11 March 2009—Materials scientists at MIT report that they’ve invented a new kind of lithium-ion battery that can fully charge or discharge in seconds instead of minutes. If commercialized, the battery could allow future hybrid cars to rapidly recharge their batteries, or it may lead to new consumer products, the scientists say.
Batteries have a high-energy but low-power density. That is, they can store a lot of energy but can’t release it quickly. Batteries, such as those used in electric cars, ”have a lot of energy, so you can drive at 55 miles per hour for a long time, but the power is low. You can’t accelerate quickly,” says Gerbrand Ceder, professor of materials science and engineering at MIT. Devices called ultracapacitors act in the opposite manner, storing less energy but releasing it in a hurry. The new battery chemistry, invented by Ceder and his graduate student Byoungwoo Kang, gives lithium-ion batteries a performance more akin to that of ultracapacitors.
For years, scientists thought that what was holding back lithium’s discharge rate was the speed of lithium ions as they migrated through the battery. But about five years ago, Ceder did some simulations that showed that lithium ions should be zipping through lithium iron phosphate, a material commonly used in batteries.
Further calculations showed that lithium ions can indeed move very quickly into the iron phosphate material, but only if they are positioned just outside tunnels accessed from the surface of the phosphate. If the ion isn’t in front of the tunnel, it can’t work its way around to get inside.
Ceder and Kang got the ions moving by creating a new surface structure that acts like a beltway to shuttle them around the outside of the material. When an ion on this beltway reaches a tunnel, it dives right in.
The researchers built the beltway by coating the lithium iron phosphate with a similar compound that’s slightly deficient in iron, phosphorous, and oxygen. After the material is heated, the coating forms a glass cover that acts as the ion beltway. Using their new processing technique, Ceder and Kang report in the 12 March issue of Nature , the pair constructed a small battery that could be fully charged or discharged in 10 to 20 seconds instead of the 300 seconds it would have taken without the process.
Calling the work ”wonderful,” Christina Lampe-Onnerud, CEO of lithium-ion battery maker Boston-Power , says that for years there had been reason to believe that lithium-ion batteries could be made to charge much more quickly. ”Fundamentally, I think we’ve known this was possible,” she says.
Fast charging is of interest to some plug-in vehicle makers. For example, using Altair Nanotechnologies’ battery technology, Phoenix Motorcars has built a 160-kilometer-range electric car that can be fully charged in just 10 minutes . But critics don’t see much of a benefit, noting that it would take 250 kilowatts of power to do so—five times as much as what your average office building consumes at its daily peak.
Other firms have been chasing the goal of a quicker charge even in the lowly lead-acid battery. Furukawa Battery, in Yokohama, Japan, and Axion Power International, in New Castle, Pa., have independently developed hybrids of lead-acid batteries and ultracapacitors , with the aim of replacing nickel-metal-hydride batteries in hybrid cars.
To Probe Further
Ceder and Kang’s findings will be published in the 12 March 2009 issue of the journal Nature .