Mechanical engineer Carol Livermore in her lab. (MIT/Patrick Gillooly)
MIT scientists have found that carbon nanotubes could be formed into tiny springs capable of storing as much energy, pound for pound, as the best lithium-ion batteries - potentially more durably and reliably.

Carol Livermore, associate professor of Mechanical Engineering, found that carbon nanotube springs can potentially store more than a thousand times more energy for their weight than steel springs.

Livermore explains that for some applications, springs can have advantages over other ways of storing energy. Unlike batteries, for example, springs can deliver the stored energy effectively either in a rapid, intense burst, or slowly and steadily over a long period. Also, unlike batteries, stored energy in springs normally doesn't slowly leak away over time.

For that reason, the springs could be ideal for use in emergency backup systems. With batteries, such devices need to be tested frequently to make sure they still have full power, and replaced or recharged when the batteries run down.

Livermore says that the springs made from these minuscule tubes might find their first uses in large devices rather than in micro-electromechanical devices. The best uses of such springs may be in cases where the energy is stored mechanically and then used to drive a mechanical load, rather than converting it to electricity first.

Any system that requires conversion from mechanical energy to electrical and back again, using a generator and then a motor, will lose some of its energy in the process through friction and other processes that produce waste heat. In addition to the direct energy losses, about half the weight of electromechanical systems is in the motor-generator used for the conversion — something that wouldn't be needed in a purely mechanical system.

The nanotube molecules have a strong tendency to stick to each other, lending themselves to being made into longer fibers that can make effective springs. Livermore says that this quality means that ultimately it may be possible to "make something that looks like a carbon nanotube and is as long as you want it to be."

Carbon nanotube springs are also relatively unaffected by differences in temperature and other environmental factors, whereas batteries need to be optimized for a particular set of conditions, usually to operate at normal room temperature.

Livermore says that to create devices that come close to achieving the theoretically possible high energy density of the material will require plenty of additional basic research, followed by engineering work. The initial lab tests used fibers of carbon nanotubes joined in parallel, but creating a practical energy storage device will require assembling nanotubes into longer and likely thicker fibers without losing their key advantages.