Smaller, lighter batteries being developed

Battery material that could speed up the time needed to recharge batteries for cell phones and other devices.

March 18, 2009 - What is just about as frustrating as a battery running out of power when a device is critically needed? It could be the requirement to charge the battery for hours instead of minutes or even seconds.

MIT engineers are working to solve that ubiquitous problem. They have created a kind of a beltway that allows for the rapid transit of electrical energy through a well-known battery material. As a result, cell phones and other devices could recharge in seconds rather than minutes or hours.

The work could also allow for the quick recharging of batteries in electric cars, although that particular application would be limited by the amount of power available to a homeowner through the electric grid.

The work, led by Gerbrand Ceder, the Richard P. Simmons Professor of Materials Science and Engineering, is reported in the March 12 issue of Nature.

Because the material involved is not new - the researchers have simply changed the way they make it - Ceder believes the work could make it into the marketplace within two to three years.

State-of-the-art lithium rechargeable batteries have very high energy densities - they are good at storing large amounts of charge. The tradeoff is that they have relatively slow power rates - they are sluggish at gaining and discharging that energy.

About five years ago, however, Ceder and colleagues made a surprising discovery. Computer calculations of a well-known battery material, lithium iron phosphate, predicted that the material's lithium ions should actually be moving extremely quickly.

"If transport of the lithium ions was so fast, something else had to be the problem," Ceder said in a press release.

Further calculations showed that lithium ions can indeed move very quickly into the material but only through tunnels accessed from the surface. If a lithium ion at the surface is directly in front of a tunnel entrance, there's no problem: it proceeds efficiently into the tunnel. But if the ion isn't directly in front, it is prevented from reaching the tunnel entrance because it cannot move to access that entrance.

Ceder and Byoungwoo Kang, a graduate student in materials science and engineering, devised a way around the problem by creating a new surface structure that does allow the lithium ions to move quickly around the outside of the material, much like a beltway around a city. When an ion traveling along this beltway reaches a tunnel, it is instantly diverted into it.

Kang is a coauthor of the Nature paper.