Study On Preparation And Properties Of Olivine Type Lithium Ion Battery Cathode Material

In this thesis,the design and synthesis of phase-pure lithium iron phosphate?LiFePO₄?nanoparticles by solvothermal method are reported,and the preparation of carbon-encapsulated Li FePO₄ nanoparticles with glucose,pitch and PVDF as the carbon source are presented.The results show the uniform dispersion of the LiFePO₄nanoparticles obtained from the solvothermal method.The prepared four types of LiFePO₄ nanoparticles?LiFePO₄,Li FePO₄/HCC,LiFePO₄/SCC,and LiFePO₄@C/F?were characterized,and the results show that these nanoparticles have good phase structures.The relative content of each element in LiFePO₄ was tested by EDS and XPS spectra,and a proper atom ratio is obtained.To characterize the electrochemical properties of the carbon-encapsulated and fluorine-doped LiFePO₄,half cells were assembled in CR2032 coin batteries.The cyclic voltammetry,charge-discharge and electrochemical impedance were tested,showing that the electrochemical performance of Li FePO₄ have been significantly enhanced after carbon encapsulation and fluorine doping.The LFP/SCC samples prepared with asphalt as carbon source have excellent electrochemical performance,and the first discharge capacity can reach as high as160.4mAhg^-1 at 0.2 discharge ratio.The electrochemical performances of fluorine-doped and carbon-encapsulated Li FePO₄ are also improved,and its specific capacity up to 147mAhg^-1.The findings indicate that carbon encapsulation and fluorine doping are of great value to enhance the energy storage performance of Li FePO₄cathode materials.The theoretical capacity of lithium iron phosphate is 170mAhg^-1,however,the commercial lithium iron phosphate capacity is normally at 150-160mAhg^-1.In order to improve the capacity of the commercial LiFePO4,the commercial LiFePO₄graphene composite is obtained,after high temperature reduction which delivers the initial discharge capacity of 171.1mAhg^-11 at 0.1C and a rate discharge capacity of 169.4mAhg^-1 at 1C.The main work and innovation of this thesis are summarized as follows:1?The nanoscale lithium iron phosphate was synthesized by solvothermal method.The effects of hydrothermal and calcining time as well as temperature on the structure and properties of lithium iron phosphate were studied by using hard carbon and soft carbon respectively.It was found that when the hydrothermal reaction occurs under 180degrees for 2 hours,with calcining at 700 degrees for 6 hours,the as-prepared samples showed the best electrochemical performance:the first discharge capacity could reach160.4mAhg^-1 at 0.2 discharge ratio.After 100 cycles,the discharge capacity could still reach 158.3m Ahg^-1,and the capacity drop rate was only 1.3%.2?For the first time,PVDF was used as a carbon source to produce lithium fluoride lithium with coated carbon.The discharge capacity at 0.1 discharge ratio was147.6mAhg^-1 and the charge/discharge efficiency was 98.46%.3?When using the commercial LFP and graphene oxide/ethylene glycol as the solvent,the reduction of high pressure and high temperature of ethylene glycol reduce graphene oxide and LFP particles into graphene/LFP layers.The discharge capacity of C-Li Fe_0.97V_0.03PO₄/rGO composite in graphene was beyond the theoretical capacity of LFP for the first time,where the discharge capacity can reach 171.1mAh g^-1 at 0.1discharge ratio.