Guiding Uniform Li Plating/Stripping through Lithium-Aluminum Alloying Medium for Long-Life Li Metal Batteries

Atom-Level Tandem Catalysis in Lithium Metal Batteries

High-energy-density lithium metal batteries (LMBs) are limited by reaction or diffusion barriers with dissatisfactory electrochemical kinetics. Typical conversion-type lithium sulfur battery systems exemplify the kinetic challenges. Namely, before diffusing or reacting in the electrode surface/interior, the Li(solvent)x + dissociation at the interface to produce isolated Li+, is usually a prerequisite fundamental step either for successive Li+ "reduction" or for Li+ to participate in the sulfur conversions, contributing to the related electrochemical barriers. Thanks to the ideal atomic efficiency (100 at%), single atom catalysts (SACs) have gained attention for use in LMBs toward resolving the issues caused by the five types of barrier-restricted processes, including polysulfide/Li2S conversions, Li(solvent)x + desolvation, and Li0 nucleation/diffusion. In this perspective, the tandem reactions including desolvation and reaction or plating and corresponding catalysis behaviors are introduced and analyzed from interface to electrode interior. Meanwhile, the principal mechanisms of highly efficient SACs in overcoming specific energy barriers to reinforce the catalytic electrochemistry are discussed. Lastly, the future development of high-efficiency atomic-level catalysts in batteries is presented.

Structural Design of 3D Current Collectors for Lithium Metal Anodes: A Review

Due to the ultrahigh theoretical specific capacity (3860 mAh g-1) and low redox potential (-3.04 V vs. standard hydrogen electrode), Lithium (Li) metal anode (LMA) received increasing attentions. However, notorious dendrite and volume expansion during the cycling process seriously hinder the development of high energy density Li metal batteries. Constructing three-dimensional (3D) current collectors for Li can fundamentally solve the intrinsic drawback of hostless for Li. Therefore, this review systematically introduces the design and synthesis engineering and the current development status of different 3D collectors in recent years (the current collectors are divided into two major parts: metal-based current collectors and carbon-based current collectors). In the end, some perspectives of the future promotion for LMA application are also presented.

A Perspective on Uniform Plating Behavior of Mg Metal Anode: Diffusion Limited Theory versus Nucleation Theory

Utilizing metal anode is the most attractive way to meet the urgent demand for rechargeable batteries with high energy density. Unfortunately, the formation of dendrites, which is caused by uneven plating behavior, always threaten the safety of the batteries. To explore the origin of different plating behavior and predict the plating morphology of anode under a variety of operating conditions, multifarious models have been developed. However, abuse of models has led to conflictive views. In this perspective, to clarify the controversial reports on magnesium (Mg) metal plating behavior, the previously proposed models are elaborated that govern the plating process. Through linking various models and clarifying their boundary conditions, a scheme is drawn to illustrate the strategy for achieving the most dense and uniform plating morphology, which also explains the seemingly contradictory of diffusion limited theory and nucleation theory on uniform plating. This perspective will undoubtedly enhance the understanding on the metal anode plating process and provide meaningful guidance for the development of metal anode batteries.

Semi-Embedded Structured Bi Nanospheres for Boosted Self-Heating-Induced Healing of Li-Dendrites

It is reported that self-heating-induced healing on lithium metal anodes (LMAs) provides a mitigation strategy for suppressing Li dendrites. However, how to boost the self-heating-induced healing of Li-dendrites and incorporate it into Li-host design remains an imminent problem that needs to be solved. Herein, a new bismuth nanosphere semi-buried carbon cloth (Bi-NS-CC) material with a 3D flexible host structure is proposed. The ultrasmall Bi nanospheres are uniformly and densely distributed on carbon fiber, providing active sites to form uniform Li3 Bi alloy with molten lithium, thereby guiding the injection of molten metallic lithium into the 3D structure to form a self-supporting composite LMAs. The ingenious semi-embedded structure with strong interfacial C─Bi ensures superior mechanical properties. Interestingly, when the current density reaches up to 10 mA cm-2 , the lithium dendrites undergo self-heating. Carbon cloth as a host can quickly and uniformly transfer heat, which induces the uniform migration of Li on anodes. The semi-embedded structure with strong C─Bi ensures Bi nanospheres guide the formation of smooth morphology even under these harsh conditions (high-temperature, high-rate, etc.). Consequently, at 10 mA cm-2 /10 mAh cm-2 , the Li/Li3 Bi-NS-CC realizes ultra-long cycles of 1500 h and ultra-low overpotential of 15 mV in a symmetric cell.

Uniform Li Deposition through the Graphene-Based Ion-Flux Regulator for High-Rate Li Metal Batteries

Lithium (Li) metal is considered an ultimate anode owing to its high specific capacity and energy density. However, uncontrolled Li dendrite growth and low Coulombic efficiency have limited the application of Li metal. Among various strategies introduced to address these limitations, the surface modification of polyolefin separators with functional materials has been widely adopted for improving the mechanical and thermal stabilities of polymer separators and to protect the separator from the penetration of Li dendrites. Herein, we report a new functional polymer separator that is surface-altered with a graphene-based Li-ion flux regulator (GLR) to homogenize the Li-ion flux and suppress the growth of sharp dendritic Li in Li metal batteries. The nanopores distributed through the GLR structure serve as channels for ion transport and junctions for electron transfer, facilitating efficient electrolyte penetration and rapid charge transfer between graphene (Gr) sheets. Owing to these favorable features of porous GLR, a Li-Cu cell with the GLR surface-altered polypropylene separator (GLR-PP) delivers excellent cycle and rate performances compared to a Li-Cu cell with a Gr surface-altered polypropylene separator. In addition, among the tested cells, Li-sulfur cells with GLR-PP exhibit the most stable cycle performance over 500 cycles. These results demonstrate that the concept of tailoring the surface of a polymer separator with porous 2D materials is an effective strategy for improving the long-term cycle stability and electrochemical kinetics of Li metal-based batteries and would trigger further relevant studies.

