Purification, processing, and separation of carbon nanotubes and application in lithium ion batteries

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Title: Purification, processing, and separation of carbon nanotubes and application in lithium ion batteries
Author: Ganter, Matthew
Abstract: Single wall carbon nanotubes' (SWCNTs) unique structural and electronic properties have made them an ideal candidate for use in electrochemical devices like lithium ion batteries, PEM fuel cells, and supercapacitors. SWCNTs were investigated as free-standing anodes, current collector supports, and conductive additives in lithium ion batteries. Before incorporation into batteries, the effect of organic solvents on SWCNT material properties and separation of SWCNTs by electronic type was studied to enable characterization and control of SWCNT properties. Variations in purity, solvent processing, and electronic type of SWCNTs are shown to have a significant effect on SWCNT characterization, optoelectronic properties, and performance in lithium ion batteries as described further below. Organic solvents from the alkyl amide and halogenated aromatic classes have been analyzed as dispersion agents for high purity single wall carbon nanotubes (SWCNTs). The resulting dispersions from two novel SWCNT solvents, N,N,N',N'-Tetramethylmalonamide (TMMA) and 1-Chloronaphthalene (1-CLN), have been compared to well-established solvents (i.e., N,N Dimethylacetamide (DMA) and 1,2 Dichlorobenzene (DCB)). The spectroscopic results for the halogenated aromatic solvents are consistent with a sonopolymerization that results in a polymer wrapping of the SWCNTs. In comparison, the alkyl amide solvents (DMA and TMMA) show similar dispersion limits with no significant change in absorbance as a function of ultrasonication. These solvents also have the additional benefit of being able to be removed without damaging the SWCNT structure. Density-gradient ultracentrifugation (DGU) has enabled separation of SWCNTs by diameter, electronic type, and chirality. The DGU method uses surfactant coated SWCNT material that rises or falls to the point in the gradient matching its density with ultracentrifugation. SWCNTs produced through laser vaporization were separated by electronic type and materials were characterized through optical absorption. The effect of purity, organic solvent processing, and DGU electronic type separation on lithium ion capacity of free-standing anodes was studied which showed improved performance for SWCNT material with high purity and uniformity. The use of a SWCNT paper as a current collector for a traditional anode coating was found to improve energy density by reducing electrode mass while retaining high capacity. As a conductive additive, SWCNTs formed an effective percolation network at extremely low mass loadings in traditional cathode and anode coatings. The SWCNT additives were shown to increase rate capability and usable capacity of the electrodes compared to higher mass loadings of typical conductive carbon additives.
Record URI: http://hdl.handle.net/1850/13746
Date: 2010-11-05

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