functional interlayer (∼55, 130, and 238 mV). The average Coulombic efficiency of lithium stripping and plating is up to ∼97.4% (94.0% for that without the interlayer) at 0.5 mA cm^-2. Meanwhile, the Li|LiFePO₄ full cell with the superlithiophilic interlayer demonstrates a high capacity retention rate of 99.6% (91.0% for that without the interlayer) over 200 cycles at 1 C. The introduction of the lithiophilic interphase could provide a convenient strategy and guidance to design the configuration for the practical application of Li metal batteries.

Reduced-Graphene-Oxide-Guided Directional Growth of Planar Lithium Layers

Rechargeable lithium (Li) metal batteries hold great promise for revolutionizing current energy-storage technologies. However, the uncontrollable growth of lithium dendrites impedes the service of Li anodes in high energy and safety batteries. There are numerous studies on Li anodes, yet little attention has been paid to the intrinsic electrocrystallization characteristics of Li metal and their underlying mechanisms. Herein, a guided growth of planar Li layers, instead of random Li dendrites, is achieved on self-assembled reduced graphene oxide (rGO). In situ optical observation is performed to monitor the morphology evolution of such a planar Li layer. Moreover, the underlying mechanism during electrodeposition/stripping is revealed using ab initio molecular dynamics simulations. The combined experiment and simulation results show that when Li atoms are deposited on rGO, each layer of Li atoms grows along (110) crystallographic plane of the Li crystals because of the fine in-plane lattice matching between Li and the rGO substrate, resulting in planar Li deposition. With this specific topographic characteristic, a highly flexible lithium-sulfur (Li-S) full cell with rGO-guided planar Li layers as the anode exhibits stable cycling performance and high specific energy and power densities. This work enriches the fundamental understanding of Li electrocrystallization without dendrites and provides guidance for practical applications.

Advances in Artificial Layers for Stable Lithium Metal Anodes

Lithium (Li) metal is considered as the most promising anode material for rechargeable high-energy batteries. Nevertheless, the practical implement of Li anodes is significantly hindered by the growth of Li dendrites, which can cause severe safety issues. To inhibit the formation of Li dendrites, coating an artificial layer on the Li metal anode has been shown to be a facile and effective approach. This review mainly focuses on recent advances in artificial layers for stable Li metal anodes. It summarizes the progress in this area and discusses the different types of artificial layers according to their mechanisms for Li dendrite inhibition, including regulation of uniform deposition of Li metal and suppression of Li dendrite growth. By doing this, it is hoped that this contribution will provide instructional guidance for the future design of new artificial layers.

Lithium Thiosilicophosphate Glassy Solid Electrolytes Synthesized by High-Energy Ball-Milling and Melt-Quenching: Improved Suppression of Lithium Dendrite Growth by Si Doping

Due to the volatility of P₂S₅, the ambient pressure synthesis of Li₂S + P₂S₅ (LPS) has been limited to planetary ball-milling (PBM). To utilize PBM of LPS to generate a solid electrolyte (SE), the as-synthesized powder sample must be pressed into pellets, and as such the presence of as-pressed grain boundaries in the SE cannot be avoided. To eliminate the grain boundaries, LPS doped with SiS₂ has been studied because SiS₂ lowers the vapor pressure of the melt and promotes strong glass formation, which in combination allows for greater ease in synthesis. In this work, we have examined the structures and electrochemical properties of lithium thiosilicophosphate 0.6Li₂S + 0.4[xSiS₂ + 1.5(1 - x)PS_5/2], 0 ≤ x ≤ 1, glassy solid electrolytes (GSEs) prepared by both PBM and melt-quenching (MQ). It is shown that the critical current density improved after incorporating SiS₂, reaching 1.5 mA/cm² for the x = 0.8 composition. However, the interfacial reaction of MQ GSE with lithium metal introduced microcracks, which shows that further research is needed to explore and develop more stable GSE compositions. These fundamental results can help to understand the interface reaction and formation and as such can provide a guide to design improved homogeneous GSEs with SiS₂ as a glass former, which have no grain boundaries and thereby may help suppress lithium dendrite formation.

