Synthesis of porous carbon Na4Fe3(PO4)2P2O7@C by sol-gel method as a high-rate cathode for sodium-ion batteries

Na₄Fe₃(PO₄)₂P₂O₇@C (NFPP) has attracted widespread attention from researchers due to its excellent structural stability, low cost, and non-toxic nature, making it one of the most promising energy storage materials for the future. The inherently poor electrical conductivity of NFPP, its actual specific capacity (around 100 mAh g⁻¹) is often lower than its theoretical capacity (129 mAh g⁻¹), which has been a key issue of concern for researchers. In this study, we adopted a mixed enhancement approach, where polyvinyl alcohol (PVA) and a carbon source were appropriately combined. The PVA uniformly adhered to the surface, and during high-temperature pyrolysis, its decomposition resulted in the formation of carbon layers with pores, creating a porous carbon framework network. The presence of a porous carbon framework allows for sufficient electrolyte infiltration, significantly increasing the reaction surface area. As a result, we successfully synthesized NFPPx (with some PVA added) material with a porous carbon framework network. The surface of NFPPx was coated by a porous carbon layer, which provides the provision of a rapid conductive network and abundant sodium-ion transport pathways. NFPP2 exhibits excellent electrochemical performance, with an outstanding reversible specific capacity of 118 mAh g⁻¹ at 0.1C, which corresponds to 91.5% of its theoretical capacity. Additionally, NFPP2 demonstrates remarkable long-term cycling stability and excellent rate performance. After 6,000 cycles at 20C, it retains 90.6% of its capacity with a reversible specific capacity of 78 mAh g⁻¹. Even at a high current rate of 40C, NFPP2 achieves a specific capacity of 69 mAh g⁻¹. This simple method of mixing and enhancing materials has significantly advanced the development of sodium-ion batteries, providing a promising pathway for the future production of porous carbon framework materials.