Lithium (Li) metal is considered a promising anode material for high energy density secondary Li metal batteries because it has the highest specific energy (3860 mAh·g^-1) and lowest redox potential (-3.04 V compared to standard hydrogen electrodes. However, the development of high-performance Li metal batteries is challenging. Firstly, Li dendrites tend to grow on the surface of Li metal foil, leading to a limited anodic coulombic efficiency (CE), poor cyclability, and even explosion hazards when an internal cell short circuit occurs. Moreover, Li metal suffers from serious surface stability problems and is easily corroded by electrolytes during cycling, further resulting in low CE, thus shortening the life cycle. We have developed a Li-carbon nanotube (Li-CNT) composite microsphere via a facile molten impregnation method. The Li-CNT composite's CNT framework can suppress volume changes during the charge/discharge process and help stabilize the solid electrolyte interphase (SEI), which is typically mechanically fragile. As a result, Li-CNT shows a high specific capacity (2000 mAh·g^-1) and can significantly suppress dendrite formation by reducing the current density, resulting in enhanced safety and cycling stability. However, the large specific surface area of the Li-CNT microspheres also enables increased reaction with the air and the electrolyte. A passivation layer is critical for the practical application of Li-CNT during the electrochemical cycling and manufacturing process. LiF is an important component of SEI in the liquid electrolyte system, and a uniform and dense LiF-rich SEI film can enable stable cycling. Moreover, LiF has been widely used as the preferred coating material to protect Li metal anodes through different methods. In this study, we improved the Li-CNT composite stability by constructing a uniform LiF-rich protecting layer on the surface through in situ polymerization of 4-fluorostyrene. The F functional group of 4-fluorostyrene, which is a lithiophilic group, reacts with the Li-CNT to produce a uniform LiF-rich layer on the surface of the Li-CNT via a facile and scalable liquid-phase reaction. The resulting passivation layer effectively suppresses the Li-CNT corrosion by the electrolyte and air, leading to better environmental and electrochemical stability. Consequently, after exposure to dry-air with a dew point -40 ℃ for 24 h, the specific capacity of the surface passivated Li-CNT is still as high as 1129 mAh·g^-1, corresponding to a capacity retention of 52.85%. When the surface passivated Li-CNT is paired with a LiFePO₄ cathode (the capacity ratio of cathode and anode is 1 : 6), a prolonged lifespan of over 280 cycles at 0.5C was reached, corresponding to a CE of 97.7%. The in situ polymerization passivation is simple and easy to be scale up; thus, it is a promising method for developing Li metal anodes towards the practical Li metal batteries.

Although traditional graphite anodes ensure the cycling stability and safety of lithium-ion batteries, the inherent drawbacks, particularly low theoretical specific capacity (372 mAh·g^-1) and Li-free character, of such anodes limit their applications in high energy density battery systems, especially in lithium-sulfur and lithium-air batteries. Lithium metal has been considered as one of the best next-generation anode materials due to its extremely high theoretical specific capacity (3860 mAh·g^-1) and low redox potential (-3.04 V vs. the standard hydrogen electrode). The first generation of commercial rechargeable lithium metal batteries were developed by Moli Energy in the late 1980s and were not widely used due to several problems, including low coulombic efficiency, poor cycle stability, and safety hazards. These problems associated with the Li metal anode are mainly caused by lithium dendrite growth, electrode volume changes, and interface instability. During the charge and discharge processes, Li deposition is not uniform across the electrode surface. Due to the low surface energy and high migration energy of Li metal, dendrites are preferentially formed during Li deposition. These dendrites proceed to grow with successive battery cycling, penetrate the separator, and eventually reach the cathode, thereby causing short circuits and thermal runaway. Additionally, the growth of the lithium dendrite is inherently correlated with the reaction interface structure, and dendrite growth results in inhomogeneity of the SEI (solid electrolyte interface) which is inevitably formed on the Li metal surfaces. Moreover, the volume change of lithium metal anodes is of importance, particularly during battery cycling and Li stripping/deposition processes which make the SEI layers considerably unstable. SEI layers usually cannot withstand the mechanical deformation caused by volume changes; such layers continuously break and repair during cycling and consume large amounts of the electrolyte. Additionally, some Li dendrites could break and become wrapped by SEI layers to form electrically isolated "dead" Li, which results in the loss of active Li in the Li metal anode. All these factors are responsible for the failure of Li metal anodes. Herein, recent investigations on the failure mechanisms of lithium metal anodes are reviewed and summarized, including the formation of SEI layers on the surface of Li metal anodes, the behavior and mechanism of lithium dendrite growth, and the mechanism of "dead" lithium formation. Additionally, some advanced characterization techniques for investigating lithium metal anodes are introduced, including in situ tools, cryo-electron microscopy, neutron depth analysis technology, and solid state nuclear magnetic resonance technology. These techniques enable researchers to gain in-depth insights into the failure mechanisms of Li metal anodes.

Solid-state Li metal batteries are considered promising next-generation energy storage systems due to its exceptional advantages in terms of safety and high energy density. The continuous process on the development of solid-state fast ionic electrolytes enables the solid-state battery to operate at room temperature. Among these, sulfide-based solid electrolytes have attracted significant attentions due to their extremely high ionic conductivity, excellent deformability, and mild low-temperature processability. However, the full demonstration of practical batteries remains challenging due to the slow lithium-ion transport kinetics at working solid-solid interfaces. The sluggish interfacial transport kinetics mainly result from the poor solid-solid contacts, resulting in poor battery performance. Especially for solid-state pouch cells, the high local current due to the poor contact is amplified by the high working current, leading to rapid failure. Constructing fast ion transport paths between the Li metal anode and solid electrolyte interface is key for the practical application of solid-state batteries. Here a simple protocol was developed to realize fast ionic transportation by wetting the solid electrolyte/Li metal anode interface with localized high salt concentration liquid electrolyte. First, 3.5 mmol lithium trifluoroalfonylimide (LiTFSI) was added into 1, 1, 2, 2-tetrafluoroethyl-2, 2, 3, 3-tetrafluoropropyl ether (HFE) and dimethoxyethane (DME) mixed solvent, and stirred to obtain uniformly dispersed localized high-concentration liquid electrolyte, denoted as HFE-DME LiTFSI. The fluidity of liquid electrolyte ensures sufficiently conformal contacts between lithium anode and liquid electrolyte, as well as solid-state electrolyte and liquid electrolyte. Thus, fast ion transportation channels were constructed between the solid electrolyte and Li metal anode by wetting HFE-DME LiTFSI at a concentration of 3.0 μL·cm^-2. After liquid phase therapy, the interfacial resistance of solid-state Li|Li₄Ti₅O₁₂ (LTO) pouch cell rapidly reduced from 4366 to 64 Ω·cm^-2 and even lower than the cell that was pressed at 3 MPa in the assemble process (340 Ω·cm^-2). This suggests that the ion transport kinetics are significantly improved by liquid phase therapy. Therefore, the solid-state Li metal pouch cell with dimensions of 30 mm × 30 mm showed excellent cycling performances with specific capacities of 107 and 96 mAh·g^-1 at 0.1C and 0.5C, respectively. Furthermore, the solid-state Li-S pouch cell delivered capacities of 1100 and 932 mAh·g^-1 at 0.01C and 0.02C, respectively. This study demonstrates the effectiveness of the novel liquid phase therapy to construct fast ionic transportation channels, which providing an effective strategy for the practical application of solid-state Li metal pouch cells.

