There have been numerous cases of lithium-ion batteries catching fire in everything from mobile phones and laptops to cars and airplanes. While the odds of this occurring are low, the fact that hundreds of millions of lithium-ion batteries are produced and sold every year means the risk is still very real. Researchers at Stanford University have now developed a "smart" lithium-ion battery that would provide users with a warning if it is overheating and likely to burst into flames.

Lithium-ion batteries consist of a carbon anode and lithium metal-oxide cathode that are separated by an ultrathin polymer. This separator is porous to allow the lithium ions to move between the electrodes that are contained within a flammable electrolyte solution through which the lithium ions flow.

But if particles of metal or dust find their way into the separator at the manufacturing stage, or if the battery is charged too fast when the battery is too cold, lithium ions can build up on the anode and form fibers known as dendrites. If these dendrites penetrate the separator and come into contact with the cathode, the battery can short circuit, igniting the flammable electrolyte solution.

While other research teams are exploring non-flammable electrolytes, Yi Cui, an associate professor of materials science and engineering, and his colleagues at Stanford set about developing an early-warning system that could be used in conventional lithium-ion batteries. They did this by applying a nanolayer of copper onto one side of the separator, which created a third electrode 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," said graduate student Denys Zhuo. "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."

Zhuo says that when the coated separator detects that the voltage has dropped to zero, a message could be displayed on the device warning that the battery needs to be replaced.

"The copper coating on the polymer separator is only 50 nanometers thick, about 500 times thinner than the separator itself," said Hui 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."

In addition to lithium-ion batteries, the Cui says the technology will work in any battery that would require the detection of a short before it explodes, including zinc, aluminum and other metal batteries.

The team's research appears in the journal Nature Communications and the smart battery is shown in the video below.

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