An Electron/Ion Dual‐Conductive Alloy Framework for High‐Rate and High‐Capacity Solid‐State Lithium‐Metal Batteries

The solid‐state Li battery is a promising energy‐storage system that is both safe and features a high energy density. A main obstacle to its application is the poor interface contact between the solid electrodes and the ceramic electrolyte. Surface treatment methods have been proposed to improve the interface of the ceramic electrolytes, but they are generally limited to low‐capacity or short‐term cycling. Herein, an electron/ion dual‐conductive solid framework is proposed by partially dealloying the Li–Mg alloy anode on a garnet‐type solid‐state electrolyte. The Li–Mg alloy framework serves as a solid electron/ion dual‐conductive Li host during cell cycling, in which the Li metal can cycle as a Li‐rich or Li‐deficient alloy anode, free from interface deterioration or volume collapse. Thus, the capacity, current density, and cycle life of the solid Li anode are improved. The cycle capability of this solid anode is demonstrated by cycling for 500 h at 1 mA cm^?2, followed by another 500 h at 2 mA cm^?2 without short‐circuiting, realizing a record high cumulative capacity of 750 mA h cm^?2 for garnet‐type all‐solid‐state Li batteries. This alloy framework with electron/ion dual‐conductive pathways creates the possibility to realize high‐energy solid‐state Li batteries with extended lifespans.     

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.     

A Natural Biopolymer Film as a Robust Protective Layer to Effectively Stabilize Lithium‐Metal Anodes

Li metal is considered as an ideal anode for Li‐based batteries. Unfortunately, the growth of Li dendrites during cycling leads to an unstable interface, a low coulombic efficiency, and a limited cycling life. Here, a novel approach is proposed to protect the Li‐metal anode by using a uniform agarose film. This natural biopolymer film exhibits a high ionic conductivity, high elasticity, and chemical stability. These properties enable a fast Li‐ion transfer and feasiblity to accomodate the volume change of Li metal, resulting in a dendrite‐free anode and a stable interface. Morphology characterization shows that Li ions migrate through the agarose film and then deposit underneath it. A full cell with the cathode of LiFPO₄ and an anode contaning the agarose film exhibits a capacity retention of 87.1% after 500 cycles, much better than that with Li foil anode (70.9%) and Li‐deposited Cu anode (5%). This study provides a promising strategy to eliminate dendrites and enhance the cycling ability of lithium‐metal batteries through coating a robust artificial film of natural biopolymer on lithium‐metal anode.     

Double‐Layer Polymer Electrolyte for High‐Voltage All‐Solid‐State Rechargeable Batteries

No single polymer or liquid electrolyte has a large enough energy gap between the empty and occupied electronic states for both dendrite‐free plating of a lithium‐metal anode and a Li⁺ extraction from an oxide host cathode without electrolyte oxidation in a high‐voltage cell during the charge process. Therefore, a double‐layer polymer electrolyte is investigated, in which one polymer provides dendrite‐free plating of a Li‐metal anode and the other allows a Li⁺ extraction from an oxide host cathode without oxidation of the electrolyte in a 4 V cell over a stable charge/discharge cycling at 65 °C; a poly(ethylene oxide) polymer contacts the lithium‐metal anode and a poly(N‐methyl‐malonic amide) contacts the cathode. All interfaces of the flexible, plastic electrolyte remain stable with no visible reduction of the Li⁺ conductivity on crossing the polymer/polymer interface.     

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.     