The Li metal anode is considered the most promising anode for next-generation high energy density batteries owing to its high theoretical capacity and low electrode potential. The development of batteries with high energy density is essential to meet the growing demand for energy storage devices in the modern world. However, the Li metal anode has operational problems. The high activity of Li causes dendritic growth during the cycling process, which leads to the cracking of the SEI (solid-electrolyte interphase), increased side reactions, and formation of dead Li. Furthermore, if the growth of Li dendrites is left uncontrolled, it can penetrate the separator and create a short-circuit accompanied by thermal runaway. Additionally, the complete utilization of active Li is challenged by the infinite volume expansion of the Li anode. To improve the application scope of Li metal batteries, it is imperative to develop advanced strategies for inhibiting Li dendritic growth, enhancing the stability of the SEI, reducing the accumulation of dead Li, and buffering the volume expansion. Understanding the mechanisms and models of Li nucleation and growth provides insight into solving these problems. This review summarizes some of the important models of Li nucleation and growth such as the surface nucleating model, charge-induced model, SEI model, and deposition/dissolution model. These models aid comprehension of the Li nucleation and growth process under various conditions. This review also discusses the strategies explored in the literature for improving the electrodes (such as three-dimensional (3D) matrix), electrolyte, SEI, and separator to realize uniform deposition of Li and improved utilization of Li. The 3D matrix strategy for improving the electrode design explores various matrices including graphene-based, carbon fiber-based, porous metal-based, and powder-based for buffering the volume expansion and reducing the local current density. To improve the electrolyte, concentrated lithium salts and functional additives are employed to stabilize the SEI and inhibit dendritic growth by regulating the chemical composition of SEI and inducing the deposition of Li. With respect to improving the design of the SEI, strategies for the construction of inorganic or organic components with high ionic conductivity and stable structure are explored for even distribution of Li ions and to avoid SEI rupture. This can reduce electrolyte consumption and dead Li formation. The modification of the separator by functional nanocarbon layer can control the direction of dendritic growth, thereby preventing the penetration of dendrites into the separator and achieving a uniform Li deposition layer. Finally, all solid state Li metal batteries (ASSLMBs) are discussed that utilize ceramic and polymer electrolytes owing to high safety of the solid state electrolyte. Therefore, reducing the interfacial resistance and suppressing dendritic growth between the Li anode and the electrolyte is key for the practical applications of ASSLMBs. Overall, this review provides a summary and outlook for promoting the practical applications of Li metal batteries.

In past decades, lithium-ion batteries (LIBs) were the dominant energy storage systems for powering portable electronic devices because of their reliable cyclability. However, further increase in the energy density of LIBs was met by a bottleneck when low-specific- capacity graphite was used at the anode. Li metal has long been regarded as the ideal anode material for building the next high-energy-density batteries due to its ultrahigh capacity of 3860 mAh·g^-1, which is ten times higher than that of graphite. However, using Li metal as an anode in rechargeable batteries is challenging due to its high uncontrolled volume expansion and aggressive side reactions with liquid electrolytes. In this study, we demonstrate the effect of a three-dimensional (3D) framework with enriched fluorinated sites for Li metal protection. This framework is obtained via a facile integration of down-sized fluorinated graphite (CF_x) particles into Li⁺ conducting channels. Thermogravimetry, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy results show that Li⁺ conducting channels rich in lithium fluoride (LiF) are formed in situ across the embedded CF_x particles during the initial lithiation process, leading to fast Li⁺ transfer. Scanning electron microscopy results show that residual CF_x particles could act as high-quality nucleation sites for uniform Li deposition inside the framework. These features could not be achieved with a 2D structure consisting of large CF_x flakes, due to the limited Li⁺ transfer paths and low utilization ratio of CF_x for conversion into LiF-based solid electrolyte interphase (SEI) layers. Consequently, better performance of Li metal anodes in a 3D framework with enriched fluorinated sites is demonstrated. Stable Li plating/stripping over 240 cycles is obtained at a current density of 0.5 mA·cm^-2 for a fixed capacity of 1 mAh·cm^-2 by maintaining a voltage hysteresis below 80 mV. Improved Li-LiFePO₄ full cell performance with a practical negative/positive capacity ratio of 3 is also demonstrated. These results show the rational combination of well-developed 3D Li⁺ transfer channels and enriched fluorinated sites as an optimized interfacial design beyond the single use of a 2D fluorinated interface, giving new insight into the protection of Li metal anodes in high-energy-density batteries.

Lithium metal batteries (LMBs) are representative systems for high-energy-density batteries. The design of LMBs with high capacity and high cycle stability is imperative. However, the development of LMBs is hindered by typical interface-related problems such as lithium dendrite growth, incompatible separator interfaces, and unstable cathode interfaces because of the inhomogeneous ionic flux and composition distribution. The intrinsic instability significantly hinders electron/ion transfer at the interface, causing serious issues such as dendrite growth, volume changes, low coulombic efficiency, dead lithium, interface deterioration, capacity degradation, and loss of safety. Metal-organic frameworks (MOFs) are organic-inorganic hybrid materials with a stable highly porous structure, which can allow for highly efficient gas adsorption, separation and purification, catalysis, etc., in addition to facilitating their application in nanomedicine and other fields. In recent years, MOFs have attracted much attention in the field of LMBs as a possible solution to the typical interface problems abovementioned. The porous structure and open metal sites (OMs) of MOFs provide an excellent interface structure for uniform and high ionic conductivity. As additional bonus, the stable structure provides high mechanical strength with different functional groups and metal sites, resulting in significant versatility of functionality for interface stabilization. MOFs are usually synthesized by hydrothermal/solvothermal, microwave-assisted, electrochemical, and spray-drying methods. The excellent properties of MOFs have prompted researchers to pursue their rational design and modification. Much progress has been made in this direction, and exemplary investigations have been performed to solve the abovementioned interfacial problems encountered with LMBs. Consequently, metallic lithium deposition frameworks, artificial solid electrolyte interface films, electrolyte additives, separator materials, cathode materials for lithium-sulfur batteries, and lithium-air batteries have been developed. However, there is a long way to go before the commercialization of batteries based on MOF materials. In practical, more complex electrochemical reactions occur at the lithium-metal interface, and the operating conditions (temperature, over charging/discharging, external stress, etc.) vary widely. Moreover, MOFs as electrode materials have intrinsic drawbacks, including structural collapse, pore blockage, and low inherent conductivity during the cycles. Based on these interfacial challenges, in LMBs, it introduces the structural characterization and optimization of MOFs and the key chemical components that determines the MOFs of structure (central atom, organic ligand, etc.). Subsequently, we summarized the growth mechanism of lithium dendrites and discussed the applications of MOFs and their derivatives to battery cathodes, separators, anodes, and electrolytes.The manuscript contents would be a guide to solve the problem of unstable interfaces in LMBs by the use of MOFs. Furthermore, the prospects and rational design of MOF-based materials are discussed.

