Preparation, Characterization And Performance Of LiFePO₄/C With Composite Carbon Sources For Lithium-ion Batteries

In recent years, LiFePO₄ material is becoming a research focus of cathode materials owning to its good properties, such as safety, environmental friendly, low cost and excellent cycle performance. However, rate performance and practical application of the material were limited by its weak conductivity. Composite carbon-coated, ion-doping and optimized preparation methods were used intending to enhance the conductivity of LiFePO₄ and improve its electrochemical properties.In this paper, olivine-type LiFePO₄/C was prepared using FeC2O4, Li2CO3, NH4H2PO₄ and sugar as raw materials. The impact of milling process, the sintering temperature, sintering time and the amount of carbon on the performance of samples was investigated by orthogonal test. The phase, morphology and carbon content analysis of LiFePO₄/C was characterized by XRD, SEM, TEM, EA and other technologies; and then electrochemical performances were tested by charge-discharge, cyclic stability, and AC impedance through assembly button batteries. The results show that via ball milling of raw materials, LiFePO₄/C capacity increased 39.2%; the sample sintered under 700℃has perfect crystal structure, uniform particle size, and high specific capacity (128.9 mAh/g), while little amounts of impurities of Li3PO₄,Fe2P and Fe are existed in the sample sintered under 800℃.Olivine-type LiFePO₄/C was firstly synthesized using sucrose and PEG1000 as composite carbon sources, and the sample possessed good electrochemical performance with a initial discharge capacity of 145.7 mAh/g at the current density 34 mA/g (0.2 C) and little attenuation after 10 cycles.LiFePO₄/C was prepared by modified coprecipitation method. The sample had discharge capacities of 149.0 and 126.3 mAh/g at 0.2 and 1 C, respectively, and fine cycle stability, at 0.2 C, charge-discharge 10 times, the capacity remained 97.2% of the initial after 10 cycles. And the influence of the amount of precipitating agent, pH of the systems and different composite carbon sources on the electrochemical properties of samples were also studied.A non-aqueous sol-gel system was developed. Using LiCl, FeCl₂, H₃PO₄ as the main raw materials and anhydrous ethanol as solvent, nano-LiFePO₄/C with a specific morphology was prepared. Sol-gel can be quickly formed in such a non-aqueous sol-gel system, which has features of easy to dry and short preparation cycle. The sample has discharge capacities of 157.7 and 130.9 mAh/g at 0.2 and 1 C, respectively. The effects of different composite carbon sources on the morphology and properties of products were also discussed.LiMxFeyPO₄/C (M=Mg, Cr, Zr) was synthesized by ion doping based on the preparation of olivine-type LiFePO₄/C by non-aqueous sol-gel method. LiZr_xFe_1-2xPO₄/C shows excellent electrochemical properties: Zr-doped LiZr_0.01Fe_0.98PO₄/C has discharge capacities of 160.6, 141.9 and 123.6 mAh/g at 0.2, 1 and 2 C, respectively. The capacity remained 98.0% of the initial after 10 cycles at the rate of 0.2 C.