They’re supposed to be fun, but self-balancing scooters—often called “hoverboards”—have caught fire while people were riding them. They’ve been banned by some universities and airlines. Their culprit? Lithium-ion batteries.
Such widely used batteries, the workhorse in consumer electronics, have also caused fires in electric cars and cargo planes. Because of their overheating risks, companies and the U.S. government have recalled thousands of batteries used in cameras, laptops, tablets, cordless tools, and even winter jackets.
Now Stanford University researchers may have a solution. In a study published Monday in Nature Energy, they say they’ve developed the first lithium-ion battery that will shut down before overheating and will restart immediately when the temperature cools.
“The potential for mass production is quite high,” says co-author Zhenan Bao, a professor of chemical engineering, noting that most of the materials involved are inexpensive plastic and nickel. She says their battery restarts without losing efficiency.
Worldwide, researchers are racing to build better and cheaper batteries, because the demand for them is increasing. As the new UN climate accord prods countries to shift away from fossil fuels, more battery-powered electric cars are entering the market, and more solar panels and wind turbines are producing power that needs to be stored for days without sun or wind.
So scientists are coming up with lots of ideas. Also Monday, for example, a team at Lawrence Berkeley National Laboratory announced that it's working on a new way to make cathodes—a key componenet in batteries—that could boost performance.
Since safety concerns linger, some research aims to lower the risk of fire. Last year, a Harvard team debuted a non-flammable flow battery, which can store grid-scale energy in external liquid tanks. MIT researchers published a paper that said lithium-ion batteries could be safer by making another key component, the electrolyte, solid rather than liquid.
Others have looked at adding flame retardants to the electrolyte, which carries charged particles between the battery’s electrodes. Last year, Stanford engineer Yi Cui built a “smart” battery that offers a warning before it gets too hot.
“Unfortunately, these techniques are irreversible, so the battery is no longer functional after it overheats,” Cui said in announcing the newest study, of which he’s a co-author.
To solve this big problem, he and colleagues went small—really small. They turned to nanotechnology, inspired by Bao’s recent invention of a wearable sensor to monitor human body temperature. The sensor is made of a plastic material embedded with tiny particles of nickel that have spikes protruding from their surface.
For the battery, the Stanford team put the spiky nickel particles in a thin film of elastic polyethylene and coated them with graphene, an atom-thick layer of carbon. To conduct electricity, the particles have to touch one another, but as temperatures rise, the polyethylene stretches so the particles spread out and electricity no longer flows through the battery. As the temperatures fall back down, the plastic shrinks and the particles come back into contact.
“Our design provides a reliable, fast, reversible strategy that can achieve both high battery performance and improved safety,” the study says.
The research is "significant," says George Crabtree, director of Argonne National Laboratory's Joint Center for Energy Storage Research, noting that "lithium-ion battery safety is a top priority for electric transportation and the grid, where the risk of even a few runaway fires could be a showstopper."
Crabtree says that while Stanford's "novel concept" is "impressive and promising," more tests are needed on larger battery systems operating over many charge-diischarge cycles to be sure there are no unanticipated problems. He adds: "Lithium-ion batteries are notorious for failure by unexpected side-reactions of the components, and the new spiky graphne-coated nickel particles embedded in a polyethylene film is a new component."
Hundreds of millions of lithium-ion batteries are made annually, and very few cause melting, fire or explosions. But some are linked to high-profile accidents. Those in the new popular “hoverboards” have been blamed for setting several houses ablaze and even burning one down in Louisiana.
While a safer battery may be on the way, entrepreneurs aren’t just waiting. They’re taking other steps to avoid battery fires. After the lithium-ion batteries in a few Teslas were damaged in collisions, and the cars caught fire, company co-founder Elon Musk announced the company would add a titanium shield to the underbody of the Model S sedan.