Computational Investigation of Halogen-Substituted Na Argyrodites as Solid-State Superionic Conductors

The discovery of promising inorganic superionic conductors for use as solid-state electrolytes can enable the design of safe and high-energy-density solid-state batteries. Li argyrodites with the composition Li–P–S–X (X = Cl, Br, or I) have been found to have an ionic conductivity of up to 14.8 mS/cm, indicating the ability of the argyrodite framework to conduct alkali-metal ions. However, the sodium version of argyrodites remains unexplored. In this work, using first-principles density functional theory calculations, we examine the phase stability, electrochemical stability, and ionic conductivity of 48 potential Na argyrodites with different pnictogen, chalcogen, and halogen chemistries and site occupancies. We find that for sulfide-based Na argyrodites, the compounds with halogens occupying both the 4a and 4c sites typically display better ionic conductivity. However, the larger size of the Se2– anion makes selenide-based Na argyrodites less-dependent on the exact halogen-site occupancy. Most of the Na argyrodites have positive zero-K formation energy but are within the typical energy where compounds, including the Li argyrodite, can be synthesized. We find that compounds within the Na–P–Se–X (X = Cl, Br, or I) chemical space may form a good compromise between ionic conductivity, phase stability, and synthesizability.