Computational Prediction and Evaluation of Solid-State Sodium Superionic Conductors Na₇P₃X₁₁ (X = O, S, Se)

Inorganic solid-state ionic conductors with high ionic conductivity are of great interest for their application in safe and high-energy-density solid-state batteries. Our previous study reveals that the crystal structure of the ionic conductor Li7P3S11 contains a body-centered-cubic (bcc) arrangement of sulfur anions and that such a bcc anion framework facilitates high ionic conductivity. Here, we apply a set of first-principles calculations techniques to investigate A7P3X11-type (A = Li, Na; X = O, S, Se) lithium and sodium superionic conductors derived from Li7P3S11, focusing on their structural, dynamic and thermodynamic properties. We find that the ionic conductivity of Na7P3S11 and Na7P3Se11 is over 10 mS cm–1 at room temperature, significantly higher than that of any known solid Na-ion sulfide or selenide conductor. However, thermodynamic calculations suggest that the isostructural sodium compounds may not be trivial to synthesize, which clarifies the puzzle concerning the experimental problems in trying to synthesize these compounds.