Solid electrolytes are key materials to enable solid-state rechargeable batteries, a promising technology that could address the safety and energy density issues. Here, we report a sulfide sodium-ion conductor, Na2.88Sb0.88W0.12S4, with conductivity superior to that of the benchmark electrolyte, Li10GeP2S12. Partial substitution of antimony in Na3SbS4 with tungsten introduces sodium vacancies and tetragonal to cubic phase transition, giving rise to the highest room-temperature conductivity of 32 mS cm−1 for a sintered body, Na2.88Sb0.88W0.12S4. Moreover, this sulfide possesses additional advantages including stability against humid atmosphere and densification at much lower sintering temperatures than those (>1000 °C) of typical oxide sodium-ion conductors. The discovery of the fast sodium-ion conductors boosts the ongoing research for solid-state rechargeable battery technology with high safety, cost-effectiveness, large energy and power densities.

We report a facile synthetic protocol from aqueous solution for Na3SbS4-Na2WS4 superionic conductors with sodium-ion conductivity of 4.28 mS cm−1 at 25 °C, which is the highest one in reported sulfide electrolytes prepared via liquid-phase methods.

To realize all-solid-state sodium-ion batteries, the ionic conductivities and stabilities of solid electrolytes must be improved. The sulfide Na 3 SbS 4 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 Na 3 SbS 4 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 Na 3+x Sb 1−x M x S 4 (M = Si, Ge, Sn) electrolytes, which were doped with excess Na, were lower than that of the Na 3 SbS 4 electrolyte; in contrast, the conductivities of the Na 3−x Sb 1−x Mo x S 4 electrolytes, which were doped with Na vacancies, were higher. The Na 2.88 Sb 0.88 Mo 0.12 S 4 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 Na 3 SbS 4 electrolyte, introducing Na vacancies instead of excess Na was found to be effective.