Even though they have around three times the energy density of lead acid and NiMH, lithium-ion batteries can be very unstable. The Li-ion batteries in electric vehicles have been known to catch fire following accidents and front-end collisions. The Federal Aviation Administration has even considered a moratorium on shipping batteries for fear that they could ignite during flight.
Over the last month, as the press have raised the questions of their overall safety, and engineers have guaranteed that failures are extremely rare, two research projects have invented Li-ion batteries with internal fail-safes to prevent the catastrophic failures that cause them to catch fire.
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When severely damaged, overcharged due to a malfunction, or contaminated by metallic dust in the production process, Li-ion batteries can short-circuit and overheat. Too much heat triggers a chain reaction known as a thermal runaway, in which the flammable electrolyte inside the battery catches fire. Low-quality separators between the battery’s electrodes can also cause thermal runaways.
The most recent research to address these problems comes out of Stanford University. Researchers have developed a temperature sensor that can shut down the battery before it overheats, preventing a thermal runaway. In addition, the sensor is capable of restarting the battery automatically once it cools down.
Writing in the journal Nature Energy, the researchers said that they had based their design on wearable sensors for measuring body temperature. The sensor is made out of a thin film of elastic polyethelene, and embedded in the film are tiny nickel particles coated in graphene.
“We attached the polyethylene film to one of the battery electrodes so that an electric current could flow through it,” says Zheng Chen, lead author of the paper. “To conduct electricity, the spiky particles have to physically touch one another. But during thermal expansion, polyethylene stretches. That causes the particles to spread apart, making the film nonconductive so that electricity can no longer flow through the battery.”
The polyethylene film expands when the battery’s temperature is above 160°F, causing the spike to separate and the battery to shut down. When the temperature falls back under that threshold, the polyethylene contracts and the battery generates electricity again. The temperature threshold can be adjusted with different materials, according to the research team.
Another study from Penn State University takes a similar approach, using sensors to shut down the Li-ion battery at high temperatures. The Penn State team worked closely with large-format batteries—the kind used in electric vehicles and other gadgets—that require more power than cell phones and computers.
The team worked with a grant from the U.S. Department of Energy’s Computer Aided Engineering for Electric Drive Vehicle batteries (CAEBAT) project, underlining the need for Li-ion batteries that can survive accidents, as well as adapt to wide changes in weather and temperature. Its research was published in the journal Scientific Reports earlier this month.
The transition to electric vehicles will serve as one of the main tests of Li-ion battery safety. Automobile and battery companies are working to increase their energy density, at the same time making them smaller and lighter, in an attempt to extend the range of electric vehicles. At these higher densities, battery failures have the potential to cause more violent thermal runaways.
Chao-Yang Wang, a professor of mechanical, chemical, and materials engineering at Penn State, notes “you are compressing more and more energy into a smaller space, and you have to careful when you do that.”