Bending‐Tolerant Anodes for Lithium‐Metal Batteries

Bendable energy‐storage systems with high energy density are demanded for conformal electronics. Lithium‐metal batteries including lithium–sulfur and lithium–oxygen cells have much higher theoretical energy density than lithium‐ion batteries. Reckoned as the ideal anode, however, Li has many challenges when directly used, especially its tendency to form dendrite. Under bending conditions, the Li‐dendrite growth can be further aggravated due to bending‐induced local plastic deformation and Li‐filaments pulverization. Here, the Li‐metal anodes are made bending tolerant by integrating Li into bendable scaffolds such as reduced graphene oxide (r‐GO) films. In the composites, the bending stress is largely dissipated by the scaffolds. The scaffolds have increased available surface for homogeneous Li plating and minimize volume fluctuation of Li electrodes during cycling. Significantly improved cycling performance under bending conditions is achieved. With the bending‐tolerant r‐GO/Li‐metal anode, bendable lithium–sulfur and lithium–oxygen batteries with long cycling stability are realized. A bendable integrated solar cell–battery system charged by light with stable output and a series connected bendable battery pack with higher voltage is also demonstrated. It is anticipated that this bending‐tolerant anode can be combined with further electrolytes and cathodes to develop new bendable energy systems.     

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.     

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.     

High‐Voltage Lithium‐Metal Batteries Enabled by Localized High‐Concentration Electrolytes

Rechargeable lithium‐metal batteries (LMBs) are regarded as the “holy grail” of energy‐storage systems, but the electrolytes that are highly stable with both a lithium‐metal anode and high‐voltage cathodes still remain a great challenge. Here a novel “localized high‐concentration electrolyte” (HCE; 1.2 m lithium bis(fluorosulfonyl)imide in a mixture of dimethyl carbonate/bis(2,2,2‐trifluoroethyl) ether (1:2 by mol)) is reported that enables dendrite‐free cycling of lithium‐metal anodes with high Coulombic efficiency (99.5%) and excellent capacity retention (>80% after 700 cycles) of Li||LiNi_1/3Mn_1/3Co_1/3O₂ batteries. Unlike the HCEs reported before, the electrolyte reported in this work exhibits low concentration, low cost, low viscosity, improved conductivity, and good wettability that make LMBs closer to practical applications. The fundamental concept of “localized HCEs” developed in this work can also be applied to other battery systems, sensors, supercapacitors, and other electrochemical systems.     

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.     

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.     

Considering Critical Factors of Li‐rich Cathode and Si Anode Materials for Practical Li‐ion Cell Applications

In order to keep pace with increasing energy demands for advanced electronic devices and to achieve commercialization of electric vehicles and energy‐storage systems, improvements in high‐energy battery technologies are required. Among the various types of batteries, lithium ion batteries (LIBs) are among the most well‐developed and commercialized of energy‐storage systems. LIBs with Si anodes and Li‐rich cathodes are one of the most promising alternative electrode materials for next‐generation, high‐energy batteries. Si and Li‐rich materials exhibit high reversible capacities of <2000 mAh g^?1 and >240 mAh g^‐1, respectively. However, both materials have intrinsic drawbacks and practical limitations that prevent them from being utilized directly as active materials in high‐energy LIBs. Examples for Li‐rich materials include phase distortion during cycling and side reactions caused by the electrolyte at the surface, and for Si, large volume changes during cycling and low conductivity are observed. Recent progress and important approaches adopted for overcoming and alleviating these drawbacks are described in this article. A perspective on these matters is suggested and the requirements for each material are delineated, in addition to introducing a full‐cell prototype utilizing a Li‐rich cathode and Si anode.     

Additive‐Assisted Novel Dual‐Salt Electrolyte Addresses Wide Temperature Operation of Lithium–Metal Batteries

In this study, self‐synthesized lithium trifluoro(perfluoro‐tert‐butyloxyl)borate (LiTFPFB) is combined with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to formulate a novel 1 m dual‐salt electrolyte, which contains lithium difluorophosphate (LiPO₂F₂) additive and dominant carbonate solvents with low melting point and high boiling point. The addition of LiPO₂F₂ into this novel dual‐salt electrolyte dramatically improves cycleability and rate capability of a LiNi_0.5Mn_0.3Co_0.2O₂/Li (NMC/Li) battery, ranging from ?40 to 90 °C. The NMC/Li batteries adopt a Li–metal anode with low thickness of 100 µm (even 50 µm) and a moderately high cathode mass loading level of 10 mg cm^?2. For the first time, this paper provides valuable perspectives for developing practical lithium–metal batteries over a wide temperature range.     

