Revealing the Reductive Decomposition at the Interface of Li₃YCl₆-Lithium Metal

Halide solid electrolytes have attracted considerable attention due to their high ionic conductivity and excellent formability. However, their limited stability under reductive conditions remains a critical issue that must be solved for the practical use of all-solid-state batteries. Li₃YCl₆ is one of the representative halide solid electrolytes and is known to possess relatively high reductive stability compared to other halide systems. Nevertheless, decomposition reactions still occur when exposed to a strongly reductive potential such as a lithium metal anode. In this study, a symmetric cell of Li metal | Li₃YCl₆ | Li metal was constructed to investigate the interfacial behavior under reductive conditions. Electrochemical evaluation, X-ray absorption spectroscopy, and computational simulations were performed to reveal the reductive decomposition mechanism of the solid electrolyte. Electrochemical impedance spectroscopy revealed a gradual decrease in impedance over time, eventually resulting in metallic electron conduction. Spectroscopic analysis revealed that metallic yttrium (Y⁰) was formed as a result of reductive decomposition of Li₃YCl₆. This metallic Y⁰ was found to form progressively from the interface with lithium metal as nanoclusters, finally bridging the electrodes and leading to a high electronic conductivity. Understanding this mechanism provides valuable insights into the future design of halide solid electrolytes.