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

Olivine LiMnPO4 cathode materials are favored for their low cost and higher operating voltage compared to those of LiFePO4. However, significant volume changes due to the Jahn–Teller effect of Mn3+, slow lithium-ion diffusion, and poor electronic conductivity limit their structural stability and electrochemical performance. Through a straightforward solid-state reaction, LiMnxFe1–xPO4/C (x = 0.7, 0.8, 0.9) cathode materials were synthesized using FePO4·2H2O and MnPO4·H2O precursors at varying calcination temperatures. Optimal results were obtained at 650 °C, leading to further investigation to identify the most suitable Mn/Fe ratio. LiMn0.7Fe0.3PO4/C exhibited a higher initial discharge capacity of 149.1 mAh g–1 at 0.1 C compared to LiMn0.8Fe0.2PO4/C (146.9 mAh g–1) and LiMn0.9Fe0.1PO4/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 LiMn0.7Fe0.3PO4/C cathodes during (de)lithiation were monitored via operando XRD. During charging, the orthorhombic LiMn0.7Fe0.3PO4 transitioned to orthorhombic Mn0.7Fe0.3PO4, maintaining the same space group Pmnb. Simultaneously, a solid-solution reaction within LixMn0.7Fe0.3PO4 and a two-phase reaction between LixMn0.7Fe0.3PO4 and Mn0.7Fe0.3PO4 were observed to occur successively.