The Study Of LiMnPO₄ Used As Cathode Material For Lithium Ion Battery

Olivine-structured LiMnP04 used as a cathode material for lithium-ion batteries has many advantages, such as high potential, high energy density, good thermal stability, low price of raw materials, benignity to the environment, etc. LiMnP04 is widely studied after the cathode material of LiFePO4. But the inherent low conductivity and low lithium ion diffusion coefficient of LiMnP04 limit its development and application. By reducing the particle size and doping carbon can effectively improve the electrochemical properties of LiMnPO4.In this thesis, LiMnPO4/C composite cathodes were synthesized by hydrothermal method and mixed solvothermal method combined with high temperature heat treatment with sucrose. The crystal structures, microstructures were characterized via XRD, FTIR, FE-SEM and HRTEM. The electrochemical properties were characterized by cyclic voltammetry measurement (CV), charge-discharge test and AC Impedance test. The effects of reductant, amount of sucrose, and pH, etc on the morphology and electrochemical properties were investigated. In addition, LiMnPO4-LiMn2O4 composite cathodes were synthesized by hydrothermal method, and the effects of the amount of LiMnP04 on the properties of the composites were also studies.The results of XRD and FTIR indicated that all the samples synthesized by hydrothermal method and mixed solvothermal method were LiMnP04 of olivine structure. FE-SEM analysis indicated that the amount of sucrose had a great impact on the morphology of the hydrothermal-synthesized LiMnPO4/C. HRTEM analysis indicated that LiMnP04 coated on the surface of LiMn2O4 in the LiMnPO4-LiMn2O4 composite cathodes. The results of electrochemical tests showed that for the samples synthesized by hydrothermal method, the one synthesized at 180℃, using ascorbic acid (Vc) as reductant, and doping 10wt.% sucrose had the best electrochemical performances. When tested at 0.05C rate, its initial discharge capacity reached 118.0mAhg-1 and retained a discharge capacity of 68.0mAhg-1 after 50 cycles. For the samples synthesized by mixed solvothermal method, the sample synthesized under the condition of pH=10 possessed the best electrochemical performances. The initial discharge capacities of this sample at 0.05C and 0.02C rate were 114.5mAhg-1 and 108.4mAhg-1, respectively. It retained a discharge capacity of 68.7mAhg-1 after 30 cycles at 0.05C rate. Among the LiMnPO4-LiMn2O4 composite materials, the sample with 15wt.% LiMnPO4 possessed the best performances. At 55℃, it delivered an initial discharge capacity of 120.1 mAh·g-1, and retained 97% of the initial capacity after 100 cycles at 1C rate.