Electrochemical Performance and In Situ Phase Transition Analysis of Iron-Doped Lithium Manganese Phosphate

Olivine LiMnPO 4 cathode materials are favored for their low cost and higher operating voltage compared to those of LiFePO 4 . However, significant volume changes due to the Jahn–Teller effect of Mn 3+, slow lithium-ion diffusion, and poor electronic conductivity limit their structural stability and electrochemical performance. Through a straightforward solid-state reaction, LiMn x Fe 1– x PO 4 /C ( x = 0.7, 0.8, 0.9) cathode materials were synthesized using FePO 4 ·2H 2 O and MnPO 4 ·H 2 O precursors at varying calcination temperatures. Optimal results were obtained at 650 °C, leading to further investigation to identify the most suitable Mn/Fe ratio. LiMn 0.7 Fe 0.3 PO 4 /C exhibited a higher initial discharge capacity of 149.1 mAh g –1 at 0.1 C compared to LiMn 0.8 Fe 0.2 PO 4 /C (146.9 mAh g –1 ) and LiMn 0.9 Fe 0.1 PO 4 /C (125.6 mAh g –1 ), and a superior capacity retention of 96.1% after 160 cycles. Additionally, it showed improved rate capability with average discharge capacities of 138.7, 131.1, and 110.6 mAh g –1 at 0.2, 0.5, and 1 C rates, respectively. Furthermore, the phase transitions of LiMn 0.7 Fe 0.3 PO 4 /C cathodes during (de)lithiation were monitored via operando XRD. During charging, the orthorhombic LiMn 0.7 Fe 0.3 PO 4 transitioned to orthorhombic Mn 0.7 Fe 0.3 PO 4, maintaining the same space group Pmnb. Simultaneously, a solid-solution reaction within Li x Mn 0.7 Fe 0.3 PO 4 and a two-phase reaction between Li x Mn 0.7 Fe 0.3 PO 4 and Mn 0.7 Fe 0.3 PO 4 were observed to occur successively.