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Design And Lithium-Storage Properties Of Carbon/Transition Metal Oxide Nanostructures

Posted on:2012-08-26Degree:DoctorType:Dissertation
Country:ChinaCandidate:J S ZhouFull Text:PDF
GTID:1111330368458912Subject:Materials Science and Engineering
Abstract/Summary:PDF Full Text Request
Up to now, lithium-ion batteries have been used widely in portable electronic devices such as cell phones, digital camera, laptop because of their high energy density, high voltage and non-pollution. Meanwhile, they have a great application-potential in the fields of energy storage for renewable energy sources and electric vehicles. However, the commercial electrode materials (such as Graphite and LiCoO2) can not support the new energy sources and electric vehicles. Therefore, the new electrode materials with high energy density, high power density, and long-term cyclic lifetime need to be developed.In this work, we designed various novel nanocarbon and carbon/metal oxide composites with suitable nanostructures for bulk storage and rapid insertion/desertion of lithium ions by utilizing the nanoscale Kirkendall effect, confinement effect, and interfacial interaction between metal/metal oxide and nanocarbons. First, carbon/metal composites (carbon-encapsulated metal nanoparticles/wires, graphene-encapsulated metal spheres, etc.) were prepared by co-pyrolysis of carbon sources and metal compounds based on our previous work. Then, the as-prepared carbon/metal composites were converted to hollow metal oxide/carbon composites with various morphologies and structures via the outward diffusion of metal atoms in the carbon-confined space induced by the nanoscale Kirkendall effect. At last, the electrochemical performances of these hollow metal oxide/carbon composites were investigated in detail. In further, we explore in-depth the interfacial interaction between Fe3O4 nanoparticles and graphene nanosheets as well as its impact on the electrochemical performance of Fe3O4/graphene anode materials for lithium-ion batteries. In addition, in order to improve the high-rate performance of carbon nanotubes, a novel kind of carbon nanotubes with quadrangular cross section was prepared in large-scale.We synthesized carbon-encapsulated metal oxide hollow nanoparticles/nanotubes and pure metal oxide hollow nanoparticles by directly oxidizing the carbon-encapsulated metal nanoparticles in air. By controlled air-oxidation, carbon-encapsulated metal nanorods can also be converted to carbon-encapsulated metal oxide nanotubes and mesoporous metal oxide nanotubes. The conversion processes of Fe3C@C nanorods/nanoparticles to nanotubes/hollow nanoparticles as special examples are investigated in detail. It was found that both oxygen and carbon play important roles in the formation of hollow nanostructures, wherein oxygen is the driving force for the outward diffusion of core species, and carbon shell not only provides the diffusion vacancies but also effectively moderates the interdiffusion rates of metal core materials and oxygen. A growth model was proposed:during the oxidation process, three diffusion processes occur including the inward diffusion of oxygen along the carbon shell, outward diffusion of core materials and inward diffusion of vacancies from carbon shell to core. The outward diffusion of core species involves two steps:the first step is the diffusion of Fe3C from core to carbon shells, which is only a physical change (single-crystal Fe3C was changed to multicrystal Fe3C); and the second one is the diffusion and chemical reactions of Fe3C in carbon shells with oxygen (the multicrystal Fe3C was oxidized to Fe3O4 and then to a-Fe2O3).Subsequently, we prepare "echinus-type" metal oxide/graphene hollow microspheres by directly oxidizing the graphene-encapsulated metal microspheres in air. It is found that the surface of metal oxide nanowires/nanosheets on the "echinus-type" metal oxide/graphene hollow microspheres is wrapped by graphene flake, which should be attributed to the strong interaction between metal atoms and graphene. The formation of "echinus-type" metal oxide/graphene hollow microspheres should also be ascribed to the nanoscale Kirkendall effect. In a word, the diffusion of metal atoms in graphene-based metal spheres and a nanospace confined by carbon shells is a great extension to the nanoscale Kirkendall effect, which not only can synthesize various novel nanostructures, but also is helpful to advance in basic and scientific studies on the diffusion of metal atoms and interaction between metal and graphene.The obtained carbon-encapsulated metal oxide hollow nanoparticles, and carbon-encapsulated metal oxide nanotubes not only possess high specific capacities, but also exhibit