With the booming growth market of electric vehicles and portable electronics, high-energy-density rechargeable lithium ion batteries are being extensively used to advance high-end devices. Lithium-ion batteries with graphite anodes approach the ceiling in energy density, but they cannot satisfy the current demand. Among the next-generation electrodes, lithium metal anodes are strong candidates because of their high theoretical capacity and the most negative electrochemical potential. However, lithium metal batteries have been abandoned because of their poor safety resulting from the growth of lithium dendrites during lithium deposition. Although several strategies have been proposed to suppress the generation of lithium dendrites as well as the side reactions between active lithium and the electrolyte, lithium metal anodes have not been practically applied so far. Various studies have been conducted on the factors influencing lithium deposition, with the aim of understanding the growth behavior of lithium dendrites. The electrolyte plays a crucial role in the performance of the working Li metal anode. In this study, a unique battery system is proposed to realize columnar lithium deposition, which is convenient for obtaining the length and diameter of lithium deposits. The influence of different electrolytes on lithium deposition was investigated by comparing the length-diameter (L/D) ratio of the lithium deposits in two kinds of electrolytes (1.0 mol·L^-1 LiPF₆-ethylene carbonate/diethyl carbonate (EC/DEC, 1 : 1 by volume) and 1.0 mol·L^-1 LiPF₆-5% (volume fraction) fluoroethylene carbonate (FEC)-EC/DEC (1 : 1 by volume)). The morphology of the lithium deposits was strongly affected by the electrolyte composition. In the electrolyte with the FEC additive, the diameter of columnar lithium increased from 0.3-0.6 μm to 0.7-1.3 μm, while the L/D ratio decreased from 12.5 to 5.6. The small L/D ratio can reduce the reactive area between the lithium metal anode and the electrolyte, which is beneficial for achieving high lithium utilization and a long lifespan. To probe the origin of this influence, the surface chemistry of the cycled lithium metal anode was investigated by X-ray photoelectron spectroscopy. The FEC additive can increase the proportion of lithium fluoride (LiF) in the solid electrolyte interphase, which is conducive for the rapid diffusion of lithium ions. As a result, fewer nucleation sites are formed, providing more space for the growth of lithium cores with a large diameter. Therefore, the addition of FEC leads to a decrease in the L/D ratio of columnar lithium.

Lithium is a promising anode material for next-generation high-energy-density rechargeable batteries owing to its high specific capacity, low density, and low electrochemical reduction potential. However, dendrite growth during cycling impedes its practical application and causes safety hazards. Extensive research has been conducted to obtain dendrite-free safe Li anodes with an extended cycle life by electrolyte or anode surface modification. In previous studies, the symmetrical Li/Li cell test was widely applied to evaluate the effect of various Li anode modification methods on the cycle stability and Li deposition overpotential. However, a general criterion has not yet been established to identify the short circuit in Li/Li cells. Some researchers have even made incorrect conclusions based on the Li/Li cycling data. The most common misjudgment is the ignorance of short circuit signals and mixing up of soft short circuit and normal potential decrease caused by electrode activation. In some studies, the fractal voltage signals were attributed to the unstable activation process of the symmetrical cell. Therefore, this study uses an in situ optical cell to demonstrate that a short circuit caused by the contact of dendrites from two opposite electrodes can cause a sudden drop in cell voltage to certain extent. According to the reversibility of the voltage, the short circuit induced by dendrite growth can be classified into unrecoverable hard short circuits and recoverable soft short circuits. Typical short circuit data were summarized and described to establish a rule to determine the different types of short circuits. The voltage profiles provide characteristic signals to distinguish between the soft short circuit, hard short circuit, and cell activation processes in symmetrical cells. Furthermore, this study provides a reference for identifying dendrite growth and cell short circuits, which is important for confirming the practical effect of different modification methods.

Li metal batteries (LMBs) have attracted worldwide attention in recent years with a focus on the extremely high theoretical energy density of 3580 Wh·kg^-1 (Li-O₂) and 2600 Wh·kg^-1 (Li-S), which benefit from the highest specific capacity (3860 mAh·g^-1) and the lowest negative potential (-3.04 V) of Li metal anodes. However, further development and practical applications are hindered by the formation of Li dendrites and a large volume expansion, which not only lowers the coulombic efficiency but also leads to many security risks, such as internal short circuits, fires, and even explosions. In this study, we selected a low-cost and commercial carbon fiber cloth (CC) as a 3D framework for accommodating Li metal and relieving the volume expansion during the Li plating/stripping process. In addition, lithiophilic SnS₂ nano-sheet arrays were grown on the surface of carbon fiber cloth via a one-step method. The SnS₂ arrays can be partially converted to Li-Sn alloy and Li₂S components during the Li plating process. The as-formed Li-Sn alloy can provide reversible sites for further Li deposition and improve the electrochemical kinetics process. As a typical component of the solid electrolyte interface (SEI), Li₂S can promote Li⁺ migration at SEI and ensure a homogeneous distribution of Li⁺-flux near the electrode surface, thereby reducing the overpotential of Li deposition and suppressing the formation and growth of Li dendrites. Meanwhile, the 3D carbon skeleton can also reduce the local current density of the electrode because of its high specific surface area to ensure uniform Li deposition. Benefiting from the design of the combination bulk and the surface, the composite SnS₂@carbon fiber cloth (SnS₂@CC) demonstrated excellent prospects for practical applications. Upon pairing with Li foils, the SnS₂@CC electrode displayed stable cycling performance with improved coulombic efficiency (> 98%) over 100 cycles at 1.0 mA·cm^-2/5.0 mAh·cm^-2. After loading 10 mAh·cm^-2 Li metal, the composite Li metal anode could run over 400 h with a low overpotential of 60 mV at a current density of 1.0 mA·cm^-2, even when the current density was increased to 2.0 mA·cm^-2; additionally, a low overpotential of 85 mV could also be maintained over 350 h, manifesting one of the most stable composite Li metal anodes to date. Moreover, when the composite Li metal anode was assembled with a LiFePO₄ cathode, the full cells exhibited a high initial specific discharge capacity of 160.6 mAh·g^-1 and high cycling stability. At a rate of 2.0C, the cell showed a high capacity retention of 80.6% after 500 cycles.We believe that the lithiophilic SnS₂@CC composite electrode offers a simple and effective strategy to suppress dendritic Li growth and relieve the volume change during the charging/discharging process.

Improvement in the energy density of conventional lithium-ion batteries (LIBs), based on the intercalation-extraction chemistry of graphite and transition metal layered oxides, has apparently lagged behind the advances in consumer electronics and electric vehicles. Secondary Li-metal batteries (LMBs), utilizing metallic Li as the anode material, have incomparable advantages in terms of energy density due to their high specific capacity (3860 mAh·g^-1) and low redox potential (-3.04 V vs. standard hydrogen electrode) of Li metal. Irrespective of whether Li anodes are coupled with intercalation-type cathodes (e.g. LiFePO₄, LiCoO₂, LiNi_xCo_yMn_zO₂, etc.) or conversion-type cathodes (S, O₂), the energy density of LMBs is much higher than that of traditional LIBs, which should solve the range concern of electric vehicles. However, the intrinsically high reactivity between metallic Li and organic electrolytes could induce the formation of a solid electrolyte interface (SEI). The heterogeneous SEI, consisting of a flexible organic outer layer and a brittle inorganic inner layer, suffers from repeated rupture and regeneration due to infinite volume expansions associated with Li deposition and dissolution reactions. Meanwhile, Li is preferentially deposited on the "hot sites" and is stripped from the root of sediments, resulting in uncontrolled dendrite growth during charging and formation of electrochemically isolated Li ("dead" Li) during discharging. Thus, the Columbic efficiency of Li metal full cells is greatly limited by interfacial side effects and continuous loss of active Li, especially in conventional carbonate-based electrolyte, viz. 1 mol·L^-1 LiPF₆-EC/DEC (ethylene carbonate/diethyl carbonate), which impedes the large-scale employment of Li metal batteries. Recently, novel electrolytes with high or localized-high salt concentrations have attracted considerable attention because of their unique physiochemical properties and excellent electrochemical performance. In high-concentration electrolytes, the reduction in the population of free solvent molecules inhibits irreversible electrolyte decomposition at the electrode-electrolyte interface. In localized-high-concentration electrolytes, the introduction of a dilute reagent retains the desired solvation structure, while improving the physicochemical properties (conductivity and viscosity) of the electrolyte. Herein, we systemically review the latest progress in high-concentration and localized-high-concentration electrolytes for use in Li metal batteries. The solvation chemistry structure, physicochemical properties, and interfacial-stabilizing mechanisms are analyzed in detail, and special attention is devoted to their superior interfacial compatibility with Li metal anodes. Finally, we briefly clarify the current problems associated with the research of high-concentration and localized-high-concentration electrolytes from the viewpoints of basic scientific research and practical applications, and some possible solutions are provided to further pave the way to practical Li metal batteries.

