Design Principles for Cation‐Mixed Sodium Solid Electrolytes

Abstract All‐solid‐state sodium‐ion batteries are highly promising for next generation grid energy storage with improved safety. Among the known sodium superionic conductors, the Na₃PnS₄ family and the recently discovered Na₁₁Sn₂PnS₁₂ (Pn = P, Sb) have garnered major interest due to their extremely high ionic conductivities. In this work, comprehensive investigation of the Na₃PnS₄‐Na₄TtS₄ (Pn = P/As/Sb, Tt = Si/Ge/Sn) phase space of superionic conductors using density functional theory calculations, as well as AIMD simulations on the promising new Na₁₁Sn₂PnS₁₂ (Pn=P/As/Sb) structures are presented. Crucial design rules on the effect of cation mixing are extracted on relative phase stability, electrochemical stability, moisture stability, and ionic conductivity. In particular, it is shown that while larger cations can substantially improve the ionic conductivity and moisture stability in these structures, there is an inherent trade‐off in terms of electrochemical stability. Na₁₁Sn₂AsS₁₂ is also identified as a hitherto unexplored stable sodium superionic conductor with higher Na⁺ conductivity and better moisture stability than the Na₁₁Sn₂PS₁₂ and Na₁₁Sn₂SbS₁₂ phases already reported experimentally.