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Early-warning system cuts fire risk for Li-ion batteries

Posted: 16 Oct 2014 ?? ?Print Version ?Bookmark and Share

Keywords:Stanford University? lithium-ion battery? electrode? copper?

To address the problem, Cui and his colleagues applied a nanolayer of copper onto one side of a polymer separator, creating a novel third electrode halfway between the anode and the cathode.

"The copper layer acts like a sensor that allows you to measure the voltage difference between the anode and the separator," Zhuo said. "When the dendrites grow long enough to reach the copper coating, the voltage drops to zero. That lets you know that the dendrites have grown halfway across the battery. It's a warning that the battery should be removed before the dendrites reach the cathode and cause a short circuit."

The buildup of dendrites is most likely to occur during charging, not during the discharge phase when the battery is being used.

"You might get a message on your phone telling you that the voltage has dropped to zero, so the battery needs to be replaced," Zhuo said. "That would give you plenty of lead time. But when you see smoke or a fire, you have to shut down immediately. You might not have time to escape. If you wanted to err on the side of being safer, you could put the copper layer closer to the anode. That would let you know even sooner when a battery is likely to fail."

In addition to observing a drop in voltage, co-lead author Hui Wu was able to pinpoint where the dendrites had punctured the copper conductor simply by measuring the electrical resistance between the separator and the cathode. He confirmed the location of the tiny puncture holes by actually watching the dendrites grow under a microscope.

"The copper coating on the polymer separator is only 50nm thick, about 500 times thinner than the separator itself," said Wu, a postdoctoral fellow in the Cui group. "The coated separator is quite flexible and porous, like a conventional polymer separator, so it has negligible effect on the flow of lithium ions between the cathode and the anode. Adding this thin conducting layer doesn't change the battery's performance, but it can make a huge difference as far as safety."

Most lithium-ion batteries are used in small electronic devices. "But as the electric vehicle market expands and we start to replace on-board electronics on airplanes, this will become a much larger problem," Zhuo said.

"The bigger the battery pack, the more important this becomes," Cui added. "Some electric cars today are equipped with thousands of lithium-ion battery cells. If one battery explodes, the whole pack can potentially explode."

The early-warning technology can also be used in zinc, aluminum and other metal batteries. "It will work in any battery that would require you to detect a short before it explodes," Cui said.

- Paul Buckley
??EE Times Europe


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