Year

2010

Degree Name

Doctor of Philosophy

Department

University of Wollongong. Institute for Superconducting & Electronic Materials

Abstract

Since the commercialization of the lithium ion battery in the 1990s, it has been rapidly developed as a novel energy storage and conversion system. So far, the lithium ion battery has dominated the global market for electronic devices such as cell phones, laptops, digital cameras, and video devices. Portable power applications continue to drive research and development of advanced battery systems. Moreover, the rechargeable lithium ion battery is considered as the most promising power system for electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicle (PHEVs).

However, the next generation battery for electric vehicles requires a high power system with outstanding cycle life. The current lithium ion battery systems available are operated at low charge/discharge currents, which can not match the demands on the next generation battery. Nanotechnology has paved the way for advanced energy materials to achieve high power performance. Herein, we have developed a series of nanostructured materials with the morphologies of nanorods, nanowires, and mesoporous (2-dimensional hexagonal and 3-dimensional cubic) shapes. Also, we have modified the surfaces of materials by using nanotechnology to prepare high performance electrode materials, including Fe3O4/C core-shell and ZrO2 nanolayer coated LiFePO4 materials.

One-dimensional materials, including Fe2O3 single crystal nanorods and polycrystalline nanowires, were synthesized by a facile hydrothermal method. These both exhibit excellent electrochemical performance as anode materials for lithium ion batteries. The discharge capacity of Fe2O3 nanorod electrode is 763 mAh/g after 30 cycles. For Fe2O3 nanowire electrode, the discharge capacity after 50 cycles is 811 mAh/g.

Novel highly ordered mesoporous structure oxides (Co3O4, NiO) were obtained by the nanocasting method (hard template). The initial discharge capacity of the cubic mesoporous Co3O4 replica was 1760 mAh/g at the testing current of 50 mA/g. After 100 cycles, the discharge capacity was still above 1200 mAh/g. Mesoporous cubic NiO prepared from the KIT-6 template also presents excellent electro chemical properties for lithium storage. The discharge capacity after 50 cycles is 680 mAh/g, which is much higher than for the commercial bulk NiO (188 mAh/g). At the highcharge/discharge rate of 2 C, the discharge capacity of mesoporous NiO is 515mAh/g with good retention. The enhancement of these mesoporous materials is attributed to their unique mesostructure. The mesopores can act as tunnels for electrolyte inflow, ensuring a high contact surface area between the electrode and electrolyte. The nanosize wall thickness ensures short pathways for lithium ion diffusion.

Core-shell structured materials were developed to achieve high electro chemical performance. The shell structure can protect the core material from volume change (pulverization) during the charge/discharge process. Furthermore, the coating can enhance the thermal stability, reduce charge transfer resistance, and depress the polarization of the electrode during electrochemical testing. Herein, Fe3O4/C coreshell and ZrO2 nanolayer coated LiFePO4 materials were prepared for lithium ion batteries. From electrochemical measurements, both of them exhibit excellent electrochemical performance for lithium storage.

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