Engineering electronic band structure of Co-doped Na₄Fe₃(PO₄)₂P₂O₇ for sodium-ion batteries

The iron-based polyanion Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) cathode material has been extensively studied for sodium-ion batteries (SIBs) owing to its excellent structural stability. However, its practical application is severely hindered by intrinsic limitations, including poor electronic conductivity and the formation of electrochemically inactive NaFePO₄ impurity phases during synthesis. To address these issues, a series of Co-doped Na₄Fe_3-xCo_x(PO₄)₂(P₂O₇) cathode materials with gradient doping concentrations were synthesized via a facile spray-drying strategy. X-ray absorption near edge structure (XANES) analysis suggests that Co doped NFPP significantly shortens the FeO bond. This result is beneficial for electron transfer during charging/discharging processes, further enhancing the electrochemical performance of the NFPP material. Density functional theory (DFT) calculations demonstrate that Co doping effectively reduces the bandgap of NFPP and mitigates lattice parameters variations during Na⁺ extraction/insertion. These synergistic effects jointly enhance high-rate capability and long-cycling stability of Na₄Fe_2.91Co_0.09(PO₄)₂(P₂O₇). The capacity retention rate reaches up to 80 % after 6000 cycles at 20C (initial discharge capacity of 89.1 mAh g^-1). This study elucidates the Co doped NFPP mechanism of polyanionic cathodes, and thereby advancing the practical application of high-performance SIBs.