Year

2012

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

Lithium-ion batteries are essential to modern life as power sources for a wide spectrum of devices ranging from portable electronic devices to electric automobiles. There is currently an enormous ongoing research effort aimed at developing ultrathin, flexible, and soft batteries to cater for the bendable modern devices. Although there is a high demand for flexible lithium-ion batteries, these must satisfy stringent requirements, including larger reversible capacity, smaller size, lighter weight, mechanical stability, and long cycle life. Advanced nanotechnology of materials provides the main solutions to these issues. Improved battery performance depends on the development of materials for the various battery components, with the key aspect of improving the performance of the active materials used to fabricate the anode and cathode. The use of nanostructured and conductive composite materials is designed to enhance both ion transport and electron transport by shortening the diffusion lengths of ions and increasing the conductivity within the whole electrode. In this doctoral study, several nanostructured and conductive composite materials were examined and characterized for possible application as electrodes for flexible lithium-ion batteries. With these aims, nanocrystalline SnO2-coated multiwall carbon nanotubes, free-standing SnO2 − single wall carbon nanotubes, polypyrrole-coated V2O5 nanowires, and stacked graphene with MoO3 nanobelts were investigated.

SnO2-coated multiwall carbon nanotube (MWCNT) nanocomposites were synthesized by a facile hydrothermal method. Field emission scanning electron microscope (FE-SEM) images show that deposition of SnO2 onto the surfaces of the MWCNTs takes place in some selected sites, while in the composites with higher content of SnO2, more SnO2 particles are deposited on the surfaces of MWCNTs.

The SnO2/MWCNT composites, when combined with carboxymethyl cellulose (CMC) as a binder, show excellent cyclic retention, with the high specific capacity of 473 mAh g-1 beyond 100 cycles, much greater than that of the bare SnO2 which was also prepared by the hydrothermal method in the absence of MWCNTs. The enhanced capacity retention could be mainly attributed to good dispersion of the tin dioxide particles in the matrix of MWCNTs, which protected the particles from agglomeration during the cycling process. Furthermore, the usage of CMC as a binder is responsible for the low cost and environmental friendliness of the whole electrode fabrication process.

Free-standing single-walled carbon nanotube/SnO2 (SWCNT/SnO2) anode paper was prepared by vacuum filtration of SWCNT/SnO2 hybrid material which was synthesized by the polyol method. From FE-SEM and transmission electron microscope (TEM) images, the CNTs form a three-dimensional nanoporous network, with the ultra-fine SnO2 nanoparticles, which had crystallite sizes of less than 5 nm, distributed predominately as groups of nanoparticles on the surfaces of single walled CNT bundles. Electrochemical measurements demonstrated that the anode paper with 34 wt.% SnO2 had excellent cyclic retention, with the high specific capacity of 454 mAh g-1 beyond 100 cycles at a current density of 25 mA g-1, much higher than that of the corresponding pristine CNT paper. The SWCNTs could act as a flexible mechanical support for strain release, as well as offering an efficient electrically conducting channel, while the nanosized SnO2 provides the high capacity. The SWCNT/SnO2 flexible electrodes can be bent to extremely small radii of curvature and still function well, despite a marginal decrease in the conductivity of the cell. The electrochemical response is maintained in the initial and subsequent cycling. Such capabilities demonstrate that this model holds great promise for applications requiring flexible and bendable Li-ion batteries.

Highly flexible, paper-like, free-standing V2O5 and V2O5-polypyrrole (PPy) films were prepared via the vacuum filtration method. The films are soft, lightweight, and mechanically robust. FE-SEM images of the pristine V2O5 film show straight nanowires ~80-120 nm in diameter and several microns in length, resulting in an aspect ratio of ~102-103. On the other hand, the V2O5-PPy film shows similar morphology to the V2O5 film, with the PPy uniformly deposited throughout entire lengths of nanowires in irregular or spherical shapes. The electrochemical performance of the free-standing pure V2O5 electrode was improved by incorporating conducting polypyrrole. A bendable cell with a novel design was fabricated, consisting of a free-standing V2O5-PPy cathode film, gel electrolyte, and a lithium foil anode. The cell was tested under repeated bending conditions for several cycles. The results show that the battery performance of the repeatedly bent cell was similar to that of a comparable conventional cell.

Highly flexible, binder-free, MoO3 nanobelt/graphene film electrode was prepared by a two-step microwave hydrothermal method. Graphene is first prepared by an ultra-fast microwave hydrothermal method and then mixed with MoO3 solution to synthesize the MoO3 nanobelt/graphene composite, which exhibits the combination of stacked graphene sheets and uniform MoO3 nanobelts with widths of 200-500 nm and lengths of 5-10 μm. In the charge-discharge measurements, the assynthesized MoO3/graphene hybrid materials demonstrated excellent rate capability, large capacity, and good cycling stability compared to the pure MoO3 film. An initial discharge capacity of 291 mAh g-1 can be obtained at 100 mA g-1, with a capacity of 172 mAh g-1 retained after 100 cycles. The results show that the MoO3/graphene designed in this study can be used as a free-standing cathode material in rechargeable and bendable lithium batteries.

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