Microwave-assisted Solid-state Synthesis And Lithium-storage Properties Of Anode Materials For Lithium Ion Batteries

At present, graphite carbon is the most used commercial lithium ion battery anodematerial, but the high irreversible capacity and low rate performance have limited thedevelopment of this material. Now lithium ion batteries are developed towards highenergy density, high rate capability and high cycle life.Li4Ti5O12is a zero strain material with almost no structural change in the charge-discharge process. It is now a hotspot of research on the lithium ion battery anodematerial because of its long cycle life and high stability. However, the electronicconductivity of the Li4Ti5O12is poor, which restricts its electrochemical propertiesespecially high rate performance. It is necessary to do some material modification.ZnCo2O4is a derivative of the metal oxide which uses Zn atom instead of Co atom inthe Co3O4. It not only has high theoretical specific capacity as the metal oxide, but alsoreduces the cost and environmental pollution. It is essential to do modification to improveits columbic efficiency.This dissertation chooses a new preparation method, namely microwave-assistedsolid-state reaction to get Li4Ti5O12and ZnCo2O4nanoparticles, and further use Nb5+todope Li4Ti5O12and add MWCNTs to modify ZnCo2O4, respectively. Compared with thetraditional heating method, microwave irradiation prompts a much more rapid solid-statereaction due to the interaction of microwave field with reactant molecules. The microwavemethod has advantages of considerable lower synthesis temperature and shorter reactiontime than that of the traditional method. Since the reaction time and temperature ofconsecutive steps can be finely programmed in the modern microwave systems, it can beuniformly and rapidly absorbed by reactants or heating medium.Thus the high-temperatureneeded in a solid-state reaction can be obtained within several minutes.Nb-dopedLi4Ti5O12(LTO) has been successfully prepared by a fast and economicalmicrowave-assisted solid-state process using TiO2, Li2CO3and Nb2O5as raw materials at750oC for20min. The size of the nanoparticles is around200to400nm.ZnCo2O4@CNTs composites have been successfully prepared by a fast and economical microwave-assisted solid-state process using Zn(NO3)22H2O,Co(NO3)26H2O andMWCNTs as raw material at450oC for20min.Results show that Nb-doped LTO by microwave irradiation exhibits enhancedelectrochemical properties in comparison to the LTO (T) powder prepared by a traditionalsolid-state process. In particular, the Nb-doped LTO with Nb-containing content of x=0.03exhibits the best electrochemical performances with a higher specific capacity,excellent cyclability and superior rate capability. Discharge capacities of174.7,157.1,141.7and130.2mAh g1after200cycles are achieved at the charge-discharge rates of1C,5C,10C,20C, respectively. This work provides an effective way for the fast preparationof Li4Ti5O12-based anode materials with superior rate performance, which holds greatpotential for large-scale industrial production. Also, the results show the electrochemicalperformance of ZnCo2O4(MW, CNTs) composite has been improved compared withZnCo2O4which is prepared through traditional solid-phase method. The columbicefficiency in the first cycle has been increased from22%to77%and the specific capacityremains a700mAh g1after20cycles..