A lithium-ion battery with a positive electrode made of carbon nanotubes delivers 10 times more power than a conventional battery and can store five times more energy than a conventional ultracapacitor. The nanotube battery technology, developed by researchers at MIT and licensed to an undisclosed battery company, could lead to batteries that improve heavy-duty hybrid vehicles and allow faster recharging for electronic gadgets, including smartphones.
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.
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.