Mn‐Rich Phosphate Cathode for Sodium‐Ion Batteries: Anion‐Regulated Solid Solution Behavior and Long‐Term Cycle Life

Abstract As a sodium superionic conductor, Mn‐rich phosphate of Na_3.4Mn_1.2Ti_0.8(PO₄)₃ is considered as one of the promising cathodes for sodium‐ion batteries owing to its good thermodynamic stability and high working voltage. However, Na_3.4Mn_1.2Ti_0.8(PO₄)₃ is faced with low electronic conductivity, poor cycling stability and complex phase transition caused by multi‐electron transfers, which limits its practical application. Herein, an anion‐regulated strategy is proposed to optimize the Mn‐rich Na_3.4Mn_1.2Ti_0.8(PO₄)₃ phosphate cathode. After introducing F anions into the lattice, the rate performance is improved from 60.5 to 72.8 mAh g⁻¹ at 20 C. Ascribed to unique structure design, the reaction kinetics of Na_3.4Mn_1.2Ti_0.8(PO₄)₃ are significantly improved, as demonstrated by cyclic voltammetry at varied scan rates and galvanostatic intermittent titration technique. The generated M‐F bond inhibits Jahn–Teller effect with an improved cycle stability (85.8 mAh g⁻¹ after 1000 cycles at 5 C with 94.3% capacity retention). Interestingly, reaction mechanism of Na_3.4Mn_1.2Ti_0.8(PO₄)₃ with the complex two‐phase and solid solution reactions changes to the whole solid solution reaction after fluorine substitution, and leads to a smaller volume change of 5.41% during reaction processes, which is verified by in situ X‐ray diffraction. This anion regulation strategy provides a new method for designing the high‐performance phosphate cathode materials of sodium‐ion batteries.