A Study On The Synthesis And Performance Of LiFePO₄ As Cathode Materials For Lithium Ion Battery

Lithium iron phosphate(LiFePO4)is a promising candidate cathode material for lithium ion batteries due to its low cost,safety and excellent cycling stability in comparision with other cathode materials.Because of its low electronic conductivity and low diffusion coefficient,It is highly desirous to improve the properties of LiFePO4 in order to realize the commercialization.In this thesis,in-situ carbon coated LiFePO4/C composites were synthesized by carbothermal reduction method.Its crystal structure,morphologies and physical properities of the composites were investigated by XRD,SEM,etc.The electrochemical performance of cathode materials were investigated by galvanostatic charge-discharge test.The results show that the carbon source has a great influence on the performance of the material.When polyethylene glycol(PEG6000)was used as carbon source and the amount was controlled with molar ratio n(Fe):n(C)=1:2,the composite product showed good carbon-coated granular morphology with uniform size distribution,and best electrochemical performance.The discharge capacities were 148mAh/g,140mAh/g and 99.5mAh/g at the charge and discharge rates of 0.1C,1C and 5C respectively.To improve the electrochemical performance of the LiFePO4/C material,LiFePO4/C samples doped with niobium oxides(Nb2O5)in order to form the lattice defects.The Nb5+ content(expressed as x=0.005,0.01,0.015)in the composite can influcence the electrochemical properties of the materials.When x=0.01,It demonstrated best charge-discharge abilities.The discharge capacities were 151.6mAh/g,143.2mAh/g and 114mAh/g at the charge and discharge rates of 0.1C,1C and 5C respectively.LiFePO4 materials were also synthesized by hydrothermal method and coated with carbon.The influences of synthetic conditions such as temperature have been investigated in detail.The sample synthesized at 170? and coated with carbon showed uniform morphology and had good electrochemical performance.The discharge capacities were 137.7mAh/g,125mAh/g and 95.1 mAh/g at the charge and discharge rates of 0.1C,1C and 5C respectively.