Electronic modulation with high-valence metal doping towards high-rate Na₄Fe₃(PO₄)₂P₂O₇ cathode in sodium-ion batteries

Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) emerges as a cost-effective structurally stable and environmentally benign cathode material, exhibiting significant potential in the energy storage of sodium-ion batteries. However, inherent low ionic mobility and electronic conductivity of NFPP have affected its power performance. In this study, high-valence transition metal cations (Mo⁶⁺, Ta⁵⁺, Nb⁵⁺, and W⁶⁺) are doped within the NFPP lattice, which induces internal electronic rearrangement and average valence state decrease of Fe cations through distortion of locally corner-sharing FeO polyhedra. The increased Fe-O bond lengths within Mo-doped NFPP crystals and altered electronic cloud distribution further validates this ionic charge compensation mechanism. High-valence metal-doping can also decrease the bandgap, enhance average electronic conductivity, as well as lower Na⁺ migration barrier, thus increasing Na⁺ diffusion coefficient by three orders of magnitude. Therefore, the optimized Na₄Fe_2.91Mo_0.09(PO₄)₂P₂O₇ cathode material demonstrates excellent rate performance and outstanding cycling stability (retaining 85.86% capacity after 1300 cycles at 1C). In addition, the universal effectiveness of the high-valence transition metal doping strategy is verified by investigation of Ta⁵⁺, Nb⁵⁺, and W⁶⁺ doping based on experimental characterizations and theoretical calculations. These findings provide a new perspective to modulate electronic structure and ionic transport pathway of NFPP, and shedding light on great application prospects of iron-based mixed polyanion cathode materials.