Researchers have been trying to make electrodes for lithium-ion batteries from carbon nanotubes because their high surface area and high conductivity promise to improve both energy and power density relative to conventional forms of carbon. But working with the material has proved challenging--most methods for assembling carbon nanotubes require a binding agent that brings down the conductivity of the electrode, and lead to the formation of clumps of the material, reducing the surface area. The electrodes made by the MIT group, however, have a very high surface area for storing and reacting with lithium. This high surface area is critical both to the high storage capacity of the electrodes, as well as their high power: because lithium is stored on the surface, it can move in and out of the electrode rapidly, enabling faster charging and discharging of the battery.
The key to the performance of the MIT electrodes is an assembly process that creates dense, interconnected, yet porous carbon-nanotube films, without the need for any fillers. The group, led by chemical engineering professor Paula Hammond and mechanical engineering professor Yang Shao-Horn, create water solutions of carbon nanotubes treated so that one group is positively charged and the other is negatively charged. They then alternately dip a substrate, such as a glass slide, in the two solutions, and the nanotubes, attracted by differences in their charge, cling to one another very strongly in uniform, thin layers. The researchers had previously demonstrated that when heated and removed from the substrate, these dense yet porous films could store a lot of charge and release it quickly--acting like an electrode in an ultracapacitor.