Rare Elements Influence On The Electrochemical Behaviour Of Li₃V₂（PO₄）₃/C Cathode Material For Lithium-ion Batteries

Lithium vanadium phosphate[Li3V2(PO4)3] is one of the new type of cathode materials for lithium-ion batteries, it has a significant potential application, especially in the research and development of the large capacity power lithium-ion battery. According to the research, lithium vanadium phosphate has the same discharge flat plateaus and capacity density as lithium cobalt oxide[LiCoO2], and has far superior thermal stability as well as safety performance to LiCoO2, LiMn2O4, LiFePO4. Furthermore, compared with LiFePO4, higher Li-ion diffusion coefficient, charge-discharge voltage plateau(3.6V,4.1V and4.6V), and capacity density(2330mWh/cm3after C-coated) are owned by Lithium vanadium phosphate, but the material has low electronic conductivity because of its three-dimensional structure, and that makes it display poor rate capacity and cycling performance. Meanwhile, there is series of problems at the synthesis of Li3V2(PO4)3/C such as complicated preparation, too big particle size and impurities of products and so on,which hinder the the commercialization for this material. Therefore, in this paper, a novel fast sol-gel assisted microwave heating route was used to get monoclinic crystal Li3V2(PO4)3/C, and combining it with the rare metals doping method to make the electrochemical performance of the materials optimal.In Chapter1, a general introduction is given on following aspects:the development and categories of lithium-ion batteries, the structure and working mechanism of lithium ion batteries, and the research progress on the Li3V2(PO4)3/C.In Chapter2, the experimental raw materials, equipments and methods used in this thesis process were introduced in detail. A detailed description on the process of making a experimental cell is presented.The structural and electrochemical analyses methods are also summarized. Furthermore, the preparation of the Li3V2(PO4)3/Csamples were mainly introduced.The third chapter introduces the fast sol-gel assisted microwave heating route to prepare Li3V2(PO4)3/C in detail. V2O5, Li2CO3,NH4H2PO4and citric acid were employed as raw materials in the molar ratio of2:3:6, here, citric acid was employed as both reduction agent and carbon source, which can prevent the oxidation of vanadium ions and afford the network structure of carbon for electronic conduction. Microwaved under400W for10minutes, material owned good crystallization and without impurities was got. More important, it displayed best performance with a first discharge capacity of166.4mAh/g,94.1%for coulombic efficiency at0.2C.In Chapter4, Ce was chosen as doping element to synthesize samples Li3V2-x Cex(PO4)3/C(x=0,0.01,0.02,0.03,0.05,0.1). To study rare elements influence on the electrochemical behaviour of Li3V2(PO4)3cathode material for lithium-ion batteries, properties of the prepared samples were investigated using thermogravimetric (TG), scanning electron microscopy(SEM), X-ray diffraction(XRD)and electrochemical methods, electrochemical workstation and constant current charge-discharge test. For which the effect of Ce doping were Fine. The sample of Li3V1.98Ce0.02(PO4)3/C derived a initial charge specific capacity of191.8mAh/g and a a initial discharge specific capacity of180.7mAh/g after charge-discharge cut-off voltages of3.0-4.8V. After charge-discharged at0.2C rate for100cycles, the sample retained a discharge specific capacity of174.6mAh/g.The next Chapter used the same method prepared the La-doped Li3V2-x Lax(PO4)3/C(x=0,0.05,0.1,0.15,0.2) to study rare elements influence on the electrochemical behaviour of Li3V2(PO4)3cathode material for lithium-ion batteries, properties of the prepared samples were investigated using thermogravimetric (TG), scanning electron microscopy(SEM), X-ray diffraction(XRD) and electrochemical methods, electrochemical workstation and constant current charge-discharge test. The optimal amount of La doping could improve the electrochemical features with respect to the undoped ones, and when x=0.10, the LisV1.9La0.1(PO4)3/C sample shows the best high-rate performance. For instance, the the initial discharge specific capacities of Li3V1.9La0.1(PO4)3/C is177.8mAh/g when the coulombic efficiency is96.3%at0.2C in the voltage range of3.0～4.8V. Furthermore,95.8%capacity retention can still be held after50cycles.The last chapter gives all overview on the originality and the deficiency in this thesis. Some prospects and suggestions of the possible future research directions are pointed out.