Vacancy-Stabilized Superionic State in Na _{3– x} Sb _{1– x} W _x S ₄

The discovery of numerous superionic conductors has accelerated the development of solid-state energy storage systems. Among all alkaline ionic conductors known thus far, Na3–xSb1–xWxS4 exhibits the highest ionic conductivity of >30 mS cm–1 at ambient temperature, even surpassing the conductivity of the best lithium superionic conductor. The origin of the peculiar high Na+ diffusivity was investigated by the in-depth crystallographic analysis of Na3–xSb1–xWxS4 through combined X-ray diffraction and molecular dynamics simulations, emphasizing the accurate three-dimensional disordered nature of sodium in the crystalline matrix. The important features induced by the aliovalent tungsten substitution were (i) tetragonal to cubic transformation, (ii) the creation of sodium vacancies that are evenly distributed throughout the crystal without the local condensation around W, and (iii) the effective formation of sodium positional disordering by sodium vacancy. These features inherent to the best alkaline-ion conducting phase reported thus far would provide invaluable insights toward the further development of superionic conductors.