Structural and Transport Properties of Na2M3X8 (M = Mg, Ca, Sr): Comparative Atomistic Insights into Bromide vs Chloride Solid-State Electrolytes

Abstract

The present theoretical study explores the Na2M3Br8 (M = Mg, Ca, Sr) family of bromide solid-state electrolytes for sodium-ion batteries, with direct comparison to their chloride analogues. Structural and electronic analyses reveal that bromide substitution expands the lattice and enhances flexibility, while defect energetics confirm NaBr Schottky defects as the most favorable across all compositions. The computed NaBr Schottky solution energies decrease from 1.2 eV in Na2Mg3Br8 to 1.1 eV in Na2Ca3Br8, reaching as low as 0.6 eV in Na2Sr3Br8, lower than the corresponding chloride values (1.2–1.0 eV). Transport properties demonstrate low migration barriers, with Na2Sr3Br8 exhibiting quasi-one-dimensional conduction characterized by a migration barrier of ∼0.11–0.14 eV, while three-dimensional transport remains blocked (∼1.5 eV). Despite this anisotropy, Na2Sr3Br8 achieves a remarkable room-temperature ionic conductivity of ∼12 mS/cm and a diffusion coefficient of 4.60 × 10–7 cm2/s, placing it firmly within the superionic regime. Furthermore, the Br-rich mixed composition at x = 0.875 surpasses the pure bromide, reaching 22.3 mS/cm with a reduced conduction activation energy of 0.13 eV and competitive diffusion properties. These findings highlight bromide frameworks, particularly Sr-based Br-rich solid solutions, as affordable, chemically resilient, and high-performance materials for the next-generation sodium solid-state batteries, though experimental validation is required to confirm these predictions.