A novel dual carbon source improvement strategy for optimizing the ultra-high rate performance of Na₄Fe₃(PO₄)₂P₂O₇ cathode for sodium-ion batteries

Iron-based mixed polyanion-type sodium-ion cathode materials like Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) typically suffer from poor electronic conductivity, resulting in capacity retention under high-rate cycling and rapid capacity degradation. In this study, we introduce an innovative dual-carbon enhancement strategy that integrates carbon nanotubes (CNTs) into the precursor mixing stage, combined with citric acid as both an organic carbon source and a dispersant. Unlike conventional methods where CNTs are added post-synthesis or during slurry preparation—often leading to uneven dispersion—we incorporate CNTs during the initial mixing process. Citric acid not only provides carbon for pyrolysis but also forms a gel-like precursor that ensures homogeneous dispersion of CNTs and raw materials. This one-step sintering approach produces NFPP particles uniformly coated with carbon layers intimately connected to well-dispersed CNTs, potentially forming chemical bonds between them. The resulting pyrolytic carbon and CNT-coated NFPP (NFPP-CNT) exhibits a dense and interconnected electron-conductive network, significantly enhancing its electronic conductivity and electrochemical performance. The precisely designed NFPP-CNT delivers a reversible capacity of 111 mAh/g at 0.1 C and maintains a reversible capacity of 78.8 mAh/g even at an ultra-high rate of 100 C. NFPP-CNT also demonstrates outstanding high-rate capacity retention, with 85.7% capacity remaining after 27,000 cycles at 100 C. This novel synthesis method and the multifaceted role of citric acid endow NFPP with superior high-rate, long-cycle, and low-temperature performance, making it a highly competitive material for large-scale electric energy storage systems (EESs).