Materials design of sodium chloride solid electrolytes Na3MCl6 for all-solid-state sodium-ion batteries

Abstract

All-solid-state sodium-ion batteries have attracted increasing attention owing to the low cost of sodium and the enhanced safety compared to conventional Li-ion batteries. Recently, halides have been considered as promising solid electrolytes (SEs) due to their favorable combination of high ionic conductivity and chemical stability against high-voltage cathode materials. Although a wide variety of lithium chloride SEs, Li₃MCl₆, have been developed for high-voltage all-solid-state batteries, only a limited number of sodium chloride SEs have been reported. This study aims to offer a material design insight for the development of sodium chloride SEs through systematic assessment of the phase stability, electrochemical stability, and transport properties of novel Na₃MCl₆ SEs. Structural calculations indicate that Na₃MCl₆ exhibits trigonal P1c, monoclinic P2₁/n, and trigonal R phases, and the stable phase of Na₃MCl₆ is dependent on the type and ionic radius of M. Na₃MCl₆ typically exhibits a high oxidation potential, demonstrating good electrochemical stability against cathodes. The bond-valence site energy and ab initio molecular dynamics calculations revealed that Na₃MCl₆ with P2₁/n and R phases showed low ionic conductivity, while the P1c phase slightly improved the ionic conductivity of Na₃MCl₆. The formation of Na vacancies by aliovalent substitution considerably increased the ionic conductivity up to four orders of magnitude for pristine Na₃MCl₆, exhibiting ∼10⁻⁵ S cm⁻¹ for trigonal P1c and R phases. The formation of defects could further enhance the ionic conductivity of Na₃MCl₆, and the optimization of defect type and ratio can be helpful in developing superionic Na chloride SEs. The material design of Na₃MCl₆ in this study will provide fundamental guidelines for the development of novel sodium halide SEs for all-solid-state sodium-ion batteries.