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Nanocomposite Silicon and Graphene Composite Negative Electrode Materials for Lithium Ion Batteries

出版	McGill University Libraries, 2017
URL	http://books.google.com.hk/books?id=0J_lswEACAAJ&hl=&source=gbs_api

註釋"Engineering electrical energy storage systems with high energy density is critical to the adoption of electric vehicles as a green transportation system. The Li-ion battery has the highest energy density of any mature technology and is currently employed in this application. Improving the energy density of the Li-ion battery system requires new electrode materials, for both the anode and cathode, with high volumetric and gravimetric capacity for Li storage. Si is a potential anode material with extraordinarily high gravimetric capacity 4200mAh/g, as compared to the theoretical limit of 373mAh/g for conventional graphite electrodes. However, Si is beset by several technical challenges, including the formation of an unstable solid-electrolyte interphase that irreversibly consumes Li, and a 400% volumetric expansion that pulverizes bulk Si. These challenges require novel solutions to realize viable Si based anode technology. In this thesis, we present a family of engineered Si / graphene composites for anode applications. These composites consist of Si nanoparticles attached to micron scale graphene flakes. We have experimentally shown that the nature of the attachment between Si nanoparticles and graphene flakes - physical, ionic, or covalent - is critical to the suppression of capacity fading in the composite anodes. A covalently coupled composite material containing 60% Si by weight has an initial capacity after formation of 1600mAh/g, more than 4 times greater than commercial carbon anodes. We also present a novel strategy for improving the overall capacity of the Li-ion battery anode by eliminating the heavy copper foil current collector. A light-weight, flash reduced graphene oxide thin film current collector is integrated with a high capacity film of graphene encapsulated Si nanoparticles. The resulting two-layered structure forms a single self-supporting, conductive anode film with improved gravimetric capacity of 1100mAh/g with 60% Si by weight loading. The specific capacity after 200 cycles is stable at greater than 600mAh/g. The final contribution of this thesis is a comparative study of thermal reduction, flash reduction, and hybrid reduction techniques for the preparation of Si / graphene composites. Flash reduced graphene oxide is open and porous to accept Li ions, but easily disintegrates upon handling. Thermally reduced graphene oxide films are mechanically robust, but exhibit poor electrolyte penetration and poor rate performance. The use of a hybrid reduction technique, first partially thermally reducing films and completing the reduction with a flash process, allows for the tuneable introduction of pores. The films are mechanically robust, have good electrical conductivity, and show much reduced initial capacity loss due solid electrolyte interphase formation. We close the thesis with a discussion of the opportunities and remaining challenges for realizing viable Si / graphene composite anodes."--