Preparation and Characterization of Cation-Substituted Na₃SbS₄ Solid Electrolytes

To realize all-solid-state sodium-ion batteries, the ionic conductivities and stabilities of solid electrolytes must be improved. The sulfide Na3SbS4 electrolyte is known to show a high sodium-ion conductivity of over 10–3 S cm–1 at room temperature. In this study, cation-substituted Na3SbS4 solid electrolytes with excess Na or Na vacancies were prepared, and the effects of substitution on the material conductivity were examined. The ionic conductivities of the Na3+xSb1–xMxS4 (M = Si, Ge, Sn) electrolytes, which were doped with excess Na, were lower than that of the Na3SbS4 electrolyte; in contrast, the conductivities of the Na3–xSb1–xMoxS4 electrolytes, which were doped with Na vacancies, were higher. The Na2.88Sb0.88Mo0.12S4 electrolyte showed the highest room-temperature ionic conductivity of 3.9 × 10–3 S cm–1 and the lowest activation energy for conduction of 21 kJ mol–1. To improve the ionic conductivity of the Na3SbS4 electrolyte, introducing Na vacancies instead of excess Na was found to be effective.