Researchers demonstrate cells that can power up in seconds.
Two researchers have developed battery cells that can charge up in less time than it takes to read the first two sentences of this article. The work could eventually produce ultra-fast power packs for everything from laptop computers to electric vehicles.
Byoungwoo Kang and Gerbrand Ceder of the Massachusetts Institute of Technology in Cambridge have found a way to get a common lithium compound to release and take up lithium ions in a matter of seconds. The compound, which is already used in the electrodes of some commercial lithium-ion batteries, might lead to laptop batteries capable of charging themselves in about a minute. The work appears in Nature1 this week.
Lithium-ion batteries are commonplace in everything from mobile phones to hybrid vehicles. "They're essentially devices that move lithium ions between electrodes," says Ceder. The batteries generate an electric current when lithium ions flow out from a storage electrode, float through an electrolyte, and are chemically bound inside the opposing cathode. To recharge the battery, the process is reversed: lithium ions are ripped from the cathode compound and sent back to be trapped in their anode store.
The speed at which a battery can charge is limited by how fast its electrons and ions can move - particularly through its electrodes. Researchers have boosted these rates by building electrodes from nanoparticle clumps, reshaping their surfaces, and using additives such as carbon. But for most lithium-ion batteries, powering up still takes hours: in part because the lithium ions, once generated, move sluggishly from the cathode material to the electrolyte.
That seemed to be the case for lithium iron phosphate (LiFePO4), a material that is used in the cathode of a small number of commercial batteries. But when Ceder and Kang did some calculations, they saw that the compound could theoretically do much better. Its crystal structure creates "perfectly sized tunnels for lithium to move through", says Ceder. "We saw that we could reach ridiculously fast charging rates."
So why hadn't anyone seen this speedy charging in practice? Ceder and Kang theorize that the lithium ions were having trouble finding their way to the crystal structure's express tunnels. The authors helped the ions by coating the surface of the cathode with a thin layer of lithium phosphate glass, which is known to be an excellent lithium conductor. Testing their newly-coated cathode, they found that they could charge and discharge it in as little as 9 seconds.
"As far as I know, this is the fastest yet for this material," comments Peter Bruce, a chemist at the University of St Andrews, UK. The researchers do not know exactly how the disordered glass helps lithium ions transfer between the electrolyte and the cathode.
Other materials, such as nickel oxide, have achieved similarly fast charging rates, says John Owen, a chemist at the University of Southampton, UK. "This is a nice demonstration of the concept in a lithium system," he says. Lithium, though, can store more energy for less weight than nickel compounds, and holds its charge better.
It's particularly important because lithium iron phosphate is already being used commercially, adds Bruce. Speeding lithium ion movement would vastly improve energy recovery in hybrid vehicles, which recharge their batteries when the vehicle brakes — a process that lasts only seconds. It could also eventually lead to fully electric vehicles that could charge reasonably quickly.
Ceder says that he thinks that improvements in modelling will allow researchers to find other candidates for ultra-fast batteries. "My guess is that there are more materials like this out there," he says.
Kang, B. & Ceder, G. Nature 458, 190–193 (2009).
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Brumfiel, G. Lithium batteries charge ahead. Nature (2009). https://doi.org/10.1038/news.2009.156