Researchers at Stanford University have discovered a solution to the lithium-ion battery’s downfall—its propensity for self-combustion.
This design flaw has caused explosions in many of the portable devices that rely on the incredibly powerful and lightweight batteries. Tesla electric cars have caught fire, and several iPods have gone up in flames. And recent hoverboard combustions have caused 29 emergency room visits and at least 11 fires throughout the US.
The new technology could prevent the kinds of fires that have prompted recalls and bans of battery-powered devices, and even prompted Amazon to urge customers to throw out their hazardous hoverboards.
The fix is no more than a thin polyethylene film that turns off the battery when it is overheated and restarts it immediately after it has cooled to a suitable temperature.
“People have tried different strategies to solve the problem of accidental fires in lithium-ion batteries,” says Zhenan Bao, a professor of chemical engineering at Stanford. “We’ve designed the first battery that can be shut down and revived over repeated heating and cooling cycles without compromising performance.”
Why use a battery that might burst into flames?
Designers use lithium-ion batteries because they pack power without adding excessive weight or bulk. Lithium is the least dense metallic element, but as a member of the alkali metal group, it is also a highly reactive substance.
A typical lithium-ion battery consists of two electrodes separated by a liquid or gel electrolyte that carries charged particles between the electrodes. Puncturing, shorting or overcharging a battery produces heat. If the temperature rises to 150° C, the electrolyte can catch fire and explode.
The key: thermal expansion
The researchers coated spiky nickel particles within a layer of carbon and embedded the particles in a thin film of elastic polyethylene. The resulting strip of plastic is placed between two battery electrodes, allowing an electrical current to flow through.
“To conduct electricity, the spiky particles have to physically touch one another,” says the study’s lead author Zheng Chen. “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.”
When heated above 70° C, the polyethylene film expands, causing the spiky particles to separate and the battery to shut down. But when the temperature drops back down to 70° C, the film shrinks, allowing the particles to touch and the battery to resume generating electricity.
The design can be customised to turn off at a chosen temperature, depending on the concentration of nickel particles and the type of polymer used.
“Compared with previous approaches, our design provides a reliable, fast, reversible strategy that can achieve both high battery performance and improved safety,” engineer and study co-author Yi Cui says. “This strategy holds great promise for practical battery applications.”