Phase-structure design for sodium chloride solid electrolytes with outstanding performance: a first-principles approach

As a promising alternative to the current commercial lithium-ion batteries, the all-solid-state sodium-ion battery (ASS-SIB) requires solid electrolytes (SEs) with high Na ionic conductivity and good stability. Recently, lithium halide SEs have attracted widespread attention owing to their outstanding performance. However, since studies on sodium halides are still in their infancy and very few compounds have been reported, it remains unclear whether sodium halides can be used as SEs. Herein, we focus on Na3MCl6 (M = In and Sc) with the same chemical formula as the reported candidate lithium halide SEs Li3MCl6 (M = In and Sc) for the solid-state lithium-ion battery, and we performed a comprehensive study to evaluate the stabilities, mechanical properties, and Na-ion diffusion mechanism by primarily using first-principles calculations. The calculated results show that the predicted C2/m and P3m1 phases of Na3MCl6 have good stabilities and reasonable electrochemical windows. Moreover, Na3MCl6 with P3m1 phases demonstrated excellent mechanical and thermal properties among all the considered phases. Both C2/m and P3m1 phases of Na3MCl6 were found to exhibit high ionic conductivities (>1 mS cm(-1)) as compared with experimental structures, and the P3m1 phase further improved the Na-ion transportation, leading to even higher ionic conductivities of 3.09 and 11.54 mS cm(-1) for Na3ScCl6 and Na3InCl6, respectively. Our study highlights sodium chloride conductors as a promising new research direction for SEs with high ionic conductivity and good stability in ASS-SIB.