Lithium ion batteries have been widely used in the fields of portable energy storage devices and electric vehicles due to their high energy density and high safety, and have a profound impact on modern society. However, the frequent occurrence of battery fire and explosion accidents has caused widespread concern of thermal runaway and thermal safety issues. Many reviews have reported the measures to mitigate thermal runaway of lithium ion batteries. Due to the use of graphite with a smaller capacity as the negative electrode material, the specific energy of lithium ion batteries has approached the theoretical limit, and there is an urgent need to develop more efficient electrode materials to meet the growing demand of the energy storage market. Lithium metal anode has smaller density, higher theoretical capacity and lower potential, which is the ideal anode material for the next generation of high energy density battery system. However, the high reactivity of lithium will cause uncontrollable lithium dendrite growth during the cycle, which may penetrate the separator and cause internal short circuit of the battery, and then cause thermal runaway, fire and even explosion. Therefore, the thermal runaway of lithium metal batteries are more complicated and serious, which hinders the commercial application of batteries. Aiming at the thermal runaway problem of lithium metal batteries, this article first introduces the causes of thermal runaway, which are mainly uncontrollable exothermic reactions caused by internal short circuits. The basic process of thermal runaway is divided into three stages. By analyzing the three characteristic temperatures and heating rate, it is proved that improving the thermal stability of electrolyte and seperator can alleviate thermal runaway. Then we investigated the influence of thermodynamics on the nucleation and growth of lithium dendrites, revealing the dual effects of temperature, and proving that a uniform thermal field is beneficial to obtain uniform lithium deposition and improve battery cycle performance and safety. Secondly, a variety of strategies to improve battery thermal safety are reviewed at the material level. In terms of liquid electrolytes, the development of non-flammable electrolyte systems includes the use of flame-retardant electrolytes and ionic liquid electrolytes with lower flammability. In addition, high-concentration electrolyte and local high-concentration electrolyte can change the solvation structure of lithium ions, and improve safety by reducing the number of free solvent molecules. In terms of separators, high thermal stability separators and thermal response separators with thermal shutdown function have been developed. The flame-retardant separators can release flame retardants to inhibit combustion. In addition, the new intelligent separators have dendrite detection, early warning and elimination functions, which effectively improve the safety and cycle life of the battery. In terms of solid electrolytes, thermally responsive polymer electrolytes have been developed to avoid thermal runaway through the strain function of polymer materials. Finally, further research on the thermal runaway of lithium metal batteries in the future is prospected.

The applications of lithium-ion batteries have been limited because their energy density can no longer meet the requirements of an emerging energy society. Lithium metal batteries (LMBs) are being considered as potential candidate for next-generation energy storage systems owing to the high theoretical specific capacity and low electrochemical potential of lithium metal. However, the commercialization of LMB is limited due to several challenges, such as uncontrollable formation of dendrites, unstable solid electrolyte interface, and infinite anode volume change, which can lead to grievous catastrophe. In this study, several typical mechanisms of lithium dendrite formation and growth are summarized. The results suggest that a smaller current density, greater Li⁺ transference number, higher mechanical strength of the electrolyte, and a more homogeneous distribution of Li⁺ on the substrate are conducive to the uniform deposition morphology of lithium metal. In view of these results, combined with the researches on LMBs conducted in recent years, composite anodes can be summarized into three level from internal to external. (ⅰ) Internal composite of lithium metal anode: the scaffolds composited with lithium metal are classified as non-conductive (NC), electron-conductive (EC), ion-conductive (IC), and mixed ion and electron-conductive (MIEC) scaffolds. Composited with NC scaffolds, the tip effect can be weakened through the interaction between polar functional groups and Li⁺. The composite of lithium metal and EC scaffolds can effectively reduce the local current density, while IC scaffolds can increase the ion flux. However, the performance of LMBs may be hindered by the insulation of electrons or Li⁺ at high rates. In comparison, MIEC scaffolds can provide fast ion/electron transfer channels for the deposition or dissolution of lithium metal, which is beneficial for the electrochemical performance of LMBs even at high rates. (ⅱ) Internal composite of LMB: Compared with liquid electrolytes, solid-state electrolytes (SSEs) and quasi-solid-state electrolytes are much safer. However, their interfacial contact with lithium metal anodes has been seriously criticized. Lithium metal anodes can be composited with SSEs or quasi-solid-state electrolytes to optimize the interface contact performance and reduce the interface resistance, thereby promoting the development of solid-state batteries. (ⅲ) Composite of internal environment and external operating conditions: Composited with external physical fields, such as electric fields, magnetic fields, and temperature fields, the distribution of Li⁺ can be homogeneous and the initial nucleation process can be regulated. Overall, this review summarizes several composite anodes that have greatly optimized the performance of LMBs and highlights the potential of multi-level composites for applications in lithium metal anodes.

Solid-state batteries have garnered significant attention, owing to their high safety and improved energy density. Among various solid-state electrolytes (SSEs), garnet-type SSEs are promising for application in solid-state batteries, owing to their high ionic conductivities (10^-4–10^-3 S·cm^-1) at room temperature and excellent stability against Li metal. However, the poor contact between the rigid ceramic and Li metal will result in high interfacial impedance and uneven lithium ion flux during cycling. Consequently, this will lead to rapid dendrite penetration along the grain boundary and eventual short circuit. Herein, inspired by the unique H⁺/Li⁺ exchange reaction of the garnet electrolyte, we propose a facile and efficient metal salt aqueous-solution-based strategy to construct an in situ lithiophilic ZnO layer on the garnet surface without employing any specific apparatus. A Zn(NO₃)₂ aqueous solution was selected to modify the garnet surface. Within one minute, LiOH spontaneously formed as a result of the H⁺/Li⁺ exchange reaction reacted with Zn(NO₃)₂ to produce homogeneous precipitates. After heat treatment, a lithiophilic ZnO layer was obtained. This was verified by the results of X-ray diffraction and attenuated total reflection Fourier transform infrared spectroscopy analyses. Furthermore, combined with scanning electron microscopy (SEM) images and corresponding elemental mapping, it was proved that a thin in situ interlayer can be successfully deposited on the garnet surface using our strategy. Moreover, the deposited ZnO nanoparticles were uniformly and densely distributed on the garnet surface. In the presence of the introduced layer, the wettability of the garnet-type SSE with molten Li was greatly improved. The introduced ZnO nanoparticles reacted with molten Li to form a LiZn alloy, achieving a tight and continuous contact at the Li–garnet interface, thereby greatly reducing the interfacial impedance to ~10 Ω·cm². In the case of the untreated SSE in contact with the molten Li, the cross-sectional SEM image shows obvious gaps at the interface, indicating poor contact with Li. This resulted in a large interfacial resistance of up to 1350 Ω·cm². Moreover, the slow ion transport at the interface reduces the capacity of the battery, and the uneven Li ion flux shortens the life of the cell. With a modified layer, the formed LiZn alloy interphase acting as a mixed ionic and electronic conductive interlayer ensures a uniform Li ion flux at the interface and an appreciable electrochemical performance. Symmetric Li cells with modified garnet-type electrolytes can achieve long cycling stability for approximately 1000 h at a current density of 0.1 mA·cm^-2 at room temperature (RT). The quasi solid-state batteries with LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) or LiFePO₄ cathodes can cycle stably for over 100 cycles at RT.