A Sustainable Solid Electrolyte Interphase for High-Energy-Density Lithium Metal Batteries Under Practical Conditions

High-energy-density Li metal batteries suffer from a short lifespan under practical conditions, such as limited lithium, high loading cathode, and lean electrolytes, owing to the absence of appropriate solid electrolyte interphase (SEI). Herein, a sustainable SEI was designed rationally by combining fluorinated co-solvents with sustained-release additives for practical challenges. The intrinsic uniformity of SEI and the constant supplements of building blocks of SEI jointly afford to sustainable SEI. Specific spatial distributions and abundant heterogeneous grain boundaries of LiF, LiN_x O_y , and Li₂ O effectively regulate uniformity of Li deposition. In a Li metal battery with an ultrathin Li anode (33 μm), a high-loading LiNi_0.5 Co_0.2 Mn_0.3 O₂ cathode (4.4 mAh cm^-2 ), and lean electrolytes (6.1 g Ah^-1 ), 83 % of initial capacity retains after 150 cycles. A pouch cell (3.5 Ah) demonstrated a specific energy of 340 Wh kg^-1 for 60 cycles with lean electrolytes (2.3 g Ah^-1 ).

Model systems for screening and investigation of lithium metal electrode chemistry and dendrite formation

The use of lithium metal as an electrode for electrochemical energy storage will provide a significant impact on practical energy storage technology. Unfortunately, the use of lithium metal is plagued with challenging chemical problems. Specifically, the formation of a solid electrolyte interphase layer and the nucleation and growth of lithium dendrites: both must be addressed and controlled in order to achieve a practically useable pure lithium metal electrode. Currently sophisticated experimental techniques and computationally expensive simulations are being used to probe these problems but these methods are arduous and time consuming which delays timely evaluation and insight into the rapidly changing field of advanced energy storage. Here, we report the use of DFT simulations of lithium nanoclusters to investigate and explore lithium metal chemistry with inexpensive computational methods to gain greater insight into electrochemical reductions and the nucleation and growth of dendrites. DME, LiTFSI, and LiFSI reduction energetics and structures with electrode effects from lithium metal are reported providing better physical description of the absolute reduction potential characterization. The electronic structure of the lithium nanoclusters were used to investigate the nucleation and growth of lithium dendrites from an ab initio perspective. The results demonstrate that kinetic processes have more control over non uniform growth than thermodynamic processes. Based on this information, a non ab initio model was created in Matlab that shows the initial stages of dendrite nucleation considering approximately 2000 atoms.

Towards practical lithium-metal anodes

Lithium-ion batteries have had a tremendous impact on several sectors of our society; however, the intrinsic limitations of Li-ion chemistry limits their ability to meet the increasing demands of developing more advanced portable electronics, electric vehicles, and grid-scale energy storage systems. Therefore, battery chemistries beyond Li ions are being intensively investigated and need urgent breakthroughs toward commercial applications, wherein the use of metallic Li is one of the most intuitive choices. Despite several decades of oblivion due to safety concerns regarding the growth of Li dendrites, Li-metal anodes are now poised to be revived because of the advances in investigative tools and globally invested efforts. In this review, we first summarize the existing issues with regard to Li anodes and their underlying reasons and then highlight the recent progress made in the development of high-performance Li anodes. Finally, we propose the persisting challenges and opportunities toward the exploration of practical Li-metal anodes.

Developing high safety Li-metal anodes for future high-energy Li-metal batteries: strategies and perspectives

Developing high-safety Li-metal anodes (LMAs) are extremely important for the application of high-energy Li-metal batteries. The recently state-of-the-art technologies, strategies and perspectives for developing LMAs are comprehensively summarized in this review.

Nitrofullerene, a C60-based Bifunctional Additive with Smoothing and Protecting Effects for Stable Lithium Metal Anode

Practical applications of lithium metal anodes are gravely impeded by inhomogeneous lithium deposition, which results in dendrite growth. Electrolyte additives are proven to be effective in improving performance but usually serve only a single function. Herein, nitrofullerene is introduced as a bifunctional additive with a smoothing effect and forms a protective solid electrolyte interphase (SEI) layer on stable lithium metal anodes. By design, nitro-C₆₀ can gather on electrode protuberances via electrostatic interactions and then be reduced to NO₂^- and insoluble C₆₀. Next, the C₆₀ anchors on the uneven groove of the lithium surface, resulting in a homogeneous distribution of Li ions. Finally, NO₂^- anions can react with metallic Li to build a compact and stable SEI with high ion transport. With a 5 mM nitro-C₆₀ additive, Li-Li symmetric cells show superior cycle stability in both carbonate and ether electrolytes, Li-sulfur batteries with a high cathode loading (10.6 mg cm^-2, 6 mAh cm^-2) can achieve improved cycle retention of 63.2% over 100 cycles in a carbonate electrolyte, and full cells paired with a high-areal-capacity LiNi_0.6Co_0.2Mn_0.2O₂ cathode (3.5 mAh cm^-2) exhibit a significantly enhanced cycle lifespan even under lean electrolyte conditions.

