Vacancy-Controlled Na+ Superion Conduction in Na11Sn2PS12

Highly conductive solid electrolytes are one key component for the development of safe and high-power all-solid-state batteries. Enormous progress has been achieved in the field of lithium solid electrolytes. Meanwhile, their ion conductivities match those of liquid electrolytes used in commercial Li+ ion batteries. However, the future availability and the price of lithium are points of concern, so that Na+ ion conductors have come into the spotlight in recent years. Here we present the superionic conductor Na11Sn2PS12 consisting exclusively of abundant elements. This material exhibits a room temperature Na+ ion conductivity close to 4 mS/cm, the highest value known to date for sulfide-based solids. Importantly, the stoichiometry of this quaternary compound differs from that of the Li analogues, in derogation from recent theoretical and experimental works. Structure determination based on synchroton X-ray powder diffraction data proves the existence of Na+ vacancies in the tetragonal structure that speed up Na+ ion transport, an untypical mechanism in superion conductors. The results indicate that sodium electrolytes are about to equal the performance of their lithium counterparts. The major target application is clean and safe energy storage, for which the lower energy density of Na+ ion batteries is not a concern, but long-term materials availability and costs are important prerequisites.