MIT researchers have found that carbon nanotubes could be formed into tiny springs capable of storing as much energy, pound for pound, as state-of-the-art li-ion batteries. The benefit of using carbon nanotubes is that these batteries would be more durable and reliable. More info at MITnews.
Imagine, for example, an emergency backup power supply or alarm system that can be left in place for many years without losing its "charge," portable mechanical tools like leaf blowers that work without the noise and fumes of small gasoline engines, or devices to be sent down oil wells or into other harsh environments where the performance of ordinary batteries would be degraded by temperature extremes. That's the kind of potential that carbon nanotube springs could hold, according to Carol Livermore, associate professor of mechanical engineering. Carbon nanotube springs, she found, can potentially store more than a thousand times more energy for their weight than steel springs.