LiFePO₄/CNF And LiFePO₄/CNT Cathode Material Fabricated By In-Situ Chemical Vapor Deposition

Olivine structured cathode material, LiFePO4, has attracted broad attention due to its advantages of low cost, rich source, environmental benign and higher stability which has great potential for application in power and storage batteries. However, the main problem that restricts the practical application of LiFePO4is its poor electronic conductivity and low Li ion diffusion rate. In this thesis, carbon nanofibers and carbon nanotubes, fabricated by in-situ chemical vapor deposition during the synthesis of LiFePO4, were used to improve its electrochemical properties.LiFePO4were synthesized by a solid-state reaction using Fe2O3, LiH2PO4and sucrose throuhg a two-step method at the temperature of750℃. This material was used as a cathode of lithium ion battery with the diachage capacity of140mAh/g at0.1C,100mAh/g at0.5C,80mAh/g at1C,52mAh/g at2C.The LiFePO4/CNF composite material synthesis process combined a high temperature solid state reaction and the chemical vapor deposition method. The catalyst was mixed with LiFePO4powder after pre-sintering., then ethanol or cyclohexane used as carbon source for the growth of CNF. We mainly studied the morphology and structure of the product influenced by the atmosphere, catalyst, speed of carbon source injection. The experimental results show that particles of LiFePO4were winded by CNF with the diameter of20-30nm. The LiFePO4/CNF composite material had a discharge capacity of125mAh/g at0.1C,96mAh/g at0.5C,77mAh/g at1C,60mAh/g at2C.The LiFePO4/CNT composites material was prepared using the same chemical vapor deposition method, with C2H4as the carbon source. CNT grown on the surface of the lithium iron phosphate, has a diameter of30nm. This material has a discharge capacity of136mAh/g at0.1C,105mAh/g at0.5C,90mAh/g at1C,75mAh/g at2C.CNF and CNT in LiFePO4/CNF and LiFePO4/CNT composite material grew among the lithium iron phosphate particles, and connected eath other forming a conductive net. This net can effectively improve the conductivity of the material, then improve the performance of lithium iron phosphate battery.