Operando Photoelectron Spectroscopy Analysis of Li6PS5Cl Electrochemical Decomposition Reactions in Solid-State Batteries

Most solid electrolytes (SEs) which are promising for all-solid-state battery (ASSB) applications are known to have a narrow electrochemical stability window. Consequently, parasitic electrolyte reactions are observed when high-energy-density electrode materials such as lithium and silicon are employed, hindering their utilization in commercial battery systems. Therefore, it is crucial to understand at which potentials such reactions start, and which chemical species are present in the subsequently formed 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. The feasibility of this approach is demonstrated using a sulfide-based Li6PS5Cl solid electrolyte (lithium argyrodite). It is shown that electrolyte reduction reactions start upon polarization of the working electrode to voltages below 1.75 V (vs. Li+/Li) and result in considerable Li2S formation, particularly in the voltage range of 1.5 – 1.0 V. The overall intensity trends confirm the heterogeneous/layered microstructure of the SEI (e.g., preferential Li2O and Li2S deposition near the current collector). The reversibility of side reactions is also observed, as Li2O and Li2S decompose in the 2–4 V potential window, generating oxidized sulfur species, sulfites and sulfates. The introduced experimental approach is promising for the spectroscopic investigation of electrolyte side reactions under dynamic conditions for various solid electrolyte and current collector combinations.