Year

2007

Degree Name

Doctor of Philosophy

Department

nstitute for Superconducting and Electronic Materials - Faculty of Engineering

Abstract

The commercially available lithium-ion cells, which are the most advanced among the rechargeable battery systems available so far, employ polycrystalline microsized powder as the electrode materials, which functions as the Li-ion insertion hosts. With the advancement of nanotechnology, there is an interest in the replacement of conventional materials by nanostructured materials. The use of nanoparticles in composite electrodes for Li-ion batteries may have considerable kinetic advantages due to the reduction of the diffusion length for lithium-ion insertion into the active mass, and also because of the reduction of the overall charge transfer resistance of the electrodes. In this doctoral work, several nanostructured materials were examined and characterized for possible application as electrode materials in Li-ion rechargeable batteries. Among the anode candidates studied were free-standing single-walled carbon nanotube (SWCNT) paper, lead oxide (PbO) and lead oxide-carbon (PbO-C) nanocomposite, and carbon-coated silicon (Si-C) nanocomposite materials. Meanwhile, several cathode candidates were also studied: nanostructured vanadium oxide (V2O5), lithium trivanadate (LiV3O8) nanoparticles, and lithium manganese oxide (LiMn2O4) thin film electrode.

Free-standing SWCNT paper electrodes have been synthesized by a simple filtration method via positive pressure. The free-standing electrode was produced without any binder or metal substrate, which reduced the weight significantly. The free-standing SWCNT paper electrodes were also flexible and had good electrical conductivity. With the addition of both carbon black and nanosized Si particles, the electrical conductivity and specific capacity of the free-standing SWCNT paper electrode were greatly enhanced, so that they retained a capacity of 400 mAh g-1 beyond 100 cycles. A new approach has been used to prepare nanostructured PbO and PbO-C composites via the spray pyrolysis technique. The prepared powders consist of fine nanocrystalline PbO homogeneously distributed within an amorphous carbon matrix with highly developed surface area. The combination of spray technology and carbon addition increased the specific surface area (above 6 m2 g-1) and the conductivity of PbO, and also improved the specific capacity, with a reversible capacity above 100 mAh g-1 retained beyond 50 cycles. An effective, inexpensive, and industrially oriented approach was applied to produce carbon-coated Si nanocomposites. Carbon-coated Si nanocomposites spraypyrolyzed in air at 400 oC showed the best cycling performance, retaining a specific capacity of 1120 mAh g-1 beyond 100 cycles, with a capacity fading of less than 0.4 % per cycle. The beneficial effect of the carbon-coating in enhancing the dimensional stability of the Si nanoparticles appears to be the main reason for this markedly improved electrochemical performance.

One-dimensional (1D) nanostructures of V2O5 have been successfully synthesized via a precipitation process followed by heating in vacuum at 300 oC. The increase in crystallinity and higher yield of one-dimensional nanostructured oxides contributed significantly to the improved capacity and enhanced cycle life. V2O5 nanoparticles were also synthesized via the flame spray pyrolysis (FSP) process in air. They showed an improved cycle life when the cut-off potential for discharging was increased from 1.5 Vto 2.5 V. The significant capacity loss when discharging to 1.5 V is possibly related to the dissolution of vanadium active mass and the structural changes upon cycling in the larger potential span. The flame spray pyrolyzed V2O5 nanoparticles show excellent cyclability when cycled between 2.5 V and 4.0 V vs. Li/Li+, retaining a discharge capacity of 120 mAh g-1 beyond 100 cycles at a cycling rate of 100 mA g-1. LiV3O8 nanoparticles (~24 nm in size) have been synthesized by FSP for the first time. The assynthesized LiV3O8 nanoparticles proved to be a promising cathode material for lithium rechargeable batteries, retaining a specific discharge capacity of 180 mAh g-1 beyond 50 cycles. A series of LiMn2O4 thin films on either Si (100) or stainless steel substrate were successfully prepared via pulsed laser deposition (PLD). The as-deposited LiMn2O4 thin films on stainless steel substrate are highly lithium- and oxygen-deficient, as confirmed by ERDA/RBS and Raman analysis. Lithium and oxygen content increased when the pulse rate was increased, leading to thicker films. However, the LiMn2O4 thin film with the lowest deposition pulse rate (or thinnest film) exhibited the best electrochemical performance, retaining a charge capacity of 48 μAh cm-2 μm-1 beyond 100 cycles.

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