Electrochemical and Spectroscopic Evaluation of Li7Si2S7I Solid Electrolyte in All-Solid-State Batteries

Abstract

All-solid-state batteries require solid electrolytes with sufficient oxidative and reductive stability; however, the interfacial behavior of emerging silicon-based sulfide electrolytes remains poorly understood. In particular, the electrochemical stability of Li7Si2S7I under practical cell operation and the separate effects of cathode and anode interfaces on performance degradation have not been clarified. Here, we systematically evaluate Li7Si2S7I in LiNi0.82Mn0.07Co0.11O2 (NMC82) | solid electrolyte | Li-In all-solid-state cells using two- and three-electrode configurations combined with impedance analysis and ex situ Raman and X-ray photoelectron spectroscopy. Three-electrode measurements decouple the interfacial reactions and reveal concurrent interfacial decomposition at both the cathode and anode sides, with dominating effect from the cathode side. Symmetric cell tests using Li-In/Li7Si2S7I composite electrodes confirm slight reductive instability against Li-In for Li7Si2S7I as an anolyte. Spectroscopic analysis identifies polysulfides, oxidized iodide, and oxygenated SiOx and SOx species at the Li7Si2S7I/NMC82 interface, as well as In2S3 formation at the Li7Si2S7I/Li-In interface. Our findings provide mechanistic insight into the interfacial reactions of Si-based sulfide electrolytes and highlight the importance of electrode-decoupled diagnostics for evaluating the compatibility with electrode materials in all-solid-state batteries.