Studies On The Synthesis And Modification Of Li₃V₂（PO₄）₃ With Sol-gel Method

This thesis describes the systematical investigation of the synthesis method of the Li₃V₂（PO₄）₃/C composite material as the cathode material for lithium ion batteries. Optimizations on chelating agent, sintering temperature, pH value or other assistant methods during the sol-gel synthesis have been conducted. The surface morphologies and structures of the synthesized materials were determined by scanning electron microscopy and power X-ray diffraction. The electrochemical performances of the materials as the cathode material for lithium ion batteries were characterized by charge-discharge test, cycle voltammogram and electrochemical impedance spectro-scopy.The Li₃V₂（PO₄）₃/C composite materials were synthesized by sol-gel method using different sintering temperatures at 650 ℃,750 ℃ and 850 ℃. The sample synthesized at 650 ℃ shows incomplete crystallization and contains some impurities. As the temperature increasing, the resulting powder exhibits better crystallinity, but when the sintering temperature reaches 850 ℃, the material appears a mass of agglomeration. It is demonstrated that sintering at 750 ℃, the material has desired crystallization and relatively small particle size.Li₃V₂（PO₄）₃ materials were synthesized by sol-gel method using 750 ℃ as the sintering temperature, and citric acid as chelating agent. During the synthesis, ball-millingof the dry gel was conducted and compared with the proceduce without ball-milling. The results show that ball-milling inhibits the agglomeration, providing materials with more uniform particle size distribution and larger specific surface area, comparing with materials synthesized without ball-milling. All these modifications can increase the electronic conductivity of the material and result in a better retention capacity under the large thr charge and discharge rates, but the EIS test indicates that ball-milling has little impact on the diffusion coefficient of lithium ions in the obtained cathode material.The ultrasonic processor was used to produce ultrasonic cavitation to assist synthesizing Li₃V₂（PO₄）₃ material during sol-gel method. The results indicate that ultrasonic cavitation can make the dispersive mixing of reactants, and prepare particles with more uniform size distribution and fewer reunion, which provides a larger specific surface area and more chances for electrons conduction. At 0.2 ℃, the discharge capacities of the material synthesized without ultrasonic （LVP/C） and that with ultrasonic （LVP/C-U） in the range of 3.0-4.3 V are 111.0 and 117.4 mAh g-1 respectively. In the range of 3.0-4.8 V at 0.2 C, the discharge capacity of LVP/C-U is 162.4 mAh g-1, about 10 mAh g-1 higher than that of LVP/C （152.0 mAh g-1）. The apparent diffusion coefficients of LVP/C and LVP/C-U to be 2.1 x 10-10 and 3.2 × 10-10 cm2 s-1, respectively. The diffusion coefficient of lithium ions （DLi+）in LVP/C-U is approximately 50%higher than that of LVP/C, which gives rise to the better rating performance of LVP/C-U than LVP/C. Moreover, LVP/C-U has smaller charge transfer resistance than LVP/C.Tartaric acid was used as the chelating agent to synthesize Li₃V₂（PO₄）₃/C material by sol-gel method, and pH value and the amount of tartaric acid during the synthesis were investigated. The results shows that the amount of tartaric affect the crystallinity of materials. Among the three investigated mole ratios, the materials resulting from mole ratio of 1:3 of V2O5 of tartaric acid shows the best crystallinity and ratio performance; The pH value influences the resulting materials in the size of the grain and reunite degree. LVP/C-4 （pH= 4） exhibits an optimum capability at low-rate and LVP/C-9 （pH= 9） possesses a desirable performance at high-rate. This phenomenon can be attributed to their microstructure:uniform particle distribution and less agglomeration of LVP/C-4 particles and small size of LVP/C-9 particles. But all the samples have lower specific capacity than the Li₃V₂（PO₄）₃ synthesized with citric acid; thus further optimization is need for the preparation conditions.