Self-Stabilized and Strongly Adhesive Supramolecular Polymer Protective Layer Enables Ultrahigh-Rate and Large-Capacity Lithium-Metal Anode

Constructing a solid electrolyte interface (SEI) is a highly effective approach to overcome the poor reversibility of lithium (Li) metal anodes. Herein, an adhesive and self-healable supramolecular copolymer, comprising of pendant poly(ethylene oxide) (PEO) segments and ureido-pyrimidinone (UPy) quadruple-hydrogen-bonding moieties, is developed as a protection layer of Li anode by a simple drop-coating. The protection performance of in-situ-formed LiPEO-UPy SEI layer is significantly enhanced owing to the strong binding and improved stability arising from a spontaneous reaction between UPy groups and Li metal. An ultrathin (approximately 70 nm) LiPEO-UPy layer can contribute to stable and dendrite-free cycling at a high areal capacity of 10 mAh cm^-2 at 5 mA cm^-2 for 1000 h. This coating together with the promising electrochemical performance offers a new strategy for the development of dendrite-free metal anodes.

Green in Situ Growth Solid Electrolyte Interphase Layer with High Rebound Resilience for Long-Life Lithium Metal Anodes

Lithium (Li) metal is one of the most promising anodes for the high-energy density lithium batteries. Nevertheless, it is still a great challenge to construct a dendrite-free Li anode with stable solid electrolyte interphase (SEI) by adopting environmentally friendly approaches. Herein, a green artificial Li polylactic acid (LiPLA) SEI layer with biodegradability and highly rebound resilience is fabricated via an in situ reaction to regulate Li metal plating/stripping behavior. Guided by this stable environmentally friendly LiPLA SEI, the Li anode shows excellent stability with suppressive dendrites as demonstrated by a stable cycling of 850 h in LiPLA-Li/LiPLA-Li symmetric batteries and a significant capacity retention rate enhancement of 18% in LiPLA-Li/LiFePO₄ full batteries and 25% in LiPLA-Li/LiNi_3/5Co_1/5-Mn_1/5O₂ full batteries. This proposed strategy provides a green and facile way to ameliorate the stability of the Li anode for safe and long-life lithium metal batteries.

Flexible Amalgam Film Enables Stable Lithium Metal Anodes with High Capacities

Dendrite formation is a critical challenge for the applications of lithium (Li) metal anodes. In this work a new strategy is demonstrated to address this issue by fabricating an Li amalgam film on its surface. This protective film serves as a flexible buffer that affords repeated Li plating/stripping. In symmetric cells, the protected Li electrodes exhibit stable cycling over 750 hours at a high plating current and capacity of 8 mA cm^-2 and 8 mAh cm^-2 , respectively. Coupled with high-loading cathodes (ca. 12 mg cm^-2 ) such as LiFePO₄ and LiNi_0.6 Co_0.2 Mn_0.2 O₂ , the protected hybrid anodes demonstrate significantly improved cell stability, indicating its reliability for practical development of Li metal batteries. Interfacial analyses reveal a unique plating-alloying synergistic function of the protective film, where Li beneath the film is actively involved in the electrode reactions upon cycling. Lithium amalgams enrich the alloy anode family and provide new perspectives for the rational design of dendrite-free anodes.

