Optimizing Metal‐Precursor Stoichiometry Toward Impurity‐Suppressed and Structurally Reversible Iron–Manganese Phosphate Cathodes for Sodium‐Ion Batteries

ABSTRACT Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) is emerging as a promising cathode material for sodium‐ion batteries owing to its robust polyanionic framework and low material cost. However, its practical energy density is restricted by the low operating potential of the Fe 2 + /Fe 3 + redox couple. Partial substitution of Fe with Mn introduces the high‐voltage Mn 2 + /Mn 3 + redox reaction, but simultaneously aggravates Jahn–Teller distortion, sluggish reaction kinetics, and the competitive formation of maricite‐NaMPO 4 impurities. Herein, the transition‐metal (TM)/Na–P precursor ratio is systematically regulated to control competitive phase formation in Na 4 Fe 1 . 2 Mn 1 . 8 (PO 4 ) 2 P 2 O 7 . The sample synthesized with a 3% reduction in the total Fe/Mn precursor content (NMFPP‐3) exhibits a substantially reduced maricite‐NaMPO 4 fraction of 2.1%. Suppression of maricite‐NaMPO 4 impurities reduces voltage polarization, facilitates Na + transport, mitigates Mn 3 + ‐induced Jahn–Teller distortion and promotes highly reversible structural evolution. Consequently, NMFPP‐3 delivers reversible discharge capacities of 105.5 and 85.0 mAh g − 1 at 0.05 and 10 C, respectively, and retains 89.6% of its initial capacity after 200 cycles at 1 C. Moreover, the NMFPP‐3//hard‐carbon full cell achieves an energy density of 237.2 Wh kg − 1 . These results demonstrate that precursor‐stoichiometry regulation provides a simple and effective strategy for suppressing competitive impurity formation and improving the structural and electrochemical reversibility of mixed‐transition‐metal phosphate cathodes.