Nanostructure materials for rechargeable lithium-ion and lithium-oxygen batteries

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Electrode materials and catalysts are key factors influencing the high power and high efficiency performances of lithium-ion batteries or lithium-oxygen batteries. In this doctoral work, a series of nanostructure materials, such as one-dimensional nanorods, two-dimensional nanoplates and nanosheets, three-dimensional microspheres and mesoporous structures, were successfully synthesized by various methods. Their electrochemical performance in lithium-ion batteries and lithium-oxygen batteries was also measured by galvanostatic charge-discharge, cyclic voltammetry and electrochemical impedance spectra. LiFePO₄ facet nanoplates/graphene hybrid materials and mesoporous nanolayer carbon coated LiFEPO₄ microspheres were synthesized by a hydrothermal method combined with high temperature treatment. The as-prepared materials exhibited both high discharge capacities and excellent high-rate performances as cathode materials for lithium-ion batteries. Mesoporous α-Fe₂O₃ was successfully synthesized by a soft template method for testing as an anode material in lithium-ion batteries. The as-prepared mesoporous α-Fe₂O₃ electrodes showed a high discharge specific capacity and stable cycleability. The excellent electrochemical performance should be attributed to the unique mesostructure, with its high surface area able to provide high surface contact with the electrolyte and decrease the current density per unit area. One-dimensional MnO/C core-shell nanorods were successfully prepared from the reduction of MnO₂ nanowires. This material exhibited good electrochemical performance as an anode material for lithium-ion batteries, which is higher than that of MnO microparticle and MnO₂ nanowire electrodes. A one-dimensional nanorod structure can greatly shorten the pathways for lithium ion diffusion. The nanoporous carbon coating layer greatly increased the electronic conductivity of the composite. Graphene nanosheets (GNSs) were prepared by a chemical reduction reaction and directly used as cathode catalysts for lithium-oxygen batteries with an alkyl carbonate electrolyte. The as-prepared GNSs electrode exhibited better cyclability and lower over-potential than that of the Vulcan XC-72 electrode. The reduced over-potential shows the as-prepared GNSs, with many carbon vacancies and defects on their surfaces, were more electrochemically active than Vulcan XC-72 in an alky carbonate electrolyte. Mesoporous CoO/CMK-3 nanocomposite was synthesised by an impregnation method using the mesoporous carbon CMK-3 as the template. When used as the cathode catalyst in lithium-oxygen batteries, the as-prepared CoO/CMK-3 nanocomposite electrode exhibied better capacity retention than that of bare mesoporous CMK-3 carbon, Super-P or CoO/Super-P nanocomposite. The mesopores inside the CoO/CMK-3 nanocomposite facilitate the diffusion of oxygen during the discharge process and the release of the charge products during the charging process. The CoO nanoparticles significantly reduce the charge over-potential and increase the round-trip efficiency.
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