Laptops, airplanes, electric cars, and even the new rolling hoverboards have all faced troubles with lithium-ion batteries bursting into flames. But now, researchers report that they’ve come up with a potential cheap and effective way to prevent batteries from overheating. They’ve added a heat-sensitive polymer sheet to conventional lithium-ion batteries that shuts down their operation if the temperature rises too high. And once the temperature drops, the polymer sheet returns to normal, allowing the battery to come back to life.
A variety of problems can lead to battery fires. If an electrical short occurs within a battery, for example, current funnels through a threadlike pathway between the battery’s two electrodes, creating a hot spot that can trigger a fire. Researchers have come up with a variety of ways to shut down overheating before it leads to catastrophe. For example, a team recently incorporated capsules containing liquid plastic precursors inside lithium-ion batteries. When the batteries started to overheat, the capsules melted, releasing the liquid, which then formed an insulating plastic layer in the center of the battery that shut off the current.
“Unfortunately, these techniques are irreversible, so the battery is no longer functional after it overheats,” says Yi Cui, a battery expert at Stanford University in Palo Alto, California.
In hopes of doing better, Cui turned to one of his long time collaborators at Stanford, chemical engineer Zhenan Bao. Bao and her colleagues had previously designed a set of plastics embedded with nickel nanoparticles that lowered their electrical conductivity when their temperature increased beyond a critical point. Bao and Cui wondered whether a similar approach could improve the safety of lithium-ion batteries. Ultimately, they settled on the idea of creating a polymer film that shut down its conductivity at a temperature above the normal operating temperature of a lithium-ion battery, but one that was still below the point where such batteries would catch fire.
To make their film, the researchers started with nanoscale nickel particles, which readily conduct current when they are packed tightly enough to touch one another. When exposed to the chemicals inside batteries, such particles can quickly degrade. So the scientists coated the nickel particles with atom-thick sheets of carbon, called graphene, which is also conductive but prevents the nickel from degrading. They then embedded their graphene coated nickel particles in polyethylene, creating thin, flexible conducting plastic sheets.
Next, the team incorporated its plastic sheets into a conventional lithium-ion battery. Such batteries consist of two electrodes, separated by an electrolyte and a membrane that allows lithium ions to flow in one direction depending on whether the battery is being charged or discharged. On the outside of the two electrodes are metal current collectors that shuttle electrons produced by the electrochemical reaction in the battery into an external circuit.
Bao, Cui, and their colleagues placed their film between one of the electrodes and the current collector. At normal operating temperatures, the electricity readily passed through the film, allowing the battery to charge and discharge. But when the temperature of the battery was raised to 70°C, the polyethylene in the film swelled, pushing the nickel particles apart from one another. This dropped the conductivity of their films as much as 100 billion–fold in as little as one second, shutting down the movement of charges in the battery, thereby causing the temperature to drop. What’s more, as they report today in Nature Energy, when the battery temperature fell below 70°C, the polymer relaxed back to its original configuration, boosting the conductivity back to normal, restoring the battery’s activity.
“This looks like something that could work,” to prevent batteries from overheating, says Vincent Battaglia, a chemical engineer and battery expert at the Lawrence Berkeley National Laboratory in California. However, he notes that the approach has so far only been demonstrated when the battery’s voltage is operating at normal levels. If an accident causes the voltage to increase, the added plastic insulating layer would act as a resistor, which could drive up the heat inside the battery to a point where it could still catch fire. Nevertheless, Battaglia says, “if it works, it’s a nice idea.” And one that could perhaps even save lives.