Li is highly attractive anode material for next-generation high-energy-density batteries, such as Li-air, Li-sulfur, and solid-state Li-based systems because of its exceedingly low electrode potential (-3.04 V vs the standard hydrogen electrode) and ultra-high theoretical capacity (3860 mAh-g^-1). However, Li metal anodes and Li-based batteries are plagued by issues, including unstable solid electrolyte interface (SEI), dead Li formation, and uncontrollable dendritic growth. These limitations result in low cycling stability and could induce short circuits, thermal runaway, and safety hazards. In recent years, a variety of efficient strategies have been proposed to alleviate the challenges faced by Li anodes. For example, the design of Li-free anodes (with Li supplied from the lithiated cathode) or Li-composite anodes has attracted significant attention. Their population can be ascribed to the use of non-excessive Li metal that could be potentially safer and easier to produce. In Li-free and Li-composite anodes, the initial nucleation sites play a crucial role in influencing the subsequent Li electroplating behavior. Stable, homogenous Li electrodeposition is crucial for improving Coulomb efficiency and inhibiting dendrite formation. Moreover, it is also desirable to explore the nucleation and growth mechanism of Li metal on substrates or current collectors. Therefore, in this article, we aim to provide an overview of the mechanism of Li nucleation and strategies to enhance Li metal batteries via substrate modification. The mechanisms of Li nucleation are discussed in terms of nucleation-driven forces and the relation between nuclei size/distribution and overpotential/current density. Heterogeneous nucleation and Chazalviel space charge models are introduced to describe the deposition behaviors of Li in the initial nucleation stage. In the heterogeneous nucleation process, the formation of Li nuclei and its kinetics depend on the nucleation barrier, which correlates with the properties of substrates, such as their crystal structure, lattice matching, facets, and defects. The space charge model can be applied to low-concentration electrolytes or rapid Li deposition, where the decrease in ion concentration on the electrode surface induces a localized space charge and polarized electric field. This subsequently affects the microstructure and morphology of the deposited Li. After discussing the nucleation mechanism and substrate effect, strategies to stabilize nucleation and suppress dendrite are highlighted, such as three-dimensional frameworks, heterogeneous crystal nuclei, Li storage buffer layers, electric field effects, and lattice matching engineering. Information gained from the perspective of Li nucleation and the substrate effect might enlighten the development of strategies to upgrade metallic Li anodes for application in Li-based batteries.

In the early 1990s, Sony launched the first commercial lithium ion battery (LIB), which achieved great success in energy storage systems. The current commercially used insertion anode, graphite, is approaching its capacity limit (~372 mAh·g^-1), and is inadequate to satisfy the ever-increasing energy demand for power grids and large-scale energy storage systems. In order to address this challenge, lithium metal anodes have been the focus of considerable research effort in recent years, and are regarded as the most promising anode materials because of their extremely high theoretical capacity (3860 mAh·g^-1), lowest electrode potential (-3.04 V vs. standard hydrogen electrode), and low density (0.534 g·cm^-3). For example, the theoretical energy densities of lithium-sulfur batteries and lithium-air batteries are as high as 2567 and 3505 Wh·kg^-1, respectively. However, the uncontrollable dendrite growth during cycling leads to low coulombic efficiency and puncture of the separator, causing a short circuit or even explosion of the battery, thereby seriously hindering the development of the lithium metal anode. Many solutions have been proposed to inhibit dendrite growth, including the use of electrolyte additives, solid electrolytes, and artificial protective films. During charging and discharging, the solid electrolyte interphase (SEI) plays an important role in lithium metal anodes. However, the infinite volume changes of the electrode during plating/stripping processes result in breakage of the SEI film, which continuously consumes the electrolyte and lithium metal. Designing an artificial interface on the surface of lithium metal anodes has been considered as a simple and efficient strategy to control lithium deposition behavior, and is achieved by precoating a protective layer on the surface of lithium metal. An ideal artificial protective film should possess high ionic conductivity, chemical stability, and excellent mechanical strength, in order to prevent side reactions between lithium metal and the electrolyte and realize dendrite-free lithium metal anodes with a long cycle life and high coulombic efficiencies. In this paper, the research progress on artificial protective films for lithium metal anodes in recent years is reviewed. Further, the structural characteristics and preparation methods of various protective films are introduced in detail, including polymer protective films, inorganic protective films, organic-inorganic composite protective films, and alloy protective films. The mechanisms of various protective films toward the suppression of dendrite growth are summarized. Existing challenges and future research directions are also proposed, which together provide a reference for promoting the use of lithium metal in high-energy batteries.

Conventional lithium-ion batteries with graphite anode have gradually ceased satisfying demand for the rapid development of modern electric commodities, such as portable electronic devices and electric vehicles. Therefore, metallic lithium is considered the ultimate alternative anode material for future high-energy-density lithium batteries because of its excellent properties, including the highest theoretical capacity and lowest potential of available materials as well as its low density. However, research on lithium metal anodes in traditional liquid batteries has encountered impediments. Numerous studies have shown that lithium dendrites, dead lithium, solid electrolyte interphase problems, and the correlating safety hazards are the main hindrances to the practical application of liquid-based lithium metal batteries. For solid-state batteries, the challenges of lithium metal anodes continue to grow. Studies on the mechanical, thermal, chemical, and electrochemical stability of solid-state electrolyte and lithium metal anode indicate that, unlike early recognition, solid-state lithium metal batteries remain far from commercialization. Unexpected issues like lithium growth along crystal boundaries, mixed-conductivity interphase generation, and interfacial contact losses have emerged that complicate the solid-state lithium metal battery. To achieve practically applicable lithium metal anodes, it is necessary to deepen our understanding of the basic scientific issues. This review systematically discusses the electrode behaviors of lithium metal and the corresponding electrode characterization techniques at multiple spatial scales. First, the basic science and technology issues of lithium metal anodes at different scales are reviewed. Lithium electrodeposition behaviors from the atomic to the macroscale are divided into ion transportation, deposition, nucleation, crystallization, expansion and growth. Various issues are also categorized among different characteristic scales. Second, advanced characterization techniques for all spatial scales are reviewed in light of recent works. Finally, the technical characteristics of various characterization techniques from the atomic to macroscale are analyzed. Features and possible directions of improvement of various characterization techniques used to examine lithium metal anodes in solid-state batteries are highlighted. In situ observation has become a common requirement for battery characterization as it can connect macroscale phenomena to microscale mechanisms. Meanwhile, non-damaging detection techniques have faced growing demand because of the urgent need to understand the complete actual reactions at the bulk and interfaces of solid-state electrolytes and lithium anodes. The combination of techniques for different scales should provide comprehensive information to characterize lithium metal anodes and identify reasonable mechanisms for their behaviors.

