Influence of Reduced Na Vacancy Concentrations in the Sodium Superionic Conductors Na11+xSn2P1−xMxS12 (M = Sn, Ge)

Exploration of sulfidic sodium solid electrolytes and their design contributes to advances in solid state sodium batteries. Such design is guided by a better understanding of fast sodium transport, for instance in the herein studied Na₁₁Sn₂PS₁₂-type materials. By using Rietveld refinements against synchrotron X-ray diffraction and electrochemical impedance spectroscopy, the influence of aliovalent substitution onto the structure and transport in Na_11+xSn₂P_1−xM_xS₁₂ with M = Ge and Sn is investigated. Whereas Sn induces stronger structural changes than Ge, the found influence on the sodium sublattice and the ionic transport properties are comparable. Overall, a reduced in-grain activation energy of Na⁺ transport can be found with the reducing Na⁺ vacancy concentration. This work explores previously unreported phases in the Na₁₁Sn₂PS₁₂ structure type that, based on their determined properties reveal Na⁺ vacancy concentrations to be an important factor guiding further understanding within Na₁₁Sn₂PS₁₂-type materials.