Synergistic Coupling of Li6.4La₃Zr1.4Ta0.6O₁₂ and Fluoroethylene Carbonate Boosts Electrochemical Performances of Poly(Ethylene Oxide)‐Based All‐Solid‐State Lithium Batteries

All‐solid‐state lithium batteries (ASSLBs) with poly(ethylene oxide) (PEO)‐based composites solid‐state electrolytes have received much attention owing to their higher energy density and better safety compared with conventional liquid electrolytes. However, ASSLBs with PEO‐based solid‐state electrolytes generally suffer from severe capacity degradation and interface transfer obstacles during the charge/discharge process. In this work, fluoroethylene carbonate (FEC) is employed as a reducing additive to in‐situ form LiF‐rich and stable solid‐state electrolyte interface (SEI). Benefiting from the integrated advantages of Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) and FEC binary additives, the number of lithium‐ion transference increases to 0.48, which facilitates the stable cycling of Li||Li symmetrical batteries over 900 h at 0.1 mA cm −2 . The synergistic interplay of LLZTO and FEC constructs a stable LiF‐rich SEI film, effectively addressing the interfacial problems caused by lithium dendrites and promoting the transport of Li ions. Therefore, the high ionic conductivity and self‐healing anode‐electrolyte interface are achieved. This study provides a facile and economical strategy to solve the problem of the lithium‐electrolyte interface. It is of great scientific significance for the development of dendrite‐free solid‐state lithium metal batteries.