LiFePO4的合成及其热分析动力学

在惰性气氛下, 以Li2CO3、FeC2O4·2H2O和NH4H2PO4为原料, 用高温固相方法合成了橄榄石型LiFePO4材料. 利用不同升温速率的热重及差热分析研究了固相合成LiFePO4的反应动力学. 研究表明, LiFePO4的高温固相合成过程可分为三个步骤, 利用Doyle-Ozawa法和Kissinger法分别计算了各个反应阶段的表观活化能. 用Kissinger法确定每个反应阶段的反应级数和频率因子, 并给出了各个阶段的动力学方程. 根据动力学研究的结果, 采用优化的固相 分段法合成了碳包覆改性的LiFePO4正极材料. 利用X射线衍射、扫描电镜及恒流充放电对材料进行了物性表征及性能测试. 结果表明, 该材料具有单一的橄榄石结构, 颗粒尺寸细小均匀, 0.1C倍率放电时表现出良好的电化学性能.

Synthesis of LiFePO4 and Thermal Dynamics of the Precursor

Olivine LiFePO4 cathode materials were synthesized by a solid state method in an inert atmosphere. The thermal decomposition processes taking place in the solid state mixture of Li2CO3, FeC2O4·2H2O, and NH4H2PO4 were investigated using TG-DTA and XRD techniques. The dynamic study of the precursor was also investigated using TG-DTA at different heating rates. The decomposition proceeded through three well-defined steps while TG curves closely corresponded to the theoretical mass loss. The apparent activation energy of each stage was calculated using the Doyle-Ozawa and Kissinger methods. The calculated results were 134.3, 122.2, 173.2 kJ·mol-1 for Ozawa method and 102.4, 128.1, 145.3 kJ·mol -1 for Kissinger method. The coefficients of reaction order, frequency factor, and dynamic equations were also determined. Based on the results of the dynamic study, the cathode material LiFePO4 was synthesized by optimized step-sintering method. The crystal structure and the electrochemical performance were characterized by X-ray powder diffraction (XRD), scan electron microscopy (SEM), and galvanostatical charge-discharge testing. The results showed that the material had a single crystal olivine structure with homogeneous grain sizes, and exhibited excellent electrochemical performance at 0.1C rate.