Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Xueliang Sun

Abstract

Lithium-ion batteries (LIBs) are promising energy storage media under serious consideration for practical applications in electric vehicle (EVs) and hybrid electric vehicles (HEVs). However, to meet the requirements for EVs and HEVs, the performance of commercially available LIBs needs to be greatly improved in terms of the energy density, cycling life, rate capability, safety and cost. It is well known that the LIB performance is highly dependent on the choice of electrode materials. Therefore, it is greatly important to develop new electrode materials as replacements for graphite/LiCoO2 used in commercial LIBs, in order to achieve high-performance LIBs desirable for EV and HEV applications.

In this thesis, to achieve the above goal, efforts made in this thesis followed into two sections. The first section was to develop novel nanostructured electrode materials, which could be directly used in LIBs. The other section was to develop various surface-modification materials, which could be applied to further improve the LIB performance of electrode materials. Various advanced characterization techniques, including field-emission scanning electron microscope (FE-SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscope (TEM), high-resolution TEM (HRTEM), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform-infrared spectroscopy (FT-IR), X-ray absorption near edge structure (XANES) spectroscopy, and electrochemical methods, have been applied to analyze the prepared nanomaterials, understand their growth mechanisms, and evaluate their battery performance.

The nanostructured electrode materials included nitrogen-doped carbon nanotubes (NCNTs), phosphorus-nitrogen doped carbon nanotubes (PNCNTs), and lithium titanate (Li4Ti5O12). A scalable method, ultrasonic spray pyrolysis, was developed inhouse to produce NCNTs with tunable structure as potential anode materials. Further attempt to incorporate P element into CNTs was made, and it was successful when P and N elements were doped together. The P doping effect on the structure of NCNTs was investigated in details. Furthermore, novel nanosctuctured Li4Ti5O12 were prepared by a microwave-assisted hydrothermal method in a fast and energy-efficient way. Their electrochemical performances were evaluated, and nanoflower-like Li4Ti5O12 showed better LIB performance than nanoparticle Li4Ti5O12.

Three different surface-modification materials, ZrO2, AlPO4 and LiTaO3 solid-state electrolyte, were developed by atomic layer deposition (ALD), for potential use to improve the chosen electrode materials. Deposition of these materials on different substrates, including NCNTs, graphene nanosheets, Si (100) and anodic aluminum oxide (AAO) template, showed that as-grown thin films of ZrO2, AlPO4 and LiTaO3 were precisely controllable in terms of film thickness, film crystallinity and film composition. These characteristics enabled by ALD promised ZrO2, AlPO4 and LiTaO3 great potentials as surface-modification materials. One application example of these materials was demonstrated by using ALD-ZrO2 coating to enhance the performance of nanoflower-like Li4Ti5O12.

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