Commercialization of high-energy rechargeable batteries can promote the rapid development of portable electronics and electric vehicles. Li metal batteries (LMBs) have attracted considerable attention owing to their high theoretical energy density. Li metal anodes (LMAs) used in LMBs suffer from the disadvantages of high reactivity, interface instability and dendrite growth, which impede the practical development of the LMBs. Coulombic efficiency (CE), which depends on the type of electrolyte used, is one of the key parameters for evaluating the reversibility of battery systems. Herein, we use atomic force microscopy (AFM) to study the initial plating stages and growth of the lithium metal in different electrolytes, such as 1 mol·L^-1 lithium hexafluorophosphate (LiPF₆)-ethylene carbonate/dimethyl carbonate (EC/DMC, 1 : 1, V/V), 1 mol·L^-1 LiPF₆-EC/DMC (1 : 1, V/V) + 5% (mass fraction, w) fluoroethylene carbonate (FEC), 1 mol·L^-1 lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-1, 3-dioxolane/dimethoxyethane (DOL/DME, 1 : 1, V/V) + 2% (w) lithium nitrate (LiNO₃), and 4 mol·L^-1 lithium bis(fluorosulfonyl)imide (LiFSI)-DME, and further investigate the correlation between the CE of LMA and Li plating morphology. There are two types of Li morphologies in these electrolytes: strip-like and particle-like morphology. Since the specific surface area of particle-like deposits is much smaller than that of strip-like deposits, the particle-like morphology facilitates higher CE. (1) In the conventional carbonate electrolyte (1 mol·L^-1 LiPF₆-EC/DMC), Li predominantly forms strip-like deposits with large specific surface area, consuming much active Li (due to the side reaction between Li and the electrolyte). The dendrite morphology of the Li deposits lead to the formation of dead Li during the stripping process, which results in low CE. (2) FEC, an effective additive often used in carbonate electrolyte, can induce the transformation of Li plating morphology from strip-like to particle-like morphology. Therefore, the CE in FEC-containing electrolytes has been significantly improved with stable electrode/electrolyte interphase and small specific surface area of deposited Li. (3) In ether electrolytes, which have better compatibility with LMAs than carbonate electrolytes, Li metal exhibits a particle-like morphology and achieves high CE. (4) In the highly concentrated electrolyte (4 mol·L^-1 LiFSI-DME), Li metal grows into large particles without dendrite formation, which hampers the parasitic side reactions, and further enhances CE.

Lithium (Li) metal anodes are critical components for next-generation high-energy density batteries, owing to their high theoretical specific capacity (3800 mAh·g^-1) and low voltage (-3.040 V versus the standard hydrogen electrode). However, their applications are hindered by dendrite growth, which potentially induces inner short circuit and leads to safety issues. Employing three-dimensional (3D) current collectors is an effective strategy to suppress dendrite growth by decreasing the local current density. However, many of the reported 3D current collectors have a lithiophobic surface, which leads to non-uniform Li⁺ ion deposition. Thus, a complicated modification process is required to increase the lithiophilic property of the current collectors. In addition, they have a large weight or volume, which greatly lowers the energy density of the entire anode. In this work, we report a lightweight 3D carbon current collector with a lithiophilic surface by employing the direct carbonization of low-cost bacterial cellulose (BC) biomass. The current collector is composed of electrically conductive, robust, and interconnected carbon nanofiber networks, which provide sufficient void space to accommodate a large amount of Li and buffer the volume changes during Li plating and stripping. More importantly, homogeneously distributed oxygen-containing functional groups on the nanofiber surface are retained by controlling the carbonization temperature. These functional groups serve as uniform nucleation sites and help realize uniform and dendrite-free Li deposition. Notably, the areal mass density of the 3D carbon current collector was only 0.32 mg·cm^-2 and its mass ratio in the whole anode was 28.8%, with a capacity of 3 mAh·cm^-2. This 3D carbon current collector facilitates the stable working of the half cells for 150 cycles under a high current density of 3 mA·cm^-2 or a high capacity of 4 mAh·cm^-2. Symmetric cells exhibit a steady cycling life as long as 600 h under a current density of 1 mA·cm^-2 and a capacity of 1 mAh·cm^-2. Moreover, appreciable cycling performance was realized in the full cells when the anodes were paired with LiNi_0.8Co_0.15Al_0.05 cathodes. Furthermore, the low-cost raw materials and the simple preparation method promise significant potential for the future applications of the proposed 3D current collectors.

The attention towards lithium (Li) metal anode (LMA) has been rekindled in recent years as it can augment the energy density of Li batteries due to its high theoretical specific capacity (3860 mAh·g^-1) and low electrochemical potential (-3.04 V versus standard hydrogen electrode), especially when paired with Li-free cathodes such as Li-oxygen and Li-sulfur. However, severe interfacial instability and safety concerns on rechargeable LMA, associated with Li dendrite formation, continuous side reactions, and infinite volume changes, extremely hinder its commercialization. Numerous strategies have been employed to modify LMA for realizing a uniform distribution of the Li ion flux through interface and dendrite-free Li deposits during repeated Li plating/stripping, which leads to a better cycling performance; however, to the best of our knowledge, a clear understanding of the Li deposition/dissolution behavior and the nucleation growth mechanism of Li dendrites is still lacking, which is conducive to more efficient modification studies on LMA. Therefore, it is critical to achieve considerable progress in the development of advanced characterization techniques. However, the high reactivity of Li metal, which leads to complexity of products and diversity in morphology, causes many difficulties in the characterization of in situ spectroscopy. Recently, some promising characterization techniques have been introduced to further investigate the evolution of LMA during cycling, such as cryo-electron microscopy, solid-state nuclear magnetic resonance technology, and neutron depth profiling (NDP) technique. Because of its high-penetration characteristics, quantitative and nondestructive merits, and highly selective sensitivity to ⁶Li via the capture reaction with neutrons, the NDP technique shows a broad application prospect for obtaining real-time information of the electrochemical behavior of Li in Li metal batteries. The NDP results contain a wealth of information about time and space for Li. Accordingly, not only can the real-time distribution and migration of Li ions be detected, but also changes in the active sites of Li deposition/dissolution can be analyzed to understand the formation principle of Li dendrites and the failure mechanism of Li metal batteries. In addition, the NDP technique has shown its potential in the diagnosis and prediction of short circuit in Li metal batteries, which is confirmed through voltage curves. This review first briefly introduces the principle of the NDP technique and the methods for improving its space/time resolution; second, it summarizes the recent use of the NDP technique in the research on LMAs based on liquid or solid cell systems. Finally, it provides a prospect for the future development of NDP technique.