A Lithium-Ion Pump Based on Piezoelectric Effect for Improved Rechargeability of Lithium Metal Anode

Lithium metal is widely studied as the "crown jewel" of potential anode materials due to its high specific capacity and low redox potential. Unfortunately, the Li dendrite growth limits its commercialization. Previous research has revealed that the uniform Li-ion flux on electrode surface plays a vital role in achieving homogeneous Li deposition. In this work, a new strategy is developed by introducing a multifunctional Li-ion pump to improve the homogenous distribution of Li ions. Via coating a β-phase of poly(vinylidene fluoride) (β-PF) film on Cu foil (Cu@β-PF), a piezoelectric potential across such film is established near the electrode surface because of its piezoelectric property, which serves as a driving force to regulate the migration of Li ions across the film. As a result, uniform Li-ion distribution is attained, and the Cu@β-PF shows coulombic efficiency around 99% throughout 200 cycles. Meanwhile, the lithium-sulfur full cell paired with Li-Cu@β-PF anode exhibits excellent performance. This facile strategy via regulating the Li-ion migration provides a new perspective for safe and reliable Li metal anode.

Protecting the Lithium Metal Anode for a Safe Flexible Lithium-Air Battery in Ambient Air

The flexible Li-air battery (FLAB) with ultrahigh energy density is a hopeful candidate for flexible energy storage devices. However, most current FLAB operate in a pure oxygen atmosphere, which is limited by safety and corrosion issues from the metallic lithium anode and has thus greatly impeded the application of FLAB. Now, inspired by the protection effect of the umbrella, a stable hydrophobic composite polymer electrolyte (SHCPE) film with high flexibility, hydrophobicity, and stability was fabricated to protect the lithium anode. The SHCPE mitigated lithium corrosion and improved the capacity, rate performance, and cycle life (from 24 cycles to 95 cycles) of a battery in the ambient air. Based on the protection of SHCPE and the catalysis of MnOOH, the prepared pouch-type FLAB displayed high flexibility, stable performances, long cycling life (180 cycles), and excellent safety; the battery can bear soaking in water, high temperature, and nail penetration.

Normalized Lithium Growth from the Nucleation Stage for Dendrite-Free Lithium Metal Anodes

Inducing uniform deposition of lithium from the stage of metal crystallization nucleation is of vital importance to achieve dendrite-free lithium anodes. Herein, using experiments and simulation, homogenization of Li nucleation and normalization of Li growth can be achieved on PNIPAM polymer brushes with lithiophilic functional groups modified Cu substrates. The lithiophilic functional groups of amide O can homogenize ion mass transfer and induce the uniform distribution of Li nucleation sites. What is more, the ultra-small space between each brush can act as the channels for Li transportation and normalization growth. Owing to the synergistic effect of homogenization and normalization of electrodeposited Li, the obtained planar columnar Li anode exhibits excellent cycle stability at an ultra-high current density of 20 mA cm^-2 .

Li Alginate-Based Artificial SEI Layer for Stable Lithium Metal Anodes

Lithium metal anodes (LMAs) are critical for high-energy-density batteries such as Li-S and Li-O₂ batteries. The spontaneously formed solid electrolyte interface on LMAs is fragile, which may not accommodate the cyclic Li plating/stripping. This usually will result in a low coulombic efficiency (CE), short cycle life, and potential safety hazards induced by the uncontrollable growth of lithium dendrites. In this study, we fabricate a Li alginate-based artificial SEI (ASEI) layer that is chemically stable and allows easy Li ion transport on the surface of LMAs, thus enabling the stable operation of lithium metal anodes. Compared to bare LMAs, the ASEI layer-protected LMAs exhibit a more stable Li plating/stripping behavior and present effective dendrite suppression. The symmetric Li∥Li cells with the ASEI layer-protected LMAs can stably run for 850 and 350 h at current densities of 0.5 and 1 mA cm^-2, respectively. Additionally, the LiFePO₄∥Li full cell with the ASEI layer-protected LMA exhibits a capacity retention of about 94.0% coupled with a CE of 99.6% after 1000 cycles at 4 C. We believe that this study of engineering an ASEI brings a new and promising approach to the stabilization of LMAs for high-performance lithium metal batteries.

A Sponge-Driven Elastic Interface for Lithium Metal Anodes

The lithium (Li) metal is one promising anode for next generation high-energy-density batteries, but the large stress fluctuation and the nonuniform Li deposition upon cycling result in a highly unstable interface of the Li anode. Herein, a simple yet facile engineering of the elastic interface on the Li metal anodes is designed by inserting a melamine sponge between Li and the separator. Driven by the good elasticity of the sponge, the modified Li anode maintains a Coulombic efficiency of 98.8% for 60 cycles and is cyclable at 10 mA cm^-2 for 250 cycles, both with a high capacity of 10 mA h cm^-2. We demonstrate that the sponge can be used to replace the conventional polypropylene as a porous yet elastic separator, showing superior cycling and rate performance as well. In addition to the efficiency of the elastic interface on the cycling stability, which is further confirmed by an in situ compression-electrochemistry measurement, the porous structure and polar groups of the sponge demonstrate an ability of regulating the transport of Li ions, leading to a uniform deposition of Li and the suppression of Li dendrites in cycling.

