Operando Photoelectron Spectroscopy Analysis of Li ₆ PS ₅ Cl Electrochemical Decomposition Reactions in Solid-State Batteries

It is crucial to understand at which potentials electrolyte decomposition reactions start and which chemical species are present in the subsequently formed decomposition films, e.g., solid electrolyte interphase (SEI). Herein, a new operando experimental approach is introduced to investigate such reactions by employing hard X-ray photoelectron spectroscopy (HAXPES). This approach enables the examination of the SEI formed below a thin metal film (e.g., 6 nm nickel) acting as the working electrode in an electrochemical cell with sulfide-based Li 6 PS 5 Cl solid electrolyte. Electrolyte reduction reactions already started at 1.75 V (vs Li + /Li) and resulted in considerable Li 2 S formation, particularly in the voltage range 1.5–1.0 V. A heterogeneous/layered microstructure of the SEI is observed (e.g., preferential Li 2 O and Li 2 S deposits near the current collector). The reversibility of side reactions is also observed, as Li 2 O and Li 2 S decompose in the 2–4 V potential window, generating oxidized sulfur species, sulfites, and sulfates.