supercapacitor

A supercapacitor stores energy not in a chemical reaction but as a physical layer of charge — picture ions crowding right up against an electrode like spectators pressing on a fence, with the gap between charge and electrode just a nanometre wide. Because that gap is so thin and the electrode surface (porous carbon) is enormous — a single gram can have the area of a basketball court — it stores thousands of times more charge than an ordinary capacitor, bridging the gulf between capacitors and batteries.

Its superpower is speed and stamina: with no slow chemistry to wear out, a supercapacitor charges and discharges in seconds and survives a million cycles, versus a battery's few thousand. The catch is energy density — it holds perhaps 1/20th the energy per kilogram of Li-ion, so it empties fast. That is why you find supercapacitors capturing a train's braking energy, smoothing power glitches, or buffering a camera flash, not running your laptop for hours.