Origin of Superionic Li₃Y_{1- x}In_xCl₆ Halide Solid Electrolytes with High Humidity Tolerance

The high ionic conductivity, air/humidity tolerance, and related chemistry of Li₃MX₆ solid-state electrolytes (SSEs, M is a metal element, and X is a halogen) has recently gained significant interest. However, most of the halide SSEs suffer from irreversible chemical degradation when exposed to a humid atmosphere, which originates from hydrolysis. Herein, the function of the M atom in Li₃MX₆ was clarified by a series of Li₃Y_1-xIn_xCl₆ (0 ≤ x < 1). When the ratio of In³⁺ was increased, a gradual structural conversion from the hexagonal-closed-packed (hcp) anion arrangement to cubic-closed-packed (ccp) anion arrangement has been traced. Compared to hcp anion sublattice, the Li₃MX₆ with ccp anion sublattice reveals faster Li⁺ migration. The tolerance of Li₃Y_1-xIn_xCl₆ towards humidity is highly improved when the In³⁺ content is high enough due to the formation of hydrated intermediates. The correlations among composition, structure, Li⁺ migration, and humidity stability presented in this work provide insights for designing new halide-based SSEs.