Study On Preparation Of LiFePO₄

Lithium ion batteries become preferred due to its good electrochemical performance and environmental protection. Olive-structured LiFePO4 is a promising cathode material for lithium ion batteries because of its low cost, remarkable thermal stability, non-toxicity and safety. However, it has poor specific capability due to its low electronic conductivity, low lithium ions diffusivity and low volume ratio of capacity, and the purity is difficult to control during the manufacture procedure, which blocked its practical applications. In order to overcome these defaults, the first way is to synthesize small fine particles with good micromorphology and size distribution of LiFePO4 through improved methods; the second way is to coat LiFePO4 with electronically conductive materials such as carbon or metal powder to enhance its electronic conductivity; the final way is to substitute Li by other metal ions to enhance the inherent electronic conductivity.The precursor FePO4·xH2O particles were obtained by oxidation co-precipitation using low price raw materials. The powder was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA) and thermogravimetry (TG) and laser grain-size analysis. The effects of pH value, Fe2+ concentration and the adding of surfactant to purity, morphology, particle size and dispersion of obtained powder were studied. The research indicated that the low concentration is more conductive to good morphology and dispersion. The powder with high purity was obtained at pH of 3.50. FePO4·xH2O particles obtained at pH of 3.50 were spherical in shape and its grain size was about 150nm.The LiFePO/C precursor was synthesized on molecular lever by a simple rheological phase reaction, and single-phase LiFePO4 was obtained sintering at 650℃.The chemical characterization of the composites included XRD, SEM and laser grain-size analysis. The results indicated that the particle size and morphology can be controlled by adjusting the molar ratio of OA/CA. The LiFePO4/C composites obtained at 1:1.33 and 1:0.67 of OA/CA had more regular morphology and better particle size distribution. The tap density is 0.81g/cm3 and 1.01g/cm3 respectively. The initial charge capacity reached 149.8mAh/g at 0.1C rate with good rate performance when OA/CA=1:1.33. The composite prepared at 1:0.67 have more high initial charge capacity of 159.3mAh/g at 0.1 C rate, with a serious decrease on greater C-rates.