Design Principles for Cation-Mixed Sodium Solid Electrolytes

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.