Protected Lithium‐Metal Anodes in Batteries: From Liquid to Solid

High‐energy lithium‐metal batteries are among the most promising candidates for next‐generation energy storage systems. With a high specific capacity and a low reduction potential, the Li‐metal anode has attracted extensive interest for decades. Dendritic Li formation, uncontrolled interfacial reactions, and huge volume effect are major hurdles to the commercial application of Li‐metal anodes. Recent studies have shown that the performance and safety of Li‐metal anodes can be significantly improved via organic electrolyte modification, Li‐metal interface protection, Li‐electrode framework design, separator coating, and so on. Superior to the liquid electrolytes, solid‐state electrolytes are considered able to inhibit problematic Li dendrites and build safe solid Li‐metal batteries. Inspired by the bright prospects of solid Li‐metal batteries, increasing efforts have been devoted to overcoming the obstacles of solid Li‐metal batteries, such as low ionic conductivity of the electrolyte and Li–electrolyte interfacial problems. Here, the approaches to protect Li‐metal anodes from liquid batteries to solid‐state batteries are outlined and analyzed in detail. Perspectives regarding the strategies for developing Li‐metal anodes are discussed to facilitate the practical application of Li‐metal batteries.     

Regulating Li-Ion Transport through Ultrathin Molecular Membrane to Enable High-Performance All-Solid-State–Battery

Solid-state lithium metal batteries with garnet-type electrolyte provide several advantages over conventional lithium-ion batteries, especially for safety and energy density. However, a few grand challenges such as the propagation of Li dendrites, poor interfacial contact between the solid electrolyte and the electrodes, and formation of lithium carbonate during ambient exposure over the solid-state electrolyte prevent the viability of such batteries. Herein, an ultrathin sub-nanometer porous carbon nanomembrane (CNM) is employed on the surface of solid-state electrolyte (SSE) that increases the adhesion of SSE with electrodes, prevents lithium carbonate formation over the surface, regulates the flow of Li-ions, and blocks any electronic leakage. The sub-nanometer scale pores in CNM allow rapid permeation of Li-ions across the electrode–electrolyte interface without the presence of any liquid medium. Additionally, CNM suppresses the propagation of Li dendrites by over sevenfold up to a current density of 0.7 mA cm⁻² and enables the cycling of all-solid-state batteries at low stack pressure of 2 MPa using LiFePO₄ cathode and Li metal anode. The CNM provides chemical stability to the solid electrolyte for over 4 weeks of ambient exposure with less than a 4% increase in surface impurities.     

Lithium–Graphite Paste: An Interface Compatible Anode for Solid‐State Batteries

All‐solid‐state batteries (ASSBs) with ceramic‐based solid‐state electrolytes (SSEs) enable high safety that is inaccessible with conventional lithium‐ion batteries. Lithium metal, the ultimate anode with the highest specific capacity, also becomes available with nonflammable SSEs in ASSBs, which offers promising energy density. The rapid development of ASSBs, however, is significantly hampered by the large interfacial resistance as a matched lithium/ceramic interface that is not easy to pursue. Here, a lithium–graphite (Li–C) composite anode is fabricated, which shows a dramatic modification in wettability with garnet SSE. An intimate Li–C/garnet interface is obtained by casting Li–C composite onto garnet‐type SSE, delivering an interfacial resistance as low as 11 Ω cm². As a comparison, pure Li/garnet interface gives a large resistance of 381 Ω cm². Such improvement can be ascribed to the experiment‐measured increased viscosity of Li–C composite and simulation‐verified limited interfacial reaction. The Li–C/garnet/Li–C symmetric cell exhibits stable plating/striping performance with small voltage hysteresis and endures a critical current density up to 1.0 mA cm^?2. The full cell paired with LiFePO₄ shows stable cycle performance, comparable to the cell with liquid electrolyte. The present work demonstrates a promising strategy to develop ceramic‐compatible lithium metal‐based anodes and hence low‐impedance ASSBs.     

