The Synthesis And Characterization Of LiVPO4F Compounds As Cathode Materials For Lithium Ion Batteries

The synthesis and electrochemical properties of lithium vanadium fluorophosphate (LiVPO4F) as cathode material for lithium-ion batteries were studied in this thesis. Triclinic LiVPO4F material with its PO43-polymerization anion has aroused extensive concern for its high plat form (4.2V) and theory specific capacity (156mAh/g). It is one of the most promising industrialization cathodes due to its excellent safety performance, good thermal stability, environment-friendly and low cost. However, LiVPO4F has low electronic conductivity and poor rate performance, which hinders its progress of commercialization.The main objective of this work is to prepare high-performance LiVPO4F materials by optimizing the synthesis conditions, coating carbon source and doping metal ions. The crystal structure, partical morphology and electrochemical performance of the product were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM) etc. The main content of this thesis includes:1. Triclinic LiVPO4F cathode materials have been synthesized by carbothermal reduction (CTR) method. The effects of sintering temperatures on the structure, morphology, and electrochemical properties of products have been studied. At750℃, the intermediates VPO4and final product LiVPO4F can be prepared by CTR method when sintering time is6h and30min, respectively. The obtained LiVPO4F has the initial capacity of82.7mAh/g and residual capacity of68.2mAh/g at50th cycle (82.5%of initial discharge capacity).2. The electrochemical performance of LiVPO4F has been enhanced by the carbon coating method. The carbon coating does not affect the triclinic structure of LiVPO4F according to the XRD patterns. When using activated carbon as carbon sources, the products present122.3mAh/g initial capacity and keep87.8%of initial discharge capacity after50cycles. By using phenolic resin as carbon sources, the initial discharge capacity of product increases to be130.8mAh/g, and residual capacity maintains92.9%of initial discharge capacity after50cycles.3. In order to modify the cycle performane of LiVPO4F, the method of doping Al is studied. The result shows the crystal structure of modified sample remains well. The results of electrochemical test indicate that the cycle performance of procuct gets better as increasing the amount of Al, although the discharge capacity is decreased slightly. When doping0.06%Al, the capacity retention rate changed from87.8%to91.6%.4. Triclinic LiVPO4F cathode materials have been synthesized by sol-gel method. SEM images show that the particle size of products was reduced, and specific surface area was increased accrodingly. The maximum discharge capacity of product is up to124.8mAh/g at1C rate, and the capacity retention rate is as high as94.3%after50cycles. Under0.1C,0.2C,0.5C,1C and2C, the coulombic efficiencies all reach about97%, showing an excellent rate performance.