Controllable Enhancement of Maricite-NaFePO₄ to Na₄Fe₃(PO₄)₂P₂O₇ With High Bulk Purity for Sodium-Ion Batteries via a Precursor Reverse-Phase Conversion

Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) cathode demonstrates promising application potential due to its inherent benefits, such as high theoretical capacity, appropriate operating voltage, low cost, and environmental benignity. However, it suffers significant capacity degradation due to the inevitable Maricite-NaFePO₄ and Na₂FeP₂O₇ impurity phases. Herein, an efficient reverse-phase conversion engineering strategy was proposed to eliminate these impurities, enabling the controlled synthesis of a high-purity bulk A-NFPP/C composite. Density functional theory calculation reveals that the Maricite-NaFePO₄ impurity phase can transform into NFPP phase, playing a crucial role in optimizing bulk purity. Reverse-phase conversion engineering enables the controlled transformation of the precursor from hexagonal to amorphous structure, further regulating the formation of the key intermediate Maricite-NaFePO₄ during the pyrolysis. In situ XRD analysis confirms that the strategy effectively converts the inert Maricite-NaFePO₄ into electroactive NFPP while simultaneously inhibiting the formation of Na₂FeP₂O₇. The resulting A-NFPP/C delivers a high reversible discharge capacity of 107.24 mAh g⁻¹ at 0.1C, excellent rate capability of 97.00 mAh g⁻¹ at 3.0C, and outstanding long-term cycling stability, maintaining nearly 100% capacity retention after 1000 cycles at 1.0C. This strategy establishes a novel pathway for designing and synthesizing high bulk purity NFPP/C electrodes, accelerating the commercialization of sodium-ion batteries.