Year

2011

Degree Name

Master of Engineering - Research

Department

University of Wollongong. Institute for Superconducting & Electronic Materials

Abstract

Lithium ion batteries are currently the best portable energy storage devices for the consumer electronics market. Large capacity, good cyclability, and no reaction with electrolytes are indispensable characteristics for lithium ion battery materials. In this Master’s research study, several materials were characterized and examined for possible applications as cathode or anode for rechargeable lithium-ion batteries. Among the cathode candidates, copper sulphur (CuS) and lithium trivanadate (LiV3O8) with polyaniline were studied. Tin with polypyrrol was also studied as an anode material candidate for use in rechargeable lithium-ion batteries.

Tin nanoparticle/polypyrrole (nano-Sn/PPy) composite was prepared by chemically reducing and coating Sn nanoparticles onto the PPy surface. The composite shows much higher surface area than the pure nano-Sn reference sample, due to the porous higher surface area of PPy and the much smaller size of Sn in the nano-Sn/PPy composite than in the pure tin nanoparticle sample. Poly (vinylidene fluoride)(PVDF) and sodium carboxymethyl cellulose (CMC) were also used as binders, and the electrochemical performance was investigated. The electrochemical results show that both the capacity retention and the rate capability are in the same order of nano-Sn/PPy-CMC > nano-Sn/PPy-PVDF > nano-Sn-CMC > nano-Sn-PVDF. Scanning electronic microscopy (SEM) and electrochemical impedance spectroscopy (EIS) results show that CMC can prevent the formation of cracks in electrodes during the charge-discharge process, despite the big volume changes, and the PPy in the composite can provide a conducting matrix and alleviate the agglomeration of Sn nanoparticles. The present results indicate that the nano-Sn/PPy composite could be suitable for the next generation of anode materials with relatively good capacity retention and rate capability.

CuS nanoparticles, including nanoflakes, microspheres composed of nanoflakes, microflowers, and nanowires have been selectively synthesized by a facile hydrothermal method using CuSO4 and thiourea as precursors under different conditions. The morphology of CuS particles was affected by the following synthetic parameters: temperature, time, surfactant, pH value, solvent, and concentration of the two precursors. The synthesized CuS nanomaterials were characterized by X-ray diffraction, Brunauer-Emmett-Teller N2 adsorption, SEM, and energy-dispersive Xray spectroscopy. The electrochemical tests, including constant current charge discharge and cyclic voltammetry, show the specific capacities of the different morphologies, as well as their cycling stability. The nanowire electrode presented here has near theoretical specific capacity and relatively stable cycling performance.

A composite, LiV3O8-polyaniline (PANi), suitable for lithium-ion battery cathodes, was synthesized by dispersing LiV3O8 and dissolving PANi powders in N-methyl-2-pyrrolidinone (NMP) followed by heating. Electrochemical impedance measurements showed that the polyaniline significantly decreased the charge-transfer resistance of LiV3O8 electrodes. Charge-discharge properties of composites as cathode materials for lithium-ion batteries were studied. The results indicated that PANi-LiV3O8 had higher discharge capacity and better cycling property. The PANi-LiV3O8 composite with 10 wt% polyaniline showed the best electro chemical performance, with a specific capacity of ~161 mAh g-1 retained after 55 cycles.

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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.