Different electrochemical responses of LiFe0.5Mn0.5PO4 prepared by mechanochemical and solvothermal methods

A comparative investigation of the crystal structure, morphology and electrochemistry of LiFe0.5Mn0.5PO4 (LFMP) prepared by the mechanochemically assisted solid-state synthesis (SS) and the solvothermal synthesis (ST) was performed. The as-prepared samples were studied by XRD, FTIR, Raman, Mӧssbauer, SEM, TEM, galvanostatic cycling, GITT, and EIS. The low-temperature carbon-free LFMP-ST displays nanoplatelets with 20–30 nm thickness in the b direction and is characterized by the low concentration of the MLi defects and more distorted MO6 octahedra compared to LFMP-SS. It shows poor conductivity and only one sloping voltage plateau at 3.4 V. Its post-coating with carbon at 750 °C leads to the appearance of two commonly observed plateaus, corresponding to the Fe2+/Fe3+ and Mn2+/Mn3+ redox couples, and improves cyclability and rate capability. Carbon-coated LFMP-SS consists of irregular-shaped submicron particles and demonstrates two two-phase plateaus and an additional one at 3.6–3.7 V upon discharge. According to the GITT study, this plateau increases with cycling rate and is most likely controlled by the slower kinetics of the Mn3+ ions reduction.