Electrochemical properties of carbon-coated LiFePO₄ and LiFe_0.98Mn_0.02PO₄ cathode materials synthesized by solid-state reaction

Abstract

Olivine structured LiFePO₄/C (lithium iron phosphate) and Mn²⁺-doped LiFe_0.98Mn_0.02PO₄/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 LiFe_0.98Mn_0.02PO₄/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.