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.