Year

2014

Degree Name

Doctor of Philosophy

Department

Institute of Superconducting and Electronic Materials

Abstract

Worldwide efforts to reduce the consumption of fossil fuels and to preserve the environment have been promoted the development of renewable energy systems which require energy storages to preserve electric energy. Lithium rechargeable battery is one of promising storage devices because of high energy and power density and reliable safety. Ti-based oxides are attractive candidates for use as an anode for lithium ion batteries due to low cost, excellent functionality, non-toxicity, thermal stability, and environmental friendliness. The morphology, phase formation and electronic structure of Ti-based oxide materials on the nanoscale definitely determine their electrochemical properties in energy storage systems. In this thesis, one-dimensional nanostructured Ti-based oxides, including nitrogen-doped TiO2, Ag-Li4Ti5O12 Zr-doped Li4Ti5O12 and a core-shell Si@TiO2 fibrous 1D Ag (1.75 at%)-Listructure, were fabricated and their morphology, phase formation, electronic structure and electrochemical properties were investigated for application in anodes of lithium ion batteries.

1D Ag (1.75 at percent)-Li4Ti5O12 composite nanofibers showed enhanced specific capacity, rate capability, and cycling stability compared to bare Li4Ti5O12 nanofibers, due to the Ag nanoparticles (

Incorporating a small fraction of Zr4+ ions in the Ti4+ sites of the Li4Ti5O12 led to enhanced lithium ion battery performance, which was due to the structural distortion through an increase in the average lattice constant, and thereby, enlarged Li+ diffusion paths, rather than changes to the electronic structure. However, insulating ZrO2 nanoparticles that were present between the Li4Ti5O12 grains due to the low Zr4+ solubility had a negative effect on the Li+ In the case of nitrogen-doped TiO extraction capacity.

In the case of nitrogen-doped TiO extraction capacity. 2 nanofibers, it was speculated that substitutional nitrogen would be spread widely in the bulk, while interstitial nitrogen atoms were mostly located near the surface. The nitrogen-doped TiO2 nanofibers showed the improved cyclic retention and rate capability compared to TiO2 nanopowders and TiO2 nanofibers. This result was due to higher conductivity and faster Li+ diffusion of the 1D nanostructure. For core-shell structured Si-nanoparticles@TiO2-x/C composite microfibers, the core-shell composite exhibits high reversible capacity, excellent rate capability and improved cycle performance were found. In addition, the exothermic behaviour was remarkable suppressed, which can prevent possible thermal runaway and safety problems of the cells. The improved electrochemical and thermal properties are attributed to the mechanically, electrically, and thermally robust architecture of the TiO2-x/C nanocomposite encapsulating the Si-nanoparticles. These findings could provide promising material architectures for robust, high performance Li4Ti5O12- and TiO2-based anodes of lithium rechargeable batteries.

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