In-situ Synthesis Of Nano-carbon/lithium Iron Phosphate Composite Material And Its Applications In Li-ion Battery

Recently, Lithium iron phosphate (LiFePO4) has become a new cathode materialfor lithium-ion batteries owing to its high theoretical capacity, stable dischargingplatform, high cycle performance and environment-friendly properties. Carboncoating is used to improve the electron/ion conductivity of LiFePO4, but the bondinginterface between carbon-coating and LiFePO4has to be improved. Therefore, it isdesirable to fabricate C/LiFePO4composite material with well interface.In this study, nano C/LiFePO4composites are prepared by chemical vapordeposition (CVD), mechanical doping method, and solvothermal synthesis. The effectof different conditions on the morphology of nano C/LiFePO4composites was studied.The performances of different C/LiFePO4composites were investigated as cathode ofLi-ion battery.The results reveal that (1) in the in-situ CVD fabrication process of nano carbonon LiFePO4, the yield of tubular product decreases with the increase of catalystcontent; nano carbon products gradually change from carbon fibers to carbonnanotubes with the increase of growth temperature and the decrease of the reactiongas proportion. The optimum parameters for in-situ CVD synthesis are0.5wt%Co,690°C, and7:350(ml/min). The morphology of products is between fishbone carbonnanotube and tubular carbon nanotube. The diameter of products is20~30nm, and thegraphite layer spacing to0.34nm.(2) The amount of the nano-carbon material addedduring solvothermal synthesis impacts the morphology of composite material. Moreor less nano-carbon material added both have a negative impact on the homogeneityof C/LiFePO4composites. The proper amount for nano-carbon addition is20mg per0.5g, and a sandwich-shaped laminated structure (or a coaxial tube structure) can beformed.(3) Compared to the composites fabricated by mechanical doping method andsolvothermal synthesis, the composite synthesized by in-situ CVD method has betterelectrochemical properties with160.1mAh g-1of initial discharge capacity,144.8mAhg-1of discharge capacity after100cycles in1C (100%retention) and97.2mAh g-1after20times in10C (94.5%retention), and the minimum electrode polarization aswell as battery impedance.