Year

2014

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

One of the great challenges in the twenty-first century is unquestionably energy storage. In response to the needs of modern society and emerging ecological concerns, it is now essential to search for new, low-cost, and environmentally friendly energy conversion and storage systems. The performance of these devices depends intimately on the properties of their materials. Nanostructured materials have attracted great interest in recent years because of the unusual mechanical, electrical and optical properties with which such materials are endowed by their specific structures. In this work, materials with unique nanostructured morphologies were prepared, which include germanium, cobalt oxide, manganese oxide, nickel oxide, and tin-antimony alloy. Carbon sources such as amorphous carbon with added graphene were also introduced into the materials to form composites. The as prepared materials have been characterized in terms of their physical properties and energy storage performance.

Mesoporous and hollow germanium@carbon nanostructures were synthesized through simultaneous carbon coating and reduction of a hollow ellipsoidal GeO2 precursor. The formation mechanism of GeO2 ellipsoids and the ratio of Ge4+ to Sn4+ in the starting materials were also investigated. Specifically, in a basic solution, the hydrolysis of Ge4+ and Sn4+ can occur simultaneously to generate insoluble GeO2 and Sn(OH)4 white precipitates. In order to minimise the free energy, the small nanoparticles gradually self-assemble to form large ellipsoids. Simultaneously, gradual dissolution of the Sn(OH)4 templates also takes place due to the basic etching under continuous ultrasonication. When the reaction time reaches 120 min, GeO2 hollow ellipsoidal structures with well-defined interiors and compact shells are eventually formed after complete dissolution of the Sn(OH)4. Sn(OH)4 was selected as the template because it has good material compatibility with GeO2 and can slowly dissolve in a basic solution under continuous ultrasonication, while the GeO2 precipitate can only dissolve in strong basic or acidic solutions; therefore, in principle, the hollow structure can be generated as a result of simultaneous etching of the Sn(OH)4 template. The volume of the hollow interior can also be adjusted by the reaction time under ultrasonication and the ratio of GeCl4 to SnCl4 in the starting materials. In order to gain more insight into the effects of the ratio of GeCl4 to SnCl4 as starting materials on the hollow ellipsoid morphology, a series of ratio-dependent experiments (Ge4+ : Sn4+ = 1:1, 1:0.5, and 1:0, with the resultant samples denoted as GeO2-1,GeO2-2, and GeO2-3, respectively) were conducted. Compared to the solid ellipsoidal Ge@carbon (Ge@C-3) sample, the hollow ellipsoidal Ge@C-1 sample exhibits better cycling stability (100% capacity retention (1285 mAh/g) after 200 cycles at the 0.2 C rate) and higher rate capability (805 mAh/g at 20 C) compared to Ge@C-3 due to its unique hollow structure; therefore, this hollow ellipsoidal Ge@carbon can be considered as a potential anode material for lithium ion batteries.

A novel hierarchical star-like form of Co3O4 was successfully synthesized from self-assembled hierarchical Co(OH)F precursor via a facile hydrothermal method and subsequent annealing in air. The as-prepared hierarchical Co3O4 material displays novel star-like microstructures with a diameter of around 11 􀈝m along every diagonal axis, which are made up of bundled porous nanoneedles. The morphological evolution process of the Co(OH)F precursor was investigated by examining the different reaction times during synthesis. Firstly, hexagonal plates are formed, and then nanodiscs grow on the surface of the plates. Subsequently, dissolution and regrowth of the Co(OH)F occurs to form the star-like hierarchical structures. The Co3O4 obtained from thermal decomposition of the Co(OH)F precursor in air at 350 oC exhibited high reversible capacity as anode material in lithium ion batteries. The specific charge capacity of 1036 mAh/g was obtained in the first cycle at the current density of 50 mA/g, and after 100 cycles, the capacity retention was nearly 100%. When the current density was increased to 500 mA/g and 2 A/g, the capacities were 995 and 641 mAh/g, respectively, after 100 cycles. In addition, a capacity of 460 mAh/g was recorded at current density of 10 A/g in the rate capability test. The excellent electrochemical performance of the Co3O4 electrodes can be attributed to the porous interconnected hierarchical nanostructures, which protect the small particles from agglomeration and buffer the volume change during the discharge/charge processes.

