An improved cathode could lead to lithium batteries that fully charge in seconds and discharge equally rapidly--providing a quick power boost for electric cars. What's more, the advance relies on materials that are already commercialized. So the high-speed refinements could make it to market within a few years.
There are several types of powerful lithium-ion batteries. Those that hold the most energy per weight, commonly used in laptops and cell phones, are made from lithium cobalt oxide. But that mixture is relatively expensive, making such lithium cells too pricey for applications such as hybrid cars. Batteries made with lithium iron phosphate (LiFePO4) are a possible replacement, because that combo is considerably cheaper. But it doesn't hold as much power and has traditionally been slow to charge and discharge.
The speed trap centers on the way lithium ions move into the LiFePO4-containing electrode, or cathode. This is the final step in converting chemical energy stored in the battery to electrical energy when the battery is discharging. The cathode is made up of tiny LiFePO4 particles, and lithium ions must wiggle deep within the particles. Gerbrand Ceder, a materials scientist at the Massachusetts Institute of Technology in Cambridge, and colleagues previously ran detailed computer models that showed that the ions quickly enter the particles once they find a passageway inside. But they often get hung up on the surface of the particles, unable to find an entrance.
Battery makers have recently improved matters by coating the LiFePO4 particles with carbon, which helps the ions move more quickly around the outside to find an entry point. Now Ceder and his student Byoungwoo Kang have made significant gains by replacing the carbon coating with an even more conductive glassy material made from lithium phosphate. The upshot, Ceder says, is that small cells made from the new material could discharge their power in as little as 9 seconds, more than 30 times faster than LiFePO4 cells without the glassy coating and 100 times faster than commercial lithium-ion batteries.
"It's a very nice concept," says Marca Doeff, a materials scientist and battery expert at Lawrence Berkeley National Laboratory in California. Because LiFePO4 is inexpensive and both chemically and thermally stable, it could prove a boon for hybrid electric cars that can quickly repower their batteries when under gasoline power. It could also lead to a new generation of rechargeable batteries that power up in minutes rather than the hours it often takes today. Ceder says his group has already licensed the advance to two companies looking to commercialize the technology.