The End of the Battery?

In the Economist:

[T]he so-called ultracapacitors on which the XH-150 is based may supplant rather than merely supplement a car's batteries. And if that happens, a lot of other batteries may be for the chop, too. For it is possible that the long and expensive search for a better battery to power the brave, new, emission-free electrical world has been following the wrong trail.

A traditional capacitor stores electricity as static charges, positive and negative, on two electrodes that are separated by an insulator. This works best when the electrodes are parallel with each other, which means they need to have smooth surfaces. The amount of charge that can be stored depends on the surface area of the electrodes, the strength and composition of the insulation between them, and how close they are together. If the electrodes are then connected by a wire, a current will flow from one to the other. A battery, by contrast, stores what is known as an electrochemical potential. Its two electrodes are made of different chemicals—ones that will release energy when they react. But because the electrodes are physically separated from one another their chemical constituents can react only by remote control...

The reason ultracapacitors may be able to bridge the gap between speed and endurance is that, like batteries, they use ions and an electrolyte rather than simply relying on the static charges.