Dual‐Layered Film Protected Lithium Metal Anode to Enable Dendrite‐Free Lithium Deposition

Lithium metal batteries (such as lithium–sulfur, lithium–air, solid state batteries with lithium metal anode) are highly considered as promising candidates for next‐generation energy storage systems. However, the unstable interfaces between lithium anode and electrolyte definitely induce the undesired and uncontrollable growth of lithium dendrites, which results in the short‐circuit and thermal runaway of the rechargeable batteries. Herein, a dual‐layered film is built on a Li metal anode by the immersion of lithium plates into the fluoroethylene carbonate solvent. The ionic conductive film exhibits a compact dual‐layered feature with organic components (ROCO₂Li and ROLi) on the top and abundant inorganic components (Li₂CO₃ and LiF) in the bottom. The dual‐layered interface can protect the Li metal anode from the corrosion of electrolytes and regulate the uniform deposition of Li to achieve a dendrite‐free Li metal anode. This work demonstrates the concept of rational construction of dual‐layered structured interfaces for safe rechargeable batteries through facile surface modification of Li metal anodes. This not only is critically helpful to comprehensively understand the functional mechanism of fluoroethylene carbonate but also affords a facile and efficient method to protect Li metal anodes.     

Anode Interface Engineering and Architecture Design for High‐Performance Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries are considered as one of the most promising options to realize rechargeable batteries with high energy capacity. Previously, research has mainly focused on solving the polysulfides' shuttle, cathode volume changes, and sulfur conductivity problems. However, the instability of anodes in Li–S batteries has become a bottleneck to achieving high performance. Herein, the main efforts to develop highly stable anodes for Li–S batteries, mainly including lithium metal anodes, carbon‐based anodes, and alloy‐based anodes, are considered. Based on these anodes, their interfacial engineering and structure design are identified as the two most important directions to achieve ideal anodes. Because of high reactivity and large volume change during cycling, Li anodes suffer from severe side reactions and structure collapse. The solid electrolyte interphase formed in situ by modified electrolytes and ex situ artificial coating layers can enhance the interfacial stability of anodes. Replacing common Li foil with rationally designed anodes not only suppresses the formation of dendritic Li but also delays the failure of Li anodes. Manipulating the anode interface engineering and rationally designing anode architecture represents an attractive path to develop high‐performance Li–S batteries.     

A Highly Reversible,Dendrite-Free Lithium Metal Anode Enabled by a Lithium-Fluoride-Enriched Interphase

Metallic lithium is the most competitive anode material for next-generation lithium (Li)-ion batteries. However, one of its major issues is Li dendrite growth and detachment, which not only causes safety issues, but also continuously consumes electrolyte and Li, leading to low coulombic efficiency (CE) and short cycle life for Li metal batteries. Herein, the Li dendrite growth of metallic lithium anode is suppressed by forming a lithium fluoride (LiF)-enriched solid electrolyte interphase (SEI) through the lithiation of surface-fluorinated mesocarbon microbeads (MCMB-F) anodes. The robust LiF-enriched SEI with high interfacial energy to Li metal effectively promotes planar growth of Li metal on the Li surface and meanwhile prevents its vertical penetration into the LiF-enriched SEI from forming Li dendrites. At a discharge capacity of 1.2 mAh cm⁻², a high CE of >99.2% for Li plating/stripping in FEC-based electrolyte is achieved within 25 cycles. Coupling the pre-lithiated MCMB-F (Li@MCMB-F) anode with a commercial LiFePO₄ cathode at the positive/negative (P/N) capacity ratio of 1:1, the LiFePO₄//Li@MCMB-F cells can be charged/discharged at a high areal capacity of 2.4 mAh cm⁻² for 110 times at a negligible capacity decay of 0.01% per cycle.     

Solid state lithium metal batteries – Issues and challenges at the lithium-solid electrolyte interface

Solid-state Li-ion batteries employing a metallic lithium anode in conjunction with an inorganic solid electrolyte (ISE) are expected to offer superior energy density and cycle life. The realization of these metrics critically hinges on the simultaneous optimization of the ISE and the two electrode/electrolyte interfaces. In this Opinion article, we provide an overview of the materials and interfacial challenges that limit the performance of solid-state lithium metal batteries (SSLMBs). Owing to the importance of the Li/ISE interface, we dedicate a large section of this article to discuss the mechanistic aspects of lithium deposition at the Li/ISE interface. We further discuss a few recently proposed mechanisms that rationalize the growth of lithium through ISEs. We conclude our review with a brief discussion on the anode-free approach for fabricating SSLMBs where metallic lithium is generated in-situ from pre-lithiated cathodes.     

