Now the MIT group has adapted these methods to make battery electrodes. Lithium-ion batteries are charged and discharged when lithium ions move from one electrode to the other, driving or being driven by an external current. The more total lithium the battery can store, the greater its total energy storage capacity. The faster the ions can move out of one electrode and into the other, the greater its power. In work published this week in the journal Nature Nanotechnology, the MIT group showed that lithium ions in a battery electrolyte react with oxygen-containing chemical groups on the surface of the carbon nanotubes in the film. Because of the huge surface area and porous structure of the nanotube electrodes, there are many places for the ions to react, and they can travel in and out rapidly, which gives the nanotube battery high energy capacity and power, says Shao-Horn.

“This work has demonstrated once more that the development of methods for careful structural control at the nanoscale leads to major improvements in materials performance,” says Nicholas Kotov, professor of chemical engineering at the University of Michigan. “I believe that it’s just the beginning of the major improvement of lithium batteries using a materials engineering approach.”

The next step, says Hammond, is to “speed things up.” Using the dipping method, the group is able to make relatively thick nanotube films, but it takes a week. “If you want to make a car battery, you need to make it thicker, and over large areas,” says Hammond. Instead of dipping a substrate in the two nanotube solutions, Hammond’s group is now making the electrodes in a few hours by alternately spraying dilute mists of the two nanotube solutions. A major advantage of this misting method is that it’s compatible with large-area printing processes that promise speed and compatibility with a wide range of substrates. For example, nanotube batteries might be printed directly onto integrated circuits.