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

Na4Fe3(PO4)2P2O7 (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 Fe2 +/Fe3 + redox couple. Partial substitution of Fe with Mn introduces the high-voltage Mn2 +/Mn3 + redox reaction, but simultaneously aggravates Jahn-Teller distortion, sluggish reaction kinetics, and the competitive formation of maricite-NaMPO4 impurities. Herein, the transition-metal (TM)/Na-P precursor ratio is systematically regulated to control competitive phase formation in Na4Fe1.2Mn1.8(PO4)2P2O7. The sample synthesized with a 3% reduction in the total Fe/Mn precursor content (NMFPP-3) exhibits a substantially reduced maricite-NaMPO4 fraction of 2.1%. Suppression of maricite-NaMPO4 impurities reduces voltage polarization, facilitates Na+ transport, mitigates Mn3 +-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.