Solvation Sheath Engineering by Multivalent Cations Enabling Multifunctional SEI for Fast-Charging Lithium-Metal Batteries

With the pursuit of high energy and power density, the fast-charging capability of lithium-metal batteries has progressively been the primary focus of attention. To prevent the formation of lithium dendrites during fast charging, the ideal solid electrolyte interphase should be capable of concurrent fast Li+ transport and uniform nucleation sites; however, its construction in a facile manner remains a challenge. Here, as Al3+ has a higher charge and Al metal is lithiophilic, we tuned the Li+ solvation structure by introducing LiNO3 and aluminum ethoxide together, resulting in the dissolution of LiNO3 and the simultaneous generation of fast ionic conductor and lithiophilic sites. Consequently, our approach facilitated the deposition of lithium metal in a uniform and chunky way, even at a high current density. As a result, the Coulombic efficiency of the Li||Cu cell increased to over 99%. Moreover, the Li||LiFePO4 full cell demonstrated significantly enhanced cycling performance with a remarkable capacity retention of 94.5% at 4 C, far superior to the 46.1% capacity retention observed with the base electrolyte.

Work-Function-Induced Interfacial Electron/Ion Transport in Carbon Hosts toward Dendrite-Free Lithium Metal Anodes

Coupled electron/ion transport is a decisive feature of Li plating/stripping, wherein the compatibility of electron/ion transport rates determines the morphology of deposited Li. Local Li+ hotspots form due to inhomogeneous interfacial charge transfer and lead to uncontrolled Li deposition, which decreases the Li utilization rate and safety of Li metal anodes. Herein, we report a method to obtain dendrite-free Li metal anodes by driving electron pumping and accumulating and boosting Li ion diffusion by tuning the work function of a carbon host using cobalt-containing catalysts. The results reveal that increasing the work function provides an electron deviation from C to Co, and electron-rich Co shows favorable binding to Li+ . The Co catalysts boost Li+ diffusion on the carbon fiber scaffolds without local aggregation by reducing the Li+ migration barrier. The as-obtained dendrite-free Li metal anode exhibits a Coulombic efficiency of 99.0 %, a cycle life of over 2000 h, a Li utilization rate of 50 %, and a capacity retention of 83.4 % after 130 cycles in pouch cells at a negative/positive capacity ratio of 2.5. These findings provide a novel strategy to stabilize Li metal by regulating the work function of materials using electrocatalysts.

Lithiophilic Zn3N2-Modified Cu Current Collectors by a Novel FCVA Technology for Stable Anode-Free Lithium Metal Batteries

Anode-free lithium metal batteries (AFLMBs) offer high-energy-density battery systems, but their commercial viability is hindered by irregular lithium dendrite growth and "dead Li" formation caused by current collector defects. This study employed filtered cathode vacuum arc (FCVA) technology to fabricate Cu current collectors (CCs) with a lithiophilic Zn3N2 film. This advanced preparation process ensures an evenly distributed film that reduces the nucleation overpotential, homogenizes the interfacial electric field during plating/stripping processes, inhibits lithium dendrite growth, and forms a stable solid-electrolyte interphase (SEI). Our results show that the advanced Zn3N2@Cu CCs exhibit superior performance with a high CE of above 99.3% after 230 cycles at a current density of 0.5 mA cm-2 and an area capacity of 1 mAh cm-2. Additionally, Li-Zn3N2@Cu||Li-Zn3N2@Cu symmetrical cells had a longer stable cycle time of over 1000 h than that of Li||Li and Li-Cu||Li-Cu symmetrical cells at a current density of 1 mA cm-2 and an area capacity of 2 mAh cm-2. Compared with bare Cu CCs, the capacity retention rate is increased from 14.9 to 63.1% after 100 cycles with a 0.5C rate in the AFLMBs with LFP as the cathode. This work provides a pioneering, eco-friendly, and effective solution for the fabrication of anode CCs in AFLMBs, addressing a significant challenge in their commercial application.

High Rate and Low-Temperature Stable Lithium Metal Batteries Enabled by Lithiophilic 3D Cu-CuSn Porous Framework

Lithium (Li) metal is regarded as the "Holy Grail" of anodes for high-energy rechargeable lithium batteries by virtue of its ultrahigh theoretical specific capacity and the lowest redox potential. However, the Li dendrite impedes the practical application of Li metal anodes. Herein, lithiophilic three-dimensional Cu-CuSn porous framework (3D Cu-CuSn) was fabricated by a vapor phase dealloying strategy via the difference in saturated vapor pressure between different metals and the Kirkendall effect. CuSn alloy sites were converted into LiSn alloy sites through the molten Li infusion method, and composite Li metal anodes (3D Cu-LiSn-Li) are achieved. Alloyed tin, as the bridge between the porous copper substrate and metallic Li, plays a critical role in optimizing Li nucleation and enhancing the fast lithium migration kinetics. This work demonstrates that lithiophilic binary copper alloys are an effective way to achieve room-temperature high rate performance and satisfied low-temperature cycling stability for Li metal batteries.

Nucleophilic deposition behavior of metal anodes

Nucleophilic materials play important roles in the deposition behavior of high-energy-density metal batteries (Li, Na, K, Zn, and Ca), while the principle and determination method of nucleophilicity are lacking. In this review, we summarize the metal extraction/deposition process to find out the mechanism of nucleophilic deposition behavior. The key points of the most critical nucleophilic behavior were found by combining the potential change, thermodynamic analysis, and active metal deposition behavior. On this basis, the inductivity and affinity of the material have been determined by Gibbs free energy directly. Thus, the inducibility of most materials has been classified: (a) induced nuclei can reduce the overpotential of active metals; (b) not all materials can induce active metal deposition; (c) the induced reaction is not changeless. Based on these results, the influencing factors (temperature, mass, phase state, induced reaction product, and alloying reactions) were also taken into account during the choice of inducers for active metal deposition. Finally, the critical issues, challenges, and perspectives for further development of high-utilization metal electrodes were considered comprehensively.

Interface Engineering of All-Solid-State Batteries Based on Inorganic Solid Electrolytes

All-solid-state batteries (ASSBs) based on inorganic solid electrolytes (SEs) are one of the most promising strategies for next-generation energy storage systems and electronic devices due to the higher energy density and intrinsic safety. However, the poor solid-solid contact and restricted chemical/electrochemical stability of inorganic SEs both in cathode and anode SE interfaces cause contact failure and the degeneration of SEs during prolonged charge-discharge processes. As a result, the increasing interface resistance significantly affects the coulombic efficiency and cycling performance of ASSBs. Herein, we present a fundamental understanding of physical contact and chemical/electrochemical features of ASSB interfaces based on mainstream inorganic SEs and summarize the recent work on interface modification. SE doping, optimizing morphology, introducing interlayer/coating layer, and utilizing compatible electrode materials are the key methods to prevent side reactions, which are discussed separately in cathode/anode-SE interface. We also highlight the constant extra stack pressure applied during ASSB cycling, which is important to the electrochemical performance. Finally, our perspectives on interface modification for practical high-performance ASSBs are put forward.

