| Olivine LiFePO4 is a new kind of cathode material for lithium-ion battery. It has a relatively large theoretical capacity of 170mAh/g. This material is cheap and environmentally friendly, thermally stable in the folly charged state. LiFePO4 has a low electronic conductivity, and its cycle ability at large charging/discharging current should be improved. This thesis focuses on olivine LiFePO4 synthesis by modified solid state reaction and electrochemical performances, the main aspects follow:1) Study on the electrochemical performances of LiFePO4 by reducing carbon solid state reactionThe effect of the amount of carbon, synthesis temperature, reaction time are discussed. The results show that the amount of carbon, synthesis temperature and time influence the electrochemical performances of the products. The products with the amount of carbon (Li: Fe: P: C=l:l:l:2) and sintered at 650°C for 8h possessed excellent electrochemical performance. The crystalline structure, granularity and morphology of particles of the best samples are investigated by X-ray diffraction(XRD), particle diameter measure system, scanning electron microscope(SEM) and transmission electron microscope(TEM).2) Study on the electrochemical performances of LiFePO4 by reducing sucrose solid state reactionThe effect of the amount of sucrose, synthesis temperature, reaction time are discussed. The crystalline structure, granularity and morphology of particles of the sample are investigated by X-ray diffraction(XRD), particle diameter measure system, scanning electron microscope(SEM) and transmission electron microscope(TEM). The results show that the amount of sucrose, synthesis temperature and time influence the electrochemical performances of the products. The products with the amount of sucrose(Li: Fe: P: C= 1:1:1:1) and sintered at 650°C for 8h possessed excellent electrochemical performance.3) Comparison electrochemical performances of LiFePO4 by reducing carbon solid state reaction and reducing sucrose solid state reactionComparing electrochemical performances, crystalline structure, granularity and morphology of particles of LiFePO4 synthesized by two different solid state reaction, we can conclude that LiFePO4 prepared by reducing sucrose solid state reaction has abetter capacity and cycle performance.4) Study on the electrochemical performances of LiFePO4 doped with Cu LiFePO4 doped with Cu was synthesized by reducing sucrose solid state reaction.The samples were characterized by their electrochemical performances, X-ray diffraction(XRD), scanning electron microscope(SEM) and transmission electron microscope(TEM). The result indicates that the Cu doped does not affect the structure of the material but considerably improves its electrochemical performances. LiFePO4 samples doped with Cu show capacities of 156mAh/g at a current density of 0.2 mA/cm2. They can show better cycle performance at a current density of lmA/cm2.5) Study on the electrochemical performances of LiFePO4 doped with Mg2+ LiFePO4 doped with Mg2+ was synthesized by reducing sucrose solid state reaction.The samples were characterized by their electrochemical performances, X-ray diffraction(XRD), transmission electron microscope(TEM). The result indicates that the Mg2+ doped does not affect the structure of the material but shows better electrochemical performances. LiFePO4 samples doped with Mg2+ show capacities of 158mAh/g at a current density of 0.2mA/cm2. They can show better cycle performance at a current density of 1 mA/cm2. |
