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

Abstract

<p>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 Na<sub>11</sub>Sn<sub>2</sub>PS<sub>12</sub>-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<sub>11+<i>x</i></sub>Sn<sub>2</sub>P<sub>1−<i>x</i></sub><i>M<sub>x</sub></i>S<sub>12</sub> with <i>M</i> = 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<sup>+</sup> transport can be found with the reducing Na<sup>+</sup> vacancy concentration. This work explores previously unreported phases in the Na<sub>11</sub>Sn<sub>2</sub>PS<sub>12</sub> structure type that, based on their determined properties reveal Na<sup>+</sup> vacancy concentrations to be an important factor guiding further understanding within Na<sub>11</sub>Sn<sub>2</sub>PS<sub>12</sub>-type materials.</p>