Synthesis And Characterization Of Li3v2(po4)3 As Cathode Materials For Lithium-ion Batteries

Since the first commercialization of Li-ion batteries by Sony in 1990, lithium-ion batteries are currently considered to be the most efficient rechargeable energy storage systems. They have found wide applications in various communication appliances, digital devices, electrical vehicles (EVs), hybrid electrical vehicles (HEVs), etc. However, with the requirements of higher power density, longer cycle life and higher battery safety, the existing commercially applied electrode materials, mainly LiCoO₂ cathode and graphite anode, can hardly meet these needs. Thus, researchers are making great efforts to search for new electrode materials to improve the performance of cells. In addition, it is well-known that the cathode material is the most important part among all of the components in a battery, because it can largely decide the cost, power density, cycle life and safety of cells. In this thesis, an exploration on a new cathode material Li3V2(PO4)3 is conducted.In Chapter 1, a general introduction is given on following aspects: the development and status of batteries including lithium-ion batteries, the structure and working mechanism of lithium-ion battery, the usual cathode, anode and electrolyte materials, and the research progress on the Li₃V₂(PO₄)₃.In Chapter 2, the author mainly introduces the experimental raw materials, equipment and methods used in the project of this thesis. A detailed description on the process of making a coin cell is presented. The structural and electrochemical analyses methods are also summarized.In Chapter 3, monoclinic Li₃V₂(PO₄)₃/C composites are successfully synthesized by the carbothermal method using sucrose as the carbon source. The effects of sintering temperature and the amount of sucrose added on the physicochemical properties of Li3V2(PO4)3/C composites are investigated. It is found that the sample prepared at 750oC and with the molar ratio n(sucrose):n(LVP)=0.4:1 displays the best electrochemical performance, especially the rate capability.Chapter 4 presents an investigation of the effect of four carbon sources (citric acid, glucose, PVDF and starch) on the electrochemical performance of Li3V2(PO4)3/C composites. Based on the residual carbon content determination, it is found that the citric acid-derived sample shows the lowest residual carbon content (1.33 wt%). The electrochemical measurements indicate that the PVDF-derived sample displays the lowest resistance values, and thus owns the best rate performance.In Chapter 5, monoclinic Li₃V₂(PO₄)₃/C composites are successfully synthesized by the sol-gel method using oxalic acid as chelating reagent and maltose as carbon source. Sintering temperature and residual carbon content are found to be two important factors to the physicochemical properties of Li₃V₂(PO₄)₃/C composites. The sample synthesized at 750oC with a carbon content of 11.6 wt% exhibits an excellent electrochemical performance. In the voltage range of 3.0-4.3 V, its discharge capacity has only 7.2% decrease from 125 mAh g-1 at 0.5C to 116 mAh g-1 at 5C, and even at a high rate of 10C it can still achieve 82 mAh g-1.In Chapter 6, three electrochemical analyses techniques, CV, GITT and EIS, have been employed to determine the chemical diffusion coefficient of Li ions ( Ltriu+eD and app Li+D ) in Li₃V₂(PO₄)₃. The lithium diffusion coefficients calculated from the CV measurement are in the range of 10-10 to 10-11 cm2 s-1. The EIS measurement gives the true diffusion coefficient in the range of 10-9 to 10-10 cm2 s-1 within the single-phase region of discharge process. And the GITT measurement provides more accurate diffusion coefficients. Depending on the voltage, Ltriu+eD is in the order of 10-9 cm2 s-1 and Lapi+pD in the range of 10-8 to 10-13 cm2 s-1. For the true chemical diffusion coefficients in the single-phase region, the results from all of these three techniques fit very well and the derived diffusion coefficient is in the range of 10-9 to 10-10 cm2 s-1.Finally, in Chapter 7, the author gives an overview on the originality and the deficiency in this thesis. Some prospects and suggestions of the possible future research directions are pointed out.