Cu Current Collector with Binder-Free Lithiophilic Nanowire Coating for High Energy Density Lithium Metal Batteries

Despite significant efforts to fabricate high energy density (ED) lithium (Li) metal anodes, problems such as dendrite formation and the need for excess Li (leading to low N/P ratios) have hampered Li metal battery (LMB) development. Here, the use of germanium (Ge) nanowires (NWs) directly grown on copper (Cu) substrates (Cu-Ge) to induce lithiophilicity and subsequently guide Li ions for uniform Li metal deposition/stripping during electrochemical cycling is reported. The NW morphology along with the formation of the Li₁₅ Ge₄ phase promotes uniform Li-ion flux and fast charge kinetic, resulting in the Cu-Ge substrate demonstrating low nucleation overpotentials of 10 mV (four times lower than planar Cu) and high Columbic efficiency (CE) efficiency during Li plating/stripping. Within a full-cell configuration, the Cu-Ge@Li - NMC cell delivered a 63.6% weight reduction at the anode level compared to a standard graphite-based anode, with impressive capacity retention and average CE of over 86.5% and 99.2% respectively. The Cu-Ge anodes are also paired with high specific capacity sulfur (S) cathodes, further demonstrating the benefits of developing surface-modified lithiophilic Cu current collectors, which can easily be integrated at the industrial scale.

Mitigating Concentration Polarization through Acid-Base Interaction Effects for Long-Cycling Lithium Metal Anodes

Lithium (Li) metal has attracted great attention as a promising high-capacity anode material for next-generation high-energy-density rechargeable batteries. Nonuniform Li⁺ transport and uneven Li plating/stripping behavior are two key factors that deteriorate the electrochemical performance. In this work, we propose an interphase acid-base interaction effect that could regulate Li plating/stripping behavior and stabilize the Li metal anode. ZSM-5, a class of zeolites with ordered nanochannels and abundant acid sites, was employed as a functional interface layer to facilitate Li⁺ transport and mitigate the cell concentration polarization. As a demonstration, a pouch cell with a high-areal-capacity LiNi_0.95Co_0.02Mn_0.03O₂ cathode (3.7 mAh cm^-2) and a ZSM-5 modified thin lithium anode (50 μm) delivered impressive electrochemical performance, showing 92% capacity retention in 100 cycles (375.7 mAh). This work reveals the effect of acid-base interaction on regulating lithium plating/stripping behaviors, which could be extended to developing other high-performance alkali metal anodes.

Intrinsic Self-Healing Chemistry for Next-Generation Flexible Energy Storage Devices

The booming wearable/portable electronic devices industry has stimulated the progress of supporting flexible energy storage devices. Excellent performance of flexible devices not only requires the component units of each device to maintain the original performance under external forces, but also demands the overall device to be flexible in response to external fields. However, flexible energy storage devices inevitably occur mechanical damages (extrusion, impact, vibration)/electrical damages (overcharge, over-discharge, external short circuit) during long-term complex deformation conditions, causing serious performance degradation and safety risks. Inspired by the healing phenomenon of nature, endowing energy storage devices with self-healing capability has become a promising strategy to effectively improve the durability and functionality of devices. Herein, this review systematically summarizes the latest progress in intrinsic self-healing chemistry for energy storage devices. Firstly, the main intrinsic self-healing mechanism is introduced. Then, the research situation of electrodes, electrolytes, artificial interface layers and integrated devices based on intrinsic self-healing and advanced characterization technology is reviewed. Finally, the current challenges and perspective are provided. We believe this critical review will contribute to the development of intrinsic self-healing chemistry in the flexible energy storage field.

Efficient Regulation of Polysulfides by MoS2 /MoO3 Heterostructures for High-Performance Li-S Batteries

The notorious shuttle effect and sluggish conversion of polysulfides seriously hinder the practical application of Lithium-sulfur (Li-S) batteries. In this study, a novel architecture of MoS₂ /MoO₃ heterostructure uniformly distributed on carbon nanotubes (MoS₂ /MoO₃ @CNT) is designed and introduced into Li-S batteries via decorating commercial separator to regulate the redox reactions of polysulfides. Systematic experiments and theoretical calculations showed that the heterostructure not only provides sufficient surface affinity to capture polysulfides and acts as an active catalyst to promote the conversion of polysulfides, but also the highly conductive CNT enables rapid electron/ion migration. As a result, Li-S batteries with the MoS₂ /MoO₃ @CNT-PP separator deliver an impressive reversible capacity (1015 mAh g^-1 at 0.2 A g^-1 after 100 cycles), excellent rate capacity (873 mAh g^-1 at 5 A g^-1 ), and low self-discharge capacity loss (94.6% capacity retention after 7 days of standing). Moreover, even at an elevated temperature of 70 °C, it still exhibits high-capacity retention (800 mAh g^-1 at 1 A g^-1 after 100 cycles). Encouragingly, when the sulfur load is increased to 8.7 mg cm^-2 , the high reversible areal capacity of 6.61 mAh cm^-2 can be stably maintained after 100 cycles, indicating a high potential for practical application.

Enhanced Kinetic Behaviors of Hollow MoO2/MoS2 Nanospheres for Sodium-Ion-Based Energy Storage

Mo-based heterostructures offer a new strategy to improve the electronics/ion transport and diffusion kinetics of the anode materials for sodium-ion batteries (SIBs). MoO₂/MoS₂ hollow nanospheres have been successfully designed via in-situ ion exchange technology with the spherical coordination compound Mo-glycerates (MoG). The structural evolution processes of pure MoO₂, MoO₂/MoS₂, and pure MoS₂ materials have been investigated, illustrating that the structureofthenanospherecan be maintained by introducing the S-Mo-S bond. Based on the high conductivity of MoO₂, the layered structure of MoS₂ and the synergistic effect between components, as-obtained MoO₂/MoS₂ hollow nanospheres display enhanced electrochemical kinetic behaviors for SIBs. The MoO₂/MoS₂ hollow nanospheres achieve a rate performance with 72% capacity retention at a current of 3200 mA g^-1 compared to 100 mA g^-1. The capacity can be restored to the initial capacity after a current returns to 100 mA g^-1, while the capacity fading of pure MoS₂ is up to 24%. Moreover, the MoO₂/MoS₂ hollow nanospheres also exhibit cycling stability, maintaining a stable capacity of 455.4 mAh g^-1 after 100 cycles at a current of 100 mA g^-1. In this work, the design strategy for the hollow composite structure provides insight into the preparation of energy storage materials.