Effects of Concentrated Salt and Resting Protocol on Solid Electrolyte Interface Formation for Improved Cycle Stability of Anode-Free Lithium Metal Batteries

The combined effect of concentrated electrolyte and cycling protocol on the cyclic performance of the anode-free battery (AFB) is evaluated systematically. In situ deposition of Li in the AFB configuration in the presence of a concentrated electrolyte containing fluorine-donating salt and resting the deposit enables the formation of stable and uniform SEI. The SEI intercepts the undesirable side reaction between the deposit and solvent in the electrolyte and reduces electrolyte and Li consumption during cycling. The synergy between the laboratory-prepared concentrated 3 M LiFSI in the ester-based electrolyte and our resting protocol significantly enhanced cyclic performances of AFBs in comparison to the commercial carbonate-based dilute electrolyte, 1 M LiPF₆. Benefitting from the combined effect, Cu∥LiFePO₄ cells delivered excellent cyclic performance at 0.5 mA/cm² with an average CE of up to 98.78%, retaining a reasonable discharge capacity after 100 cycles. Furthermore, the AFB can also be cycled at a high rate up to 1.0 mA/cm² with a high average CE and retaining the encouraging discharge capacity after 100 cycles. The fast cycling and stable performance of these cells are attributed to the formation of robust, flexible, and tough F-rich conductive SEI on the surface of the in situ-deposited Li by benefiting from the combined effect of the resting protocol and the concentrated electrolyte. A condescending understanding of the mechanism of SEI formation and material choice could facilitate the development of AFBs as future advanced energy storage devices.

Single-Atom Iron as Lithiophilic Site To Minimize Lithium Nucleation Overpotential for Stable Lithium Metal Full Battery

High lithium nucleation overpotential on a lithiophobic matrix results in uncontrollable growth of lithium dendrites and thus restricts the wide application of lithium-metal batteries. Herein, the single-atom iron in a N-doped carbon matrix (Fe_SA-N-C) is first reported as a lithiophilic site to minimize Li nucleation overpotential from 18.6 mV to a very low value of 0.8 mV. Molecular dynamics simulations quantitatively confirmed the excellent affinity between Li ions and Fe_SA-N-C in the atomic level. Induced by the homogeneously distributed Fe_SA-N in the carbon substrate, uniform and stable metallic Li plating/stripping behaviors are realized and lithium dendrite growth is greatly suppressed. The proposed strategy of using single-atom iron as a lithiophilic site to minimize lithium nucleation overpotential opens a promising avenue for solving intrinsic problems of Li-metal-based batteries.

Electrochemical Diagram of an Ultrathin Lithium Metal Anode in Pouch Cells

Lithium (Li) metal is regarded as a "Holy Grail" electrode for next-generation high-energy-density batteries. However, the electrochemical behavior of the Li anode under a practical working state is poorly understood, leading to a gap in the design strategy and the aim of efficient Li anodes. The electrochemical diagram to reveal failure mechanisms of ultrathin Li in pouch cells is demonstrated. The working mode of the Li metal anode ranging from 1.0 mA cm^-2 /1.0 mAh cm^-2 (28.0 mA/28.0 mAh) to 10.0 mA cm^-2 /10.0 mAh cm^-2 (280.0 mA/280.0 mAh) is investigated and divided into three categories: polarization, transition, and short-circuit zones. Powdering and the induced polarization are the main reasons for the failure of the Li electrode at small current density and capacity, while short-circuit occurs with the damage of the separator leading to safety concerns being dominant at large current and capacity. The electrochemical diagram is attributed from the distinctive plating/stripping behaviors of Li metal, accompanied by dendrites thickening and/or lengthening, and heterogeneous distribution of dendrites. A clear understanding in the electrochemical diagram of ultrathin Li is the primary step to rationally design an effective Li electrode and render a Li metal battery with high energy density, long lifespan, and enhanced safety.