The Salt Matters: Enhanced Reversibility of Li–O2 Batteries with a Li[(CF3SO2)(n‐C4F9SO2)N]‐Based Electrolyte

The safety hazards and cycle instability of lithium metal anodes (LMA) constitute significant barriers to progress in lithium metal batteries. This situation is worse in Li–O₂ batteries because the LMA is prone to be chemically attacked by O₂ shuttled from the cathode. Notwithstanding, efforts on LMA are much sparse than those on the cathode in the realm of Li–O₂ batteries. Here, a novel lithium salt of Li[(CF₃SO₂)(n‐C₄F₉SO₂)N] (LiTNFSI) is reported, which can effectively suppress the parasitic side reactions and dendrite growth of LMA during cycling and thereby significantly enhance the overall reversibility of Li–O₂ batteries. A variety of advanced research tools are employed to scrutinize the working principles of the LiTNFSI salt. It is revealed that a stable, uniform, and O₂‐resistive solid electrolyte interphase is formed on LMA, and hence the “cross‐talk” between the LMA and O₂ shuttled from the cathode is remarkably inhibited in LiTNFSI‐based Li–O₂ 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.     

High‐Coulombic‐Efficiency Carbon/Li Clusters Composite Anode without Precycling or Prelithiation

Lithium metal has attracted much research interest as a possible anode material for high‐energy‐density lithium‐ion batteries in recent years. However, its practical use is severely limited by uncontrollable deposition, volume expansion, and dendrite formation. Here, a metastable state of Li, Li cluster, that forms between LiC₆ and Li dendrites when over‐lithiating carbon cloth (CC) is discovered. The Li clusters with sizes in the micrometer and submicrometer scale own outstanding electrochemical reversibility between Li⁺ and Li, allowing the CC/Li clusters composite anode to demonstrate a high first‐cycle coulombic efficiency (CE) of 94.5% ± 1.0% and a stable CE of 99.9% for 160 cycles, which is exceptional for a carbon/lithium composite anode. The CC/Li clusters composite anode shows a high capacity of 3 mAh cm^?2 contributed by both Li⁺ intercalation and Li‐cluster formation, and excellent cycling stability with a signature sloping voltage profile. Furthermore, the CC/Li clusters composite anode can be assembled into full cells without precycling or prelithiation. The full cells containing bare CC as the anode and excessive LiCoO₂ as the cathode exhibit high specific capacity and good cyclic stability in 200 cycles, stressing the advantage of controlled formation of Li clusters.     

Extended Electrochemical Window of Solid Electrolytes via Heterogeneous Multilayered Structure for High‐Voltage Lithium Metal Batteries

In response to the call for safer high‐energy‐density storage systems, high‐voltage solid‐state Li metal batteries have attracted extensive attention. Therefore, solid electrolytes are required to be stable against both Li anode and high‐voltage cathodes; nevertheless, the requirements still cannot be completely satisfied. Herein, a heterogeneous multilayered solid electrolyte (HMSE) is proposed to broaden electrochemical window of solid electrolytes to 0–5 V, through different electrode/electrolyte interfaces to overcome the interfacial instability problems. Oxidation‐resistance poly(acrylonitrile) (PAN) is in contact with the cathode, while reduction tolerant polyethylene glycol diacrylate contacts with Li metal anode. A Janus and flexible PAN@Li_1.4Al_0.4Ge_1.6(PO₄)₃ (80 wt%) composite electrolyte is designed as intermediate layer to inhibit dendrite penetration and ensure compact interface. Paired with LiNi_0.6Co_0.2Mn_0.2O₂ and LiNi_0.8Co_0.1Mn_0.1O₂ cathodes, which are rarely used in solid‐state batteries, the solid‐state Li metal batteries with HMSE exhibit excellent electrochemical performance including high capacity and long cycle life. Besides, the Li||Li symmetric batteries maintain a stable polarization less than 40 mV for more than 1000 h under 2 mA cm^?2 and effective inhibition of dendrite formation. This study offers a promising approach to extend the applications of solid electrolytes for high‐voltage solid‐state Li metal batteries.     

