Synthesis And Modification Study On LiFePO₄ Used As Cathode Materials For Lithium-ion Batteries

Olivine structure lithium iron phosphate (LiFePO₄) has the advantages of low cost, environmental benignity, long cycle life and thermal stability when it was used as cathode materials for lithium-ion batteries. However, its low electron conductivity and slow Li-ion diffusion velocity limit its electrochemical properties badly, which prevents its applications. Therefore, how to enhance its electron conductivity and lithium-ion diffusion coefficient is studied in this thesis.In order to prepare pure LiFePCO₄ via high temperature solid-state reaction, the mechanism and process of solid-state reaction was studied. First, LiOH·H₂O, FeC₂O₄·2H₂O and NH₄H₂PO₄ were selected as the raw materials based on their characteristics. The results show that LiOH·H₂O was easily reacted with FeC₂O₄·2H₂O when they were directly mixed. The resultant Fe(OH)₂ was generally oxidized to the impurity (Fe³⁺). An improved high temperature solid-state reaction method for synthesizing pure LiFePO₄ was introduced. In addition, the effect of reaction temperature of synthesis on the particle granularity, morphology and the electrochemical properties of LiFePO₄ was investigated, the results showed that 750℃ was optimum temperature to synthesize LiFePO₄.Based on the investigation of the effect of carbon choice on LiFePO₄, P-cyclodextrin was chosen to synthesize superfine LiFePO₄/C. The electron conductivity of LiFePO₄ was enhanced owning to the carbon coating from the pyrolysate of β-cyclodextrin by a factor of 10⁶. The LiFePO₄/C has a higher discharge specific capacity of 157mAh/g at 0.1C, compared to about 85mAh/g of pure LiFePO₄. It has a particle size about 200nm due to the effect of P-cyclodextrin in the synthesis process. The diffusion path of lithium ion shortened owing to the reduced particle size, and then the lithium ion...