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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 Na3−xIn1−xZrxCl6 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.

Development of oxide solid electrolytes for all-solid-state batteries is attracting increasing attention. In this study, amorphous Li2O–LiI materials are prepared via a mechanochemical process to achieve high lithium ionic conductivity and good compatibility to lithium metal. Amorphous 66.7Li2O·33.3LiI (mol%) electrolyte shows a high ionic conductivity of 3.1 × 10−5 S cm−1 at 25 °C with a relative density of 96 %. An all-solid-state Li symmetric cell (Li/66.7Li2O·33.3LiI/Li) operates without an increase in overvoltage. A simple combination of lithium oxide and lithium iodide exhibits high ionic conductivity, ductility, and stability to lithium metal.

The development of solid electrolytes is necessary for practical applications of all-solid-state Na-ion batteries. In this study, we systematically developed sulfide Na-ion conductors with central In cations, prepared by solid-state reaction and mechanochemical methods. Thermodynamically stable crystals of Na5InS4 and NaInS2 were obtained by the solid-state reaction method, whereas amorphous and/or metastable phases were produced by the mechanochemical method. Two new metastable phases, Na5InS4 and NaInS2, are expected to be examined in the future as end-members of sulfide Na-ion conductors with central In cations.