Study On Synthesis And Modification Of Li₃V₂（PO₄）₃ As Cathode Materials For Lithium-ion Battery

Li₃V₂（PO₄）₃ cathode material has attracted much attention because of its various advantages, such as high operating voltage, high theoretical capacity, high ion mobility and good stability. But it also exists some disadvantages caused by itself’s structure. For example, the low electronic conductivity and the poor rate performance, which is hindering the development and application of Li₃V₂（PO₄）₃ composite. According to these shortcomings of Li₃V₂（PO₄）₃ cathode materials, this paper explores the new synthesis method to improve the electrochemical performance of Li₃V₂（PO₄）₃ material, and using different valence state of cation, anion and cation/anion co-doping method to modify it and increase the electronic conductivity and rate performance of Li₃V₂（PO₄）₃ composite.The two-step synthesis method can improve the discharge capacity and tap density of the material. When the charge/discharge rate is 0.2C, the first discharge capacity is 132.8m Ah·g^-1, almost close to the theoretical capacity. After 160 cycles with different rates, its initial capacity content is 92.2%, delivers a good stability. Using sol gel method that can effectively control particle size of material to synthesize the spherical Li₃V₂（PO₄）₃/C materials（particle size distribute in 50-200nm）, when the charge/discharge rate is 0.2C or 5C, the first discharge capacity is 127.9 and 50.6m Ah·g^-1, indicate that capacity and rate performance needs to be improved.Aiming at the problem of the poor rate performance of the materials, the materials were modified by different valence state cation-doping methods. In the doping with divalent cations(Zn²⁺, Mg²⁺, Pd²⁺), the optimal doping ratio is x=0.05, the Li₃V_1.95Mg-0.05（PO₄）₃/C material possesses the best electrochemical performance. When the rate is 0.2C or 5C, the special discharge capacity is 125.5 and 111.5m Ah·g^-1, respectively; trivalent cation(Al³⁺, Cr³⁺, Y³⁺) doping, the optimal doping ratio is x=0.09, the best performance related to Li₃V_1.91Al_0.09（PO₄）₃/C material. When the rate is 0.2C or 5C, the first discharge capacity is 118.2 and 95.7m Ah·g^-1; tetravalent cations,(Ti⁴⁺, Mo⁴⁺, Ge⁴⁺) doping, the optimal doping ratio is x=0.01, the most excellent performance is own to Li₃V_1.99Ge_0.01（PO₄）₃/C composite. When the charge/discharge rate is 0.2C or 5C, the first discharge capacity is 137.2m Ah·g^-1 and 118.4m Ah·g^-1. It can be seen that, three kinds of cation doping can effectively improve the rate performance of material.Although the different valence cation doping can effectively improve the rate performance of the material, but decreased the active substance in the material, thereby reducing the discharge capacity of the material. In order to improve the discharge capacity of the material, different valence of anions(F-, SO₄^2-, BO₃^3- were used to modify the electrode material. The optimal doping ratio for F- and SO42- are x = 0.03; BO₃^3- optimal doping ratio is x = 0.05, when the rate of 0.2C or 5C, the specific capacity are 132.8, 132.4, 140.1m Ah·g^-1 and 95.1, 97.3, 118.9m Ah·g^-1. Indicating that anion doping method not only improves the discharge capacity, but also improve the rate performance of the material.The results show that cation doping can improve the rate performance of the material, the discharge capacity and the rate capability can be simultaneously improved by anions doping. The different valence state cations and anion possess the best electrochemical properties were co-doped and explored the effect on performance of the material. When the rate is 0.2C or 5C, the specific discharge capacity of Li₃V_1.95Mg₀.05（PO₄）_2.95（BO₃）_0.05/C, Li₃V_1.91Al_0.09（PO₄）_2.95（BO₃）（0.05）/C and Li₃V_1.91Ge_0.01（PO₄）_2.95BO_30.05/C is 135, 142, 147.7m Ah·g^-1 and 96.6, 109.1, 113.6m Ah·g^-1, respectively. Research results show that co-doping with Mg-BO3, Al-BO3 and Ge-BO3, can effectively improve the electrochemical properties of material.