of serious interfacial reactions, Li dendrite growth, large interfacial resistance, etc. [2] To overcome these issues, a chemically/ electrochemically stable, mechanically compatible, Li + -conductive and electronically insulating solid electrolyte interphase (SEI) layer needs to be formed on electrolyte/Li interface.Here in this work, a sulfide and polymer composite solid electrolyte (CSE) is developed to overcome these issues. More specifically, lithium polysulfidophosphate (LPS) is introduced as an additive for poly(ethylene oxide) (PEO)-based CSEs to synergistically react with lithium metal for in situ formation of an ideal SEI layer in Li-metal-anode ASSB. PEO itself has various drawbacks, including low Li + conductivity (10 −6 S cm −1 at room temperature), [3] narrow electrochemical window, [4] and intrinsic soft mechanical properties causing small critical current densities (CCDs). [5] Multiple approaches have been employed to solve these problems, including co-polymerization, [6] polymer blend, [7] introduction of secondary phases as inert fillers (e.g., BaTiO 3 /Al 2 O 3 /TiO 2 /SiO 2 / CeO 2 ), [8] Li + conducting active fillers (e.g., NASICON, garnet, perovskite, and sulfides), [9] and plasticizers (e.g., propylene carbonate, ethylene carbonate, and succinonitrile). [10] For the PEO-lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) system, the interfacial stability issue between polymer-salt and lithium metal cannot be ignored because of the unstable SEI at the interface. [11] During cycling, SEI grows continuously An ultrastable and kinetically favorable interface is constructed between sulfide-poly(ethylene oxide) (PEO) composite solid electrolytes (CSEs) and lithium metal, via in situ formation of a solid electrolyte interphase (SEI) layer containing Li 3 PS 4 . A specially designed sulfide, lithium polysulfidophosphate (LPS), can distribute uniformly in the PEO matrix via a simple stirring process because of its complete solubility in acetonitrile solvent, which is advantageous for creating a homogeneous SEI layer. The CSE/Li interface with high Li + transportation capability is stabilized quickly through in situ formation of a Li 3 PS 4 /Li 2 S/LiF layer via the reaction between LPS and lithium metal to inhibit lithium dendrite growth. A Li/Li symmetric cell with the LPS-integrated CSE exhibits constant and small CSE/Li resistance of 10 Ω cm 2 during cycling, delivering stable cycling for 3475 h at a current density of 0.2 mA cm −2 and a high critical current density of 0.9 mA cm −2 at 60 °C. Impressive electrochemical performance is also demonstrated for LiFePO 4 /CSE/Li all-solid-state batteries with capacity of 127.6 mAh g −1 after 1000 cycles at 1 C.

Sulfide all‐solid‐state batteries (ASSBs) have been widely acknowledged as next‐generation energy‐storage devices due to their improved safety performance and potentially high energy density. Among the various fabrication methods of sulfide ASSBs, solvent‐free dry‐film processes have unique advantages including reduced costs, suppressed film delamination, thick electrodes, and high compatibility with sulfide solid electrolytes (SEs). However, the currently dominating binder for dry‐film process polytetrafluoroethylene suffers from poor voltage stability and low viscosity, which leads to low Coulombic efficiency and poor cycling stability of sulfide ASSBs. Herein, a specially‐designed treatment is developed to obtain a new type of dry binder, styrene‐butadiene rubber (SBR), exploiting paraxylene and a NaCl substrate to dissolve and re‐precipitate SBR for controlling its stacking state, micro‐structure/morphology, density, and dispersion performance. The SE membrane prepared using this processed SBR exhibits ultra‐high ionic conductivity (2.34 mS cm‐1), contributing to excellent cycle stability of the corresponding sulfide ASSB (>84% capacity retention after 600 cycles at 0.3C).