Connecting Local Structure, Strain and Ionic Transport in the Fast Sodium Ion Conductor Na₁₁₊xSn₂₊xP₁−xS₁₂

On the road to highly performing solid electrolytes for solid state batteries, aliovalent substitution is a powerful strategy to improve the ionic conductivity. While the substitution allows optimization of the charge carrier concentration, effects on the local structure are often overlooked. Here, by pair distribution function analyses is shown that partial substitution of PS 4 3− by SnS 4 4− polyanion in the fast sodium ionic conductor Na 11+ x Sn 2+ x P 1− x S 12 results in discrepancies between the local and average structure. The significantly larger SnS 4 4− polyanions lead to inhomogeneities in the local environments of sodium ions and induce micro strain in the material. The combination of nuclear magnetic resonance spectroscopy and quasi‐elastic neutron scattering reveals a decrease in the activation energy of fast local ionic jumps. The substitution widens the bottleneck size of some diffusion pathways, and a correlation between the increased strain and improved local ionic transport is observed. Local frustrations caused by the induced inhomogeneities may flatten the energy landscape and lead to the detected decrease in the activation barrier. Understanding these effects of cationic substitution on the local structure, induced crystallographic strain and ionic transport can open up new possibilities to design fast conducting solid electrolytes.