Mechanochemical Synthesis and Characterization of Na_3−xIn_1−xZr_xCl₆ Solid Electrolyte (Supporting Information)

All-solid-state batteries (ASSBs) have attracted significant attention as alternatives to Li-ion batteries. In ASSBs, solid electrolytes (SEs) play a key role. While many halide Li-ion conductors have been reported, only a few Na-ion conductors have been reported. In this study, a new phase of Na₃InCl₆ with a cryolite-type monoclinic structure was prepared using a mechanochemical method. The new phase showed higher conductivity than the previously reported trigonal Na₃InCl₆ and underwent a phase transition to trigonal phase when heat-treated at 90 °C. A Zr-substituted system of Na_3−xIn_1−xZr_xCl₆ was mechanochemically prepared. The obtained solid solutions with monoclinic structures based on Na₃InCl₆ were formed in the compositions of x = 0.1–0.9. The Rietveld refinement results showed a decrease in Na occupancy at the octahedral sites and slight change at the prismatic sites. Bond valence sum mapping results showed that Na ions diffused alternately through two types of sites, suggesting that the introduction of Na vacancies at either site had a positive effect on Na-ion conduction. The ionic conductivity increased to approximately 10⁻⁵ S cm⁻¹ with an increase in the number of Na vacancies when x was greater than 0.6. This report describes one of the few Na-ion conducting chlorides with high conductivity.