Stable lithium metal anode enabled by in situ formation of a Li3N/Li-Bi alloy hybrid layer

A hybrid protective layer containing a Li₃N and Li-Bi alloy is fabricated on a Li-metal anode as an artificial SEI layer to guide dendrite-free Li deposition. Noteworthily, the hybrid interface could not only facilitate homogeneous Li plating but also provide rapid Li⁺ transportation, enabling a long-term stability of ∼2400 h at 0.5 mA cm^-2 with a low steady overpotential of 10 mV.

Toward Dendrite-Free Metallic Lithium Anodes: From Structural Design to Optimal Electrochemical Diffusion Kinetics

Lithium metal anodes are ideal for realizing high-energy-density batteries owing to their advantages, namely high capacity and low reduction potentials. However, the utilization of lithium anodes is restricted by the detrimental lithium dendrite formation, repeated formation and fracturing of the solid electrolyte interphase (SEI), and large volume expansion, resulting in severe "dead lithium" and subsequent short circuiting. Currently, the researches are principally focused on inhibition of dendrite formation toward extending and maintaining battery lifespans. Herein, we summarize the strategies employed in interfacial engineering and current-collector host designs as well as the emerging electrochemical catalytic methods for evolving-accelerating-ameliorating lithium ion/atom diffusion processes. First, strategies based on the fabrication of robust SEIs are reviewed from the aspects of compositional constituents including inorganic, organic, and hybrid SEI layers derived from electrolyte additives or artificial pretreatments. Second, the summary and discussion are presented for metallic and carbon-based three-dimensional current collectors serving as lithium hosts, including their functionality in decreasing local deposition current density and the effect of introducing lithiophilic sites. Third, we assess the recent advances in exploring alloy compounds and atomic metal catalysts to accelerate the lateral lithium ion/atom diffusion kinetics to average the spatial lithium distribution for smooth plating. Finally, the opportunities and challenges of metallic lithium anodes are presented, providing insights into the modulation of diffusion kinetics toward achieving dendrite-free lithium metal batteries.

Lithium Atom Surface Diffusion and Delocalized Deposition Propelled by Atomic Metal Catalyst toward Ultrahigh-Capacity Dendrite-Free Lithium Anode

Lithium metal anode possesses overwhelming capacity and low potential but suffers from dendrite growth and pulverization, causing short lifespan and low utilization. Here, a fundamental novel insight of using single-atomic catalyst (SAC) activators to boost lithium atom diffusion is proposed to realize delocalized deposition. By combining electronic microscopies, time-of-flight secondary ion mass spectrometry, theoretical simulations, and electrochemical analyses, we have unambiguously depicted that the SACs serve as kinetic activators in propelling the surface spreading and lateral redistribution of the lithium atoms for achieving dendrite-free plating morphology. Under the impressive capacity of 20 mA h cm^-2, the Li modified with SAC-activator exhibits a low overpotential of ∼50 mV at 5 mA cm^-2, a long lifespan of 900 h, and high Coulombic efficiencies during 150 cycles, much better than most literature reports. The so-coupled lithium-sulfur full battery delivers high cycling and rate performances, showing great promise toward the next-generation lithium metal batteries.

Induction/Inhibition Effect on Lithium Dendrite Growth by a Binary Modification Layer on a Separator

In lithium metal batteries (LMB), unrestricted growth of lithium dendrites will pierce the separator and cause an internal short circuit. Therefore, we designed modified separator with an InN thin layer, which could be in situ converted into a binary mixed-modified layer of Li-In alloy and Li₃N during the lithium plating/stripping process. Among them, Li-In alloy induces the lateral growth of lithium dendrites and prevents the separator from being pierced; Li₃N balances ion distribution at the lithium anode/separator interface, which is beneficial to inhibit the growth of lithium dendrites. Under the synergistic effect of the two phases, the performance of LMBs was obviously improved. In addition, the separator modification does not need to be carried out in a protective atmosphere and is suitable for large-scale roll-to-roll processing.

Limited Lithium Loading Promises Improved Lithium-Metal Anodes in Interface-Modified 3D Matrixes

Confining Li metal in a three-dimensional (3D) matrix has been proven effective in improving the Li-metal anodes; however, in most studies, the loading of Li in the 3D matrix is far excessive, resulting in a dense bulk Li-metal anode with a low Li-utilization rate, forfeiting the effect of the 3D matrix. Here, we show that limiting the loading of Li metal within an interface-modified 3D carbon matrix not only increases the Li-utilization rate but also improves the electrochemical performance of the Li-metal anode. We use lithiophilic Fe₂O₃ granules anchored on a 3D carbon fiber scaffold to guide molten Li dispersion onto the fibers with controlled Li loading. Limiting Li loading maximizes the interface lithiophilic effect of the Fe₂O₃ granules while preserving sufficient space for electrolyte infusion, collectively ensuring uniform Li deposition and fast Li⁺ transport kinetics. The Li anode with limited Li dosage achieves remarkably improved Li-anode performances, including long lifespan, low voltage polarization, and low electrochemical resistance in both the symmetric cells and full cells. The improved electrochemical performance of the limited Li anode substantiates the importance to reduce Li loading from a fresh perspective and provides an avenue for building practical Li-metal batteries.

Interface Crystallographic Optimization of Crystal Plane for Stable Metallic Lithium Anode

Li metal, the ideal anode material for rechargeable batteries, suffers from the inherent limitations of uneven interface kinetics and dendrite growth. Herein, we tackle this issue by applying an interface crystallographic optimization strategy. We demonstrate a promising metallic Li anode design by introducing a customized magnetron sputtering layer of preferred orientation copper coating on the surface of a current collector. The sputtered Cu layer employed is stable against the highly reactive robust Li metal to render the surface lithiophilic and achieve promoted interface kinetics due to the perfect interface-crystal plane matching between the sputtered copper layer and premier Li metal. The dendrite-free Li anode sustains stable interface kinetics and achieves a stable life span of 200 cycles during the plating and stripping process in commercial carbonate electrolytes. This design based on crystallographic optimization provides important insights into the design principles of the Li metal anode as well as other alkali metal anodes (Na, K, Zn, Mg, and Al).

