Controllable Enhancement of Maricite‐NaFePO 4 to Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 With High Bulk Purity for Sodium‐Ion Batteries via a Precursor Reverse‐Phase Conversion

Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (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‐NaFePO 4 and Na 2 FeP 2 O 7 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‐NaFePO 4 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‐NaFePO 4 during the pyrolysis. In situ XRD analysis confirms that the strategy effectively converts the inert Maricite‐NaFePO 4 into electroactive NFPP while simultaneously inhibiting the formation of Na 2 FeP 2 O 7 . 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.