Superion Conductor Na11.1Sn2.1P0.9Se12: Lowering the Activation Barrier of Na plus Conduction in Quaternary 1-4-5-6 Electrolytes

We report on the first quaternary selenide-based Na+ superionic solid electrolyte, Na11.1Sn2.1P0.9 Se-12 (further denoted as NaSnPSe), which shows virtually the same room temperature Na + ion conductivity (3.0 mS/cm) as the current record holder for sulfide-based systems, Na11Sn2PS12 (denoted as NaSnPS), but with a considerably lower activation energy of 0.30 eV. Both electrolytes belong to the currently highly topical class of solids comprising group I, IV, V, and VI atoms, which we summarize as 1-4-5-6 electrolytes. Herein, they are compared to each other with regard to their structural characteristics and the resulting ion transport properties. The lower activation energy of Na+ ion transport in NaSnPSe is well in line with the results of speed of sound measurements, Raman spectroscopy, bond-valence site energy calculations, and molecular dynamics simulations, all of which point to a lower lattice rigidity and to weaker Na-chalcogen interactions as compared to NaSnPS.