To prevent battery fires, researchers wrap 16340 battery cathodes with graphene

Lithium batteries, which allow electric vehicles to travel hundreds of miles, are well known for their ability to store energy, but they are also known for their risk of catching fire, a phenomenon battery people call "thermal runaway" (when the battery accumulates too much heat). Fires occur most frequently when batteries overheat or cycle rapidly. As more and more electric vehicles are on the road every year, battery technology needs to evolve to reduce the likelihood of dangerous and catastrophic fires.

Researchers at the University of Illinois at Chicago College of Engineering have published a study showing that graphene, a wonder material of the 21st century, may absorb oxygen from lithium-ion batteries when they catch fire, thereby reducing the risk of fire.

16340 battery fires are caused by rapid cycling, or charging and discharging, of the battery, and the high temperatures that build up inside the battery. This causes the cathode inside the battery (in most cases, the cathode is an oxide containing lithium, usually lithium cobalt oxide) to decompose and release oxygen. Electrolytes decompose at high temperatures to release flammable products, and if oxygen combines with such flammable products, spontaneous combustion can occur.

"We thought that if there was a way to prevent the oxygen released by the cathode from combining with other flammable products in the battery, we could reduce the possibility of fire," said Reza Shahbazian-Yassar, associate professor of mechanical and industrial engineering in the University of Illinois Chicago College of Engineering and corresponding author of the paper.

It turned out that graphene, an ultra-thin layer of carbon atoms with unique properties, was the perfect solution. Shahbazian-Yassar and his colleagues had previously used graphene to regulate the accumulation of lithium on the electrodes of lithium metal batteries.

Shahbazian-Yassar and his colleagues knew that oxygen atoms cannot seep out of graphene sheets. In addition, graphene has the advantages of high strength, good flexibility, and can conduct electricity. They thought that if they wrapped the small particles of lithium cobalt oxide cathodes in lithium batteries with graphene, they might be able to prevent oxygen from escaping.

First, the researchers chemically changed the graphene to make it conductive. Next, they wrapped tiny particles of lithium cobalt oxide cathode electrodes in conductive graphene. When the researchers used an electron microscope to observe the graphene-wrapped lithium cobalt oxide particles, they found that at high temperatures, significantly less oxygen was released compared to particles without graphene.

Next, the researchers combined the graphene-wrapped particles with a binding material to form a cathode that was integrated into a lithium metal battery. When the researchers measured the oxygen release during battery cycling, they found that almost no oxygen escaped from the cathode even at very high voltages. Even after 200 charge and discharge cycles, the lithium metal battery still performed well.

"Compared with conventional lithium metal batteries, the batteries with graphene-wrapped cathodes lost only about 14% of their capacity after rapid cycling, while the performance of conventional lithium metal batteries dropped by about 45% under the same conditions," Sharifi-Asl said.

"Graphene is an ideal material for blocking oxygen from entering the electrolyte. It is oxygen-impermeable, electrically conductive, flexible, and strong enough to withstand the conditions inside a battery. And it's only a few nanometers thick, so it doesn't add any extra mass to the battery," said Shahbazian-Yassar. "Our research suggests that using graphene in the cathode to reduce oxygen release could be a way to significantly reduce the risk of fire in batteries that power our phones and cars."

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