Sodium-Ion Conducting Solid Electrolytes in the Na<sub>2</sub>S–In<sub>2</sub>S<sub>3</sub> System

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.