Synthesis and performance of carbon-modified LiFePO₄ using an in situ PVA pyrolysis procedure

LiFePOa/carbon composite cathode material was prepared by granulating and subsequent pyrolysis processing in N₂ atmosphere with polyvinyl alcohol (PVA) as the carbon source. The influences of carbon content on the microstructure and battery performance were investigated. Single LiFePO₄ phase and amorphous carbon can be found in the products. A special micro-morphology of the optimum sample was observed. The discharge capacity of the cell with the optimum cathode was 135 mAh·g^-1, close to the charge capacity of 153 mAh·g^-1 at 17 mA·g^-1. The influence of ambient temperature on the cell capacity was investigated. The temperature dependence of its electrochemical characteristic was evaluated by using AC impedance spectroscopy. A new equivalent circuit based on the charge and mass transfer control process in an electrode was proposed to fit the obtained AC impedance spectra. The tendency of every element in the equivalent circuit was used to interpret the temperature dependence of the capacity of the optimum cathode.