Research On The Synthesisof LiFePO₄/C Composite Cathode Material Using Mixedi Ron Sources

In recent years the research of lithium-ion battery focuses on development of low cost, high performance electrode materials. The choice of the iron sources has a great influence on the performance ofLiFePO4/C composite cathode material which is synthesized by solid state reaction method.LiFePO4/C composite cathode material which is synthesized by using FeC2O4·2H2O as iron source would not need a reducing agent because FeC2O4·2H2O contains Fe2+.This method makes LiFePO4/C have a good performance of carbon coating, but the cost could be higher and it would emit a large of gas which can decrease the material density.LiFePO4/C composite cathode material which is synthesized by using FePO4 as iron source can increase material density, but the price of FePO4 is higher, and preparation need large numbers of reducing agent because FePO4 contains Fe3+. In order to overcome the disadvantages of bivalent iron source or ferric iron source as a iron source alone, LiFePO4/C is synthesized by solid state reaction method using mix iron sources.This thesis mainly studied different iron source combinations, calcination temperatures and amount of carbon coated synthesis process for synthesis of LiFePO4/C composite cathode material properties. By thermogravimetric analysis(TG), scanning electron microscope(SEM) analysis, X-ray diffraction(XRD) analysis, charging and discharging test, cyclic voltammograms respectively on the composites microstructure, crystal structure and electrochemical performance characterization and testing.FeC2O4·2H2O and Fe2O3 had been selected as mixed source of iron, LiFePO4/C composite cathode material was synthesized by carbon thermal reduction process, the effects of different temperatures and molar ratio of mixed iron sources on the electrochemical performance of as-synthesized cathode materials were investigated and analyzed. The experimental results demonstrated that the LiFePO4/C composite cathode material synthesized at 710?C and with 1/2(FeC2O4·2H2O/Fe2O3) molar ratio of mixed iron sources which was characterized by SEM and XRD, had the relatively uniform particle size, no obvious phenomenon of reunion, and good carbon coated effect, charging and discharging test show that material had good discharge specific capacity and cycle performance. The initial discharge specific capacity was 142.8 mAh/g at 0.2C rate and room temperature and after 20 cycles, charging and discharging capacity remain at a rate of 99.57%FeC2O4·2H2O:FePO4 = 1:1 had been selected as a mixed source of iron, LiFePO4/C composite cathode material was synthesized by carbon thermal reduction process. The effects of different temperatures on the electrochemical performance of as-synthesized cathode materials were investigated. The experimental results demonstrated that the LiFePO4/C composite cathode material synthesized at 700?C and with 1/1(FeC2O4·2H2O/FePO4) molar ratio of mixed iron sources had the better electrochemical performance. The initial discharge specific capacity was 154.9 mAh/gat 0.2C rate and room temperature and after 20 cycles, the discharge specific capacity was up to 165.9 mAh/g, capacity retention rate was as high as 107.1%. At the same time, the initial discharge specific capacitywere138.2mAh/g and 135.3mAh/g at 1C and 2C; after cycle 20 times the discharge specific capacity increased, respectively was 148.6mAh/g and 141.9mAh/g, capacity rate was 107.5% and 104.9% respectively.