Alloy-Type Anodes for High-Performance Rechargeable Batteries

Alloy-type anodes are one of the most promising classes of next-generation anode materials due to their ultrahigh theoretical capacity (2-10 times that of graphite). However, current alloy-type anodes have several limitations: huge volume expansion, high tendency to fracture and disintegrate, an unstable solid-electrolyte interphase (SEI) layer, and low Coulombic efficiency. Efforts to overcome these challenges are ongoing. This Review details recent progress in the research of batteries based on alloy-type anodes and discusses the direction of their future development. We conclude that improvements in structural design, the introduction of a protective interface, and the selection of suitable electrolytes are the most effective ways to improve the performance of alloy-type anodes. Furthermore, future studies should direct more attention toward analyzing their synergistic promoting effect.

Rationally Designed Fluorinated Amide Additive Enables the Stable Operation of Lithium Metal Batteries by Regulating the Interfacial Chemistry

A fluorinated amide molecule with two functional segments, namely, an amide group with a high donor number to bind lithium ions and a fluorine chain to expel carbonate solvents and mediate the formation of LiF, was designed to regulate the interfacial chemistry. As expected, the additive preferably appears in the first solvation sheath of lithium ions and is electrochemically reduced on the anode, and thus an inorganic-rich solid electrolyte interphase is generated. The morphology of deposited lithium metal evolves from brittle dendrites into a granular shape. Consequently, the Li||LiFePO₄ cell shows an excellent capacity retention of 92.7% at a high rate of 5 C after 800 cycles. Besides, the Li||LiNi_0.8Co_0.1Mn_0.1O₂ cell succeeds to maintain 98.1% of the initial capacity after 100 cycles at 1 C. Our designing of N,N-diethyl- 2,3,3,3-tetrafluoropropionamide (denoted as DETFP) highlights the importance of a "high donor number" and may shed light on the design principles of electrolytes for high performance batteries.

Advanced Current Collector Materials for High-Performance Lithium Metal Anodes

Lithium metal, as the "Holy Grail" of lithium battery anodes, is promising to be used in the next-generation of high-energy-density storage devices. However, serious safety risk and poor cycle performance are inevitable when bare lithium foil is used as the anode material, due to the uncontrolled growth of lithium dendrites, unstable solid electrolyte interface, and infinite volume expansion of lithium during cycling, which largely hinder the further commercial application of lithium metal batteries (LMBs). The utilization of up-to-date current collectors with specific composition and structure is believed to be effective to overcome these shortcomings. However, a systematic evaluation of the merit of different current collector materials for realizing high-performance lithium metal anodes is still lacking. This review summarizes the fashionable advanced current collector materials for long-life LMBs in recent years. The superiorities and related electrochemical performances by using these current collector materials are discussed in detail. It is expected that this review may promote the rational choice of appreciatory current collector materials with unique structure designs to extend the cycle life of lithium metal anodes for achieving the next-generation of high-energy-density LMBs.

Freestanding Metal-Organic Frameworks and Their Derivatives: An Emerging Platform for Electrochemical Energy Storage and Conversion

Metal–organic frameworks (MOFs) have recently emerged as ideal electrode materials and precursors for electrochemical energy storage and conversion (EESC) owing to their large specific surface areas, highly tunable porosities, abundant active sites, and diversified choices of metal nodes and organic linkers. Both MOF-based and MOF-derived materials in powder form have been widely investigated in relation to their synthesis methods, structure and morphology controls, and performance advantages in targeted applications. However, to engage them for energy applications, both binders and additives would be required to form postprocessed electrodes, fundamentally eliminating some of the active sites and thus degrading the superior effects of the MOF-based/derived materials. The advancement of freestanding electrodes provides a new promising platform for MOF-based/derived materials in EESC thanks to their apparent merits, including fast electron/charge transmission and seamless contact between active materials and current collectors. Benefiting from the synergistic effect of freestanding structures and MOF-based/derived materials, outstanding electrochemical performance in EESC can be achieved, stimulating the increasing enthusiasm in recent years. This review provides a timely and comprehensive overview on the structural features and fabrication techniques of freestanding MOF-based/derived electrodes. Then, the latest advances in freestanding MOF-based/derived electrodes are summarized from electrochemical energy storage devices to electrocatalysis. Finally, insights into the currently faced challenges and further perspectives on these feasible solutions of freestanding MOF-based/derived electrodes for EESC are discussed, aiming at providing a new set of guidance to promote their further development in scale-up production and commercial applications.

A dual-function liquid electrolyte additive for high-energy non-aqueous lithium metal batteries

Engineering the formulation of non-aqueous liquid electrolytes is a viable strategy to produce high-energy lithium metal batteries. However, when the lithium metal anode is combined with a Ni-rich layered cathode, the (electro)chemical stability of both electrodes could be compromised. To circumvent this issue, we report a combination of aluminum ethoxide (0.4 wt.%) and fluoroethylene carbonate (5 vol.%) as additives in a conventional LiPF₆-containing carbonate-based electrolyte solution. This electrolyte formulation enables the formation of mechanically robust and ionically conductive interphases on both electrodes’ surfaces. In particular, the alumina formed at the interphases prevents the formation of dendritic structures on the lithium metal anode and mitigate the stress-induced cracking and phase transformation in the Ni-rich layered cathode. By coupling a thin (i.e., about 40 μm) lithium metal anode with a high-loading (i.e., 21.5 mg cm⁻²) LiNi_0.8Co_0.1Mn_0.1O₂-based cathode in coin cell configuration and lean electrolyte conditions, the engineered electrolyte allows a specific discharge capacity retention of 80.3% after 130 cycles at 60 mA g⁻¹ and 30 °C which results in calculated specific cell energy of about 350 Wh kg⁻¹.

Lithium metal batteries suffer from poor (electro)chemical stability of the electrodes during prolonged cycling. Here, the authors report a dual function liquid electrolyte additive to form protective interphases on both electrodes to produce lab-scale high energy lithium metal batteries.

