Study Of Preparation Modification And Electrochemiacl Properties Of Li₃V₂（PO₄）₃

Lithium-ion batteries have become the best secondary battery system in new energy storage equipment because of their high specific capacity,excellent cycle performance and high power density.Lithium vanadium phosphate material has become a promising lithium because of its high specific capacity,good low-temperature discharge performance,better rate performance and better safety performance compared with other materials.Cathode materials for ion batteries.In this paper,the effects of different preparation methods and modification methods on the electrochemical properties of lithium vanadium phosphate materials are discussed.In this paper,two different synthesis methods of vanadium phosphate materials,carbothermal reduction method and sol-gel method,were explored.The materials prepared by sol-gel method had no obvious agglomeration phenomenon,and showed excellent electrochemical performance.The charge discharge test was carried out at the rate of 1 C,and the discharge capacity of the material under the first cycle was 120.9mAhg^-1.After 200 cycles,the specific discharge capacity is 116.1 mAhg^-1,but the capacitance of the material decreases obviously at 55 C,only 60%at room temperature.After modification of lithium vanadium phosphate material by graphene composite and magnesium ion doping,the material exhibits monoclinic structure.The SEM,XRD and other microstructural characterization tests show that the lithium vanadium phosphate material retains its original structure after graphene coating and magnesium ion doping.However,when the graphene content exceeds 5%,the agglomeration of graphene becomes more and more obvious,and the agglomeration of graphene to phosphoric acid becomes more and more obvious.The growth of Li₃V₂（PO₄）₃ particles was inhibited to some extent.Lithium vanadium phosphate material with graphene composite content of 5%has the best performance at room temperature.The first discharge specific capacity of this material is 123.8 mAhg^-1 at 0.5 C ratio,the discharge specific capacity after 200 cycles is109.5 mAhg^-1,and the capacity retention rate is 90.87%.The first discharge specific capacity of Li₃(V_0.9Mg_0.1)₂（PO₄）₃ material at 0.5 C ratio is 123.2 mAhg^-1,and after 100cycles.The specific discharge capacity is 119.9 mAhg^-1 and the capacity retention rate is93.4%.The first discharge specific capacity of lithium vanadium phosphate material with graphene composite content of 5%is 89.1 mAhg^-1 at 55℃,which is 17.7%higher than that of pure lithium vanadium phosphate material.After 55 cycles,the discharge specific capacity of lithium vanadium phosphate material with graphene composite content of 5%is 83.7 mAhg^-1 at 55℃,and the capacity retention rate is 93.9%at 55℃,which is higher than that of pure lithium vanadium phosphate material at 55℃.The first discharge specific capacity of Li₃(V_0.9Mg_0.1)₂（PO₄）₃ material increased by 6.1%,but the capacity retention rate decreased by 2.7%.The cyclic performance of Li₃(V_0.9Mg_0.1)₂（PO₄）₃material decreased seriously.In summary,the lithium vanadium phosphate material with graphene composite content of 5%showed better electrochemical performance.