Batteries With A Less Fiery Future

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The plastic sheet invented by Stanford University chemical engineer Zhenan Bao and her colleagues can be inserted in lithium-ion batteries to avoid overheating. (Courtesy of Zhenan Bao)
The plastic sheet invented by Stanford University chemical engineer Zhenan Bao and her colleagues can be inserted in lithium-ion batteries to avoid overheating. (Courtesy of Zhenan Bao)

Lithium-ion batteries are extremely popular because they are lightweight and pack a lot of power.

But when they overheat, they can catch fire. This has been a problem in airplanes, cars and now the popular gift item, "hoverboards." The Consumer Product Safety Commission says it's investigating 37 reports of fires associated with hoverboards.

Stanford University chemical engineer Zhenan Bao says she and her colleagues think they have "a very simple solution" that can prevent the catastrophic failure of batteries.

To understand how Bao's solution works, you have to understand a bit about batteries.

Batteries are essentially a sandwich: two electrodes with a substance called an electrolyte sandwiched between them. A chemical reaction inside the battery causes electrons to collect at one electrode. When you attach an electrical device to the battery, the electrons produce a current that powers the device.

The microstructure of the plastic sheet where the spiky nanoparticles of nickel can be observed.
The microstructure of the plastic sheet where the spiky nanoparticles of nickel can be observed.

Bao developed a plastic sheet that can be placed between an electrode and the electrolyte like a slice of cheese on a sandwich.

"Looks just like a normal plastic bag, but it's black in color," Bao says.

There are carbon-coated nanoparticles of nickel embedded in Bao's plastic sheet that allow the plastic to conduct electricity.

Under normal circumstances, the plastic sheet won't affect the battery's operation.

But the plastic has a remarkable property: It expands when it heats up, and when it expands, the carbon-coated nickel nanoparticles in the plastic are pulled apart so they can no longer conduct electricity.

If the battery starts to overheat, "the plastic will sense the temperature rise and basically stops the battery from operating," Bao says. Once the battery shuts off, it cools down all by itself.

Bao describes her research in the journal Nature Energy. Bao says others have tried similar approaches, but she says the plastic film she and her colleagues developed reacts more quickly than other plastics.

Another feature of her plastic sheet is that when it cools down, the battery that shuts off can conduct electricity and start operating again. Of course, a battery with a tendency to overheat is probably a battery that you want to recycle.

It's unclear when products with these batteries will be on the market — Bao and her colleagues' research is merely a laboratory demonstration. Whether manufacturers will adopt the technology for their products is another question.

Copyright 2016 NPR. To see more, visit http://www.npr.org/.

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Transcript

STEVE INSKEEP, HOST:

Spend some time around elementary-school-aged kids, and you will hear these two things. First, someone will say, I really want a hoverboard - you know, those things that are like scooters but without a handle. You just kind of hover above the ground. Second, you will hear this - I want a hoverboard as soon as they stop catching fire. The Consumer Product Safety Commission says it's been investigating dozens of reports of fires associated with hoverboards, fires likely caused by problems with lithium batteries. NPR's Joe Palca has been exploring new inventions as part of his series, Joe's Big Idea. And today, he has the story of a chemical engineer who invented a way to prevent lithium battery fires.

JOE PALCA, BYLINE: Lithium batteries are popular because they're lightweight and can store a lot of energy. They're used in everything from computers to jet aircraft. But lithium can be nasty. Get it hot enough, and it'll catch fire. Now, under normal operating conditions, lithium batteries are perfectly safe. But if they overheat, if, say, the battery is overcharged or develops a short circuit, then watch out.

ZHENAN BAO: The problem we're trying to solve is to come up with a very simple solution that can prevent the catastrophic failure of battery before such accident will happen.

PALCA: That's Zhenan Bao. She's a chemical engineer at Stanford University. Bao's simple solution involves a thin plastic sheet.

BAO: Looks just like a normal plastic bag, but it's black in color. And we insert this sheet of plastic inside the battery on top of the electrode.

PALCA: Batteries are essentially a sandwich - two electrodes with a substance called an electrolyte sandwiched in between. Bao's thin plastic sheet goes between the electrodes and they electrolyte, like a slice of cheese on a sandwich. There are carbon-coated nanoparticles of nickel embedded in Bao's plastic sheet. These allow the plastic to conduct electricity so the battery can operate normally. But the plastic has a remarkable property - it expands when it heats up. And when it expands, the carbon-coated nickel nanoparticles in the plastic sheet are pulled apart, and they can no longer conduct electricity. So if the battery starts to overheat...

BAO: ...The plastic will sense the temperature rise and basically stops the battery from operating.

PALCA: And when the battery stops operating, it cools down all by itself. Bao describes her research in the journal Nature Energy. Bao says others have tried similar approaches, but she says the plastic she and her colleagues have developed reacts more quickly than other plastics, making it that much safer. Joe Palca, NPR News. Transcript provided by NPR, Copyright NPR.