Revealing the Phase Evolution in Na 4 Fe x P 4 O 12+ x (2 ≤ x ≤ 4) Cathode Materials

Although great achievements have been gained on a series of Na 4 Fe x P 4 O 12+ x (2 ≤ x ≤ 4) materials such as Na 2 FeP 2 O 7 (NFPO), Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP), and NaFePO 4 (NFP), the phase evolution characteristics on these Na 4 Fe x P 4 O 12+ x materials are still lacking. Herein, 17 Na 4 Fe x P 4 O 12+ 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 Na 4 Fe x P 4 O 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 highest specific capacity (110.1 mAh g –1 ) among the 17 Na 4 Fe x P 4 O 12+ 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.