A Novel Na₄MnCr(PO₄)₂F₃ Cathode with High Energy Density for Sodium-Ion Batteries

Na₃V₂(PO₄)₂F₃ (NVPF) is a representative cathode material of sodium-ion batteries (SIBs) due to its high working voltage and high structural stability. However, its specific capacity is limited to the reversible inserting and extracting of two sodium ions per formula unit, which hampers the improvement of its energy density. In this study, we propose a new NASICON-type Na₄MnCr(PO₄)₂F₃ (NMCPF) cathode and systematically investigate its key properties using first-principles calculations. NMCPF exhibits the ability to extract/insert three sodium ions per formula unit, resulting in a high specific capacity of 180.34 mAh/g, and demonstrates three-electron redox reactions involving three redox couples of Mn^2+/3+ (3.05 V), Mn^3+/4+ (4.11 V), and Cr^3+/4+ (4.64 V). Consequently, its energy density can reach 709.33 Wh/kg. In addition, NMCPF exhibits a small volume change of 8.2% during the charging/discharging process and sodium ion diffusion properties comparable to those of NVPF. This study demonstrates the potential of NMCPF as a promising cathode material with high energy density for SIBs.