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Molybdenum Oxide Nanoarchitectures:Fabrication And Lithium Storage Properties

Posted on:2013-01-07Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y M SunFull Text:PDF
GTID:1111330371480795Subject:Materials science
Abstract/Summary:PDF Full Text Request
MoO2 holds a great promise as an anode material in lithium-ion batteries due to its low, metallic electrical resistivity, high chemical stability, and high theoretical capacity. However, MoO2 suffers a low actual capacity, poor cyclability and rate capability as anode material in lithium-ion batteries. The main solution is to prepare various MoO2 nanostructures and MoO2-based nanohybrids. The research in this dissertation mainly focused on the development of novel synthetic strategies to prepare hierarchical MoO2 nanostructures, MoO2/carbon nanocomposites, and MoO2/graphene nanohybrids with high electrochemical performances. The conditions for the synthesis of these nanostructured materials were systematically explored. The possible reasons for enhanced lithium-ion storage performances were discussed in detail. Moreover, MMn2Mo3O8/graphene and Mn2Mo3O8/graphene nanocomposites were firstly explored as anode materials for high-performance lithium-ion storage in this dissertation.A simple and cost-effective morpho-genetic route has been developed for the fabrication of hierarchically nanostructured "cellulose" MoO2 monoliths in large qualities, whereby the cotton texture acts as both a template and a stabilizer. The MoO2 monolith possesses hierarchical porosity and inter-connected framework, which is demonstrated to be useful as a binder-free anode in rechargeable lithium-ion batteries with both high specific capacity of 719 mAh g-1 and good reversibility. Our single-component anode for lithium-storage devices also benefits from the simplified fabrication process and reduced manufacturing cost, in comparison to conventional multi-component electrodes that are fabricated from a mixture of polymer binders and active materials. The present morph-genetic strategy is facile but effective, and therefore it is very promising for large-scale industrial production. It can be extended to prepare other metal oxides with elaborate textural characteristics.A composite of ultrafine MoO2 nanoparticles homogeneously distributed in a carbon matrix has been fabricated on a large scale by an easy impregnation-reduction-carbonization route. Firstly, a cotton/H3PMo12O40 composite was formed by incorporating phosphomolybdate clusters into a cotton framework. Then it was treated in a H2/Ar atmosphere at 500℃for 5 h and in situ reduced/carbonized into a black monolith comprising ultrafine MoO2 nanoparticles (< 2 nm) embedded in the carbon matrix. The electrochemical tests demonstrate that the as-formed MoO2/C hybrid exhibits high capacity and excellent capacity retention as an anode material for lithium-ion batteries. The specific discharge capacity is as high as 1207 mAh g-1 in the first cycle and 734 mAh g-1 over 350 cycles at a current density of 50 mA g-Self-assembled hierarchical MoO2/graphene nanoarchitectures have been fabricated on a large scale through a facile solution-phase process and subsequent reduction of the Mo-precursor/graphene oxide. The as-formed MoO2/graphene nanohybrid as an anode material for lithium-ion batteries exhibits not only a highly reversible capacity but also an excellent cycling performance as well as good rate capability. Results show that the hierarchical rods made of primary MoO2 nanocrystals are uniformly encapsulated within the graphene sheets. The synergistic effect of the hierarchical nanoarchitecture and the conducting graphene support may contribute to the enhanced electrochemical performances of the hybrid MoO2/graphene electrode. This work presents a facile synthetic strategy that is potentially competitive for scaling-up industrial production. Besides, the MoO2/graphene hybrids with a well-defined hierarchical topology not only provide flexible building blocks for advanced functional devices, but are also ideal candidates for studying their nanoarchitecture-dependent performances in catalytic and electronic applications. Hierarchically nanostructured Mn2Mo3O8/graphene nanocomposites have been successfully synthesized in large quantities through a facile two-step reduction approach. The unique Mn2Mo3O8/graphene nanohybrids are composed of graphene-wrapped secondary microspheres of~3-5μm in diameter that are built from many Mn2Mo3O8 nanosheets with thicknesses of 10-15 nm and widths of 80-120 nm. The as-formed Mn2Mo3O8/graphene nanohybrids have been applied as anodes for lithium-ion batteries, and show better lithium storage performance compared to the bare Mn2Mo3O8 nanostructures. The enhanced capacity and cycling performance of the Mn2Mo3O8/graphene nanoarchitectures may benefit from the synergistic effects of the nanohybridization. Graphene layers can hinder the agglomeration and enhance the electronic conductivity of the active materials. This facile strategy may be extended to fabricate other nanostructured graphene-encapsulated ternary metal oxides with elaborate secondary architectures.Fe2Mo3O8/graphene nanocomposites have been successfully prepared on a large scale through a facile solution-phase process and subsequent reduction. The obtained hybrid comprises Fe2Mo3O8 nanoparticles (~10 nm) dispersed on graphene substrate. Benefiting from the synergistic effect of Fe2Mo3O8 nanoparticles and conductive graphene, the Fe2Mo3O8/graphene nanocomposites exhibit high specific capacity, good cycling performance, and excellent rate capability as an anode material for lithium-ion batteries. The specific reversible capacity is as high as 965 mAh g-1 in the first cycle and 939 mAh g-1 over 40 cycles at a current density of 200 mA g-1 Even at the current density as high as 1500 mA g-1, the specific reversible capacity is still as high as 770 mAh g-1.
Keywords/Search Tags:Lithium-ion batteries, Anode materials, MoO2, Mn2Mo3O8, Fe2Mo3O8, Graphene, Specific capacity, Cycling performance, Rate capability
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