A nanocomposite of Mn3O4 wrapped in graphene sheets was successfully synthesized via a facile, effective, energy-saving, and scalable microwave hydrothermal technique. The reduction of graphene oxide (GO) and the loading of Mn3O4 nanoparticles on the graphene sheets occurred simultaneously, which can avoid the introduction of additional molecular cross-linkers to bridge the nanoparticles and the graphene matrix. The morphology and microstructures of the fabricated reduced GO (rGO)-Mn3O4 nanocomposite were characterized using various techniques. The nanocomposite was then tested in lithium ion batteries and supercapacitors. The results indicate that the particle size of the Mn3O4 particles in the nanocomposite markedly decreased to nearly 20 nm, significantly smaller than for the bare Mn3O4. Electrochemical measurements demonstrated, in terms of lithium energy storage, a high specific capacity of more than 900 mAh/g at 40 mA/g, and excellent cycling stability with no capacity decay could be observed up to 50 cycles. For the supercapacitors, the capacitance value of the rGO(31.6%)-Mn3O4 nanocomposite reached 153 F/g, much higher than that of the bare Mn3O4 (87 F/g) at a scan rate of 5 mV/s in the potential range from -0.1 V to 0.8 V. A 200% increase in capacitance was observed for the nanocomposite with cycling at 10 mV/s due to electrochemical activation and the oxidization of Mn(II,III) to Mn(IV) during cycling, as verified by X-ray photoelectron spectroscopy. There was no observable capacitance fading up to 1000 cycles. All of these properties are also interpreted by experimental studies and theoretical calculations. The facile synthesis method and good electrochemical properties indicate that the nanocomposite could be an electrode candidate for energy storage.

Nanocuboid shaped nickel oxide was synthesized using an optical floating zone furnace. It was found that the nanocuboids exhibit a single crystalline nature and have clean and sharp edges. Furthermore, the nickel oxide nanocuboids were tested for their electrochemical performance as anode material for lithium-ion batteries in coin-type half-cells. The effects of fluoroethylene carbonate additive on the lithium storage performance were also investigated, which is the first such study for transition metal oxides. It was found that fluoroethylene carbonate has a positive effect on the cycling stability and also improves the rate performance of the nanocuboids. The capacity recorded at 0.1 C (100 mA g-1) after 50 charge/discharge cycles is 1400 mAh g-1. The nickel oxide nanocuboids could also achieve a very high rate capability of 12 C (12 A g-1) with capacity of 312 mAh g-1.

SnSb-core/carbon-shell nanotubes directly anchored on graphene sheets (GS) were synthesized by the hydrothermal technique and chemical vapour deposition (CVD). The simultaneous carbon coating and the encapsulation of SnSb alloy is effective for alleviating the volume-change problem both in lithium ion batteries and in sodium ion batteries. After optimizing the electrolyte for SnSb in the sodium ion batteries, the optimized coaxial SnSb/carbon nanotube/GS (SnSb/CNT@GS) nanostructures demonstrated stable cycling capability and rate performance in 1 M NaClO4 with propylene carbonate (PC) + 5% fluoroethylene carbonate (FEC). The SnSb/CNT@GS electrode can retain the capacity of 360 mAh/g for up to 100 cycles, which is 71% of the theoretical capacity. This is higher than in the other three electrolytes tested, and higher than that of the sample without the addition of graphene. The good electrochemical performance can be attributed to the efficient buffer provided by the outer carbon nanotube layer and the graphene protection from the agglomeration of SnSb particles, as well as its high conductivity.

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