Synthesis Of LiFePO₄ Cathode Materials By A Chemical Methods

Compared to LiCoO2, LiMn2O4 and the other lithium-ion cathode material the olivine-type LiFePO4 became an alternative cathode material for lithium ion batteries due to its outstanding advantages such as high theoretical capacity (170 mAh/g), high energy density, safety, good thermal stability, low raw materials cost, long cycling life and good environmental compatibility. However, the key barriers to limit LiFePO4 application are extremely low electronic conductivity and low ion diffusive rate. Several methods were put forward to overcome these problems such as suitably preparing procedures to minimize the particle of the sample effectively coating particles with carbon and doping supervalent metals ions to Li+ sites or Fe2+ sites. LiFePO4 cathode material is usually prepared by conventional solid-state reaction in industry at present. However, the method has difficulty in controlling particle size and microstructure which influence on the electrochemical performance of LiFePO4 cathode material. Generally, the chemical method was used to achieve these goals. Thus, in the paper, the single-phase LiFePO4/C cathode material was obtained by oxidation co-precipitation with coating method and sol-gel method. The effects of structure and performance of material through the technics parameter were discussed.FePO4·2H2O powder as a main iron source material of synthesis of LiFePO4 cathode materials was prepared by the oxidation co-precipitation method. Used FeSO4·7H2O, H3PO4, H2O2 and NH4HCO3 as raw material. Saturated FeSO4·7H2O and NH4HCO3 were used in experimentation in order to preparation large of FePO4·2H2O. The effects of pH value on synthesized FePO4·2H2O were investigated. The results suggest that with the increasing pH value the while yellow powder will turn to brown and suppress particle aggregation. We chose the synthesized specimen at pH value of 2～4 as raw materials that synthesized LiFePO4/C cathode material through comprehensive analyzing.A novel coating process to synthesize the LiFePO4/C powders by addition of uncalcined FePO4·2H2O in pH of 2.05 and 3.50 as iron source LiOH as lithium source and different polymer as carbon source has been developed in this paper. The XRD result showed that there were impurity phases in synthesized powder of pH in 2.05 at vacuum atmosphere. However, the powder synthesized by glucose carbon source has better crystallize but the iron phase indicated that Fe(OH)3 would turn into iron otherwise there was single phases in pH of 3.50 synthesized pwoder. Furthermore, The SEM result suggested that the synthesized LiFePO4/C particle with glucose which in spherical in shape and good dispersion. Otherwise the precursor would decomposition expansion and extravasation while calcined in the vacuum stove which would affect the stove life and wasted powders. So sol-gel method and calcined in argon was used in synthesized LiFePO4/C.The results of sol-gel method synthesized LiFePO4/C suggested that the single phased LiFePO4/C would synthesized at 550℃and the pores in the particles decrease with increasing calcination temperature which used FePO4·2H2O as iron source at pH value above 3.50. By different carbon source investigated when the quantity of PEG was 5 wt% of the final products as carbon source and calcined at 650℃in argon atmosphere the obtained has the best electrochemical performance with a specific capacity of 138 mAh/g.