Preparation And Modification Of LiFePO₄ By Carbothermal Reduction Method

Carbothermal reduction route(CTR) stems from traditional solid-state method. Expensive divalent iron compounds, such as FeC2O4.H2O, were not used as the starting material, and it is easier to carry out when compared with products synthesized by traditional Solid-state method. In recent years,various iron-containing compounds have been used as iron sources for LiFePO4/C, such as Fe(OH)3, Fe2O3, Fe3O4, FePO4 and so on. FePO4 can provide both iron and phosphorus sources needed in the process of the synthesizing. Since LiFePO4 and FePO4 have the same space group, the size and shape of the products could be effectively controlled by adjusting the size and shape of the precursor.In this work, fine precursor FePO4 were prepared by precipitation method and then LiFePO4 were synthesized by carbothermal reduction. The process conditions were optimized. In addition, the effects of process conditions on the structure and morphology of FePO4 and LiFePO4 were also investigated. The structure, compositions and morphology of the prepared samples were characterized by XRD, ICP, SEM and TEM, respectively, they were used to investigate the relation between structures and properties. The galvanostatic charge-discharge, cyclic voltammetry and electrochemical impedance of materials were tested by Land and Electrochemical workstation, they were used to investigate the kinetics of lithium ion insertion/deinsertion.Precursor FePO4 were prepared by precipitation method. The effects of reaction compounds, temperature, pH, concentration, stirring speed and surfactant on FePO4 were investigated. The results show that using FeSO4.7H2O-NH4H2PO4 as raw materials is favor for FePO4 with spherical-likely particles. In this system, under these conditions, SDS as surfactant,90℃, pH=1.7,400r/min, the fine and homogenous LiFePO4 in the range of 20-80nm can be obtained.Li2CO3, C6H12O6 and FePO4 are used as starting materials to synthesize LiFePO4 by carbothermal reduction. The effects of process conditions such as sintering process parameters, molar ratio of Li:Fe, carbon content on the structure and morphology of the LiFePO4 were investigated. The sample has the hightest first discharge capacity, excellent rate performance and small polarization which prepared under these conditions,the Li:Fe=1.02, glucose content about 19wt%, water as dispersing agents sintering at 750℃ for 12h.The initial discharge capacity at 0.1C, 0.2C,0.5C of LiFePO4 is 151,150,144 mAh/g, respectively.Lithium iron phosphate composite was prepared using cation as dopant. Three types of transitional metal such as Mg, V and Ni modified LiFePO4 were investeigated in this work. V2O5 modified LiFePO4 has high discharge specific capacity and excellent cycle stability. LiFePO4 with different concentrations of V2O5 were prepared via carbothermal reduction. V content about 3wt% were the optimization concentration, the initial discharge capacity is up to 151mAh/g at 0.1 C and without any loss after 100 cycles.