Degradation rate at the Solid–Solid interface of sulfide-based solid Electrolyte–Cathode active material

All-solid-state lithium-ion batteries with sulfide-based solid electrolytes (SEs) are leading candidates for next-generation batteries because of the high lithium-ion conductivity and thermoplasticity of the SE. However, the cathode slowly degrades in harsh environments. Therefore, the degradation rate of the cathode is analyzed in this study using the “root law” for the half-cell. An argyrodite-type sulfide SE, LiNi0·5Co0·2Mn0·3O2 with LiNbO3 coating as a working electrode, and In–Li alloy as a counter electrode is used. The floating tests help determine the degradation under various potentials (4.25–4.55 V vs. Li/Li+) and temperatures (45–100 °C). The “root law”, explained by the decomposition layer growth on active material, is applicable under all durability test conditions. The results of the secondary-ion mass spectrometry showed that the increasing tendency of the normalized POx− fragment changes depending on the cathode potential. X-ray photoelectron spectroscopy revealed that the electrochemical decomposition of the SE is dominant at low cathode potentials, and the LiNbO3 coating is involved in a side reaction, which produces decomposed species such as phosphates and sulfates at higher potentials. These two side reactions control the cell performance of the cathode.