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.