Phosphorus-catalyzed graphitization strategy toward superior Na-storage performance of Na₄Fe₃(PO₄)₂P₂O₇/C cathodes

The Na₄Fe₃(PO₄)₂P₂O₇/C composite (NFPP-E) featuring a highly graphitized N- and P-co-doped carbon coating and hierarchical pore structure was successfully synthesized using a facile ball-milling-calcination approach. Ethylenediamine tetra (methylene phosphonic acid) (EDTMPA) acts as a multifunctional precursor and concurrently serves as a chelating, phosphorus, carbon, nitrogen, and pore-forming agent. The unique “phosphorus-catalyzed graphitization” effect during EDTMPA pyrolysis plays a pivotal role; the released active phosphorus oxides facilitate the ordered transformation of the carbon matrix, thereby yielding a highly graphitized N- and P-co-doped carbon coating with a low defect density. Electrochemical measurements demonstrated significant performance enhancements of NFPP-E relative to the sample prepared with conventional NH₄H₂PO₄. Specifically, NFPP-E delivered a high reversible capacity of 103.09 mAh·g^–1 at 0.1 C (1 C = 129 mAh·g^–1) and retained 77.41 mAh·g^–1 even at an elevated rate of 10 C, demonstrating excellent rate capability. Furthermore, it exhibited superior long-term cycling stability, with a capacity retention rate of 94.5% after 500 cycles at 1 C. When assembled into a full cell with hard carbon as the anode, the NFPP-E achieved a capacity retention rate of 93.3% over 200 cycles, highlighting its great potential for practical applications. This study not only uncovers the universal principle for regulating the structure and properties of carbon layers via the “phosphorus-catalyzed graphitization” mechanism using organic phosphonic acid precursors but also offers a novel strategy for the development of high-performance and low-cost cathode materials for sodium-ion batteries.