Graphene-coated nickel nanoparticles allow electricity to flow through the battery when they're touching. But as they heat up, they separate and the circuit shuts off. Photo by Stanford
PALO ALTO, Calif., Jan. 11 (UPI) -- Scientists at Stanford have designed a lithium-ion battery that doesn't overheat. The battery shuts itself off when temperatures get too high, and powers back on once it's sufficiently cooled.
Researchers say the new technology will help prevent battery fires in electronics like laptops and hoverboards.
"People have tried different strategies to solve the problem of accidental fires in lithium-ion batteries," Zhenan Bao, a professor of chemical engineering at Stanford, explained in a press release. "We've designed the first battery that can be shut down and revived over repeated heating and cooling cycles without compromising performance."
Your average rechargeable lithium-ion battery is made up of two lithium electrodes, one positive and one negative, and an intermediary electrolyte liquid or gel that enables ionic movement -- the transfer of electrons from one end to the other.
Under certain conditions, the cycle of charging and discharging can push the temperature of the battery's electrolyte material into unsafe territory. When the electrolyte's temperature rises above 300 degrees Fahrenheit, it can catch on fire and cause an explosion of flames.
Hoverboards were a popular gift over the holidays, and in their wake, dozens of YouTube videos have documented the contraption's tendency to spontaneously combust. The problem was enough to move some college campuses to ban the vehicle.
To solve this battery problem, researchers installed a thin film of elastic polyethylene between the electrodes and the electrolyte. The film acts like a governor. Inside the film are spiky nanoparticles made of nickel. The spikes are coated with graphene to enhance conductivity.
"We attached the polyethylene film to one of the battery electrodes so that an electric current could flow through it," lead study author Zheng Chen, a postdoctoral scholar at Stanford, said in a press release. "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 film can be calibrated so that the particles don't separate until a specific temperature is reached. In their prototype battery, researchers designed the film to expand at just the right pace for the nanoparticles to separate and the battery to shut down above 160 degrees Fahrenheit.
Scientists tested the new technology -- described in the journal Nature Energy -- using a hot air gun, and found the safety mechanism is accurate, effective and reversible. The battery's electricity quickly resumes after it cools below 160 degrees Fahrenheit.
"Compared with previous approaches, our design provides a reliable, fast, reversible strategy that can achieve both high battery performance and improved safety," Stanford engineer Yi Cui said. "This strategy holds great promise for practical battery applications."