Facile synthesis of LiMn_0·75Fe_0·25PO₄/C composite cathode material and electronic conductivity of carbon coating

LiMn_0·75Fe_0·25PO₄/C powders were prepared by a facile hydrothermal and ball-milling method, and then calcined. X-ray diffraction and scanning electron microscopy revealed that the calcination temperature had a strong effect on the structure and morphology of the as-prepared powder, and thus, on the subsequent electrochemical performance of the material. When the calcination temperature was increased from 550 to 700°C, the grain size of LiMn_0·75Fe_0·25PO₄/C increased greatly. The electronic conductivities of the carbon coatings, which were obtained from sucrose and calcined at 550 and 700°C, were investigated. The discharge capacities of LiMn_0·75Fe_0·25PO₄/C decreased with increasing calcination temperature, but electrochemical impedance spectroscopy showed that the charge-transfer resistances of LiMn_0·75Fe_0·25PO₄/C electrodes decreased owing to the increase in the electronic conductivity of the carbon coating. Grain growth played a more important role than the electronic conductivity of the carbon coating in the discharge capacity of the electrode. The powder calcined at 550°C exhibited the best electrochemical performance, with a discharge capacity of 131·9 mAh g⁻¹ at a rate of 0·1 C.