Synthesis And Modification Of Lifepo₄ Cathodic Materials For Lithium Ion Batteries

As a kind of cathode material for lithium ion batteries,olivine-structured LiFePO4 hasmany advantages such as abundant sources,low cost,environmental compatibility,highthermal stability and safety. However,low electronic conductivity,low volumetric densityand low lithium ion mobility are the main obstacles for the commercial application ofLiFePO4. In the present work,electronic and ion conductivity of LiFePO4 is improved bydoping,metal particles coating and carbon coating.In Chapter 3,LiFe1-xNixPO4/C(x =0,0.01,0.02,0.03,0.04)were prepared by two-stepsolid state method using LiOH·H2O,FePO4·4H2O,(CH3COO)2Ni·4H2O) and asphalt asreactants. TEM results showed that spherical LiFePO4 grains can be synthesized by Nidoping. These spherical particles were connected together through carbon nano-interconnectstructures. As the increase of the Ni doping amount,flake-shaped and spherical LiFe1-xNixPO4/C were formed. The electrochemical tests indicated that LiFePO4/C with 1 mol% Nidoping showed the best electrochemical performance. Its initial discharge capacity at 0.1 Crate was about 150 mAhg-1 and the coulombic efficiency was about 94.7% during the firstcycle. It also had excellent rate performances. It remained initial discharge capacity of 132mAhg-1 and the capacity retention of 100% after 100 cycles at 1 C rate. The effects ofsintering temperature on electrochemical performances of LiFe0.99Ni0.01PO4/C were alsoinvestigated. The results showed that the preferred sintering temperature was 650℃. Therole and the corresponding mechanism of Ni doping and C coating were discussed.In Chapter 4,LiFePO4 with Ag coating and Ag/C co-coating were synthesized usingglucose as a reducing agent. The SEM results showed that Ag/C coated LiFePO4 particle hadlow surface roughness and narrow particle size distribution. The electrochemical tests resultsrevealed that Ag coating could improve the initial discharge capacity of LiFePO4 from 80 to121 mAhg-1. By comparison, Ag/C coated LiFePO4 had a higher initial discharge capacity ofabout 132 mAhg-1 and better cycling performances. The results proved that Ag coating alsodid contribute to the decrease of the impedance of LiFePO4. The charge transfer resistanceof LiFePO4 was decreased to 54.8Ωafter Ag and C co-coating. The role and thecorresponding mechanism of the Ag coating and the Ag/C co-coating were discussed. In Chapter 5,LiFePO4/Ni composites were synthesized via precipitation and reductionmethod using LiFePO4,Ni(NO₃)₂·6H₂O and NH4HCO3 as reactants. TEM and HRTEMresults indicated that there were Ni and NiFe₂P nanocrystals on the surface of LiFePO₄. Theelectrochemical tests reaveled that these nanocrystals could improve the initial dischargecapacity,decrease the cathode resistance and improve the electronic conductivity. Theeffects of Ni/C coating on physical and electrochemical properties of LiFePO4 were alsodiscussed.