Year

2013

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

Global warming caused by the excessive use of fossil fuels has become a severe problem in the modern world. Increasing energy demand worldwide and mandates to minimize greenhouse gas emissions require the production of energy in a sustainable manner and efficient usage of that energy. Lithium ion batteries (LIBs) have demonstrated themselves to be one of the most promising electrochemical energy storage approaches. During the past two decades, LIBs have become the dominant power source for a wide range of portable electrical devices. The large-scale potential lithium ion battery applications, however, such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and stationary energy storage for solar and wind electrical energy generation, require batteries exhibiting higher rate capability, higher power, and longer cycle life. The performance of rechargeable lithium ion batteries must continue to be improved to meet these requirements. In this doctoral work, several nanostructured materials were synthesized and characterized for possible applications as electrode materials for lithium ion batteries.

Graphene nanosheets have been synthesized by using a time-efficient microwave autoclave method. The effects of the reaction time and temperature on the graphene characteristics were explored. Field emission electron microscopy and transmission electron microscopy analysis demonstrated the different morphologies of graphene nanosheets synthesized under various reaction conditions. Raman spectroscopy further revealed that the layer d-spacing of graphene nanosheets became smaller with increasing reaction time or temperature. These results have broadened our understanding of the synthesis conditions for graphene nanosheets and provided useful information for the synthesis of graphene based composite materials.

Germanium-graphene nanocomposite material with three-dimensional nanostructures has been synthesized by an efficient one-step, in-situ, and aqueous-based method. The electrochemical properties of germanium-graphene nanocomposite have been evaluated by galvanostatic discharge-charge cycling, cyclic voltammetry, and electrochemical impedance spectroscopy. Results show that the germanium-graphene nanocomposite has a much more stable cycling performance than that of the pure germanium, with capacity of about 832 mAh g-1 after 50 cycles. The rate capability is also improved significantly. The superior performance is attributed to the graphene content, which increases the material’s conductivity, enlarges the specific surface area, delivers enough sites to allow dispersion of the Ge nanoparticles without excessive agglomeration, and provides void space to buffer the volume change during discharge/charge cycles.

Fe3O4-graphene composites with three dimensional laminated structures have been synthesised by a simple in-situ hydrothermal method. According to the field emission electron microscopy and transmission electron microscopy analysis, the Fe3O4 nanoparticles, around 3-15 nm in size, are highly encapsulated in a graphene nanosheet matrix. The reversible Li-cycling properties of Fe3O4-graphene have been evaluated by galvanostatic discharge-charge cycling, cyclic voltammetry, and impedance spectroscopy measurements. Results show that the Fe3O4-graphene nanocomposite with graphene content of 38.0 wt % exhibits a stable capacity of about 650 mAh g-1, with no noticeable fading up to 100 cycles in the voltage range of 0.0-3.0 V. The superior performance of Fe3O4-graphene is clearly established by comparison of the results with those from bare Fe3O4. The graphene nanosheets in the composite materials could act not only as lithium storage active materials, but also as an electronically conductive matrix to improve the electrochemical performance of Fe3O4.

SnO2-graphene composites have been synthesized by an ultra-fast and environmentally friendly microwave autoclave method. From field emission scanning electron microscopy and transmission electron microscopy, it can be determined that the SnO2 nanoparticles, around 4-5 nm in diameter, are uniformly sandwiched in between graphene nanosheets in stacks of only a few layers. The successful synthesis demonstrates that in-situ loading of SnO2 nanoparticles can be an effective way to prevent graphene nanosheets from being restacked during the reduction. The Li-cycling properties of the materials have been evaluated by galvanostatic discharge-charge cycling and impedance spectroscopy. Results show that the SnO2-graphene composite with graphene content of 33.3 wt% exhibits a very stable capacity of about 590 mAh g-1 without noticeable fading for up to 200 cycles.

Finally, the application of microwave autoclave synthesized graphene nanosheets has been extended to the formation of composites with single-walled carbon nanotubes as free-standing film electrodes that can serve as flexible lithium ion battery anodes. According to the transmission electron microscopy and X-ray diffraction characterizations, the average d-spacing of the graphene–single-walled-carbonnanotube composites reached 0.407 nm, which was obviously larger than that of the as-prepared pure graphene (0.364 nm).The reversible Li-cycling properties of the free-standing films have been evaluated by galvanostatic discharge-charge cycling and electrochemical impedance spectroscopy. Results showed that the free-standing composite film with 70 wt% graphene exhibited the lowest charge transfer resistance and the highest capacity of about 308 mAh g-1 after 50 cycles, without any noticeable fading.

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