Year

2011

Degree Name

Doctor of Philosophy

Department

Faculty of Engineering

Abstract

Lithium ion batteries have served as power sources for portable electronic devices for the past two decades. To date, they have employed polycrystalline microsized powder electrode materials. However, many next-generation electronic devices or wireless communication devices demand thin and flexible electrodes with higher energy density than ever before. Moreover, the large-scale potential lithium ion battery applications, such as in electric vehicles, (plug-in) hybrid electric vehicles, or energy storage systems in smart grids, require batteries exhibiting high rate capability, high power, and long cycle life. Due to the advantages of nanostructured electrode materials, i.e., high surface area, more lithium active sites, and shorter lithium diffusion length, the electrochemical performance of nanomaterial electrodes are more likely to meet the specific requirements in the potential new applications. In this doctoral work, various nanostructured materials were synthesized, characterized by different physical techniques and tested as potential anode electrode materials for lithium ion batteries. The nanomaterials include porous SnO2, SnO2/C composite, one-dimensional SnO2/carbon nanotube (CNT) composite, SnO2 nanofibre, SnO2/C composite nanofibre, restacked MoS2, MoS2/SnO2 composite, one-dimensional TiO2(B) nanowire, three-dimensional TiO2 nanotube arrays, SnO2 nanocrystal/TiO2 nanotube array composite, and nanosized polycrystalline Li4Ti5O12.

Various nanostructured SnO2 and SnO2/C composites were prepared by the molten salt, solvothermal, and electrospinning techniques. The porous SnO2 and SnO2/C nanocomposite prepared by the molten salt method exhibit high surface area, giving more contact area between the active material and the electrolyte, as well as a decreased lithium diffusion length. At the same time, the pores could accommodate the volume expansion. Porous SnO2 electrode delivers a reversible capacity of 410 mAh g-1 after 100 cycles in the voltage range of 0.05-1.5 V, while the composite shows better capacity retention (85.3 wt%) than bare nano-SnO2 (64.8 wt%) after 100 cycles. SnO2/CNT composite synthesized by the solvothermal method consists of a conductive CNT core and SnO2 nanocrystals about 5 nm in size that are deposited and pinned onto the CNTs. Very large area uniform SnO2 and SnO2/C composite nanofibre with fibre diameters around 80 nm, consisting of orderly bonded SnO2 nanoparticles ~10 nm in size have been obtained by the electrospinning technique and a thermal pyrolysis process under optimized synthesis conditions. The uniformly distributed carbon greatly improved the electrochemical performance even at high rate.

Restacked MoS2 with an enlarged c-axis parameter was prepared by exfoliation and then restacking in a hydrothermal process. The enlarged c parameter and the increased surface area are favourable to the intercalation reaction. The restacked MoS2 anode exhibited large reversible capacity of about 800 mAh g-1 and stable cycling performance, as well as good rate capability. A similar strategy was applied to prepare MoS2/SnO2 composite by exfoliation and restacking of commercial MoS2 with SnO2 nanocrystals ~ 5 nm in size deposited between the MoS2 layers.

One-dimensional TiO2(B)/anatase nanowires were synthesized by the hydrothermal method, which delivered a high reversible capacity of 196 mAh g-1 up to 100 cycles at 30 mA g-1 (0.1 C). It also exhibited a reversible discharge capacity as high as 125 mAh g-1, even when cycled at 4500 mA g-1 (15 C). Three-dimensional TiO2 nanotube arrays were prepared by anodization, and then SnO2 nanocrystals were deposited into/onto the TiO2 nanotube array to make a composite anode in which the TiO2 could serve both as an electroactive material, as mechanical support, and as a buffer to accommodate SnO2 volume expansion during the charge/discharge process. The much increased capacity is due to the SnO2. The total capacity depends on the TiO2 tube length and the amount of SnO2 loading.

Carbon-incorporated Li4Ti5O12 nanocrystals were synthesized by spray pyrolysis and examined as a promising anode material. The molecular level mixing of the organic lithium and the titanium precursor allows a shorter annealing time afterwards at high temperature, which is energy saving in large-scale production. Furthermore, annealing in N2 atmosphere preserved the carbon from the organic precursor and distributed it uniformly, which could improve the conductivity. The Li4Ti5O12 electrode exhibits excellent cyclability and rate capability, as well as stable cycling in full battery tests. Br-doped Li4Ti5O12 was investigated by synchrotron X-ray diffraction to understanding the structural and impurity effects on the electrochemical behaviour. Because TiO2 impurity always appears in the Li4Ti5O12 preparation process, Li4Ti5O12/TiO2 composite anode was investigated by in-situ neutron diffraction to understand the electrochemical behaviour.

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