Vacancy‐Controlled Na+ Superion Conduction in Na11Sn2PS12

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 Na11Sn2PS12, 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.