The Study Of LiFePO₄ Material As Cathode In Lithium-ion Batteries

Compared with commercial Li-ion cathode material at present,lithium iron phosphate with an ordered olivine structure has many excellences,such as good charge-discharge character,high reversible capacity, good cyclic stability and long working voltage which is a most promising cathode material due to iron's abundant resources,non-toxicity,environmental friendly.Based on the reviews for research progress of LiFePO4 cathode material,the structure,working principle,major preparation methods and existy mostly problem were further introduced.Solid-reaction was used to synthesize LiFePO4.With FeC2O4·2H2O, NH4H2PO4,Li2CO3 which were activated by sintered at high temperature under the protection of the inert gases. In order to improve the conductivity of materials and increase the electrochemical performaces, The 3%,6%,10% of glucose were added respectively to form pure LiFePO4 of carbon coating on crystallographic theory, The SEM shows the dimension of granule was smaller after carbon coating,and the 6% rate of glucose added sample was 300nm and uniformly dispersed. Characterized by X-Ray diffraction,the sample was found with an order olivine structure,no impurity and single phase. Using LiFePO4 nano particals as active material,prepared the anode of lithium-ion batteries by casting. The impact of different kinds of binders and conductive additives were investigated by testing the performances of half-cells.The cycle performance show the 6% glucose added sample is the optimum,the first cycle capacity is 141mAh/g,and afer 50 cycle number,keep on 131.5mAh/g,decline 7%.Carbon coating only increase the electrical conductivity between paticles but has little impact on paticles internal.And the effect of metal-doped can improve the electrical' conductivity. So 5%,10%,15%,20% of Mn-doped with the 6% glucose are added to raw material respectively in this thesis in synthesizing LiFePO4 which are characterized by X-Ray diffraction.The sample are found with an order olivine structure,no impurity and single phase because of the forming LiMnxFe1-xPO4 mischcrystal. And the LiMn0.1Fe0.9PO4/C composites exhibit the most excellent electrochemical proterties.Their initial discharge capacities are 145mAh/g at 0.5C rate,132mAh/g at 1C rate,100mAh/g at 2C rate.The second platform of charge curve at 4.1 voltage occurred when the Mn doping is 20% because of the charge platform of LiMnPO4.