Synthesis And Characterization Of Metal Phosphate Composite Cathode Materials For Lithium Ion Battery

Lithium-ion battery has been a popular application in mobile devices for two decades. Nowadays it is extended to high power application areas such as energy, transportation and military fields, where higher energy density and power density are constantly demanded. Cost, energy density, cycle life and safety of the lithium-ion battery mainly depend to a high degree on cathode material. LiFePO4has been commercialized as cathode material for power batteries due to its great structural stability and safety. However, its low electronic conductivity and potential are always the obstacles to the large-scale application. In this thesis, detailed studies, including preparation processes, structure and morphology analysis and electrochemical performance, were focused on a new phosphate composite xLiFePO4·yLi3V2(PO4)3.xLiFePO4-yLi3V2(PO4)3composites were synthesized via mechanical activation, spray drying and solid-state reaction approach. Firstly the effect of Fe/V ratio on the structure, morphology and electrochemical performance of composites were systematically studied. The X-ray diffraction (XRD) results indicate that a single phase of olivine-type LiFePO4is present when y=0.05,0.1. With y increasing to0.2, two phases, including olivine LiFePO4and monoclinic Li3V2(PO4)3, coexist. Electrochemical measurements show that the cathode material of y=0.05delivers the best performance. It can perform162mAh/g,147mAh/g,122mAh/g and87mAh/g at a rate of0.1C,1C,5C and IOC, respectively.In order to improve conductivity of xLiFePO4·yLi3V2(PO4)3composites, carbon coating has been used via glucose as carbon source. The results show that the capacity of the carbon coating sample is higher than that of non-carbon coated composite. The composite of y=0.3got an excellent high-rate performance; it can deliver114mAh/g at a high charge-discharge rate of10C.It is the first time that0.7LiFePO4·0.3Li3V2(PO4)3/C composites were prepared using different molecular weight of polyethylene glycol (PEG) as carbon sources. The dependence of the graphitized degree, electronic conductivity and molecular weight of PEG has been studied. Raman spectroscopy results show that ratio of the Asp3/Asp2and AD/AG increased with the molecular weight. The composite using PEG-200as carbon source shows the highest degree of graphitized carbon and rate capability. It can deliver120mAh/g at a high charge-discharge rate of10C. X-ray Absorption Fine Structure (XAFS) technique was used to investigate local structure of0.7LiFePO4·0.3Li3V2(PO4)3/C and LiFePO4/C. The results show that they share the same local structure of Fe sites; the addition of V does not incorporate into the olivine LiFePO4. Rietveld structure refinement shows that the phase content of0.95LiFePO4·0.05Li3V2(PO4)3/C is92.9%of LiFePO4and7.1%of Li3V2(PO4)3. Transmission Electron Microscopy (TEM) analysis of0.95LiFePO4·0.05Li3V2(PO4)3/C further confirms that the two phases coexist independently. Diffusion coefficient of Li+is measured by Cyclic Voltammetry (CV) method. The results show that Li+diffusion coefficient of0.7LiFePO4·0.3Li3V2(PO4)3/C is obviously higher than that of LiFePO4/C and at the same potential peaks, indicating the higher kinetics performance.