Alleviating anti-site defects and Jahn-Teller effects in Na4Fe1.5Mn1.5(PO4)2(P2O7) for high-power and long-life Na-ion batteries

High-safety and low-cost Na₄Fe_1.5Mn_1.5(PO₄)₂(P₂O₇) (NFMPP) is a highly promising cathode for Na-ion batteries (SIBs), offering a higher energy density than typical NFPP. However, its performance is hindered by Na/Mn anti-site defects and Jahn-Teller (J-T) distortion, resulting in voltage hysteresis and sluggish Na-ion diffusion kinetics. Herein, we demonstrate a high-rate and long-life NFMPP cathode by synergistically optimizing the initial molar ratio of Na-ion to transition metal (Na/TM) and vanadium (V) doping. The Na/Mn anti-site defects have been greatly alleviated from 5.9 % to 2.2 %, effectively suppressing the voltage hysteresis of Mn²⁺/Mn³⁺ redox plateau. Meantime, the electron-rich V doping with strong V-O bonds (541 kJ/mol) exhibits strong interaction with Mn, significantly reducing the effect of J-T distortion on Na-ion diffusion. As a consequence, the optimized NFMPP cathode delivers a superior initial charge capacity of 115.1 mAh/g at 0.1C and achieves a high reversible energy density of 334 Wh/kg, surpassing many reported phosphate-based cathodes. It also exhibits exceptional cycling stability, retaining 94.1 % of its capacity after 500 cycles, making it a highly competitive candidate for next-generation SIBs.