Dual-iron source engineering for optimised solid-state reaction kinetics towards high-performance Na4Fe3(PO4)2P2O7 cathodes

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, an FePO₄–FeC₂O₄ synergistic strategy is hereby proposed to precisely regulate the reaction kinetics during the solid-state synthesis of NFPP. The 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 13 Ah pouch cells. Collectively, this work offers both theoretical insights and experimental validation for the scalable synthesis of high-performance cathode materials for sodium-ion batteries.