The Synthesis Of LiFePO₄/C Composite Cathode Material By Solvothermal Method And The Study Of Its Performance

Lithium ion battery cathode materials developed very fast in recent years due to its advantages such as high energy density, long cycle life, low cost, environmentally friendly and so on. As the most potential "green" cathode material in the 21 st century, olivine LiFePO₄ has attracted a group of researchers’ attention, because of its high theoretical capacity of 170 m Ah/g, 3.45 V high charge and discharge plateau, excellent cycle stability, outstanding performance under high temperature, as well as the environmental raw materials, which are rich in amount and low in price, and a series of other advantages. This article is based on the goal of synthesizing LiFePO₄ cathode material with excellent performance, using an one step solvothermal method, and the synthesis process was optimized. In order to further improve the electrochemical performance of this material, metal ion doping was adopted. XRD, SEM, TEM, Raman spectroscopy, constant current charge and discharge technology, cyclic voltammetry（CV） and electrochemical impedance tests were carried out to characterize the morphology, structure and electrochemical properties of the material.The single phase and uniform of LiFePO₄/C composite cathode material was prepared using an one step solvothermal method. The crystal structures of the material are complete, and the morphologies and particle sizes change with the various concentration of glucose concentration, and controlled by reaction temperature and time at the same time. It was systematically studied that the effects of reaction conditions on the final morphologies, structures and electrochemical properties of the material. During the preparation of the precursor, we optimized the process conditions by changing the concentration of glucose, reaction time and temperature, and finally found the optimum conditions were the glucose concentration of 0.05 mol/L, the reaction temperature of 180 ℃, and the reaction time of 6 h. The initial discharge specific capacity is 137 m Ah/g at 0.1 C, and increase to 147 m Ah/g after a few cycles, the capacity retention remained 100% after 50 cycles.After the precursor synthesis conditions were determined, we further explored the effects of sintering time and temperature on the materials’ electrochemical performances. Combined with XRD, SEM and constant current charge and discharge tests, we found that the crystals crystallized poorly under lower sintering temperature and time, and the particle size would increase by increasing the sintering temperature, and the increase of sintering time would increase the particle aggregation. The best comprehensive performance of LiFePO₄/C composite cathode material was obtained under the sintering condition of 750 ℃ and 10 h.As the rate performance of the LiFePO₄/C composite cathode material by this method needed to be improved, we selected the transition metal Ni, Co, Mn for doping modification on the basis of previous work. It is found that the morphology of doping samples had different degree of change, but the XRD images indicated that doping didn’t change the crystal structure of LiFePO₄. All these three doped elements had positive effect on the rate performance, and the influence of Ni on the high ratio property was most obvious. Compared with CV curves at a scanning rate of 1 m V/s, the sample doped with Ni had the lower polarization, and the higher peak current. As a result, the sample doped with 8% Ni had the best electrochemical properties, and the initial discharge specific capacity was 137 m Ah/g at 0.1 C, and decreased to75 m Ah/g at 10 C, respectively.