In recent experiments, highly purified batches of single-wall carbon nanotubes (SWCNTs) have shown promise as superior alternatives to the graphitic carbon-black anode materials heretofore used in rechargeable thin-film lithium power cells. The basic idea underlying the experiments is that relative to a given mass of graphitic carbon-black anode material, an equal mass of SWCNTs can be expected to have greater lithium-storage and charge/discharge capacities. The reason for this expectation is that whereas the microstructure and nanostructure of a graphitic carbon black is such as to make most of the interior of the material inaccessible for intercalation of lithium, a batch of SWCNTs can be made to have a much more open microstructure and nanostructure, such that most of the interior of the material is accessible for intercalation of lithium. Moreover, the greater accessibility of SWCNT structures can be expected to translate to greater mobilities for ion-exchange processes and, hence, an ability to sustain greater charge and discharge current densities.
For the experiments, soot containing carbon nanotubes was produced by laser vaporization of a graphite target containing 1.2 atomic percent Ni/Co in an argon atmosphere at a pressure of 500 torr (≈ 67 kPa) at a temperature of 1,200 °C. The soot was purified by refluxing in nitric acid at a temperature of 125 °C followed by annealing in oxygen at 550 °C for 30 minutes. By means of scanning electron microscopy, transmission electron microscopy, ultraviolet-visible spectrophotometry, and thermogravimetric analysis, 99 weight percent of the purified soot was found to consist of SWCNTs. The specific surface area of the purified material, as measured by use of the Brunauer, Emmett, and Teller (BET) technique based on adsorption of nitrogen, was found to be 1,200 m2/g — much greater than the specific surface areas of several other carbonaceous anode materials that were also subjected to the BET test.
Anodes were fabricated by casting, onto copper foils, thin films of the purified SWCNT material dispersed at a concentration of 5 weight percent in poly(vinylidiene fluoride). The anodes were incorporated into standard three electrode test cells along with lithium-foil counter and reference electrodes and an electrolyte comprising 1.0 M LiPF6 in a solution of ethylene carbonate (2 parts by volume) and dimethyl carbonate (1 part by volume). Over-potentials for interaction of the anodes with lithium and lithium capacities of the anodes were measured by use of cyclic voltammetry and galvanostatic cycling, respectively.
The figure shows representative charge and discharge capacities determined from the measurements. The plotted values show a high degree of reversibility after the initial cycle. The specific capacity after 30 cycles was found to be about 1.33 Ah/g — almost 3 times the generally accepted value (0.45 Ah/g) for graphite.
This work was done by Aloysius F. Hepp of Glenn Research Center; Ryne Raffaelle and Tom Gennett of Rochester Institute of Technology; Prashant Kumta and Jeff Maranchi of Carnegie-Mellon University; and Mike Heben of National Renewable Energy Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www. techbriefs.com/tsp under the Materials category.
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