Study On Synthesis And Performance Of Li₃V₂(PO₄)₃ Cathode Materials For Lithium-ion Batteries

Nasicon-structured Li₃V₂(PO₄)₃ is considerd as one of the most potential cathode materials for lithium-ion batteries, due to its advantages such as the friendly environment, low cost, stable structure , high safety and good electrochemical performance. The synthesis method, appearance, element doping, electrochemical behavior and kineties of lithiumin sertion-extraction process of Li₃V₂(PO₄)₃ as cathode materials were studied in detail in this dissertation. Research results are summed up as follows:1. The Li₃V)2(PO₄)₃ sample of monoclinic crystal was synthssized by microwavecarbon-thermal reduction method, with the molar ratio of n( Li): n(V): n(P) to 3.05: 2: 3 heated for 10 min by microwave. After charge-discharge cut-off voltages of 3.0-4.8V ,the sample derived a initial charge specific capacity of 160.83 mAh/g and a initial discharge specific capacity of 145.38mAh/g. And the sample retained a discharge specific capacity of 127.21 mAh/g after charge-discharged at 0.2 C rate for 30 cycles.2. La,Y,Ce oxides were doped to synthssize the samples by microwave carbon-thermal reduction method , for which the effect of La,Ce doping were fine.The sample of Li_3.05V_1.98La_0.02(PO₄)₃ derived a initial charge specific capacity of 168.75 mAh/g and a a initial discharge specific capacity of 153.28mAh/g after charge-discharge cut-off voltages of 3.0-4.8V. and the cycle performance had been improved a certain extent.The sample of Li_3.05V_1.98Ce_0.02(PO₄)₃ derived a initial charge specific capacity of 168.32 mAh/g and a a initial discharge specific capacity of 150.56 mAh/g after charge-discharge cut-off voltages of 3.0-4.8V.After charge-discharged at 0.2 C rate for 30 cycles,the sample retained a discharge specific capacity of 142.58 mAh/g ,the attenuation ratio of discharge specific capacity was 5.3%.3. Orthogonal experiments were used to investigate the effective factors in the synthesis process of the samples by microwave sol-gel method. The optimized Li₃V₂(PO₄)₃ samples had a monoclinic crystal ,which had integrative crytal lattice and were ordered.The sample derived a initial charge specific capacity of 172.42 mAh/g and a a initial discharge specific capacity of 154.38mAh/g after charge-discharge cut-off voltages of 3.0-4.8V.After charge-discharged at 0.2 C rate for 50 cycles,the sample retained a discharge specific capacity of 150.90 mAh/g, had almost no attenuation.4. Chemical diffusion coefficient of Li⁺ in these materials were studied for contrast,by the method of electrochemical impedance spectrum and theory deduction. Chemical diffusion coefficient of Li⁺ in the sample synthssized by microwave sol-gel method was the biggest and reached 1.434x10^-8 cm²/s.5. The advantages and disadvantages of the three synthesis methods were comparedby disposed .parameters and electrochemical performance. The result showed that the microwave carbon-thermal reduction method with rare earth doped was the best.