| Lithium ion battery has a bright future as one of new power source technologies for its superior performance. But the global lithium-ion battery industry is far from developing an electric energy storage component suitable in both energy and power that will satisfy the demands of high power application, especially battery electric and hybrid vehicles. LiFePO4is widely regarded as one of the best candidate cathode materials for battery-using of electric or hybrid vehicles because of its advantages of non-toxicity, high thermal stability, low cost, good cycling performance.However, the pure LiFePO4material has very poor rate performance because of its both low electronic and ionic conductivity which limits its practical application as high power density and long-life batteries. In this dissertation, following works are carried out:(1.) To synthesis Li(Fe1-2XTix)PO4/C by solid-state reaction methodTi as the alien element was brought into the LiFePO4with two different methods to improve the performance of LiFePO4. Various Ti-doping LiFePO4of Li(Fe1-2XTix)PO4(x=0.02,0.04,0.06) were prepared by the two methods separately and the effect of Ti-doping to the electrochemical performances of the samples was systematically studied. The results indicate that although both methods can improve the electrochemical properties of the LiFePO4cathode material, the effects of them are obviously diverse.(2.) To synthesis Li(Fe1-2XZrx)PO4/C by carbon thermal reduction methodA series of Li(Fe1-2xZrx)PO4/C (x=0.01,0.02,0.03,0.04,0.06) were prepared by carbon thermal reduction method. With this strategy, the impurity Li3PO4phase can be obviously reduced in the Zr-doped samples which result in the obvious improvement of their electrochemical performance comparing with the un-doped one. The best electrochemical performances were observed in Li(Feo.92Zro.o4)PO4/C as well as good cycle stability.(3.) Novel core-shell structure of LiFePO4shows good electrochemical properties Two novel core-shell structures of LiFePO4as nonstoichiometric C@LiFePO4core-shell and Ag@LiFePO4/C core-shell composites were fabricated with two different methods:Some kinds of nonstoichiometric C@LiFePO4core-shell composites are synthesized by a novel solid-state reaction method. All samples by this method show outstanding coulombic efficiency and good battery cycleability, along with high tap density, Which might meet the needs of high power applications.Ag@LiFePO4/C core-shell particles were prepared by a facile and one-step ultrasonic electrodeposition method for the first time. Due to the high electronic conductivity of the networks composited of Ag nanoparticles, high charge/discharge rates and cyclability of the LiFePO4/C electrodes were observed. As a result, the Ag@LiFePO4/C core-shell cathode material displays excellent electrochemical performances, indicating the potential application in high power situation of this core-shell composite cathode material.(4.) LiFePO4-graphene composites show excellent electrochemical propertiesTwo hierarchical superstructure graphene-sheets based LiFePO4composites of porous LiFePO4/graphene and nanosheet LiFePO4/graphene were prepared for the first time. Due to the high-conductivity3D network formed by the evenly distributed graphene-sheets and special microstructures, these two novel composites show high-performance which might meet the demands of high-power and high-energy battery applications.
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