Toward Practical High-Energy and High-Power Lithium Battery Anodes: Present and Future

Lithium batteries are key components of portable devices and electric vehicles due to their high energy density and long cycle life. To meet the increasing requirements of electric devices, however, energy density of Li batteries needs to be further improved. Anode materials, as a key component of the Li batteries, have a remarkable effect on the increase of the overall energy density. At present, various anode materials including Li anodes, high-capacity alloy-type anode materials, phosphorus-based anodes, and silicon anodes have shown great potential for Li batteries. Composite-structure anode materials will be further developed to cater to the growing demands for electrochemical storage devices with high-energy-density and high-power-density. In this review, the latest progress in the development of high-energy Li batteries focusing on high-energy-capacity anode materials has been summarized in detail. In addition, the challenges for the rational design of current Li battery anodes and the future trends are also presented.

A Novel Dendrite-Free Lithium Metal Anode via Oxygen and Boron Codoped Honeycomb Carbon Skeleton

Lithium (Li) metal is an excellent anode of Li ion batteries because of its high theoretical capacity and the low redox potential compared to other anodes. However, the uncontrollable growth of Li dendrites still incurs serious safety issues and poor electrochemical performances, leading to its limited practical application. An oxygen and boron codoped honeycomb carbon skeleton (OBHcCs) is reported and a stable Li metal-based anode is realized. It can be coated on a copper foil substrate to be used as a current collector for a dendrite-free Li metal anode. OBHcCs effectively reduces the local current density owing to the high surface area and inhibits Li dendrite growth, which is explored by scanning electron microscopy and an X-ray photoelectron spectra depth profile. The abundant lithiophilic oxygen and boron-containing functional groups reduce the potential barrier of nucleation and lead to the homogeneous Li ions flux as confirmed by the density functional theories. Therefore, the Li metal anode based on OBHcCs (OBHcCs@Li) stably runs for 700 h in a symmetric cell with a Li stripping capacity of 1 mAh cm^-2 at 1 mA cm^-2 . Furthermore, the OBHcCs@Li|LiFePO₄ full cell shows a good capacity retention of 84.6% with a high coulombic efficiency of 99.6% at 0.5 C for 500 cycles.

Lithium reduction reaction for interfacial regulation of lithium metal anode

The lithium metal anode (LMA) is regarded as a very promising candidate for next-generation lithium batteries. The interfacial issue plays a pivotal role in affecting the lithium plating/stripping behavior, Coulombic efficiency and cycling lifespan of an LMA. The lithium reduction reaction (LRR) is an advanced regulating technique for optimizing the LMA interphase, which intelligently utilizes lithium metal itself as an interphase precursor. This strategy also possesses moderate operating conditions, high efficiency, great convenience and scalability. In this review, the latest developments of LRRs in interfacial regulation for LMAs are summarized, focusing on the interfacial regulation mechanism and the construction of various inorganic/organic interfaces in lithium metal liquid/solid batteries. The target interface properties and corresponding influence factors during LRRs are investigated in detail. Besides this, the superiority and insufficiency of LRRs are discussed and possible directions for LRRs are presented. This review highlights in situ modification characteristics for anode interface regulation during the LRR and can be extended to other metal anodes such as sodium, potassium and zinc.

The LiTFSI/COFs Fiber as Separator Coating with Bifunction of Inhibition of Lithium Dendrite and Shuttle Effect for Li-SeS2 Battery

The safety problem caused by lithium dendrite of lithium metal anode and the rapid capacity decay problem caused by the shuttle effect of polysulfide and polyselenide during the charge and discharge of selenium disulfide cathode limit the application of lithium selenium disulfide batteries significantly. Here, a fibrous ATFG-COF, containing rich carbonyl and amino functional groups, was applied as the separator coating layer. Density Functional Theory (DFT) theoretical calculations and experimental results showed that the abundant carbonyl group in ATFG-COF had a positive effect on lithium ions, and the amino group formed hydrogen bonds with bis ((trifluoromethyl) sulfonyl) azanide anionics (TFSI−), which fixed TFSI− in the channel, so as to improve the transfer number of lithium ions and narrow the channels. Therefore, ATFG-COF fiber coating can not only form a rapid and uniform lithium-ion flow on the lithium anode to inhibit the growth of lithium dendrites, but also effectively screen polysulfide and polyselenide ions to suppress the shuttle effect. The Li-SeS2 cell with ATFG-COF/polypropylene (ATFG-COF/PP) separator exhibited good cycle stability at 0.5 C and maintained a specific capacity of 509 mAh/g after 200 cycles. Our work provides insights into the design of dual-function separators with high-performance batteries.

Coupling Water-Proof Li Anodes with LiOH-Based Cathodes Enables Highly Rechargeable Lithium-Air Batteries Operating in Ambient Air

Realizing an energy-dense, highly rechargeable nonaqueous lithium-oxygen battery in ambient air remains a big challenge because the active materials of the typical high-capacity cathode (Li2 O2 ) and anode (Li metal) are unstable in air. Herein, a novel lithium-oxygen full cell coupling a lithium anode protected by a composite layer of polyethylene oxide (PEO)/lithium aluminum titanium phosphate (LATP)/wax to a LiOH-based cathode is constructed. The protected lithium is stable in air and water, and permits reversible, dendrite-free lithium stripping/plating in a wet nonaqueous electrolyte under ambient air. The LiOH-based full cell reaction is immune to moisture (up to 99% humidity) in air and exhibits a much better resistance to CO2 contamination than Li2 O2 , resulting in a more consistent electrochemistry in the long term. The current approach of coupling a protected lithium anode with a LiOH-based cathode holds promise for developing a long-life, high-energy lithium-air battery capable of operating in the ambient atmosphere.

Reversing the Dendrite Growth Direction and Eliminating the Concentration Polarization via Internal Electric Field for Stable Lithium Metal Anodes

Lithium (Li) dendrite growth is a long-standing challenge leading to short cycle life and safety issues in Li metal batteries. Li dendrite growth is kinetically controlled by ion transport, the concentration gradient, and the local electric field. In this study, an internal electric field is generated between the anode and Au-modified separator to eliminate the concentration gradient of Li⁺. The Li–Au alloy is formed during the first cycle of Li plating/stripping, which causes Li⁺ deposition on the Au-modified side and lithium anode electrode, reversing the lithium dendrite growth direction. The electrically coupled Li metal electrode and Au-modified film create a uniform electric potential and Li⁺ concentration distribution, resulting in reduced concentration polarization and stable Li deposition. As a result, the Au-modified separator improves the lifespan of Li‖Li batteries; the Li‖LiFePO₄ cells show excellent capacity retention (>97.8% after 350 cycles), and Li‖LiNi_0.8Co_0.1Mn_0.1O₂ cells deliver 75.1% capacity retention for more than 300 cycles at 1C rate. This strategy offers an efficient approach for commercial application in advanced metallic Li batteries.

