Because of the high capacity of lithium (Li) metal and the intrinsic safety of solidstate electrolyte, solid-state Li-metal batteries are regarded as a promising candidate for next-generation energy storage. However, uncontrollable dendrite growth and large interfacial resistance severely hamper the practical applications. This review summarizes the issues generated by the marriage of Li-metal anodes and solid-state electrolytes. First, the current challenges are underscored. Specific attention is paid to the large interfacial resistance, uncontrolled dendrite growth, and low operation current or capacity. The second section is dedicated primarily to understanding the ionic channels in the composite electrolyte and the space charge layers in the interfacial region. Based on these dilemmas and working principles, emerging strategies to render solid-state Li-metal batteries are summarized. Finally, the general conclusion and perspective on the current limitations and recommended research of solid-state Li-metal batteries are presented.

Lithium (Li) metal‐based battery is among the most promising candidates for next‐generation rechargeable high‐energy‐density batteries. Carbon materials are strongly considered as the host of Li metal to relieve the powdery/dendritic Li formation and large volume change during repeated cycles. Herein, we describe the formation of a thin lithiophilic LiC6 layer between carbon fibers (CFs) and metallic Li in Li/CF composite anode obtained through a one‐step rolling method. An electron deviation from Li to carbon elevates the negativity of carbon atoms after Li intercalation as LiC6, which renders stronger binding between carbon framework and Li ions. The Li/CF | Li/CF batteries can operate for more than 90 h with a small polarization voltage of 120 mV at 50% discharge depth. The Li/CF | sulfur pouch cell exhibits a high discharge capacity of 3.25 mAh cm−2 and a large capacity retention rate of 98% after 100 cycles at 0.1 C. It is demonstrated that the as‐obtained Li/CF composite anode with lithiophilic LiC6 layers can effectively alleviate volume expansion and hinder dendritic and powdery morphology of Li deposits. This work sheds fresh light on the role of interfacial layers between host structure and Li metal in composite anode for long‐lifespan working batteries.

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density. [3] Among them, Li metal anode offers great promise due to its ultrahigh capacity of 3860 mAh g −1 and the most negative electrochemical potential of −3.040 V versus standard hydrogen electrode. [4] However, dendrite growth on Li metal anode along with its unstable interface against electrolytes result in safety concerns, low utilization, and a short lifespan of Li metal anode, severely impeding the implementation of Li metal batteries (LMBs). [5] The current investigations of Li metal anode focus on the feasible strategies to inhibit dendrite growth and stabilize electrode/electrolyte interfaces. [6] Various methods have been proposed, such as nonaqueous electrolyte optimization (solvent, [7] Li salt, [8] anion, [9] and additives [10] ), ex situ interfacial modifications, [11] highly concentrated electrolyte, [12] solid-state electrolyte, [13] Li metal hosts, [14,15] and 3D current collectors. [16] However, relative to the extensive researches on exploring emerging methods in protecting Li metal anode, the researches on understanding the electrochemical principles in plating and stripping process are less involved. [17] Classical models including space-charge [18] and Sand's time models [19] demonstrate the scenarios of dendrite nucleation and growth during Li plating, while few models are aimed at illustrating the evolution of dead Li during Li stripping. [20] The plating and stripping of Li are necessary to complete the transformation between chemical and electric energy in a working rechargeable battery.There is no difference in a perfectly reversible Li metal battery whether the electrode is plated or stripped first. [15] However, an actual Li metal anode is a seriously irreversible electrode due to its high reactivity and dendrite growth. This leads to a significant difference between the initially stripped or plated Li anodes. Actual Li metal anodes with very limit areal capacities when matching Li-containing cathodes (such as LiFePO 4 (LFP), LiCoO 2 , and LiCo 1/3 Ni 1/3 Mn 1/3 O 2 ) and Li-free (such as sulfur and oxygen) cathodes are initially plated and stripped, respectively. The initially stripping or plating behavior of Li metal not only renders cathodes with various utilizations [21] but also leads to different cycling performance of Li metal anode. It is very important to understand the evolution of dendrites and dead Li as well as their internal relationship to clarify the initial plating or stripping on the cycling behavior of an actual Li metal anode, which is beneficial to construct an efficient Li metal battery. Lithium (Li) metal anodes exhibits the potential to enable rechargeable Li batteries with a high energy density. However, the irreversible plating and stripping behaviors of Li metal anodes with high reactivity and dendrite growth when matching different cathodes in working cells are not fully understood yet. Herein the working manner of very thin Li metal anodes (50 µm, 10 mAh cm −2 ) is probed with different sequences of Li plating and stripping at 3.0 mA cm −2 and 3....

potential (−3.040 V vs standard hydrogen electrode). [8][9][10] Objectively, the energy density of Li batteries should take all parts of a battery into considerations instead of only anode. In order to achieve high-energydensity Li batteries, practical conditions including limited Li (<10 mAh cm −2 ), low negative/positive capacity (N/P) ratio (<3), and lean electrolyte are necessary. [11,12] In addition, fast charge is imperative with the popularization of portable electronics and electrical vehicles. [13] Consequently, high charge current density (>3 mA cm −2 ) is a confronted issue.Nowadays, the practical applications of Li batteries are still hindered by the formation of Li dendrites, rapid pulverizations of Li metal, and volume fluctuation. [14][15][16] Tremendous efforts have been devoted to disclosing the mechanism of Li plating/stripping and developing strategies to regulate its behaviors thus extending the lifespan of Li batteries during the past half century, including electrolyte formulations, [17][18][19][20][21][22][23][24][25][26][27] interfacial modifications, [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] solid-state electrolytes, [45][46][47][48][49][50][51][52][53][54][55] composite anodes, [56][57][58][59][60][61][62][63] and so on. [64] Thereinto, composite Li anode emerges as a promising strategy to regulate Li plating/stripping behaviors and promote the advances of Li batteries.Generally, a composite Li anode is composed of a 3D host and fresh Li metal. The 3D host is endowed with the unique surface chemistry and interconnecting structure. [65] The superiorities contributed by composite Li anode mainly include:(1) improving electronic conductivity, such as decreasing areal current density and avoiding the generation of "dead Li." Conductive hosts with high specific surface area, such as carbon-based and metal-based hosts, can decrease areal current density to prolong the time for depletion of ion at the anode surface to induce dendritic Li according to Sand's time model. [66] Therefore, the formation of Li dendrites will be effectively inhibited at low areal current density and the cyclability can be improved effectively.(2) Homogenizing Li-ion flux over the surface of Li metal. The distribution of Li ions over the surface of Li anode impacts the subsequent Li plating/stripping behaviors significantly. Uniform Li plating/ stripping can be improved by regulating the interactions between lithiophilic sites on 3D host surface, such as polar functional groups, [67,68] and Li ion to homogenize Li-ion flux. [69] (3) Maintaining dimensional stability of anode. 3D hosts can confine plating Li in the inside pores to avoid the Lithium (Li) metal is considered as a promising anode candidate for nextgeneration batteries owing to its extremely high specific capacity and low reduction potential. However, Li metal anode is still hindered by uncontrolled Li dendrites and extreme volume fluctuation. Composite Li metal anode emerges as an attractive strategy to suppress Li dendrites and ...