Activating Na4 site via crystalline-phase-engineering enables charge redistribution and stress mitigation in Na4Fe3(PO4)2P2O7 cathodes.

Na4Fe3(PO4)2P2O7 (NFPP) has emerged as a low-cost and durable cathode material for practical sodium-ion batteries. Nevertheless, the intrinsic presence of inactive NaFePO4 (NFP) impurities greatly limits its high-rate capability and long-cycle stability. Herein, we report the synthesis of a high-purity NFPP by customizing the NFP intermediate during synthesis, in which the intermediate with weak crystallinity and low Fe-O coordination enables its complete conversion to NFPP with negligible NFP residue. The high-purity NFPP promotes charge redistribution within Fe-O polyhedra, narrowing the electronic band gap by 15% and lowering the Na4-Na4 migration barrier by 5.5-fold, which facilitates Na4-site utilization at high voltages. Meantime, a 20.8% suppression of lattice distortion is achieved during deep (de)sodiation. Consequently, the high-purity NFPP delivers a near-theoretical charge capacity of 126.6 mAh g-1 at 0.1 C and retains 88.4 mAh g-1 at 50 C in half-cells, while full cells exhibit a 96.9% capacity retention after 3000 cycles at 5 C. This study provides a way to unlock ultrahigh-rate and long-life mixed-polyanion cathodes, accelerating the large-scale applications in grid-scale energy storage.