Study On Preparation Technology Of LiFePO₄Used As Cathode Materials For Li-ion Batteries

Olivine-structured LiFePO4is considered as one of the most promising cathode material for lithium-ion batteries and attracts a lot of researchers’attention due to its advantages such as high specific capacity, low cost, abundant raw materials, remarkable thermal stability, preferable safety performance and environmentally friendliness. At present, solid-state method is the main way to prepare LiFePO4in industry, it has simple preparation technology,but it also has some defects such as hard to mix raw material, low tap density, limited specific capacity and unsteady electrochemical performance. Well wet-chemical method to prepare LiFePO4has attracted more and more attention because of well-distributed raw mixed, higher tap density and stable electrochemical properties.A novel method, solid-liquid combination method is designed and proposed to prepare LiFePO4in this thesis. At the first step, prepare the precursor of FePO4by co-precipitation method, then mix fully the precursor, lithium and carbon source, after that sinter the compound. Then the target product LiFePO4is obtained. The product prepared by this method shows better electrochemical performance than co-precipitation product and solid-state product, effectively solves some problems of traditional solid-state method such as uneven of mixed material, low tap density and poor electrochemical performance. The physical properties of the synthesized materials are studied by using X-ray diffraction (XRD) and scanning electron micrography(SEM), and electrochemical performances are investigated by using galvanostatic charge-discharge tests and cyclic voltammetry.In the process of preparing LiFePO4materials, we investigate the effects of reactant concentration, temperature, pH and the molar ratio of Li/Fe, calcination temperature.calcination time and carbon content on the property of the target product. The results of study show:when the solution concentration of1mol/L. the reaction temperature60℃, pH=2; the ratio of Li/Fe is1:1,the calcination temperatures of650℃, the calcination time of12h, the carbon content of10%. the initial discharge capacity of LiFePO4at0.1C is the highest, up to143mAh/g.Through XRD and electrochemical performance testing we found that the material has some shortcomings such as impure product, poor rate performance and unsteady cycle performance. Aimed at the shortcomings, we choose to change the way of coating carbon and doping metal to modify the material. The experimental results indicate that the discharge specific capacity of modified Li0.98Al0.02FePO4reaches157mAh/g at0.1C rate, and138mAh/g at1.0C rate, and the tap density is about1.35g/cm3.The material was not only detected no impurities, but also has higher tap density and better rate capability and cycle stability when it is compared with the material prepared by tradional solid-phase method.