Recent progress in designing electrolytes for high-voltage lithium-ion batteries and solid-state lithium batteries is summarized.

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high interfacial impedance between SSEs and electrodes, and relatively high fabrication cost, which impede their applications. SSEs with high ionic conductivity, wide electrochemical window and low interfacial impedance are critical in developing ASSBs with high specific energy and power density. [2,3] Currently, among various SSEs, sulfide SSEs have a Li-ion conduction capability comparable to that of organic liquid electrolytes (≈10 -2 S cm -1 at room temperature). [4][5][6][7][8][9][10] Various sulfide materials with a high Li-ion conductivity of 10 -3 -10 -2 S cm -1 at room temperature, such as Li 10 GeP 2 S 12 (LGPS), [5] Li 10 SnP 2 S 12 , [6] and Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 , [7] have been investigated. Among them, a family of sulfide SSEs, lithium argyrodites Li 6 PS 5 X (X = Cl, Br), [8] are attracting more attention due to high Li-ion conductivity (e.g., Li 5.3 PS 4.3 ClBr 0.7 : 2.4 × 10 -2 S cm -1 ; [9] Li 6 PS 5 Cl (denoted as LPSCl): 3.15 × 10 -3 S cm -1 [10] ) and relatively good electrochemical compatibility. The ASSB cells using argyrodites have demonstrated good cycling and rate performance. For example, recently, a sandwiched SSE separator of LPSCl-LGPS-LPSCl has been designed to prevent the growth of Li dendrites and thus enable superior cycling performance of ASSB cells; [11] and LPSCl SSE has also been matched with the silicon anode, capable of operating with high current densities and achieving a long cycle. [12] One drawback of pressed sulfide SSE separator layers is that micro-cracks easily appear and expands during Li plating/stripping in sulfide electrolytes due to the rigidness of sulfide powders, leading to short circuit in ASSB cells. [13] Thus a thicker sulfide SSE layer (e.g., ≈0.5-1.2 mm) via pressing sulfide powder was usually used in laboratory-type cells to guarantee the long-term cycling performance of the ASSBs, [13a,b,14] but this way reduces the cell-level energy density and is detrimental to scalable fabrication. Thus, it is desired to prepare sulfide SSE membranes with a small thickness and compact structure for advanced ASSBs.To obtain a thin, free-standing sulfide SSE membrane, a soft polymeric component is often used. Recently, much progress in sulfide-polymer composite solid electrolytes (CSEs) has been made. [15][16][17][18] Among them, Luo et al. prepared a 65 µm-thick bendable sulfide SSE using LPSCl and poly(ethylene oxide) (PEO), [16] and their ASSB cell of LiNi 0.7 Co 0.2 Mn 0.1 O 2 (LiNi x Co y Mn 1-x-y O 2 , denoted as NCM)||CSE||lithium-indium All-solid-state batteries (ASSBs) using sulfide electrolytes have attracted everincreasing interest due to high ionic conductivity of the sulfides. Nevertheless, a thin, strong solid-state sulfide electrolyte membrane maintaining high ionic conductivity is highly desired for ASSBs. Here, a thin, flexible composite solid electrolyte membrane composed of argyrodite sulfide Li 6 PS 5 Cl and a polar poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) framework is prepared via an electrospinning-infil...

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Solid polymer electrolytes with large ionic conductivity, high ionic transference number, and good interfacial compatibility with electrodes are highly desired for solid‐state batteries. However, unwanted polarizations and side reactions occurring in traditional dual‐ion polymer conductors hinder their practical applications. Here, single‐ion polymer conductors (SIPCs) with exceptional selectivity for Li‐ion conduction (Li‐ion transference number up to 0.93), high room‐temperature ionic conductivity of about 10−4 S cm−1, and a wide electrochemical stability window (>4.5 V, vs Li/Li+) are prepared by precisely regulating the ion–dipole interactions between Li+ and carbonyl/cyano groups. The resulting SIPCs show an excellent electrochemical stability with Li metal during long‐term cycling at room temperature and 60 °C. LiFePO4‐based solid‐state cells containing the SIPCs exhibit good rate and cycling performance in a wide temperature range from −20 to 90 °C. By the same way of ion–dipole interaction regulation, sodium‐ and potassium‐based SIPCs with both high ionic conductivity and high cationic transference numbers are also prepared. The findings in this work provide guidance for the development of high‐performance SIPCs and other metal‐ion systems beyond Li+.

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The uncontrolled growth of Li dendrites on Li metal anodes may cause short circuit of a working cell, even electrolyte combustion and cell explosion. [4] Li dendrites may be converted to electrochemically inactive dead Li covered by SEI due to loss of electrical contact with the anode. The accumulation of dead Li may even lead to anode pulverization. [5] All these behaviors lead to a low Coulombic efficiency (CE), fast capacity fading and rapid cell failure. [6] To tackle the issues regarding Li metal anodes, various approaches have been proposed to optimize Li anodes in recent years, [7] including Li host design, [8] composite anode engineering, [9] electrolyte composition adjustment, [10] separator modification, [11] solid electrolyte employment, [12] SEI generation, [13] and artificial protective layer construction. [14] A three-dimensional hybrid host consisting of silver nanoparticles and carbon macroporous fibers enabled a dendrite-free Li plating/stripping behavior with a high CE. [8a] A molecular-level SEI was designed by using a reactive polymer composite, which effectively suppressed electrolyte consumption in the formation and maintenance of the SEI. [13a] An SEI with high inorganic components was stabilized in highly corrosive cyclic carbonates by tuning polymer-solvent interactions. [13c] Recently, artificial protective layers have attracted much interest due to the following advantages: a) the composition, structure, mechanical strength and ionic conductivity of the protective layer can be tuned and optimized; b) the ion flux through the protective layer can be homogenized at the anode surface, resulting in uniform Li deposition; c) the side reactions between the electrolyte and Li metal are reduced since solvent or anion molecules are expelled by the protective coating. [15] Yu and coworkers prepared a mixed Li 2 S/Li 2 Se protection layer based on the gas-solid reaction between SeS 2 and Li metal. The cycle performance of symmetric cells with an ether-based electrolyte was greatly improved due to the high Li + conductivity of the protection layer. [16] Huang and co-workers obtained a dual-layered protective layer with an inorganic inner layer and an organic outer one to protect Li anodes by immersing Li foils in fluoroethylene carbonate (FEC). Through this facile process, uniform Li deposition and a stable cycle performance were achieved. [17] Song and co-workers coated Li metal anodes with a LiF-polymer composite layer by a simple roll-press method. Such a composite coating Li metal is the next-generation anode material for high-energy-density Li-ion batteries. Unfortunately, its practical application is hampered by drawbacks such as an unstable solid electrolyte interphase and undesirable Li dendrites. Herein, an Fe-based protective layer with a valence gradient is constructed on Li anodes, which consists of an Fe 3+ /Fe 2+ -rich outer layer and an Fe 0containing inner layer. The protective layer not only isolates the underlying Li metal from the corrosive carbonate electrolyte, but al...

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