Using nanotechnology, they created a flexible film of spiky nickel particles coated with graphene and an atom-thick layer of carbon. The film is conductive to electricity so it can be placed around the Lithium-ion battery and connected to its electrodes. When the temperature is within the normal range, the particles connect through their spikes and let the current through. When the temperature rises, the film expands, taking the spikes out of contact and stopping the current. The video below explains the idea better than I could, and there's a demo at the end showing how the rise in temperature (when the blowtorch is used) causes the current to halt and how the return to normal temperature brings it back to life, which demonstrates the reversibility of the process
his could be used as a fail-safe mechanism in batteries, forcing them to shut down when they overheat. And the researchers think that by changing the amount of particles and the polymers used, they can fine tune the functional temperature of the film. That's super cool. It's nowhere near being implemented in our current devices, but I hope it's added soon.