Building a better battery
Posted: Wednesday, March 11, 2009 2:30 PM by Alan Boyle
Donna Coveney / MIT
A material called lithium iron phosphate, shown here in a lab dish, could soon be
used in batteries that can be charged up in a matter of seconds rather than hours.
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How many hours does it usually take to charge up a battery pack? Researchers have tweaked a material already being used in lithium-ion batteries to cut that time down to a fraction of the usual wait. They say the technology could be used to juice up batteries in seconds rather than hours.
The tweak has the potential to change the way we use gizmos ranging from mobile phones and laptops to plug-in electric vehicles over the next couple of years. But as usual with these kinds of innovations, there's a catch or two.
MIT Professor Gerbrand Ceder and graduate student Byoungwoo Kang explain the technique in Thursday's issue of the journal Nature. It involves processing lithium iron phosphate, also known as LiFePO4, in a different way. This type of lithium-ion battery has been targeted for use in a variety of plug-in vehicles, including the Chevy Volt, but one of its shortcomings has been that the batteries are sluggish when it comes to taking in and pushing out electrical energy.
"They have a lot of energy, so you can drive at 55 mph for a long time, but the power is low," Ceder explained in a news release from MIT. "You can't accelerate quickly."
Ceder and his colleagues took a close look at the chemistry involved with lithium-ion transport, using computer simulations, and determined that there was no intrinsic reason why the ions should be moving so slowly through the battery's energy-storing material. The problem was that the ions had to move in specific directions across the surface of the material in order to enter nanoscale "tunnels" leading into the material itself. It was as if the ions had to follow a maze of surface streets to enter a transit tunnel.
Ceder and Kang found a way to cook the surface of the LiFePO4 material into a glassy structure that let the ions move around quickly, as if they were traveling on a beltway that bypassed surface streets. That sped the ions toward the tunnels and increased the charge/discharge rate by a factor of about 100. The researchers built a small test battery in the lab that usually needed six minutes for a full charge or discharge. When the material was tweaked, that time was cut to 10 or 20 seconds.
Further tests showed that the tweaked material doesn't degrade as much as unprocessed materials during repeated chargings and dischargings. Another plus is that LiFePO4 batteries don't go up in flames, as has sometimes been the case with other types of laptop batteries.
Changing lifestyles
"The ability to charge and discharge batteries in a matter of seconds rather than hours may make possible new technological applications and induce lifestyle changes," the researchers observed. Based on their experiments, they estimate that the typical cellphone battery would take 10 seconds to charge.
If recharging becomes less of a pain, power-hogging mobile applications such as full-screen video might look more attractive to the devices' users. (iPhone owners, are you listening?)And laptop users might not have to hunt around so frequently for a wall outlet to plug into.
Batteries for small devices could be the first to benefit from the beltway tweaking, the researchers said. But in time, faster-charging batteries could change the way we think about plug-in vehicle power, and could provide backup storage for solar- or wind-generated electricity. MIT says that two companies have already licensed the technology, and Ceder thinks the first tweaked batteries could hit the market in two to three years. (The researchers declined to name the two companies, citing concerns about proprietary information.)
So what's the catch? Kang told me that it remains to be seen whether laptop and mobile-device batteries can be processed to build in the surface-level "beltways" while keeping them small enough to fit the required space. "In terms of volume, this material is not that good in comparison to commercial material," he said.
A plus for plug-ins
Kang said the technology would be well-suited for the bigger batteries used in plug-in hybrid electric vehicles.
"We think the plug-in hybrid is more proper for our material, because the material is quite stable and it discharges quickly, and you can achieve faster acceleration of the car," he said.
The catch for that application is that you wouldn't be able to pour in electrical power as quickly as you'd want to from your outlets at home. "Home does not have that kind of power," Kang said. "We need more power. ... The point is, with the battery, there's no limitation. The limitation comes from an external source."
The ideal situation would be to have a network of high-power electric charging stations, which would allow you to juice up your electric vehicle on the road much as people fuel up their gas guzzlers today. Ceder and Kang estimate that an 180-kilowatt power source could give a full charge to the typical plug-in car's 15-kilowatt-hour battery in five minutes.
The researchers say their tweaked material would provide the advantages of supercapacitors (high discharge rates) without the disadvantages (relatively low energy density). But scientists are coming up with a number of different technologies for better batteries - including silicon-based batteries, which I wrote about last year. How do all these battery breakthroughs compare? Feel free to share your thoughts in the comment section below.
