Computational Screening of Na₃MBr₆ Compounds as Sodium Solid Electrolytes

All-solid-state batteries (ASSBs) based on metallic Na have received an upsurge of interest in the past few years owing to the low cost of Na resources combined with enhanced safety compared to conventional liquid lithium-ion batteries. Solid electrolytes (SEs) serve as the central of ASSBs as they, together with the composite cathode layer, largely determine the power density, energy density, and cycling performance of ASSBs. Among the several categories of SE materials, halides are considered one of the most promising candidates due to their favorable combination of high ionic conductivity and chemical/electrochemical stability against high-voltage cathode materials. Excellent examples have been demonstrated by lithium halides, while their sodium counterparts have been less explored. Herein, we evaluate the phase stability, electrochemical stability, and conducting property of a series of Na-based bromides Na3MBr6 with the joint utilization of density functional theory calculations, the bond valence site energy (BVSE) method, and ab initio molecular dynamics (AIMD) simulations. Our computations reveal that Na3MBr6 could crystallize in three structures, trigonal P3̅1c, monoclinic P21/n, and trigonal R3̅ phases, with the structural preference highly related to the type and ionic radii of the M cations. The BVSE and AIMD suggest that the P3̅1c phase exhibits a higher conductivity and lower migration barrier than P21/n and R3̅ phases. With the introduction of Na vacancies, a significant enhancement in the conductivity was achieved, giving rise to a high extrapolated conductivity of 0.70 mS cm–1 at room temperature. In addition, the thermodynamic stability and chemical stability against cathode materials were also assessed. Our work provides insights into the fundamental understanding of sodium bromides and points to the importance of this family of materials to be used as potential candidates for SE materials in ASSBs.