Revealing the phase evolution in Fe-based polyanionic cathode materials for low-cost and high-performance sodium-ion batteries: the case of Na4FexP4O12+x (2≤x≤4)

Fe-based polyanionic materials are one of the most promising cathode materials for practical sodium-ion batteries due to their rich-resource, low cost, and excellent electrochemical performance. Although great achievements have been gained on a series of Na₄Fe_xP₄O_{12 + x} (2 ≤ x ≤ 4) materials such as Na₂FeP₂O₇ (NFPO), Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) and NaFePO₄ (NFP), the structure and phase evolution characteristics on these Na₄Fe_xP₄O_{12 + x} are still lacking, making it difficult to synthesize these materials with pure phase and optimal electrochemical performance. Herein, seventeen Na₄Fe_xP₄O_{12 + x} samples with varied x are investigated via both experimental and computational methods to disclose the phase evolution properties. It reveals that only three phases of NFPO, NFPP, and NFP exist in the Na₄Fe_xP₄O_{12 + x} system, and Fe-defects tend to form at Fe2 sites in NFPP, resulting in a highly pure phase of Fe-defective NFPP (x = 2.91). The NFPP (x = 2.91) exhibits the best electrochemical performance among the seventeen Na₄Fe_xP₄O_{12 + x} samples. The pouch cells assembled with the NFPP (x = 2.91) cathode and hard carbon anode show excellent rate capability, superior low-temperature performance, high over-discharge endurance, and decent cycling stability. We believe that this work not only clarifies some important issues regarding the phase evolution in Fe-based polyanionic materials, but also serves as an indispensable reference to promote the practical application of low-cost sodium-ion batteries.