Superionic Conduction in the Plastic Crystal Polymorph of Na <sub>4</sub> P <sub>2</sub> S <sub>6</sub>
Tanja Scholz, Christian Schneider, Maxwell W. Terban, Zeyu Deng, Roland Eger, Martin Etter, Robert E. Dinnebier, Pieremanuele Canepa, Bettina V. Lotsch
Abstract
Sodium thiophosphates are promising materials for large-scale energy storage applications benefiting from high ionic conductivities and the geopolitical abundance of the elements. A representative of this class is Na4P2S6, which currently shows two known polymorphs−α and β. This work describes a third polymorph of Na4P2S6, γ, that forms above 580 °C, exhibits fast-ion conduction with low activation energy, and is mechanically soft. Based on high-temperature diffraction, pair distribution function analysis, thermal analysis, impedance spectroscopy, and ab initio molecular dynamics calculations, the γ-Na4P2S6 phase is identified to be a plastic crystal characterized by dynamic orientational disorder of the P2S64– anions translationally fixed on a body-centered cubic lattice. The prospect of stabilizing plastic crystals at operating temperatures of solid-state batteries, with benefits from their high ionic conductivities and mechanical properties, could have a strong impact in the field of solid-state battery research.