An internal electric field is built between the anode and the Au-modified separator to eliminate the concentration gradient of Li⁺ and reverse the dendrite growth direction.

Phenoxy Radical-Induced Formation of Dual-Layered Protection Film for High-Rate and Dendrite-Free Lithium-Metal Anodes

The uncontrollable dendrite growth of Li metal anode leads to poor cycle stability and safety concerns, hindering its utilization in high energy density batteries. Herein, a phenoxy radical Spiro-O8 is proposed as an artificial protection film for Li metal anode owing to its excellent film-forming capability and remarkable ionic conductivity. A spontaneous redox reaction between the Spiro-O8 and Li metal results in the formation of a uniform and highly ionic conductive organic film in the bottom. Meanwhile, the phenoxy radicals on surface of Spiro-O8 facilitate the decomposition of Li salt upon exposed to the ether electrolyte and lead the formation of LiF film on the top. Arising from the synergistic effects of inner high ionic conductive film and outer rigid film, stable Li plating/stripping can be realized at a high current density (4000 cycles at 10 mA cm^-2 ) and a high areal capacity of 5 mAh cm^-2 for 550 h with an ultrahigh Li utilization rate of 54.6 %. As a proof of concept, this work shows a facile strategy to rationally fabricate dual-layered interfaces for Li metal anodes.

In Situ Construction of Aramid Nanofiber Membrane on Li Anode as Artificial SEI Layer Achieving Ultra-High Stability

Achieving uniform Li deposition is vital for the construction of a safe but also efficient Li-metal anode for Li-metal batteries (LMBs). Herein, a facile coating strategy is used for forming an ultra-thin aramid nanofiber (ANF) membrane, with a network structure, on a Li anode (ANF-Li) as an artificial layer inhibiting Li dendrite's growth. The results show that under an ultra-high current density of 50 mA cm^-2 , the ANF-Li|ANF-Li symmetric cells can be kept stably cycled for a period exceeding 300 h. The ANF-Li|LiFePO₄ full cells exhibit a high-capacity retention of 80.1% after 1200 cycles at 1 C, showing a promising potential for LMBs application. Combined experimental results with theoretical calculations, the excellent performance of the ANF-Li anode is explored. Lithiophilic polar functional groups (CO, NH) appear in the surface and structure of ANF membrane, which offer high-concentration functional sites for the Li ions to realize an effective adhesion at the molecular level. This work also finds fiber-shaped lithium deposition for the first time. Furthermore, the nanoscale porosity of the ANF membrane not only provides fast pathways and channels for the diffusion of the electrolyte and Li transportation, but also eliminates the "weak links" of micron-scale Li dendrites penetrating the membrane.

Dual-Functional MgO Nanocrystals Satisfying Both Polysulfides and Li Regulation toward Advanced Lithium-Sulfur Full Batteries

Lithium-sulfur battery (LSB) is regarded as a preferential option for next-generation energy-storage system, but the lithium polysulfides (LiPSs) shuttling effect and the uncontrollable growth of dendritic Li in the anode impede its commercial viability. To address both of the issues simultaneously, a well-designed hybrid of MgO ultrafine nanocrystals dispersed on graphene-supported carbon nanosheets (MCG) is developed via a facile self-template strategy as dual-functional host for both sulfur and lithium. Relying on the coordination of strong LiPS-capturing capability, the shuttling effect is inhibited. Furthermore, the lithiophilic configuration with high specific surface area induce homogenous Li deposition, thus preventing the formation of disordered lithium dendrite. Integrating all these advantages, a full cell based on S@MCG cathode and Li@MCG@Cu anode exhibits a stable capacity at 0.5 C for 150 cycles with a low capacity fading rate. Furthermore, the full cell achieves a high capacity retention of 85.5% at a high S areal loading of 3.82 mg cm^-2 under the condition of a low electrolyte/sulfur ratio (E/S) of 6.5 µL mg^-1 and negative/positive capacity ratio (N/P) of 3. This strategy satisfying both cathode and anode host provides a viable approach to realize high-energy-density and dendrite-free LSBs.

In Situ Formed Li-Ag Alloy Interface Enables Li10GeP2S12-Based All-Solid-State Lithium Batteries

All-solid-state lithium-metal batteries (ASSLMBs) have received great interest due to their high potential to display both high energy density and safety performance. However, the poor compatibility at the Li/solid electrolyte (SE) interface and penetration of lithium dendrites during cycling strongly impede their successful commercialization. Herein, a thin Ag layer was introduced between Li and Li₁₀GeP₂S₁₂ for the in situ formation of a Li-Ag alloy interface, thus tuning the interfacial chemistry and lithium deposition/dissolution behavior. Superior electrochemical properties and improved interfacial stability were achieved by optimizing the Ag thicknesses. The assembled symmetric cell with Li@Ag 1 μm showed a steady voltage evolution up to 1000 h with an areal capacity of 1 mAh cm^-2. Moreover, a high reversible capacity of 106.5 mAh g^-1 was achieved in an all-solid-state cell after 100 cycles, demonstrating the validity of the Ag layer. This work highlights the importance of the Li/SE interface re-engineering and provides a new strategy for improving the cycle life of ASSLMBs.

A Novel Filler for Gel Polymer Electrolyte with a High Lithium-Ion Transference Number toward Stable Cycling for Lithium-Metal Anodes in Lithium-Sulfur Batteries

Although the lithium metal is considered as the most promising anode for high energy density batteries, uncontrolled lithium dendrite growth and continuous side reactions with electrolyte hinder its practical applications for rechargeable batteries. Herein, we prepared a gel polymer electrolyte by synthesizing a novel 250 nm filler (KMgF₃), which is greatly beneficial to the formation of a uniformly deposited lithium-metal anode. This is due to the regulation effect of KMgF₃ that double the lithium-ion transference number up to 0.63 and adjust the solid electrolyte interphase layer full of dense LiF and flexible polycarbonates, which greatly reduces the side reactions on the lithium-metal surface and inhibits the growth of lithium dendrites. Consequently, the composite gel polymer electrolyte guarantees a stable long cycle performance of more than 1400 h with 1 mA h cm^-2 for symmetric cells. Moreover, the composite gel polymer electrolyte demonstrates high compatibility and great promise for rechargeable lithium-sulfur (Li-S) batteries.

