Influence Mechanism of Interfacial Oxidation of Li₃YCl₆ Solid Electrolyte on Reduction Potential

Halide-based solid electrolytes are promising candidates for all solid-state lithium-ion batteries (ASSLBs) due to their high ionic conductivity, wide electrochemical window, and excellent chemical stability with cathode materials. However, when tested in practice, their intrinsic electrochemical stability windows do not well match the conditions for stable operation of ASSBs. Existing literature reports halide-based ASSBs that still operate well outside the electrochemical stability window, while ASSBs that do not operate within the window are not well studied or the studies are based on the cathode material interface. In this study, we aim to elucidate the mechanism behind all-solid-state battery failure by investigating how the reduction potential of Li₃YCl₆ solid-state electrolyte itself changes under overcharging conditions. Our findings demonstrate that in Li-In|Li₃YCl₆|Li₃YCl₆-C half-cells during the first state of charge, Cl ions participate in charge compensation, resulting in a depletion of ligands. This phenomenon significantly affects the reduction potential of Y³⁺, causing it to be reduced to Y₂Cl₃ and ultimately to Y⁰ at conditions far exceeding its actual reduction potential. Furthermore, we analyze the interfacial impedance induced by this process and propose a novel perspective on battery failure.