Tuning the Anode–Electrolyte Interface Chemistry for Garnet-Based Solid-State Li Metal Batteries

Lithium (Li) metal is a promising candidate as the anode for high-energy-density solid-state batteries. However, interface issues, including large interfacial resistance and the generation of Li dendrites, have always frustrated the attempt to commercialize solid-state Li metal batteries (SSLBs). Here, it is reported that infusing garnet-type solid electrolytes (GSEs) with the air-stable electrolyte Li₃PO₄ (LPO) dramatically reduces the interfacial resistance to ≈1 Ω cm² and achieves a high critical current density of 2.2 mA cm⁻² under ambient conditions due to the enhanced interfacial stability to the Li metal anode. The coated and infused LPO electrolytes not only improve the mechanical strength and Li-ion conductivity of the grain boundaries, but also form a stable Li-ion conductive but electron-insulating LPO-derived solid-electrolyte interphase between the Li metal and the GSE. Consequently, the growth of Li dendrites is eliminated and the direct reduction of the GSE by Li metal over a long cycle life is prevented. This interface engineering approach together with grain-boundary modification on GSEs represents a promising strategy to revolutionize the anode–electrolyte interface chemistry for SSLBs and provides a new design strategy for other types of solid-state batteries.     

Mixed Ionic and Electronic Conductor for Li‐Metal Anode Protection

Li‐metal is the optimal choice as an anode due to its highest energy density. However, Li‐anodes suffer safety problems from dendritic Li‐growth and continuous corrosion by liquid electrolytes. Here, an effective strategy of using ultrathin and conformal mixed ionic and electronic ceramic conductor (MIEC) is proposed to stabilize Li‐anodes. An ultrathin Li_0.35La_0.52[V]_0.13TiO₃ (LLTO) ceramic film with superior ionic conductivity is first obtained by sintering single‐crystal LLTO nanoparticles, which have controlled surface facets and particle sizes. Then the MIEC property is developed in the LLTO film by introducing toluene as catalyst, which triggers the chemical reactions between LLTO and Li‐metal, leading to high electronic conductivity in the LLTO film. After evaporating toluene, a hybrid LLTO/Li anode with a conformal and stable interface is formed. When applying the hybrid anodes in Li‐metal batteries, the MIEC ceramic film blocks Li‐corrosion from electrolyte and the formation of Li‐dendrites by buffering the Li‐ion concentration gradient and leveling secondary current distribution on Li‐metal surface. At the same time, the Coulombic efficiency of batteries reaches to 98%. This finding will impact the general approach for tailoring the properties of Li‐metal anodes for achieving better Li‐metal battery performance.     

Recent research progress of alloy-containing lithium anodes in lithium-metal batteries

Lithium metal is regarded as one of the most ideal anode materials for next-generation batteries, due to its high theoretical capacity of 3860 mAh g⁻¹ and low redox potential (−3.04 V vs standard hydrogen electrode). However, practical applications of lithium anodes are impeded by the uncontrollable growth of lithium dendrite and continuous reactions between lithium and electrolyte during cycling processes. According to reports for decades, artificial solid electrolyte interface (SEI), electrolyte additives, and construction of three-dimensional (3D) structures are demonstrated essential strategies. Among numerous approaches, metals that can alloy with lithium have been employed to homogenize lithium deposition and accelerate Li ion transportation, which attract more and more attention. This review aims to summarize the lithium alloying applied in lithium anodes including the fabricating approaches of alloy-containing lithium anodes, and the action mechanism and challenges of fabricated lithium anodes. Based on summarizing the literature, shortcomings and challenges as well as the prospects are also analyzed, to impel further research of lithium anodes and lithium-based batteries.     

