Investigations On The Electrochemical Properties Of Li₃V₂（PO₄）₃as Cathode Materials For Lithium Ion Secondary Battery Prepared By Sol-Gel Method

The Li3V2(PO4)3cathode materials are prepared by sol-gel method. The microstructures of these materials have been characterized by XRD, SEM, EDS and positron annihilation techniques. And then the electrochemical properties of the materials have been tested by electrochemical workstation and battery test system. The aim of present work is to investigate the influences of sintering temperature, annealing time, Li:V, CA:V on the microstructures and the electrochemical properties of Li3V2(PO4)3materials; the effects of Al3+and Cr3+dopant on the microstructures and the electrochemical properties of Li3V2(PO4)3materials;the electrochemical properties of Li3V2(PO4)3·x%LiFePO4composite materials. The experimental results are listed as follows:(1) The Li3V2(PO4)3/C cathode material sintering at temperature of850℃for12h, with a Li:V (mol ratio) of3.1:2and a CA:V (mol ratio) of1:1shows a quite good electrochemical properties. The electrical impedance of this material is relatively small, the initial charge-discharge specific capacity of the material is196.6/150.6mAh·g-1and195.9/115.1mAh·g-1under the discharge rate of0.25C and5C, respectively. After100cycles, the discharge specific capacity retention rate of the material is86.6%and76.2%under the discharge rate of 0.25C and5C, respectively. That is, the charge-discharge cycle property of the material is quite good.(2) The NASICON crystal structure of Li3V2(PO4)3/C remains unchanged after being doped with Al3+and Cr3+.(3) With the addition of small amount of Al3+into Lithium vanadium phosphate, the electron density and defect density of the sample increases. It has been found that the electron density of Li3V1.90Al0.10(PO4)3/C is relatively high, corresponding to good electrochemical property of the material:the initial charge-discharge specific capacity are195.0/167.6mAh·g-1and195.3/126.7mAh·g-1, and the discharge specific capacity retention rate are89.4%and80.4%after100charge-discharge cycles under the discharge rate of0.25C and5C, respectively.(4) With the addition of small amount of Cr3+into Lithium vanadium phosphate, the electron density and defect density of the sample increases. The experimental results indicate that the electron density of Li3V1.90Cr0.10(PO4)3/C is relatively high, the initial charge-discharge specific capacity of Li3V1.90Cr0.10(PO4)3/C is193.2/165.6mAh·g-1,and its efficiency is88.0%after100charge-discharge cycles under the discharge rate of0.25C. That is, the electrochemical property of Li3V1.90Cr0.10(PO4)3/C is better than that of undoped sample (Li3V2(PO4)3) under the low discharge rate. The initial charge-discharge specific capacity of Li3V1.90Cr0.10(PO4)3/C is192.8/125.4mAh·g-1under the discharge rate of5C, but its efficiency is only74.4%after100charge-discharge cycles which is slight lower than that of the undoped sample.(5) The results of XRD measurements of the Li3V2(PO4)3·x%LiFePO4composite materials show that there are two sets of characteristic peaks correspongding to Li3V2(PO4)3and LiFePO4, respectively, and several small peaks due to impurities in the XRD pattern of the composite materials.(6) The results of the electrochemical property measurements of the Li3V2(PO4)3·X%LiFePO4composite materials indicate that the electrical impedance of the Li3V2(PO4)3·30%LiFePO4composite materials is relatively small and the charge-discharge cycle volt-ampere characteristics, or the electrochemical properties of this composite material, are quite good. The first charge-discharge specific capacity of Li3V2(PO4)3·30%LiFePO4composite material is196.4/141.0mAh·g-1, and the efficiency of the composite material is86.2%after100charge-discharge cycles under the discharge rate of5C. The Li3V2(PO4)3·30%LiFePO4composite material shows a good circulation charge-discharge properties even under the high discharge rate.