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

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 LiNi_0.90Co_0.05Mn_0.05O₂ electrodes in LiNi_0.90Co_0.05Mn_0.05O₂||(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 LiNi_0.90Co_0.05Mn_0.05O₂||Li ASSB is demonstrated, highlighting the scalability of the proposed approach.