Fluorine Substitution Promotes Air-Stability of P'2-Type Layered Cathodes for Sodium-Ion Batteries

As one of the most promising cathode materials in sodium-ion batteries, manganese-based layered oxides have aroused wide attention due to their high specific capacity and plentiful reserves. However, they are plagued by poor air stability rooting in water/Na⁺ exchange and adverse structural reconstruction, hindering their practical applications. Herein, it is demonstrated that utilizing fluorine to substitute oxygen atoms can narrow the interlayer spacing of novel P'2-Na_0.67 MnO_1.97 F_0.03 (NMOF) cathode material, which resists the attack of water molecules, significantly prolonging exposure time in air. Density functional theory (DFT) calculation results indicate that fluorine substitution alleviates the insertion of water molecules and spontaneous extraction of Na⁺ effectively. Benefiting from the structural modulation, NMOF can deliver a high specific capacity of 227.1 mAh g^-1 at 20 mA g^-1 and a promising capacity retention of 84.0% after 100 cycles at 200 mA g^-1 . This facile and available strategy provides a feasible way to strengthen the air-stability and expands the scope of practical applications of layered oxide cathodes.