Electrochemical properties of carbon-coated LiFePO4 and LiFe0.98Mn0.02PO4 cathode materials synthesized by solid-state reaction

Olivine structured LiFePO4/C (lithium iron phosphate) and Mn2+-doped LiFe0.98Mn0.02PO4/C powders were synthesized by the solid-state reaction. The effects of manganese partial substitution and different carbon content coating on the surface of LiFePO4 were considered. The structures and electrochemical properties of the samples were measured by X-ray diffraction (XRD), cyclic voltammetry (CV), charge/discharge tests at different current densities, and electrochemical impedance spectroscopy (EIS). The electrochemical properties of LiFePO4 cathodes with x wt.% carbon coating (x= 3, 7, 11, 15) at γ =0.2C, 2C (1C= 170 mAh·g−1) between 2.5 and 4.3 V were investigated. The measured results mean that the LiFePO4 with 7 wt.% carbon coating shows the best rate performance. The discharge capacity of LiFe0.98Mn0.02PO4/C composite is found to be 165 mAh·g−1 at a discharge rate, γ = 0.2C, and 105 mAh·g−1 at γ =2C, respectively. After 10 cycles, the discharge capacity has rarely fallen, while that of the pristine LiFePO4/C cathode is 150 mAh·g−1 and 98 mAh·g−1 at γ=0.2 and 2C, respectively. Compared to the discharge capacities of both electrodes above, the evident improvement of the electrochemical performance is observed, which is ascribed to the enhancement of the electronic conductivity and diffusion kinetics by carbon coating and Mn2+-substitution.