Surface Fluorination Shielding of Sulfide Solid Electrolytes for Enhanced Electrochemical Stability in All‐Solid‐State Batteries

Abstract

Despite their high Li+ conductivity and deformability, sulfide solid electrolytes suffer from limited electrochemical stability, which prevents all‐solid‐state batteries (ASSBs) from reaching their full performance potential. Herein, a facile surface fluorination strategy is presented for Li6PS5Cl using XeF2 as a solid‐state fluorinating agent, enabling a scalable dry process at moderate temperatures. An ≈37.3 nm‐thick uniform fluorinated layer is coated on an Li6PS5Cl surface, preserving 82.8% of the initial Li+ conductivity (from 2.9 × 10⁻3 only to 2.4 × 10⁻3 S cm⁻¹ at 30 °C). The underlying fluorination mechanism, deduced through systematic investigations using X‐ray photoelectron spectroscopy, X‐ray Rietveld refinement, nuclear magnetic resonance, and density functional theory calculations, involves the formation of surface oxidative byproducts and F substitution within the lattice. When applied to LiNi0.90Co0.05Mn0.05O2 electrodes in LiNi0.90Co0.05Mn0.05O2||(Li‐In) half cells at 30 °C, the fluorinated Li6PS5Cl substantially improves the electrochemical performance, delivering superior discharge capacities (e.g., 186.9 vs 173.6 mA h g−1 at 0.33C), capacity retention, and safety characteristics compared to unmodified Li6PS5Cl. This enhancement is attributed to the formation of a robust fluorinated cathode electrolyte interphase that mitigates Li6PS5Cl oxidation. Finally, the stable operation of a pouch‐type LiNi0.90Co0.05Mn0.05O2||Li ASSB is demonstrated, highlighting the scalability of the proposed approach.