Lithium (Li) metal is considered as the most promising anode material for high-energy-density batteries owing to its ultra-high theoretical capacity (3860 mAh·g^-1) and the lowest negative electrochemical potential (-3.040 V versus standard hydrogen electrode). However, the unstable solid electrolyte interphase (SEI) layers, uncontrollable dendrite growth, and huge volume changes during the plating/stripping processes significantly limit the practical applications of Li metal anodes. Since the unstable SEI layers can promote the nucleation and growth of Li dendrites, they play a crucial role in the decay process of Li metal anodes. The fracture and regeneration of SEI layers continuously consume electrolytes and Li metal anodes during plating/ stripping processes, and the accumulation of SEI layers can increase the interface impedance. Therefore, building artificial interphase layers is one of the most effective strategies to construct a stable SEI, reduce dendrite growth, accommodate large volume changes, and thus obtain excellent cycling performance. In this review, artificial interphase layers have been summarized into three parts based on the conductive properties of interphase, including artificial SEI layers (electronically insulating while ionically conducting), mixed ionic and electronic conductor interphase layers, and nanostructured interphase passivation layers (both ionically and electronically insulating). Artificial SEI layers with high ionic conductivity and low electronic conductivity can be classified into inorganic, organic, and organic/inorganic complex SEI according to the composition of artificial SEI layers. The artificial inorganic SEI layers with a high Young's modulus can suppress the dendrite growth. The artificial organic SEI layers with flexible features can accommodate large interface fluctuations and improve the interphase wettability. The artificial organic/inorganic complex SEI layers with a rigid-flexible structure can restrain dendrite growth and buffer volume change. The mixed ionic and electronic conductor layers possess high ionic conductivity and high Young's modulus, which are beneficial for enhancing the interphase stability and reducing dendrite growth. The artificial alloy mixed conductor layers can improve the Li diffusion coefficient and reduce Li nucleation overpotential, guiding uniform Li plating/stripping. Furthermore, the artificial mixed conductor layers comprising inorganic and organic matter have commendable flexibility and excellent interface compatibility, thereby enhancing the interphase stability and reducing dendrite growth. The nanostructured interphase passivation layers with high chemical stability can deliver Li ion through a confined electrolyte in a uniform porous structure, thereby achieving homogeneous Li plating/stripping. In addition, the structure-effective relationship of artificial interphase layers has been analyzed, and methods for improving the performance of artificial interphase layers, such as physical and chemical stability, ion transportation, interface strength and flexibility, and interfacial compatibility, have been discussed in this review. Finally, we present the main challenge and perspectives of artificial interphase layers.

The ever-increasing demand for high-energy Li-ion batteries has acted as a powerful stimulus for the development of Li metal as an anode material. Li metal has long been regarded as a "Holy Grail" in Li-ion batteries due to its high discharge capacity (3860 mAh·g^-1) and low electric potential (-3.04 V). However, the formation of unstable solid electrolyte interphases (SEIs) and Li dendrites, as well as the resultant safety issues initiated by catastrophic dendrite growth, have greatly impeded further application. High ion conductivity, surface electron insulation, and favorable mechanical strength are essential properties for an ideal SEI film, which can allow for uniform Li deposition, providing a fast transfer path for Li ions and suppressing Li dendrite growth. Therefore, designing a functional SEI layer is an effective strategy to solve the problems encountered with Li metal anodes. So far, a variety of inorganic, organic, and inorganic/organic hybird SEI layers have been designed and fabricated. Inorganic SEIs are characterized by mechanical strength and ion conductivity; organic SEIs are flexible and have electron insulation properties. Inorganic/organic composite SEIs show favorable ion conductivity derived from the inorganic components, electron insulation properties originating from the organic components, and mechanical strength benefiting from the reinforcing effect between the inorganic and organic components. Oxides, metal sulfides, lithium nitride (Li₃N) and its derivates, lithium halide (LiX, X = F, Cl), two-dimensional (2D) layered structure materials, lithium phosphate and "Janus" composite are the representative examples of inorganic SEIs. The design principle of various SEI layers is based on the inhibition of Li dendrite formation and growth. Therefore, it is a prerequisite to better understand the relevant intrinsic mechanisms. Despite past investigations, further studies are still required to fully elucidate the related mechanisms by providing more broadly accepted evidence combined with theoretical calculations and offer reliable guidance for the design of multifunctional SEI layers, boosting the performance of Li metal anodes. In this review, on the basis of the mechanisms underlying Li dendrite formation and growth, strategies for constructing various functional SEI films, highlights in structure and property of the films, and their effects on the performance of Li metal anodes are summarized. Moreover, some challenges encountered with the practical applications of Li metal anodes and the future direction for the development of Li metal anodes are addressed. This review can reveal possible strategies for the commercialization of high-energy, safe and stable Li-ion batteries.

As an ideal negative electrode material for next-generation high-energy-density batteries, lithium (Li) metal has received extensive attention from the global research community. However, the safety hazards and short cycle life caused by the growth of Li dendrites have seriously hampered the application of Li metal batteries. Based on electrochemical phenomena and theory, this paper discusses the mechanism of dendritic growth, dead Li formation, and full battery failure from the perspective of concentration polarization. During the electrodeposition process, the consumption of Li ions on the surface induces concentration polarization. After the initial deposition, a relatively loose dendrite layer appears on the Li metal surface; the electrolyte can penetrate this dendrite layer to reach the dense Li metal surface. When the grown dendrites penetrate the concentration polarization layer, the interface concentration battery is short-circuited. In this case, the concentration difference battery tends to release all stored power and reach a potential balance between the high- and low-concentration regions, which causes the deposition of Li ions over the dendrites to reduce the ion concentration in the surrounding electrolyte. Meanwhile, the dissolution of Li ions that occurs at the roots of the dendrites increases the local ion concentration. This process accelerates the formation of a dead Li layer. A similar electrochemical process often occurs in columnar Li, as reported in other studies. When columnar Li is cycled several times, each Li column degenerates into a matchstick shape with a large head and thin neck. Therefore, eliminating concentration polarization is necessary for the application of columnar Li. Furthermore, in this work, concentration polarization and dendrite suppression in state-of-the-art porous host electrodes are analyzed. The larger specific surface area of the porous electrode greatly reduces the local current density on the electrode surface, which can reduce the interface concentration polarization and thus prevent dendrite growth. In charge-discharge cycling, a constant-voltage charging or shelving step is often inserted in each cycle in order to eliminate the influence of concentration polarization. However, if a dendritic layer has been formed on the Li metal surface after charging, in addition to the self-diffusion of ions, the self-discharge process of the interface concentration battery causes the detachment of the dendrite layer, thus resulting in the above-mentioned dead Li. Therefore, a larger amount of deposited Li yields a thicker Li dendritic layer, thus accelerating the capacity decay and failure of the battery, especially to those with high-capacity, high-voltage positive electrodes. The conclusions obtained in this paper can provide a theoretical basis for researchers to further explore Li metal protection strategies.

With the rapid development of electric vehicles and portable electronic devices, traditional lithium-ion batteries with graphite anodes cannot satisfy demands for increased energy density. Lithium metal, with a high theoretical specific capacity (3860 mAh·g^-1), low density (0.534 g·cm^-3), and the lowest potential (-3.040 V vs. standard hydrogen electrode), has received much attention as an ideal anode material for next-generation energy storage devices. However, the uncontrolled growth of lithium dendrites and low Coulombic efficiency caused by negative side reactions have severely hindered the development of lithium metal batteries. Here, we propose a strategy based on the synergistic effect between a porous copper foam and thiourea, which uses the "super-filling" effect of thiourea molecules to achieve the uniform deposition of lithium metal on the surface of the porous copper foam. The unique curvature enhance coverage mechanism of thiourea molecules can accelerate Li deposition rate in grooves and achieve "super-filling" growth. The porous copper foam was obtained through simple multi-step processing. Scanning electron microscopy images showed many small pores evenly distributed on the surface; these pores acted as nucleation sites for lithium deposition. With the effect of thiourea, lithium was preferentially deposited in the small pores and then filled to the top, and finally deposited uniformly on the surface of the porous copper foam. The morphologies of the different electrodes deposited with capacities of 1, 3, and 10 mAh·cm^-2 demonstrated the synergistic effect between the porous copper foam and thiourea, which can inhibit the growth of lithium dendrites. Through this strategy, stable lithium plating/stripping over 500 h was achieved at a current density of 1 mA·cm^-2 with a fixed capacity of 1 mAh·cm^-2 while maintaining a voltage hysteresis below 20 mV. Meanwhile, greatly enhanced Coulombic efficiency and longer cycle life times were achieved: the Li||LiFePO₄ full cell maintained 94% capacity after 300 cycles at 5C. Exploiting the synergy between the electrolyte and framework provides a novel approach for fabricating advanced lithium metal batteries. This work thus details a novel strategy for lithium anode protection that may also be extended to other metal anodes, thereby facilitating the development of next-generation energy storage devices.