Early Lithium Plating Behavior in Confined Nanospace of 3D Lithiophilic Carbon Matrix for Stable Solid‐State Lithium Metal Batteries

Considerable efforts are devoted to relieve the critical lithium dendritic and volume change problems in the lithium metal anode. Constructing uniform Li⁺ distribution and lithium “host” are shown to be the most promising strategies to drive practical lithium metal anode development. Herein, a uniform Li nucleation/growth behavior in a confined nanospace is verified by constructing vertical graphene on a 3D commercial copper mesh. The difference of solid‐electrolyte interphase (SEI) composition and lithium growth behavior in the confined nanospace is further demonstrated by in‐depth X‐ray photoelectron spectrometer (XPS) and line‐scan energy dispersive X‐ray spectroscopic (EDS) methods. As a result, a high Columbic efficiency of 97% beyond 250 cycles at a current density of 2 mA cm^?2 and a prolonged lifespan of symmetrical cell (500 cycles at 5 mA cm^?2) can be easily achieved. More meaningfully, the solid‐state lithium metal cell paired with the composite lithium anode and LiNi_0.5Co_0.2Mn_0.3O₂ (NCM) as the cathode also demonstrate reduced polarization and extended cycle. The present confined nanospace–derived hybrid anode can further promote the development of future all solid‐state lithium metal batteries.     

Reviving Lithium‐Metal Anodes for Next‐Generation High‐Energy Batteries

Lithium‐metal batteries (LMBs), as one of the most promising next‐generation high‐energy‐density storage devices, are able to meet the rigid demands of new industries. However, the direct utilization of metallic lithium can induce harsh safety issues, inferior rate and cycle performance, or anode pulverization inside the cells. These drawbacks severely hinder the commercialization of LMBs. Here, an up‐to‐date review of the behavior of lithium ions upon deposition/dissolution, and the failure mechanisms of lithium‐metal anodes is presented. It has been shown that the primary causes consist of the growth of lithium dendrites due to large polarization and a strong electric field at the vicinity of the anode, the hyperactivity of metallic lithium, and hostless infinite volume changes upon cycling. The recent advances in liquid organic electrolyte (LOE) systems through modulating the local current density, anion depletion, lithium flux, the anode–electrolyte interface, or the mechanical strength of the interlayers are highlighted. Concrete strategies including tailoring the anode structures, optimizing the electrolytes, building artificial anode–electrolyte interfaces, and functionalizing the protective interlayers are summarized in detail. Furthermore, the challenges remaining in LOE systems are outlined, and the future perspectives of introducing solid‐state electrolytes to radically address safety issues are presented.     

A Novel Organic “Polyurea” Thin Film for Ultralong‐Life Lithium‐Metal Anodes via Molecular‐Layer Deposition

Metallic Li is considered as one of the most promising anode materials for next‐generation batteries due to its high theoretical capacity and low electrochemical potential. However, its commercialization has been impeded by the severe safety issues associated with Li‐dendrite growth. Non‐uniform Li‐ion flux on the Li‐metal surface and the formation of unstable solid electrolyte interphase (SEI) during the Li plating/stripping process lead to the growth of dendritic and mossy Li structures that deteriorate the cycling performance and can cause short‐circuits. Herein, an ultrathin polymer film of “polyurea” as an artificial SEI layer for Li‐metal anodes via molecular‐layer deposition (MLD) is reported. Abundant polar groups in polyurea can redistribute the Li‐ion flux and lead to a uniform plating/stripping process. As a result, the dendritic Li growth during cycling is efficiently suppressed and the life span is significantly prolonged (three times longer than bare Li at a current density of 3 mA cm^?2). Moreover, the detailed surface and interfacial chemistry of Li metal are studied comprehensively. This work provides deep insights into the design of artificial SEI coatings for Li metal and progress toward realizing next‐generation Li‐metal batteries.