Regulating Chemical Bond to Stabilize Deep Desodiation of Iron-Based Phosphate Cathodes for Durable Sodium Storage

Iron-based polyanionic compounds Na₄Fe₃(PO₄)(P₂O₇) (NFPP) have attracted attention for its crystal structure and 3D sodium ion transport channels. However, structural degradation caused by [P₂O₇] dimer distortion during deep desodiation severely compromises its electrochemical performance. In this study, the Aluminum (Al) atoms are selected to precisely enhance the bond covalency and modulate the electronic rearrangement of Fe─O chemical bonds, significantly strengthening the FeO₆ octahedron and therefore mitigating the structural distortion of [P₂O₇] dimer in deep desodiation. This similarly promotes the electronic conductivity and the intrinsic sodium ion transport dynamics in the sodium-poor states. As expected, the optimized Na_3.9Fe_2.9Al_0.1(PO₄)₂(P₂O₇) (NFPP-Al) cathode demonstrates a remarkable specific capacity of 115.2 mAh g^-1 at 0.2 C and exceptional cycling stability with 80.4% capacity retention after 4000 cycles at 10 C. Furthermore, the minimal volume change (2.8%) within the solid solution reaction of sodium storage mechanism emphasizes the stabilized octahedra and attenuated dimer aberration. This work provides insights into the relationship between chemical bond regulation and structural stability under deep desodiation, offering theoretical guidance for developing long-term stability iron-based phosphate cathodes.