A Nonstoichiometric Pure-Phase Na3.4Fe2.4(PO₄)1.4P₂O₇ Cathode for High-Performance Sodium-Ion Batteries

Iron-based polyanionic cathode materials are expected to be extensively utilized in large-scale energy storage applications. Nevertheless, the synthesis of these stoichiometric polyanions generally introduces impurities, restricting their commercial viability. In this study, a pure-phase Na3.4Fe2.4(PO4)1.4P2O7/C (1.4-NFPP/C) material is synthesized through an accurate design of molecular structure. The removal of impurity phases results in excellent electrochemical performance for 1.4-NFPP/C. 1.4-NFPP/C demonstrates a discharge-specific capacity of 112.2 mAh g–1 at 0.1 C, with a rate performance reaching 70 mAh g–1 at 30 C. 1.4-NFPP/C demonstrates a remarkable long-term cycle performance, with a capacity retention of 99.7% after 1000 cycles at 10 C. Electrochemical kinetic tests reveal the superior kinetic characteristics of 1.4-NFPP/C. Ex-situ X-ray diffraction (XRD) tests confirm the involvement of a single-phase solid solution reaction in the sodium storage process of 1.4-NFPP/C. Additionally, the full cell demonstrates competitive electrochemical performance and holds potential application value. Thus, the pure-phase Na3.4Fe2.4(PO4)1.4P2O7/C material has a great potential application in high-performance sodium-ion batteries.