Commercialization-Driven Electrodes Design for Lithium Batteries: Basic Guidance, Opportunities, and Perspectives

Current lithium-ion battery technology is approaching the theoretical energy density limitation, which is challenged by the increasing requirements of ever-growing energy storage market of electric vehicles, hybrid electric vehicles, and portable electronic devices. Although great progresses are made on tailoring the electrode materials from methodology to mechanism to meet the practical demands, sluggish mass transport, and charge transfer dynamics are the main bottlenecks when increasing the areal/volumetric loading multiple times to commercial level. Thus, this review presents the state-of-the-art developments on rational design of the commercialization-driven electrodes for lithium batteries. First, the basic guidance and challenges (such as electrode mechanical instability, sluggish charge diffusion, deteriorated performance, and safety concerns) on constructing the industry-required high mass loading electrodes toward commercialization are discussed. Second, the corresponding design strategies on cathode/anode electrode materials with high mass loading are proposed to overcome these challenges without compromising energy density and cycling durability, including electrode architecture, integrated configuration, interface engineering, mechanical compression, and Li metal protection. Finally, the future trends and perspectives on commercialization-driven electrodes are offered. These design principles and potential strategies are also promising to be applied in other energy storage and conversion systems, such as supercapacitors, and other metal-ion batteries.

In Situ Chemical Lithiation Transforms Diamond-Like Carbon into an Ultrastrong Ion Conductor for Dendrite-Free Lithium-Metal Anodes

Lithium (Li)-metal anodes are of great promise for next-generation batteries due to their high theoretical capacity and low redox potential. However, Li-dendrite growth during cycling imposes a tremendous safety concern on the practical application of Li-metal anodes. Herein, an effective approach to suppress Li-dendrite growth by coating a polypropylene (PP) separator with a thin layer of ultrastrong diamond-like carbon (DLC) is reported. Theoretical calculations indicate that the DLC coating layer undergoes in situ chemical lithiation once assembled with the lithium-metal anode, transforming the DLC/PP separator into an excellent 3D Li-ion conductor. This in situ lithiated DLC/PP separator can not only mechanically suppress Li-dendrite growth by its intrinsically high modulus (≈100 GPa), but also uniformly redistributes Li ions to render dendrite-free lithium deposition. The twofold effects of the DLC/PP separator result in stable cycling of lithium plating/stripping (over 4500 h) at a high current density of 3 mA cm^-2 . Remarkably, this approach enables more than 1000 stable cycles at 5 C with a capacity retention of ≈71% in a Li || LiFePO₄ coin cell and more than 200 stable cycles at 0.2 C in a Li || LiNi_0.5 Co_0.3 Mn_0.2 O₂ pouch cell with cathode mass loading of ≈9 mg cm^-2 .

Facile Production of Phosphorene Nanoribbons towards Application in Lithium Metal Battery

Like phosphorene, phosphorene nanoribbon (PNR) promises exotic properties but unzipping phosphorene into edge-defined PNR is non-trivial because of uncontrolled cutting of phosphorene along random directions. Here a facile electrochemical strategy to fabricate zigzag-edged PNRs in high yield (>80%) is reported. The presence of chemically active zigzag edges in PNR allows it to spontaneously react with Li to form a Li⁺ ion conducting Li₃ P phase, which can be used as a protective layer on Li metal anode in lithium metal batteries (LMBs). PNR protective layer prevents the parasitic reaction between lithium metal and electrolyte and promotes Li⁺ ion diffusion kinetics, enabling homogenous Li⁺ ion flux and long-time cycling stability up to 1100 h at a current density of 1 mA cm^-2 . LiFePO₄ |PNR-Li full-cell batteries with an areal capacity of 2 mAh cm^-2 , a lean electrolyte (20 µl mAh^-1 ) and a negative/positive (N/P) electrodes ratio of 3.5 can be stably cycled over 100 cycles.

Confronting the Challenges in Lithium Anodes for Lithium Metal Batteries

With the low redox potential of -3.04 V (vs SHE) and ultrahigh theoretical capacity of 3862 mAh g-1 , lithium metal has been considered as promising anode material. However, lithium metal battery has ever suffered a trough in the past few decades due to its safety issues. Over the years, the limited energy density of the lithium-ion battery cannot meet the growing demands of the advanced energy storage devices. Therefore, lithium metal anodes receive renewed attention, which have the potential to achieve high-energy batteries. In this review, the history of the lithium anode is reviewed first. Then the failure mechanism of the lithium anode is analyzed, including dendrite, dead lithium, corrosion, and volume expansion of the lithium anode. Further, the strategies to alleviate the lithium anode issues in recent years are discussed emphatically. Eventually, remaining challenges of these strategies and possible research directions of lithium-anode modification are presented to inspire innovation of lithium anode.

Mixed ionic/electronic conducting nanosheet arrays for stable lithium storage

Lithium metal batteries (LMBs) have received extensive attention and research interest as high specific energy systems. However, the issues of Li dendrites growth in LMBs restrict their practical applications. The development of lithiophilic collectors can effectively solve the issues of Li dendrites growth. This study reports excellent lithium storage performance of lithiophilic nanosheet arrays which consist of electronic conductor Ni and ionic conductor Li₂O (Ni-LONSs) on Ni foil (NF) fabricated via a simple preparation method for LMBs. The ionic conductor Li₂O of the Ni-LONSs layer is lithiophilic and can induce uniform Li deposition on the Ni-LONSs collector. In addition, the nanosheet array structure of the Ni-LONSs collector is beneficial to slow down the volume change of the Li plating/stripping. In comparison with the NF collector, due to the specific nanosheet array structure of Ni-LONSs collector, the Ni-LONSs collector demonstrates excellent coulombic efficiency of 97.2% after 280 cycles (95.7% after 100 cycles of NF collector) and satisfactory cycling lifespan of 340 h (about 120 h of NF collector) at 0.5 mA cm^-2with 1.0 mAh cm^-2. Furthermore, the Ni-LONSs collector shows superior electrochemical performance in Ni-LONS/Li∣LiFePO₄full cells. The excellent lithium storage performance of Ni-LONSs collector with mixed ionic/electronic conductor is conducive to the development and practical applications of LMBs.