Dual‐Phase Single‐Ion Pathway Interfaces for Robust Lithium Metal in Working Batteries

The lithium (Li) metal anode is confronted by severe interfacial issues that strongly hinder its practical deployment. The unstable interfaces directly induce unfavorable low cycling efficiency, dendritic Li deposition, and even strong safety concerns. An advanced artificial protective layer with single‐ion pathways holds great promise for enabling a spatially homogeneous ionic and electric field distribution over Li metal surface, therefore well protecting the Li metal anode during long‐term working conditions. Herein, a robust dual‐phase artificial interface is constructed, where not only the single‐ion‐conducting nature, but also high mechanical rigidity and considerable deformability can be fulfilled simultaneously by the rational integration of a garnet Al‐doped Li_6.75La₃Zr_1.75Ta_0.25O₁₂‐based bottom layer and a lithiated Nafion top layer. The as‐constructed artificial solid electrolyte interphase is demonstrated to significantly stabilize the repeated cell charging/discharging process via regulating a facile Li‐ion transport and a compact Li plating behavior, hence contributing to a higher coulombic efficiency and a considerably enhanced cyclability of lithium metal batteries. This work highlights the significance of rational manipulation of the interfacial properties of a working Li metal anode and affords fresh insights into achieving dendrite‐free Li deposition behavior in a working battery.     

Overcoming the Interfacial Limitations Imposed by the Solid–Solid Interface in Solid‐State Batteries Using Ionic Liquid‐Based Interlayers

Li‐garnets are promising inorganic ceramic solid electrolytes for lithium metal batteries, showing good electrochemical stability with Li anode. However, their brittle and stiff nature restricts their intimate contact with both the electrodes, hence presenting high interfacial resistance to the ionic mobility. To address this issue, a strategy employing ionic liquid electrolyte (ILE) thin interlayers at the electrodes/electrolyte interfaces is adopted, which helps overcome the barrier for ion transport. The chemically stable ILE improves the electrodes‐solid electrolyte contact, significantly reducing the interfacial resistance at both the positive and negative electrodes interfaces. This results in the more homogeneous deposition of metallic lithium at the negative electrode, suppressing the dendrite growth across the solid electrolyte even at high current densities of 0.3 mA cm^?2. Further, the improved interface Li/electrolyte interface results in decreasing the overpotential of symmetric Li/Li cells from 1.35 to 0.35 V. The ILE modified Li/LLZO/LFP cells stacked either in monopolar or bipolar configurations show excellent electrochemical performance. In particular, the bipolar cell operates at a high voltage (≈8 V) and delivers specific capacity as high as 145 mAh g^?1 with a coulombic efficiency greater than 99%.     

The ionic interphases of the lithium anode in solid state batteries

Solid state battery (SSB) performance is largely governed by processes occurring at electrolyte–electrode interfaces. At the Li metal anode, where the overwhelming majority of solid electrolyte (SE) are unstable against Li metal, the interface can readily react to form emergent Li-solid electrolyte interphases (SEI) with ionic, electronic, chemical, mechanical, and electrochemical properties substantially distinct from the parent phase. Facing similar challenges with liquid electrolytes, the Li battery community underwent a half century-long effort, still in progress, to illuminate fundamental properties of the Li SEI—including chemistry, morphology, transport, and sources of Li loss upon cycling—from which guiding principles have emerged to drive improvement in electrolyte and interface design. The Li metal SEI with solid electrolytes presents both similarities and differences to that in liquid electrolytes, with differences defining unique research needs. Here, we examine current understanding of the Li-SE interface as well as learnings from the liquid electrolyte community that we propose might be adopted to help rationalize and improve SE integration with Li anodes. Through this lens, we inspect current state-of-understanding of Li SEI composition, structure, and properties, along with Coulombic efficiency values reported so far for Li cycling with SE. We also highlight potential Li modification strategies for SSB, which are informed by and exploit understanding of the ionic SEI phases; in some instances, engineering strategies utilize a liquid electrolyte SEI directly, making liquid-derived SEI knowledge of immediate relevance.     

固态锂电池用MOF/聚氧化乙烯复合聚合物电解质(英文)

固态聚合物电解质具有柔韧性好和易于加工的优势,可制备各种形状的固态锂电池,杜绝漏液问题。但固态聚合物电解质存在离子电导率低以及对锂金属负极不稳定等问题。本研究以纳米金属–有机框架材料UiO-66为聚合物电解质的填料,用于改善电解质的性能。UiO-66与聚氧化乙烯(poly(ethylene oxide), PEO)链上醚基的氧原子的配位作用以及与锂盐中阴离子的相互作用,可显著提高聚合物电解质的离子电导率(25℃,3.0×10^–5 S/cm;60℃,5.8×10^–4 S/cm),并将锂离子迁移数提高至0.36,电化学窗口拓宽至4.9V。此外,制备的PEO基固态电解质对金属锂具有良好的稳定性,对称电池在60℃、0.15mA·cm^–2电流密度下可稳定循环1000h,锂电池的电化学性能得到显著改善。     

