The Study Of Carbon Source In LiFePO₄/C Composite Cathode Material

LiFePO4 has several advantages, such as wide range of sources of raw materials, benign for environment, good thermal stability, high specific capacity, good cycle performance, outstanding safety performance, and so on. Its theoretical capacity is 170mAh/g, and the working voltage is about 3.4V. LiFePO4 is considered as an ideal cathode material for aerospace, electric vehicle and hybrid electric vehicle. But there are two disadvantages of LiFePO4 cathode material by now, one is low conductivity and low ion diffusion rate, the other is low density, especially composited with carbon material, the density of LiFePO4 is further reduced, which influences the volume energy density of corresponding battery. In recent years, research work has been focused on improving the conductivity of lithium iron phosphate material, in which carbon coats on the lithium iron phosphate material to improve its electrochemical properties is the most simple and effective way.In the paper, the impacts of different carbon sources on the properties of LiFePO4/C material were studied systematically; DSC-TGA has been used to analyze the decomposition mechanism of different carbon sources. Select a representative of organic carbon source and inorganic carbon source to coat on the LiFePO4 to enhance the material properties of conductivity and rate performance.Using Fe2O3 as the iron source, the influences of different carbon sources on the electrochemical performance of LiFePO4/C were investigated in detail. The result shows that LiFePO4/C composite material carbon-coated by polypropylene, with tap density of 1.35 g/cm3, had a capacity of 155.2 mAh/g at 0.1 C and 131.6 mAh/g at 2C, showing the best electrochemical performance.Most carbon from oxygen free pyrolysis of organic precursors exhibits a fluey morphology and forms an effective conducting connection between LiFePO4 particles. LiFePO4/C of good electrochemical properties could be prepared using polypropylene, glucose and phenolic resin as carbon precursor respectively. Little improvement to the electrochemical performance could be obtained by modifying LiFePO4 with only graphite, acetylene black and carbon black.The working mechanism of carbon in the LiFePO4/C compound was studied by Raman method. The result shows that three necessary conditions determined the improvement of LiFePO4/C(1). The particles of LiFePO4 need to be coated by sp2 type carbon; (2). The good electronic conductive network among particles need to be formed; (3). The uniformity of carbon on the LiFePO4 particle is also necessary.