Year

2014

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

Lithium ion batteries (LIBs) have been regarded as the most successful
electrochemical power sources for a wide range of applications, including consumer
devices, portable electronics, electric vehicles, and renewable energy storage, due to
their potential for high power density and high energy density. Tremendous efforts
have been made towards even further improving their high capacity, excellent rate
capability, and cycling stability by developing novel cathode and anode materials to
meet the increasing power-supply requirements. In this thesis, a series of electrode
materials, including germanium, germanate, cobalt oxide, titanium dioxide, and
lithium iron phosphate have been synthesized, and their physical and electrochemical
performances were investigated.

Ultra-fine Co3O4 nanocrystals homogeneously attached to nitrogen-doped reduced graphene oxide (rGO) by the hydrothermal reaction method have been demonstrated as promising anode materials for the lithium ion battery. Transmission electron microscope images revealed that the crystal size of Co3O4 in Co3O4/N-rGO and Co3O4/rGO is 5-10 nm, much smaller than that of bare Co3O4, indicating that the reduced graphene oxide sheets with Co3O4 nanocrystals attached could hinder the growth and aggregation of Co3O4 crystals during synthesis. The graphene sheets can also effectively buffer the volume changes in Co3O4 upon lithium insertion/extraction, thus improving the cycling performance of the composite electrodes. The doped nitrogen on the reduced graphene oxide can not only improve the conductivity of the graphene sheets, but also introduces defects to store lithium and enhance the connection of the Co3O4 nanocrystals to the graphene sheets, leading to better distribution of Co3O4 on the graphene sheets and enhanced rate performance. The nitrogen doping combined with these unique structural features is a promising strategy for the development of electrode materials for lithium ion batteries with high electrochemical performance. Anatase TiO2 nanoparticles grown in situ on nitrogen-doped reduced graphene oxide have been successfully synthesized as an anode material for the lithium ion battery. The nanosized TiO2 particles were homogeneously distributed on the reduced graphene oxide and inhibited the restacking of the neighboring graphene sheets. The obtained TiO2/N-rGO composite exhibits improved cycling performance and rate capability, indicating the important role of reduced graphene oxide, which not only facilitates the formation of uniformly distributed TiO2 nanocrystals, but also increases the electrical conductivity of the composite material. The introduction of nitrogen on the reduced graphene oxide has been proved to increase the conductivity of the rGO and leads to more defects. A disordered structure is thus formed to accommodate more lithium ions, thereby further improving the electrochemical performance.

A unique sandwich-structured C/Ge/graphene composite with germanium nanoparticles trapped between graphene sheets was prepared by microwave-assisted solvothermal reaction, followed by carbon coating and thermal reduction. The graphene sheets are found to be effective in hindering the growth and aggregation of GeO2 nanoparticles. More importantly, the graphene sheets, coupled with the carbon coating, can buffer the volume changes of germanium in electrochemical lithium reactions. The unique sandwich structure features a highly conductive network of carbon, which can improve both the conductivity and the structural stability of the electrode material, and exemplifies a promising strategy for the development of new high performance electrode materials for lithium ion batteries. The C/graphene/Ge composite displayed a high-rate capability of 746.3 mA h g-1 at a high rate of 20 C and high reversible specific capacity of 992.8 mA h g-1 after 160 cycles at the rate of 1 C. Another novel germanium-carbon composite, consisting of hollow carbon spheres with encapsulated germanium (Ge@HCS), was synthesized by introducing germanium precursor into the porous-structured hollow carbon spheres. The carbon spheres not only function as a scaffold to hold the germanium and thus maintain the structural integrity of the composite, but also increase the electrical conductivity. The voids and vacancies that are formed after the reduction of germanium dioxide to germanium provide free space for accommodating the volume changes during discharging/charging processes, thus preventing pulverization. The obtained Ge@HCS composite exhibits excellent lithium storage performance, as revealed by electrochemical evaluation. The Ge@HCS showed good cycling stability at the 0.4 C rate for 100 cycles and a high rate capability up to 20 C. Furthermore, urchin-like Ca2Ge7O16 hierarchical hollow microspheres have been obtained through a facile solvothermal method. The growth mechanism is proposed based on our experimental results on the growth process. Analysis of the electrochemical performance in different electrolytes shows that ethylene carbonate/dimethyl carbonate/diethyl carbonate (3/4/3 by volume) with 5 wt% fluoroethylene carbonate additive is the most suitable solvent for the electrolyte. From the electrochemical evaluation, the as-synthesized Ca2Ge7O16 hollow microspheres exhibit high reversible specific capacity of up to 804 mA h g-1 at a current density of 100 mA g-1 and remarkable rate capability of 341 mA h g-1 at a current density of 4 A g-1. These excellent lithium storage properties are attributed to the unique urchin-like morphology and hollow structure, due to the short lithium ion diffusion distance, large surface area, high density of active sites, very good permeability, and good structural integrity.

A composite cathode material for lithium ion battery applications, Mo-doped LiFePO4/C, was obtained through a facile and fast microwave-assisted synthesis method. Rietveld analysis of LiFePO4-based structural models using synchrotron X-ray diffraction data shows that Mo-ions substitute onto the Fe sites and displace Fe-ions to the Li sites. Supervalent Mo6+ doping can act to introduce Li ion vacancies due to the charge compensation effect and thereby facilitates lithium ion diffusion during charging/discharging. Transmission electron microscope images demonstrated that the pure and doped LiFePO4 nanoparticles were uniformly covered by an approximately 5 nm thin layer of graphitic carbon. Amorphous carbon on the graphitic carbon-coated pure and doped LiFePO4 particles forms a three-dimensional (3D) conductive carbon network, effectively improving the conductivity of these materials. The combined effects of Mo-doping and the 3D carbon network dramatically enhance the electrochemical performance of these LiFePO4 cathodes. In particular, Mo-doped LiFePO4/C delivers a reversible capacity of 162 mA h g-1 at a current density of 0.5 C and shows enhanced capacity retention compared to that of un-doped LiFePO4/C. Moreover, the electrode exhibits excellent rate capability, with an associated high discharge capacity and good electrochemical reversibility.

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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.