Revealing the Phase Evolution in Na₄Fe_xP₄O_12+x (2 ≤ x ≤ 4) Cathode Materials

Although great achievements have been gained on a series of Na4FexP4O12+x (2 ≤ x ≤ 4) materials such as Na2FeP2O7 (NFPO), Na4Fe3(PO4)2P2O7 (NFPP), and NaFePO4 (NFP), the phase evolution characteristics on these Na4FexP4O12+x materials are still lacking. Herein, 17 Na4FexP4O12+x samples with varied x are investigated via both experimental and computational methods. It is revealed that only three phases of NFPO, NFPP, and NFP exist in the Na4FexP4O12+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 highest specific capacity (110.1 mAh g–1) among the 17 Na4FexP4O12+x samples. The pouch cells assembled with the NFPP (x = 2.91) cathode and hard carbon anode show a good comprehensive electrochemical performance. We believe that this work can serve as an indispensable reference to promote the practical application of sodium-ion batteries.