Lithium metal has the highest theoretical specific energy density (3860 mAh∙g⁻¹) and the most negative redox potential (−3.04 V vs standard hydrogen electrode) among all alkali metals. These features have attracted the interest of battery researchers to put lithium metal into practical use in rechargeable batteries. However, lithium metal tends to deposit as dendritic or mossy morphology during the charging process, and such non-uniform deposition induces low Coulombic efficiency and poor cycling stability. In addition, dendritic metallic lithium can easily penetrate the separator, which causes internal short circuit and leads to severe safety issues. Thus it is important to control the electrodeposition process of lithium to inhibit the formation of Li dendrites. Surface modification of lithium is a widely adopted strategy that can induce uniform deposition of Li. In this paper, a LiC₆ heterogeneous interfacial layer is decorated on the surface of lithium metal anode. It is prepared in a simple manner by mechanically rolling graphitized carbon nanospheres on a Li foil. The increase in surface area by this LiC₆ layer can homogenize the current density on the surface of the lithium foil. Simultaneously, the electronegativity of LiC₆ can also homogenize the lithium ion flux. The effect of heterogeneous interface on the electrochemical plating and stripping behavior of lithium in carbonate electrolyte is also studied. Morphological characterization and electrochemical performance tests reveal that the LiC₆ heterogeneous interface can significantly improve the reversibility and uniformity of the electrochemical plating and stripping of Li, thereby inhibiting dendritic growth and maintaining the stability of the anode/electrolyte interface. Alternating current electrochemical impedance spectroscopy analysis determines that the solid electrolyte interface (SEI) impedance of bare lithium decreases from the initial 275 to 100 Ω as the deposition capacity increases, suggesting that severe rupture of the SEI is caused by the huge volume change after lithium deposition. On the contrary, the SEI impedance of the lithium foil with the LiC₆ heterogeneous interface layer remains nearly constant (from the initial 26 to 25 Ω after electrodeposition) indicates that the LiC₆ layer is able to inhibit dendrite growth and stabilize the interface. Thus, stable lithium plating/stripping over 300 h is achieved at a current density of 1 mA∙cm⁻² and at a fixed capacity of 1 mAh∙cm⁻² with a voltage hysteresis of less than 50 mV. The Li-LiFePO₄ full cell test demonstrates that the cycling stability of the modified lithium metal anode is superior to that of the bare one.

The emerging market for consumer electronics and electric vehicles has stimulated intensive research on lithium metal batteries (LMBs) with high energy densities and large cycle lifetimes. A metallic Li anode has a high theoretical specific capacity of 3860 mAh·g^-1 and lowest redox potential of -3.04 V (vs. the standard hydrogen electrode) and is generally considered an ideal electrode for next-generation high-energy-density LMBs. However, their deployment in practical batteries is severely hindered by the formation of unsafe dendrites and fast capacity decay due to the uncontrollable formation of fragile solid electrolyte interfaces (SEIs). Herein, we describe the stable cycling of carbon paper (Cp)-supported Li-Sn alloy anodes in carbonate electrolytes modified with 1 mol·L^-1 bis(2, 2, 2-trifluorotoluene) carbonate (DTFEC). The molten Li-Sn alloy with 8% (mass fraction) Sn was synthesized through thermal treatment at 400 ℃ in an atmosphere of Ar. The Li-Sn-alloy-coated carbon paper (SnLi/Cp) was obtained after the molten alloy was conformally loaded onto the surface of a carbon paper under the action of capillarity. The as-synthesized interconnected SnLi/Cp composite was characterized by X-ray diffraction, energy-dispersive spectrometry, and scanning electron microscopy. The porous SnLi/Cp composite consisted of only Li and Sn₅Li₂₂ phases supported by the mechanically strong carbon paper with a good conductivity; no impurity was observed in the XRD results. The synergy of the DTFEC additive and alloying with Sn provided composite anodes with significantly improved rate capability and remarkable stability owing to the formation of a dense fluorinated SEI layer with high mechanical strength and ion penetration. Moreover, with the porous SnLi alloy covered by a fluorinated protection layer, lithium avoids the intrinsic issues of uncontrollable volume expansion and dendrite growth, which restrict the practical application of Li metal, exhibiting a stabilized over-potential of only 90 mV after 100 cycles at 8 mA·cm^-2. Notably, stable cycling with a 12 μL lean electrolyte was also observed at 5 mA·cm^-2. Overall, the prototype full cell assembled with the SnLi/Cp anode and NMC811 cathode exhibited a high Coulombic efficiency (98.1%) and remarkable cycling stability for 300 cycles at 1C (1.5 mA·cm^-2). The rate capability was evaluated at various rates of 0.5C to 5C. Compared to pure Li, the SnLi/Cp anode in the full cell exhibited a higher capacity, particularly at a high rate (~126 mAh·g^-1 at 5C). Our approach provides integrated Li metal electrodes with effectively improved cycle stabilities and is very attractive for practical high-energy-density lithium batteries.

Lithium-metal anode batteries have the potential to serve as next-generation, high energy density batteries with high specific capacity and low electrode potential. However, due to the high reactivity of lithium, complex interfacial reactions and uncontrollable dendrite growth obstruct their application. These lithium-metal anode interfacial reactions are often accompanied by the organic electrolyte spontaneously decomposing and combustible gas subsequently escaping, which is a safety concern. It also affects the form of the solid electrolyte interphase (SEI), which is important for stabilizing the interface between the Li-metal anode and electrolyte. Uncontrollable Li dendrite growth could penetrate the separator or electrolyte, creating the risk of a short circuit. Therefore, it is necessary to optimize the lithium nucleation and deposition processes. Solid state electrolytes (SSEs) have also attracted attention for improving the energy density and safety of Li-ion batteries; however, problems such as poor ionic conductivity still exist. Computational simulations, such as molecular dynamics (MD) simulations and first-principles calculations based on density function theory (DFT), can help elucidate reaction mechanisms, explore electrode materials, and optimize battery design. In this review, we summarize the theoretical perspective gained from computational simulation studies of lithium-metal anodes. This review is organized into four sections: interfacial reactions, SEIs, lithium nucleation, and SSEs. We first explore organic-electrolyte interfacial reaction mechanisms that were revealed through MD simulations and how electrolyte additives, electrolyte concentration, operating temperature affect them. For SEI, DFT can provide an in-depth understanding of the surface chemical reaction, surface morphology, electrochemical properties, and kinetic characteristics of SEI. We review the developments in SEI transmission mechanisms and SEI materials' properties alteration by lithium metal. We further explore artificial SEI design requirements and compare the performances of artificial SEIs, including double-layer, fluorine-, and sulfur-SEIs. Lithium dendrite growth as a result of lithium nucleation and deposition is then discussed, focusing on computational studies that evaluated how doped graphene, 3D carbon fibers, porous metals, and other matrix materials regulated these processes and inhibited dendrite growth. Computational simulations evaluating transport phenomena and interface reactions between SSEs and lithium-metal anodes are then explored, followed by ideas for further design optimization. Finally, potential research directions and perspectives in this field are proposed and discussed.