Year

2011

Degree Name

Doctor of Philosophy

Department

University of Wollongong. Institute for Superconducting & Electronic Materials

Abstract

The objectives of the PhD project were: (1) to prepare powder and thin film transition metal oxide active materials and examine their electrochemical performance in term of capacity and cycle life for rechargeable lithium ion batteries; (2) to understand the relationship between the morphology and the electrochemical and physical properties of the as-prepared materials; (3) to find the relationship between the synthesis conditions and the morphology of the as-prepared materials. In this study, several transition-metal oxides were fabricated by the hydrothermal, electrospinning, and electrostatic spray deposition methods.

The starting point of the study was to review the literature pertaining to both anode and cathode materials for lithium ion batteries. The synthesis methods, including electrospinning and electrostatic spray deposition, were also studied. The details of the current research status and developments are described in this thesis.

A variety of anode materials have been fabricated and investigated. Α series of α-Fe2O3 compounds with different morphologies have been synthesized by the hydrothermal method under different conditions, including temperature, pressure and precursor concentration. It was found that the electrochemical properties are highly related to the morphology. Morphologies with high specific surface areas can significantly improve the performance of batteries. By selecting the hydro thermal conditions, submicron-size cuboidal form of α-Fe2O3 has been synthesized and tested.

The outstanding electrochemical results are related to its unique morphology. NiO/Co3O4/C composite was also fabricated by the hydrothermal method to examine the influences of the Ni:Co ratio and of carbon on its electrochemical properties in lithium ion batteries. It was found that better performance is achieved by a Ni/Co ratioof 1:2 and that carbon plays an important role in increasing the conductivity of the electrode material and favouring the diffusion of ions.

Electrostatic spray deposition (ESD) was applied to form three-dimensional thin film electrode. The porous reticular structures of Li2O-NiO-CoO composite and tin-cobaltoxide composite are responsible for the high reversible capacity, excellent rate capability, and long cycle life. The morphology of Li2O-NiO-CoO composite is maintained after 25 cycles.

One dimensional cobalt oxide/carbon nanowires were synthesized by the electrospinning method. The electrochemical properties of the composite were characterized using cyclic voltammetry and galvanostatic methods. The result shows a remarkably improved electrochemical performance in term of reversible capacity, ratecapability, and cycling performance.

X-ray diffraction, scanning electron microscopy, and transmission electron microscopy were carried out to investigate the physical properties of the as-prepared materials. The capacity and cycle life of the electrode materials were examined bycharge/discharge cycling tests. The kinetic characteristics of lithium insertion and extraction were determined by AC impedance spectroscopy and cyclic voltammograms.

In summary, the project has been focused on the synthesis and characterization of transition metal oxides as anode materials in lithium ion batteries. The synthesis methods were the hydrothermal method, the electrospinning method, and electrostaticspray deposition (ESD). By controlling the synthesis conditions, different morphologies can be obtained, which result in different electrochemical performances. All of these studies provide a fundamental basis for the development of high performance lithium ion batteries.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.