First-Principles Study of Layered Anti-Perovskite Cathode Materials for Sodium-Ion Batteries

Sodium-ion batteries have emerged as promising alternatives to lithium-ion batteries due to their abundant raw material reserves, low cost, enhanced safety, and environmental sustainability. Na_2Fe_2OS_2, featuring a layered anti-perovskite structure, has attracted significant interest for its high capacity and facile synthesis. In this study, density functional theory calculations were performed to systematically investigate the phase stability, ionic conductivity, and voltage characteristics of Na_2Fe_2OS_2 as a model system for anti-perovskite layered cathode materials. The compound exhibits excellent phase stability, and its equilibrium potential was calculated for the series Na_xFe_2OCh_2 (0 < x < 2) (where Ch represents chalcogenides). Na ion transport analysis using the climbing image nudged elastic band method reveals a relatively low migration barrier (\sim0.47 eV) along a diagonal pathway, indicating efficient Na^+ mobility. To expand the materials design space, we systematically explored the effects of substituting Fe with various transition metals and replacing S with Se in Na_2TM_2OCh_2 structures. Among the variants studied, Na_2Mn_2OS_2 demonstrates the most favorable combination of high voltage (\sim2.51 V), robust phase stability, and superior energy density (\sim427 W\cdoth/kg). This comprehensive comparison of transition metal substitutions provides valuable insights for the rational design and experimental development of next-generation anti-perovskite layered cathode materials for sodium-ion batteries.