面向可充电电池的锂金属负极的枝晶生长:理论基础、影响因素和抑制方法

在全球能源与环境问题日趋紧迫的大背景下,可再生能源的获取与利用途径及高效安全的储能技术的研发一直是工业界和科学界关注的热点之一。锂离子二次电池作为能量存储器件,拥有高比能量、长循环寿命等优点,近十几年来其研究取得了长足进展,并在各类便携式电子设备和电动交通工具中获得了广泛应用。然而,随着各种高性能设备的不断涌现,商业化的锂离子电池越来越难以满足其在能量密度、循环稳定性和安全性等方面的要求。为了进一步提高锂离子电池的能量密度,需要开发出高比容量的负极材料(硅、锡和锂等)以取代传统石墨负极。硅、锡等新式负极材料通过与锂离子反应形成含锂化合物的原理来存储与释放锂离子,完成电池的一个充放电过程。这个过程往往伴随着负极材料体积的剧烈变化,经历较长时间循环使用后会导致负极材料的粉化甚至从集流体上剥离,引起电池容量迅速衰减甚至失效。而锂负极通过锂在负极上的溶解和沉积来完成电池的充放电过程,该过程不存在反应相变所导致的体积变化。另外,锂金属负极材料具有极高的质量比容量(3 860mAh/g)、低密度(0.59g/cm3)和低的还原电位(-3.04V,相比于氢标准电极),被认为是一种理想的可充电电池负极材料。然而,锂的枝晶生长、锂金属电池低的库伦效率和锂的无主体沉积引起的体积膨胀等一些关键问题长期以来制约着锂负极的商业应用。锂的每次沉积都会产生枝晶,在充放电循环中,锂枝晶会导致电池内部短路甚至发生爆炸,带来严重的安全问题。除此之外,锂枝晶还会增加负极表面积,新暴露的锂金属会与电解液反应生成固态电解质膜(Solid electrolyte interface,SEI),这会损耗活性材料以及降低电池的库伦效率。为了解决以上问题,研究者们对锂金属电极进行了许多探索,尤其是在锂枝晶生长的机理及其抑制方法方面。一些理论模型如扩散模型、SEI保护模型、电荷诱导生长模型和薄膜生长模型等,以及与这些模型相对应的一些抑制方法如均匀锂离子流法、SEI膜保护法、稳定沉积主体法和静电屏蔽保护法等被提出。这些抑制方法能够在一定程度上缓解锂枝晶的生长问题,但都未能达到商业化应用的要求。本文总结了近几年研究人员针对锂离子电池锂金属负极的一些重要研究,系统地介绍了业内较为认同的枝晶生长模型和影响因素,并着重叙述了抑制枝晶生长的方法及成效,最后就锂金属负极将来的研究方向给出一些建议。     

Reducing Interfacial Resistance between Garnet‐Structured Solid‐State Electrolyte and Li‐Metal Anode by a Germanium Layer

Substantial efforts are underway to develop all‐solid‐state Li batteries (SSLiBs) toward high safety, high power density, and high energy density. Garnet‐structured solid‐state electrolyte exhibits great promise for SSLiBs owing to its high Li‐ion conductivity, wide potential window, and sufficient thermal/chemical stability. A major challenge of garnet is that the contact between the garnet and the Li‐metal anodes is poor due to the rigidity of the garnet, which leads to limited active sites and large interfacial resistance. This study proposes a new methodology for reducing the garnet/Li‐metal interfacial resistance by depositing a thin germanium (Ge) (20 nm) layer on garnet. By applying this approach, the garnet/Li‐metal interfacial resistance decreases from ≈900 to ≈115 Ω cm² due to an alloying reaction between the Li metal and the Ge. In agreement with experiments, first‐principles calculation confirms the good stability and improved wetting at the interface between the lithiated Ge layer and garnet. In this way, this unique Ge modification technique enables a stable cycling performance of a full cell of lithium metal, garnet electrolyte, and LiFePO₄ cathode at room temperature.     

