Dual-Iron Source Engineering for Optimised Solid-State Reaction Kinetics Towards High-Performance Na4Fe3(PO4)2P2O7 Cathode

Abstract

Na4Fe3(PO4)2P2O7 (NFPP), boasting high operating voltage and robust structural stability, has been extensively considered one of the most promising cathode materials for sodium-ion batteries. Nevertheless, its solid-state synthesis is often complicated by multiple competing reaction pathways, which readily give rise to impurity phases. To this end, a FePO4-FeC2O4 synergistic strategy was hereby proposed to precisely regulate the reaction kinetics during the solid-state synthesis of NFPP. Results reveal that the cooperative interplay between rigid and flexible Fe-O octahedra considerably facilitates the Fe-P-O framework construction and thus lowers the topological rearrangement barrier during NFPP crystallisation. When the FePO4/FeC2O4 ratio is optimised to 1.5, a homogeneous reaction layer featuring continuous Na-Fe-P-O transport pathways is established, effectively suppressing the formation of impurity phases. With enhanced phase purity, the PC-1.5 sample delivers initial charge and discharge capacities of 115.5 and 108.0 mAh·g-1 at 0.1C. At 50C, it retains a reversible capacity of 74.3 mAh g⁻¹ and exhibits an excellent capacity retention of 88.7% after 6800 cycles. Moreover, kilogram-scale products obtained through scaled-up synthesis exhibit stable cycling performance over 1000 cycles in 13Ah pouch cells. Collectively, this work offers both theoretical insights and experimental validation for the scalable synthesis of high-performance cathode materials toward sodium-ion batteries.