Year

2017

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials, Mechatronic and Biomedical Engineering

Abstract

Lithium-ion batteries (LIBs) have become an essential commodity ever since their commercialization in the 1990s to power portable electronic devices such as laptop computers, mobile phones, etc. This is mainly due to the LIB‘s ability to store and deliver high energy and power densities more competitively than or equivalently to the fast depleting, non-recyclable fossil fuels. Nevertheless, they require a paradigm shift to make them suitable for powering plug-in electric vehicles and as an alternative to power grids to minimize the energy loss by transmission. The present state-of-the-art LIB containing ‘graphite’ anode and ‘layered LiCoO2’ cathode, with Li-ions mobilized by organic electrolyte, has limited energy density, however, and raises serious safety issues. So, the demand for high energy density and power density anode and cathode materials with a solid electrolyte layer sandwiched between them could be an ideal engineering design for future safe plug-in electric vehicles.

To date, layered graphite has been widely used as an anode material in LIBs ever since its launch in the 1990s, but its limited theoretical capacity of 372 mA h g-1 and the very low diffusion coefficient of lithium in graphite (10-9 to 10-7 cm2 s-1) restrict its use in high energy applications such as plug-in electric vehicles. Therefore, the anode of the battery is the key component in a rechargeable battery with such high energy density. Alternatively, metallic lithium would be an ideal anode, but it has safety problems resulting from anode dendrite formation. This growth from the metallic-lithium anode, when it is used in conjunction with an organic-liquid electrolyte, has resulted in the development of 'conversion-reaction' based non-layered compounds (such as transition metal oxides, nitrides, fluorides, sulphides, phosphides, and even hydrides), as they offer numerous advantages, including multiple electron transfer, the ability to tune the redox centre based on anions of transition metal compounds, and most importantly, their capability to recover their original phase upon reversing the polarity. This reaction results in fast capacity fade, however, due to the stress induced by accommodating the volume changes during cycling and the sluggish reaction kinetics upon charge transfer, while the intrinsic structural changes could damage the electrode when it is cycled at high current densities. Enormous efforts were made in past decades to circumvent these disadvantages by tuning their morphologies and particle size, but even so, fabricating a durable conversion electrode exhibiting superior reversible energy and power densities remains a great challenge. The use of blended nanostructures, wherein nanostructured active electrode materials are chemically or non-covalently bonded to conductive materials, has proved to be an effective method for achieving high performing electrode materials for LIBs by improving their electrical conductivity and electron transfer. Although the results have been encouraging, there are still issues that haunt the electrochemical performance of these composites. This is mainly due to the random/improper distribution of active materials (AM) with uneven particle sizes over carbonaceous materials, leading to poor synergy with no change in electrical conductivity and, therefore, no effect on their overall electrochemical performance. There are also limits to the high loading of AM into the composites. As the composites have had a high weight ratio of carbonaceous materials to AM, the operating voltage was reduced to a level similar to that of traditional graphite, further impeding understanding of the AM mechanism of energy storage and its contributions towards overall electrochemical performance. Therefore, in this thesis, the work is built on a strategy that could transform bulk AM into well-defined two-dimensional (2D) nanostructured AM to increase the edge density of its inert basal planes for use as the sole active anode material, followed by construction of electrodes with a three-dimensional (3D) architecture consisting of 2D nanostructured AM sandwiched between low/negligible quantities (≤20 wt.%) of conductive reduced graphene oxide (rGO) for long-term stable lithium storage.

FoR codes (2008)

0904 CHEMICAL ENGINEERING, 0912 MATERIALS ENGINEERING, 1007 NANOTECHNOLOGY

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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.