Vacancy-Controlled Na+ Superion Conduction in Na11 Sn2 PS12.

Highly conductive solid electrolytes are crucial to the development of efficient all-solid-state batteries. Meanwhile, the ion conductivities of lithium solid electrolytes match those of liquid electrolytes used in commercial Li+ ion batteries. However, concerns about the future availability and the price of lithium made Na+ ion conductors come into the spotlight in recent years. Here we present the superionic conductor Na11 Sn2 PS12 , which possesses a room temperature Na+ conductivity close to 4 mS cm-1 , thus the highest value known to date for sulfide-based solids. Structure determination based on synchrotron X-ray powder diffraction data proves the existence of Na+ vacancies. As confirmed by bond valence site energy calculations, the vacancies interconnect ion migration pathways in a 3D manner, hence enabling high Na+ conductivity. The results indicate that sodium electrolytes are about to equal the performance of their lithium counterparts.