Preparation And Electrochemical Properties Of Lithium Vanadium Phosphate As Cathode Materials

Lithium vanadium phosphate（Li₃V₂（PO₄）₃）,a polyanion-type NASICON structure cathode material,owning to the advantages of high operating voltage,stable structure,outstanding cycling stability,safety and environmental protection,is expected to become the most promising cathode material for next-generation lithium-ion batteries.However,the relatively low electronic conductivity restricts its practical applications for lithium-ion batteries.The research on modification of Li₃V₂（PO₄）₃ has become a hotspot to promote the application of Li₃V₂（PO₄）₃ lithium-ion battery.Carbon-coating is a simple and effective method to improve the electronic conductivity.In this thesis,hence,carbon coated Li₃V₂（PO₄）₃（Li₃V₂（PO₄）₃/C）cathodes were prepared via solid phase method,sol-gel method and hydrothermal method.The crystal structure and morphology of the composites were tested by XRD and SEM,parts of which were characterized by the constant current charge-discharge tests.The main contents of this paper are as follows:（1）Li₃V₂（PO₄）₃/C composites were prepared by solid phase method.The crystal structure and morphology of the composites were investigated by synthesizing through two carbon sources.The results show that the sample synthesized using sucrose and citric acid as the two carbon sources was more homogeneous than the sample synthesized from starch and sucrose as the two carbon sources.（2）Li₃V₂（PO₄）₃/C composites were prepared by sol-gel method.The composites have perfect purities,good crystallinity,and uniform particle size distribution.（3）The effects of different experimental conditions on the physicochemical and electrochemical properties of Li₃V₂（PO₄）₃/C composites were investigated by hydrothermal method.The results show that the type of chelating agent had great effects on the micro-morphology of the composites.The sample prepared with polyethylene glycol as a chelating agent shows a sheet structure,while the sample prepared with glycerol as a chelating agent is granular.Among them,in the voltage region of 3.0-4.8 V,the sample prepared with glycerol as a chelating agent delivers the specific discharge capacity of 44.5 m Ah/g at the rate of 30 C,and it exhibits an excellent high-rate capability.In addition,hydrothermal temperature affects the size of the composite,with the increase of hydrothermal temperature;the sample shows greater size,and weaker uniformity.In the voltage region of 3.0-4.3 V,the sample prepared with the hydrothermal temperature of 150 ℃ shows good cycling performance with 98.6% of capacity retention after 100 cycles at 0.5 C rate,and it has almost no capacity decay.Ultimately,the hydrothermal time also had influences on the morphology of the material.With the prolonged of hydrothermal time,the samples become more homogeneous,and the agglomeration effects gradually become weakened.The sample with the hydrothermal time of 20 h has the best cycle performance.