| The lithium ion batteries are the attractive green energy resources in energy field, especially for upcoming electric vehicles. In spite of the commercialization of the carbon material as anode, the capacity is still relatively low, which limits its application for energy storage system. Therefore, it is a significant mission that anode materials with high capacity, low cost, long cycling performance and higher safety are explored to instead of graphite. Transition metal oxides (TMOS) have shown promising applications in lithium ion batteries due to their high theoretical capacity, high abundance, low cost, environmental benignity. Nevertheless, the large volume changes and the poor electrical conductivity of the TMOS nanoparticles are observed during the Li+insertion-extraction process, resulting in capacity fade and poor cycle performance. To improve their electrochemical performance, efforts have been devoted on several aspects as follows:fabrication of nano-sized TMOS, optimizing the architecture of TMOS to improve the kinetics and combining TMOS with carbonaceous materials to improve its electronic conductivity. Among different kinds of carbonaceous materials, graphene nanosheets can efficiently improve the lithium storage properties of TMOS due to good chemical stability, excellent electronic conductivity, large specific surface area, and good structural flexibility.In this work, utilizing the interfacial interaction and confinement effect, we explored varied synthesis methods with simple and low cost and designed novel structure to prepare various anode materials with high specific capacity, high rate capability and long cycling life, such as Fe3O4/graphene, CoO/graphene, carbon-encapsulated CoFe2O4/graphene and three-dimensional CoO/graphene nanocomposite. In order to further understand the effect of special structure, interfacial interaction and graphene on improving lithium storage properties, the morphologies and structures of the synthesized-samples were characterized by different measurements, and the electrochemical performance of metal oxides based nanocomposites were researched as anode materials for lithium ion batteries. The achieved results are as followings:Graphite oxide was prepared from natural graphite power by a modified Hummer’s method, and the obtained graphite oxide colloidal suspension could keep a long time at room temperature. Fe32O4/Reduced graphene oxide nanocomposite was prepared by a facile gas-liquid interface reaction and subsequent in suit reduction process. The average size of Fe3O4nanoparticles is50nm, which uniformly disperse on the graphene nanosheets with thin, curve, crumpled and stacked morphology. The as-prepared Fe3O4/graphene nanocomposite deliveres high specific capacity of1025mAhg-1at100mAg-1 after50cycles. It still retains specific capacity of959.4mAhg"1at100mAg"1, even after800cycles at various current densities from100to4000mAg"1. The excellent performance is ascribed to stable structure derived from interface reaction and good electrical conductivity due to the introduction of graphene.The Fe3O4/graphene nanocomposite was synthesized from Fe2(C2O4)3·5H2O and hydrophilic graphite oxide via a simple and nontoxic one-spot in situ method. The method is benefit to forming nanocomposite with small-size nanoparticles, uniform morphologies, high crystallography and few impurities, ensuring strong interfacial interaction between metal oxide and graphene to protect the whole electrode structure stability and electrochemical activity. The electrochemical tests show that over1000cycles at various densities between100and6000mAg-1, the high specific capacity of1188mAhg-1is maintained when the current is back to100mAg-1, displaying high specific capacity and excellent cycle stability. The specific capacity increases gradually with the increasing of cycle number. The phenomenon is ascribed to the well-designed structure of large quantities small-sized Fe3O4nanoparticles on graphene nanosheets, and the interfacial interaction (Fe-O-C linkage) between Fe3O4and graphene. In addition, the as-synthesized CoO/RGO nanocomposite by same method delivers a high specific capacity of701.7mAhg-1at100mAg-1after50cycles.We synthesized various mixed transition-metal oxides CoFe2O4based high-performance anode materials. Using Fe2(C2O4)3·5H2O and Co(C2H3O2)·4H2O as raw materials, the rod-like CoFe2O4-Co composite was synthesized by a facile in-situ method. The rod-like and porous structures of the CoFe2O4-Co composite were optimized by controlling the temperature. But, the composite don’t show high specific capacity and good cycling stability. Therefore, we prepared CoFe2O4-Co/graphene nanocomposite with no reductant by a facile hydrothermal strategy. The CoFe2O4-Co/graphene nanocomposite exhibits high specific capacity of907.3mAhg-1at100mAg-1even over425cycles at various current densities between100and9000mAg"-, displaying higher performance than that of the rod-like CoFe2O4-Co composite.The outstanding electrochemical performance of metal oxides/graphene nanocomposite is mainly due to the covalent force between nanoparticles and graphene. But the nanoparticles are partially dispersed on graphene nanosheets with noncovalent force and some of them may fall off from graphene substrate during discharge-charge process, lead to low specific capacity. Therefore, we provided two protect strategies based on carbon shell and graphene substrate to fabricate core-shell structure, mitigating large volume changes and aggregation of nanoparticles. Using graphite oxides and glucoses as raw materials, the carbon-encapsulated CoFe2O4/graphene nanocomposite, including of33%graphene and5%amorphous carbon, was prepared by a facile hydrothermal strategy and subsequently in-situ pyrolysis process. The specific capacity of the carbon-encapsulated CoFe2O4/graphene nanocomposite is925.6mAhg-1at100mAg-1after50cycles. And it exhibits high specific capacity after135cycles at various current densities between100and2400mAg-1. The SEM and HRTEM images of the nanocomposite after135cycles show the integrity and good stability of the electrode, which is similar with that before charge-discharge test.Based on confinement effect of core-shell structure, we intend to fabricate three-dimensional (3D) structure that a large number of nanoparticles are encapsulated within3D graphene. Such a geometric confinement of nanoparticles within graphene layers could enhance their interfacial interaction and mitigate the agglomeration of nanoparticles as well as shorten Li+diffusion length.3D graphene hydrogel anchored with ultrafine CoO nanoparticles was prepared by a facile hydrothermal strategy and subsequently heat-treatment process. The presence of CoO nanoparticles is benefit to the retention of3D structure, ensuring large active specific surface area to improve lithium storage properties. After82cycles at various current densities between100and2400mAg-1, the capacity increases to1025.8mAhg"1when the current density is back to100mAg-1, which is larger than that of the as-synthesized2D CoO/graphene nanocomposite. The structure and morphology of3D CoO/RGO nanocomposite are provided after82cycles at various current densities between100and2400mAg-1, which is similar with that before charge-discharge test. The average sizes of the nanoparticles have no obvious changes and uniformly anchor on3D graphene, exhibiting strong interfacial interaction and outstanding structure stability. |
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