Fabrication of onion-like carbon frameworks and uniform carbon coating layer at Na4Fe3(PO4)2P2O7 for synergistically improving conductivity and structural stability

Abstract

Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) materials are promising candidates as cathodes in sodium-ion batteries (SIBs) due to their low cost, environmental friendliness, and high specific capacity. However, the electronic conductivity of this polyanionic material is constrained by weak electron transfer within the Fe-O-P-O-Fe pattern, limiting the high-rate performance of batteries. Herein, inexpensive bitumen is selected as a coating precursor to prepare NFPP@C composite cathode materials. After low-temperature carbonization at 480 °C, a protective layer forms with onion-like carbon as the framework and an ultrathin carbon coating uniformly cladding the NFPP. This significantly enhances the electronic conductivity while protecting the material from interfacial side reactions, thereby stabilizing the structure. It exhibits a low charge transfer resistance (Rct, 57 Ω at 3.25 V), high discharge specific capacity (115.8 mAh g⁻¹), good high-rate capability (59.8 mAh g⁻¹ at 20 C), and excellent cycling stability (91.0 % capacity retention after 1600 cycles at 10 C). This strategy provides an effective approach for optimizing the performance of sodium-ion polyanionic cathode materials.