LAUSR

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 Li3PS4. 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 Li3PS4/Li2S/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 cm2 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 LiFePO4/CSE/Li all‐solid‐state batteries with capacity of 127.6 mAh g−1 after 1000 cycles at 1 C.