Improvement of electrochemical and electrical properties of LiFePO4 coated with citric acid

LiFePO4 was synthesized using hydrothermal method and coated with different amounts of citric acid as carbon source. The samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), surface area measurement-Brunauer-Emmett-Teller (BET), discharge capability, cyclic voltammetry (CV), and electro-chemical impedance spectroscopy (EIS). The results show that the quality and thickness of the carbon coating on the surface of LiFePO4 particles are very important. The optimum carbon content (about 30 wt%) can lead to a more uniform carbon distribution. Electrochemical results show that the samples containing 20 wt%, 30 wt%, 40 wt%, and 50 wt% carbon deliver a discharge capacity of 105, 167, 151, and 112 mAh.g(-1), respectively, at the rate of 0.1C. The increase of carbon content leads to the decrease of discharge capacity of LiFePO4/C, owing to the fact that excess carbon delays the diffusion of Li+ through the carbon layers during charge/discharge procedure. The LiFePO4/C with low carbon content exhibits poor electrochemical performance because of its low electrical conductivity. Therefore, the amount of carbon must be optimized in order to achieve excellent electrochemical performance of LiFePO4/C for its application in a lithium ion battery.