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.