| With the increasingly serious problems of energy consumption and environmental pollution, more and more national governments around the world start to pay attention to the development of low-carbon economy and green power sources. Lithium-ion batteries, as one kind of environment friendly energy storage and conversion devices, are becoming "the darling" of the new times and developing rapidly in the field of new energy vehicles. However, the properties of cathode materials of lithium-ion batteries are still the most important bottle neckfor the development and applications. This has resulted in the lithium-ion batteries still not to be applied widely in the field of new energy vehicles.The research of cathode materials with high specific capacity has become one of the most active research area for lithium-ion batteries in recent years. Monoclinic Li3V2(PO4)3and Li1+xV3O8have attracted extensive interests as potential cathode materials for lithium-ion batteries due to their relatively high theoretical specific capacity. Unfortunately, the poor intrinsic electronic conductivity and Li-ion diffusion coefficient of these two materials have greatly affected the electrochemical properties of the materials and hindered their practical application.Therefore, the aim of this thesis is to ameliorate intrinsic electronic conductivity and Li-ion diffusion coefficient of monoclinic Li3V2(PO4)3and Li1+xV3O8materials. The improvement measures in this thesis mainly include optimizing the synthesis conditions, doping with heterogeneous metal cations, adding some special conductive materials and so on. The main research contents and results are summarized as follows:1. We design a new synthetic route based on the sol-gel method to fabricate the Li3V2(PO4)3/C cathode materials under static reducing atmosphere. Compared with traditional synthesis method with dynamic inert atmosphere, the advantages of using this new method to synthesize Li3V2(PO4)3/C are as follows:(?) creating a relatively suitable environment for crystal growth;(?) preventing the carbon from drifting resulting from high gas flow rate, and (?) reducing the consumption of inert gas. The physical characterization and electrochemical performance results indicate that the optimal reaction temperature and time are750?and4h, respectively, for the synthesis of Li3V2(PO4)3/C under static reducing atmosphere via sol-gel method. The obtained Li3V2(PO4)3/C can deliver an initial specific capacity of129mAh g-1 between3.0-4.3V at0.1C, which is close to the theoretical specific capacity of133mAh g-1. However, the specific discharge capacity is only95mAh g-1at5C and obvious capacity deterioration is observed.2. In order to improve the rate performance of Li3V2(PO4)3, the study of Sn doping for Li3V2(PO4)3material was performed. The XRD patterns demonstrate that Sn-doping affects the preferential crystal growth direction of Li3V2(PO4)3. SEM results show that particles of all Sn-doped samples have polyhedron shape and present high crystallinity. The obtained optimal sample shows initial discharge capacities of122.7and117.2mAh g-1at0.2and5C between3.0and4.3V, respectively. And it also shows an excellent cycling performance at high rate of5C. We think Sn4+play a important role in stabilizing crystal structure, decreasing contraction amplitude of the crystal lattice, and enhancing the Li-ion diffusion coefficient for Li3V2(PO4)3material during the Li+de-intercalation and intercalation processes, which greatly improves the rate performance and cycling performance of Li3V2(PO4)3, In addition, the Li+de-intercalation and intercalation processes in Sn-doped Li3V2(PO4)3/C material was studied via potential step method.3. The effect of adding graphite in the preparation of Li3V2(PO4)3/C on the physical and electrochemical properties of Li3V2(PO4)3/C was investigated. The physical characterization results indicate that graphite could act as crystal growth substrate and influences the direction of the Li3V2(PO4)3/C crystal growth during the synthesis of Li3V2(PO4)3/C. As a result, the Li3V2(PO4)3/C particles show polyhedron shape and the2%-graphite/Li3V2(PO4)3/C composite with high surface area presents good intrinsic electronic conductivity. The electrochemical performance tests results indicate that2%-graphite/Li3V2(PO4)3/C composite show smaller charge transfer resistance, has initial discharge capacities of128.7and124mAh g-1at0.2and5C rates in the potential range of3.0-4.3V, respectively, and presents excellent cycling performance. In addition, a simple and convenient method for the determination of carbon content of Li33V2(PO4)3/C sample is put forward.4.The effect of adding carbon nanodots in the preparation of Li3V2(PO4)3/C on the physical and electrochemical properties of Li3V2(PO4)3/C was studied. Carbon nanodots with diameter of about50nm were successfully prepared by constant voltage and constant current methods, and it is verified that the formation of carbon nanodots is actually the process of anode dissolution on graphite surface by salt bridge method. Based on the experiments phenomena, the formation mechanism of carbon nanodots is preliminarily inferred. On the basis of these works, the carbon nanodots suspensoid, which was obtained by constant current method, was employed as mother solution to synthesize C-dots/Li3V2(PO4)3/C composite. The physical characterization results indicate that the composite has plenty of micropores and shows higher specific surface area. The electric conductivity of the composite is higher than that of the sample without adding carbon nanodots, however, it is lower than that of the2%-graphite/Li3V2(PO4)3/C composite. The cathode electrode membrane material was prepared by coating method. The electrochemical performance tests results indicate that C-dots/Li3V2(PO4)3/C composite has initial discharge capacities of128.4,127.3,126.2and124.4mAh g-1at0.5,1,2, and5C rates in the potential range of3.0-4.3V, respectively, and presents good cycling performance, which delivers125.8,123.6,121.5,and119.6mAh g-1after100circles at room temperature, respectively.5. Monoclinic Li1+xV3O8cathode materials were synthesized by sol-gel method. The effects of different initial x value (x=0,0.2,0.4) on the physical and electrochemical properties of Li1+xV3O8cathode materials were studied. The results showed that the Li1.2V3O8sample, which has initial discharge and charge specific capacities of279.7and305mAh g-1at the current of50mA g-1between1.5and4.0V, respectively, shows the best electrochemical performance among obtained samples. However, after30cycles, the electrodes can only deliver specific capacities of259.7and270.6mAh g-1correspondingly, showing obvious capacity deterioration. At the current densities of100and200mA g-1, the pristine Li1.2V3O8sample exhibits an initial discharge capacity of230.9and211.8mAh g-1in the voltage range of1.5-4.0V, respectively, which indicates that its large current capability was not good at high current density. In order to ameliorate the electrochemical performance of the Li1.2V3O8sample, the study of Sn doping for Li1.2V3O8material was also performed. When the doping content x=0.03, the obtained Li1.2V2.97Sno.03O8sample displays lower initial specific capacity than the Li1.2V3O8material at50mA g-1, however, the cycling performance and large current discharge capability were both greatly enhanced. At the current densities of50,100, and200mA g-1, the Li1.2V2.97Sn0.03O8sample presents an initial discharge capacity of271.8,268, and257.2mAh g-1in the voltage range of1.5-4.0V, respectively. And the specific discharge capacities remain272.7,272.7, and249.4mAh g-1after30circles at room temperature, respectively. |
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