Enhanced Ionic Conductivity at the Solid Electrolyte Interphase of Oxygen‐Doped Li ₆ PS ₅ Cl

ABSTRACT Understanding the formation and transport properties of the solid electrolyte interphase (SEI) at the Li metal | solid state electrolyte (SSE) interface remains a central challenge for all‐solid‐state batteries, as this buried interphase governs interfacial resistance yet is largely inaccessible to direct experimental characterization. In this work, we combine machine‐learned interatomic potential molecular dynamics (MLIP–MD) simulations with a machine–learning phase identification framework to resolve kinetically formed SEI phases at the Li ₆ PS _5‐x ClO _x | Li interface and to elucidate the role of oxygen doping. Our simulations reveal that the dominant SEI formed at both the undoped and oxygen‐doped interfaces is a Li ₂ S _1‐x‐y P _0.5x Cl _0.5x O _y phase, corresponding to an anion‐substituted Li ₂ S structure. Moderate oxygen incorporation into the SEI enhances ionic conductivity, whereas excessive oxygen doping degrades both bulk electrolyte conductivity and SEI thermodynamic stability. This trade‐off provides an atomistic explanation for the experimentally observed non‐monotonic dependence of interfacial performance on oxygen content.