Carbon Gel Assisted Low Temperature Liquid-phase Synthesisand Modification Of Nano LiFePO₄: Towards Electrochemical Performance

Climate change and fluctuations in fossil fuel prices have lead society to reduce the use of such fuels and to discover new green energy resources, to limite carbon dioxide emissions. Since the olivine-type structure Li FePO4 was first reported by Padhi in 1997, it has been recognized as the most ideal material for lithium ion batteries, due to its high theoretical capacity(170.0 mAh g-1), superior cycle life, low cost, thermal stability and environmental friendliness. However, the large-scale application of LiFePO4 in electric vehicles(EVs) and hybrid electric vehicles(HEVs) was hindered by its low electrical conductivity and lithium ion diffusivity. As such, the dissertation has mainly been focusing on three aspects to make improvements:(1) Minimizing the particle size and customizing particle morphologies by appropriate synthetic methods.(2) The application of conductive agents coating on LiFePO4 surface to improve the electrical conductivity between the material particles.(3) Metal ions doping to improve the conductivity within LiFe PO4 grains. In order to improve its electrochemical properties, in this paper, LiFePO4, LiFePO4/GO and LiFe0.9Mn0.1PO4 were synthesized by carbon gel assisted low temperature liquid-phase method, After high temperature carbon treatment, the LiFePO4/C, C-LiFePO4/G and LiFe0.9Mn0.1PO4/C composites were obtained. The phase structure and morphology of products were characterized by XRD, SEM, TEM, Raman, and the electrochemical performance of the samples was measured.(1) Nano-scale LiFePO4 was synthesized by carbon gel assisted low temperature liquid-phase method, after high temperature carbon treatment, the nano-scale LiFe PO4/C composite was obtained. LiFePO4/C composite exhibits excellent discharge capacities of 160.0 mAh g-1 at rate of 0.2 C, greatly improved the discharge specific capacity of LiFePO4.(2) LiFePO4/GO was synthesized by carbon gel assisted low temperature liquid-phase method, after high temperature carbon treatment, the C-LiFePO4/G composite was obtained. C-LiFePO4/G composite exhibits excellent discharge capacities of 68 mAh g-1 at rate of 60 C, showing excellent high-rate discharge capability and cycle performance.(3) LiFe0.9Mn0.1PO4 was synthesized by carbon gel assisted low temperature liquid-phase method, after high temperature carbon treatment, the LiFe0.9Mn0.1PO4/C composite was obtained. LiFe0.9Mn0.1PO4/C composite exhibits excellent energy density of 547.2 Wh/kg, and the discharge capacities can reach 152.4 mAh g-1 at rate of 0.2 C.