Microwave-Assisted Systhesis Of Metal Oxide/Graphene Nanocomposite And Their Application As Anode Material For Li-Ion Batteries

In the past decades, transition metal oxides have been extensively investigated for anode materials in Li-ion batteries (LIBs) due to high theoretical capacity (500-1000 mAh/g). Nevertheless, the application of transition metal oxides in practical LIBs is seriously hindered by their low electronic conductivity, relatively large initial irreversible loss and the large volume change during the insertion and deinsertion cycles of lithium ions. To overcome these problems, in this thesis, we study the enhanced electrochemical performance of copper oxide and cobalt oxide nanostructures dispersed on graphene sheets as anode maerials for li-ion battery. The main contents and results are as following:(1) Graphene (GNS) sheets were synthesized by oxide the graphite and heat treatment, then, CuO nanoleaf on graphene sheets (CuO/GNS) composite was synthesized by a facile microwave-assisted method using Cu(NO3)2·3H2O, NaOH and ammonia as raw materials and absolute ethanol as solvent. The SEM and TEM results revealed that the leaf-like CuO were uniformly dispersed on GNS with a width of 40 nm in the middle and a length of 100-140 nm. Electrochemical properties were evaluated using coin cells. The reversible capacity of the CuO/GNS composites retained 600 mAh/g after 50 cycles at 100 mA/g. When cycled at various rate for 50 cycles, the capacity could recover to 600 mAh g-1 at the current of 100 mA/g, showing excellent electrochemical performance than pristine CuO. The improved electrochemical performance is ascribed to the coordination ability of the uniform distribution of rhombic CuO and graphene. On the one hand, graphene nanosheets can greatly improve the conductivity of metal oxide electrode; On the other hand, the CuO between the graphene sheets effectively keeps the neighboring graphene sheets separated, and graphene effectively prevents aggregation and pulverization of CuO nanoparticles, keeps the overall electrode highly conductive, and maintains the stability of the structure.(2) CuO-Cu2O/graphene composite is prepared by the combination of a microwave-assisted process and subsequent annealing using Cu(AC)2·H2O, NH3·H2O and GO as raw materials. The SEM and TEM results reveal that uniform copper oxide nanospheres of 100-170 nm are dispersed on graphene sheets. The nanospheres are composed of many CuO and Cu2O nanocrystals of about 5 nm, and the nanospheres are hollow with a shell thickness of 50-70 nm. Electrochemical properties were evaluated using coin cells. The reversible capacity of the composite retains 487 mAh/g after 60 cycles at 200 mA/g. When cycled at various rate (200,500,1000, 2000,5000 mA/g) for 60 cycles, the capacity could recover to 520 mAh/g at the current of 200 mA/g. The enhanced electrochemical performances are ascribed to the hollow spherical architectures, excellent conductivity of graphene sheets, and possible synergistic effects between CuO, Cu2O and graphene which can enhance the intrinsic properties of each component.(3) Graphene based Co3O4 quantum dots were synthesized by a facile, fast and efficient microwave irradiation method for 5 mins and investigated with XRD, TEM, HRTEM, TG. The results reveal that uniform Co3O4 nanocrystals of about 3-8 nm with a high density are homogeneously dispersed on graphene nanosheets and the content of graphene was about 40%., The Co3O4 quantum dots/GNS composite electrode for li-ion batteries shows high reversible capacity (1785 mAh/g at 0.1 C for 90 cycles), as well as high rate capability (485 mAh/g at 5 C), and when cycled at various rate, the capacity can recover to 2000 mAh/g at 0.1 C. The reversible capacities are much higher than the theoretical capacity of Co3O4 quantumdots/graphene composite (Ctheoretical=Cco3O4 ×mass percentage of Co3O4+ x mass percentage of graphene=890 mAh/g ×60%+744 mAh/g × 40% =831.6 mAh/g). The superior performance could be attributed to the interfacial lithium-storage and the quantum and size effects of quantum dots that lead to high activity during the lithiation/delithiation process. In addition, the flexible and conductive graphene nanosheets and well dispersed Co3O4 nanodots as well as the synergetic effect between them also benefit the electrochemical performance by endowing a superior highsurface area and shortening the diffusion pathway of lithium ions.