Polymer electrolytes and interfaces in solid-state lithium metal batteries

The polymer electrolyte based solid-state lithium metal batteries are the promising candidate for the high-energy electrochemical energy storage with high safety and stability. Moreover, the intrinsic properties of polymer electrolytes and interface contact between electrolyte and electrodes have played critical roles for determining the comprehensive performances of solid-state lithium metal batteries. In this review, the development of polymer electrolytes with the design strategies by functional units adjustments are firstly discussed. Then the interfaces between polymer electrolyte and cathode/anode, including the interface issues, remedy strategies for stabilizing the interface contact and reducing resistances, and the in-situ polymerization method for enhancing the compatibilities and assembling the batteries with favorable performances, have been introduced. Lastly, the perspectives on developing polymer electrolytes by functional units adjustment, and improving interface contact and stability by effective strategies for solid-state lithium metal batteries have been provided.     

In Operando Probing of Lithium‐Ion Storage on Single‐Layer Graphene

Despite high‐surface area carbons, e.g., graphene‐based materials, being investigated as anodes for lithium (Li)‐ion batteries, the fundamental mechanism of Li‐ion storage on such carbons is insufficiently understood. In this work, the evolution of the electrode/electrolyte interface is probed on a single‐layer graphene (SLG) film by performing Raman spectroscopy and Fourier transform infrared spectroscopy when the SLG film is electrochemically cycled as the anode in a half cell. The utilization of SLG eliminates the inevitable intercalation of Li ions in graphite or few‐layer graphene, which may have complicated the discussion in previous work. Combining the in situ studies with ex situ observations and ab initio simulations, the formation of solid electrolyte interphase and the structural evolution of SLG are discussed when the SLG is biased in an electrolyte. This study provides new insights into the understanding of Li‐ion storage on SLG and suggests how high‐surface‐area carbons could play proper roles in anodes for Li‐ion batteries.     

All‐Solid‐State Batteries with a Limited Lithium Metal Anode at Room Temperature using a Garnet‐Based Electrolyte

Metallic lithium (Li), considered as the ultimate anode, is expected to promise high‐energy rechargeable batteries. However, owing to the continuous Li consumption during the repeated Li plating/stripping cycling, excess amount of the Li metal anode is commonly utilized in lithium‐metal batteries (LMBs), leading to reduced energy density and increased cost. Here, an all‐solid‐state lithium‐metal battery (ASSLMB) based on a garnet‐oxide solid electrolyte with an ultralow negative/positive electrode capacity ratio (N/P ratio) is reported. Compared with the counterpart using a liquid electrolyte at the same low N/P ratios, ASSLMBs show longer cycling life, which is attributed to the higher Coulombic efficiency maintained during cycling. The effect of the species of the interface layer on the cycling performance of ASSLMBs with low N/P ratio is also studied. Importantly, it is demonstrated that the ASSLMB using a limited Li metal anode paired with a LiFePO₄ cathode (5.9 N/P ratio) delivers a stable long‐term cycling performance at room temperature. Furthermore, it is revealed that enhanced specific energies for ASSLMBs with low N/P ratios can be further achieved by the use of a high‐voltage or high mass‐loading cathode. This study sheds light on the practical high‐energy all‐solid‐state batteries under the constrained condition of a limited Li metal anode.     

Facile Synthesis of Ant‐Nest‐Like Porous Duplex Copper as Deeply Cycling Host for Lithium Metal Anodes

Suppressing the dendrite formation and managing the volume change of lithium (Li) metal anode have been global challenges in the lithium batteries community. Herein, a duplex copper (Cu) foil with an ant‐nest‐like network and a dense substrate is reported for an ultrastable Li metal anode. The duplex Cu is fabricated by sulfurization of thick Cu foil with a subsequent skeleton self‐welding procedure. Uniform Li deposition is achieved by the 3D interconnected architecture and lithiophilic surface of self‐welded Cu skeleton. The sufficient space in the porous layer enables a large areal capacity for Li and significantly improves the electrode–electrolyte interface. Simulations reveal that the structure allows proper electric field penetration into the connected tunnels. The assembled Li anodes exhibit high coulombic efficiency (97.3% over 300 cycles) and long lifespan (>880 h) at a current density of 1 mA cm^?2 with a capacity of 1 mAh cm^?2. Stable and deep cycling can be maintained up to 50 times at a high capacity of 10 mAh cm^?2.