Common beliefs about the role of carbon nanotubes in lithium-ion batteries and other electrochemical composites have been turned on their head. A team of researchers led by Philip Collins at Nanostructures for Electrical Energy Storage (NEES) showed that one single-walled carbon nanotube connects to other materials with a surface resistivity of only 30 mΩ cm2, about four times smaller than a multi-walled carbon nanotube, and a staggering 50 times smaller than typical films of platinum or graphite.
Carbon nanotubes have many beneficial attributes when used in composites, such as high strength, high conductivity, and low mass density, but being defect-free, they were predicted to have unusually poor surface resistivities, just like pristine graphite. As researchers created electrochemical composites with carbon nanotube additives, they found power enhancements instead of decreases. The improved power rates imply excellent charge transfer, exactly the opposite of what electrochemists expected. This enhancement may result from a number of factors; for example, the curvature of the tubular surfaces, contaminants, and disorder, or perhaps a side effect of carbon nanotubes' unusual electronic properties. After all, the way to make graphite electrochemically active is to mechanically and chemically introduce defects.
Because they are edge-free, and in some cases defect-free, single-walled carbon nanotubes offered a model system for studying this further. Collins' group coated single-walled nanotubes with a generic charge-storage cathode material and built transistors (see figure) to characterize the impedance of carbon-metal oxide interfaces.
Remarkably, they found no discernible difference between devices made of pristine or damaged single-walled nanotubes. They determined that the material itself (single/multiwalled nanotubes or platinum) did not make any difference, and the whole improvement in interfacial resistance resulted from rules of electrochemistry that only kick in at the nanoscale. The unmatched size of single-walled nanotube surfaces, especially compared to any other type of conductor, takes maximum advantage of these rules. When randomly dispersed in an active material, carbon nanotubes can create a percolative network through a large volume of material, yet still maintain a disproportionately small surface resistivity. This combination helps explain the improvements observed in carbon nanotube composites.
By revealing the rules that matter, combinations can now be designed and optimized to make better batteries and more efficient solar cells, taking full advantage of the carbon nanotube additives.
