Superionic Conduction in the Plastic Crystal Polymorph of Na4P2S6

Scholz T., Schneider C., Terban M., Deng Z., Eger R., Etter M., Dinnebier R., Canepa P. and Lotsch B.; ACS Energy Lett., 7, 1403−1411 (2022).


Sodium thiophophates 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 ℃, 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 dynamic 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.