The world's smallest battery is so tiny that thousands could fit inside a human hair, and holds nearly twice the energy of conventional lithium-ion cells.

 It was created by Jianyu Huang of Sandia National Laboratories in Albuquerque, N.M. The battery is a tiny filament of tin oxide and lithium cobalt oxide, suspended in an electrolyte like a conventional battery cell.

Unlike a conventional battery, this one expands to more than twice its normal length when it is recharged. When it is discharged, it shrinks. Huang says studying this behavior will help design new kinds of batteries, with greater power capacity.

The battery expands because as electrons are added to it, the lithium atoms get forced into the lattice between tin oxide atoms. Some of the lithium forms lithium oxide, while the tin oxide reduces to tin. Lithium oxide is a much larger molecule than Lithium by itself, so the filament gets longer and also twists under the stress.

Huang says this phenomenon could be explored to see why it is that batteries fail. Previously, battery manufacturers and engineers had thought that the expansion was in the diameter of the battery, not along its length. Huang's experiments proved otherwise.

He also notes that the nanometer-sized wire can handle a lot more stress than the bulk material - if one were to apply current to a centimeter-sized block of tin oxide the expansion would make it crack.

Leaning how this phenomenon works is important for designing batteries that won't fail, Huang says. Expansion when current is applied is a major problem when designing machines with small tolerances, and the stress generated in the battery materials can eventually causes failures.

The battery material Huang used also has a much greater capacity for power. Typical lithium ion batteries have 300 milliampere-hours per gram, meaning a 500-gram battery (about a pound) is about 150 Watt-hours - enough for a few hours at most in a laptop. Huang says the tin oxide combination he used has more than twice that. If the expansion and contraction were better understood, it might be possible to make batteries with that kind of power capacity.

Huang was inspired to study batteries precisely because there is a lot of demand for energy, especially in electronics. Energy is a hot topic right now, he said. The same technique could be used on other materials besides tin oxide, he adds.