Influence of Reduced Na Vacancy Concentrations in the Sodium Superionic Conductors Na_{11+ x}Sn₂P_{1- x}M_xS₁₂(M = Sn, Ge)

Exploration of sulfidic sodium solid electrolytes and their design contributes to advances in solid-state sodium batteries. Such a 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 on the structure and transport in Na11+xSn2P1-xMxS12 with M = Ge and Sn is investigated. Although Sn induces stronger structural changes than Ge, the influence on the sodium sublattice and the ionic transport properties is 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 based on their determined properties revealing Na+ vacancy concentrations to be an important factor providing a further understanding of Na11Sn2PS12-type materials.