Structural design strategies for superionic sodium halide solid electrolytes

Abstract

Sodium all-solid-state batteries (ASSBs) with superionic solid electrolytes (SEs) show substantial potential for large-scale energy-storage applications. Recently, lithium halide SEs have attracted attention owing to their potential compatibility with high-voltage cathode materials and high ionic conductivity. Although sodium halide SEs are believed to exhibit good electrochemical stability, very few compounds have been reported. This study provides design principles for superionic sodium halide SEs through systematic theoretical investigations of Na₃MX₆ (X = Cl, Br, and I). The Na₃MX₆ structures depend on the types and sizes of M and X: Na₃MCl₆ and Na₃MBr₆ prefer the P1c, P2₁/n and R phases, whereas Na₃MI₆ prefers the C2/m phase. The Na₃MI₆C2/m phase is found to exhibit reasonably high ionic conductivity (∼10⁻⁴ S cm⁻¹) and anion mixing with Br further improve Na-ion migration, leading to an even higher ionic conductivity (∼10⁻³ S cm⁻¹) for Na₃MBr₃I₃. The material design principles in this study provide fundamental guidelines for the development of superionic Na halide SEs for high-voltage Na ASSBs.