Synthesis And Performance Of LiFePO₄ Cathode Materials By Liquid-State Co-Precipitation Method

The cathode materials of lithium ion battery are developing towards the direction of high specific energy, high specific power, long last cycling life, low cost, benign environment, good security and so on. While lithium iron phosphate (LiFePO4) with an ordered olivine structure become the new pop research among the cathode materials of lithium ion battery for its advantaged superiority such as abundant resources, low cost, non-toxicity and environmental benignity. Liquid-state coprecipitation method was used to synthesize LiFePO4. The crystalline structure, morphology of particle and electrochemical performance were investigated by x-ray diffraction (XRD), atomic absorption spectra (AAS), scanning electron microscopy (SEM), galvanostatic charge-discharge and cyclic voltammetry (CV). The effects on the physical characteristics and electrochemical performances of samples by different experimental conditions were investigated systematically.The reaction mechanism of material and effect of the sintered temperature on the crystal structure and the electrochemical performance were characterized via TG-DSC, AAS, SEM, XRD and electrochemical performance testing. The results showed that the best molar proportion of lithium,iron and phosphor is 3 : 1 : 1. The material sintered for 12h at 650℃under nitrogen gas had a complete olivine structure, uniform surface morphology, high tapped density and excellent electrochemical performances. The initial discharge capacity, namely 124.6 mAh/g, was obtained at 0.05C charge-dischange rate. After 20 cycles, there was no obvious capacity fade. With the increasing of the discharge rate, the capacity of the sample had a rapid attenuation.In order to improve the electrochemical performance of products, LiFePO4/C by doping glucose was prepared. The effect of sintered temperature, sintered time and carbon amount on LiFePO4/C was systemically investigated. The experimental result showed that the addition of carbon reduced the LiFePO4 grain size and enhanced performance in term of improved discharge capacity and cyclability. The material coated 5% carbon sintered at 650℃for 12 hours had the best electrochemical performances. The initial discharge capacity, namely 161.2mAh/g, was obtained at 0.05C charge-dischange rate. Even at 0.1C, LiFePO4/C also had high discharge capacitiy which was above 150.0 mAh/g and well cyclablity.In order to improve the natural conductivity of materials to reform the electrochemical performances, the effect of the doping Mn2+ was studied, which showed that the addition of Mn2+ not only improved the electrochemical performances but also reformed the performance of high current rate. LiFe0.8Mn0.2PO4 showed the best electrochemical performances which delivered a discharge capacity of more than 135mAh/g at a current rate of 0.05C and the capacity loss was only 1.48% after 20 times charge-discharge cycles. Even at 0.1C and 0.5C, LiFe0.8Mn0.2PO4 also had excellent cyclablity.