Study On Coating Characteristics Of Lithium Iron Phosphate

For the existing problems in the modification of LiFePO₄ cathode materia, such as coating process complex, carbon-coated uneven and particle size too large and so on, using starch as carbon source, which can be heated to form a stable colloid achieving the uniform coated precursor. Superfine LiFePO₄/C composite material was synthesized by carbothermal reduction. The electrochemical performances were evaluated by the programmable constant-current charge-discharge test system. The structure and morphology of material were characterized by means of X-ray diffraction (XRD), Scanning electron microscope (SEM), Transmission electron microscope (TEM), and Thermogravimetry/differential thermal analysis (TG-DTA). The effect of calcining temperature and time synthetic process parameters was investigated. The best synthetic conditions were determined by the orthogonal experiment. And the study was done on the high-performance LiFePO₄/C composite material synthesized by carbothermal reduction with using composite carbon sources.It was found that LiFePO₄/C composite material synthesized by using starch as carbon source not only had superfine spherical particle morphology, a particle size of about 500nm, but also the particle surface of which was coated by the network filaceous conductive carbon evenly. Compared with traditional solid-phase coating process, not only it reduced the particle size, but also effectively reduced polarization to improve the electrochemical performance of LiFePO₄.The best synthetic process conditions determined by the orthogonal experiment are as follows: calcining temperature: 750℃, calcining time: 4h, the ratio of doped carbon: FePO₄: Li2CO3: C=2: 1.02: 2. The LiFePO₄/C composite material synthesized under the best synthetic process conditions had good crystalline, high specific conductance (5.12×10-2S/m) and tap density (1.25g/cm3). Meanwhile, the result indicated that it had excellent electrochemical performances. The discharge tests were taken at 0.2C rate and 3C rate. The first discharge specific capacity is 155.8mAh/g and 129.2mAh/g, while the capacity retention at 3C ratio in 100 times circulation reach as high as 97.8%.In order to form comprehensive compact and uniform carbon conductivity system on the surface of material, the specific conductance of LiFePO₄/C composite material is further improved. With starch and polyethylene glycol (PEG) as composite carbon sources, LiFePO₄/C composite material was synthesized by carbothermal reduction. The result indicated that the synthesized material had superfine spherical particle morphology, a particle size of about 500nm. Not only the surface of material was coated by compact conductive carbon evenly, but the gap of particle was filled by the network filaceous conductive carbon. Such a comprehensive conductive system had effectively increased the conductivity of material on the surface and between the particles, up to 2.38×10-1S/cm. The discharge tests found that the synthesized samples had excellent electrochemical performances. The discharge tests were taken at 0.2C rate and 1C rate. The first discharge specific capacity is 156.8mAh/g and 140.8mAh/g.