better high-rate performance, when they are used as anode materials for lithium-ion batteries. The electrochemical measurements indicated that the first reversible capacity of carbon-encapsualtedα-Fe2O3 hollow nanoparticles is ca.929 mAhg-1 at 20 mAg-1, and the capacity retention is ca.83% after 30 cycles. At 1 Ag-1, the reversible capacity is ca.626 mAhg-1, which is ca.67% of that at 20 mAg-1. The reversible capacity of carbon-encapsualted CuO hollow nanoparticles is ca.644 mAhg-1 at 50 mAg-1 without capacity fading after 20 cycles. At 1 Ag-1, the reversible capacity of carbon-encapsualted CuO nanoparticles is ca.430 mAhg-1, which is ca.57% of that at 50 mAg-1. The reversible capacity of carbon-encapsualted iron oxide nanotubes is ca. 878 mAhg-1 at 50 mAg-1, and the capacity retention is ca.89% after 20 cycles. It is reasonable to ascribe their excellent electrochemical performances to the encapsulation of carbon and hollow interior space. In addition, "echinus-type" metal oxide/graphene hollow microspheres also exhibit both high specific capacity and better high-rate performance in despite of their big size. The reversible capacity is ca.799 mAhg-1 at 50 mAg-1 without capacity fading after 20 cycles. At 1 Ag-1, the reversible capacity of graphene/iron oxide is ca.450 mAhg-1, which is ca.56%of that at 50 mAg-1. The excellent electrochemical performance of graphene/iron oxide should also be attributed to outstanding properties of graphene.The hollow CuO/graphene composites were successfully prepared via a nanoscale Kirkendall-effect-based approach. The hollow CuO nanoparticles with an average diameter of 50 nm were dispersed homogeneously on the graphene surface. The composite electrode exhibits excellent high-rate performance and durable cycle lifetime, when used as the anode material in lithium-ion batteries. The reversible capacity attains 640 mAhg-1 at 50 mAg-1, and the retention of capacity is ca.96% when the current density is increased by 10 times. At 1 Ag-1 (ca. 1.7 C), the reversible capacity reaches 485 mAhg-1 and remains 281 mAhg-1 after 500 cycles, indicating that the capacity fading is less than 0.4 mAhg-1 per cycle. The enhancement of electrochemical performance should be attributed to the hollow interior of CuO nanoparticles and interfacial interaction between CuO and graphene.In order to explore in-depth the interfacial interaction between metal oxide and graphene nanosheets as well as its impact on the electrochemical performance of graphene-based metal oxide anode materials for lithium-ion batteries, two kinds of Fe3O4/graphene hybrid materials used as a special example are prepared by in situ growth and ultrasonic mixing, respectively. It was found that strong covalent bond interaction (Fe-O-C bond) between and graphene nanosheets exists in Fe3O4/graphene sheets obtained by in situ growth, while no strong interaction in composites prepared by ultrasonic mixing. When they are used as anode materials, the strong covalent links ensure the high specific capacity and long-period cyclic stability of Fe3O4/graphene hybrid electrodes for lithium-ion batteries at high current density. The capacity keeps as high as 796 mAhg-1 after 200 cycles without any fading in comparison with the first reversible capacity at the current density of 500 mAg-1 (ca.0.6C). At 1 Ag-1 (ca.1.3C), the reversible capacity attains ca. 550 mAhg-1 and 97% of initial capacity is maintained after 300 cycles. This work reveals an important factor affecting the high-rate and cyclic stability of metal oxide anode, and provides an effective way to the design of new anode materials for lithium-ion batteries.The novel quadrangular carbon nanotubes (q-CNTs) with the length of 0.5-3μm, average outer diameter of 100 nm, and average inner diameter of 20 nm were synthesized in large-scale by pyrolysis of durrene and ferrocene. Besides the quadrangular cross sections, it was found that the q-CNTs also possess one open end and "herringbone"-like walls. The unique nanostructures of q-CNTs endow the shorter diffusion length and higher chemical diffusion coefficient of lithium-ion, so the q-CNTs exhibit higher specific capacity and much better high-rate performance when they are used as anode materials for lithium-ion batteries. At a current density of 20 mAg-1, the reversible capacity is ca.387 mAhg-1. when the current density increase to 500 and 1000 mAg-1 (ca.3 C), the reversible capacity remains 226 and 181 mAhg-1, respectively, which is much higher than that of traditional carbon nanotubes.
Keywords/Search Tags:Lithium-ion batteries, anode materials, carbon-encapsulated metal nanocrystals, Kirkendall effect, metal oxide, carbon nanotubes, graphene, hollow nanostructures, oxidation
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