Lithium-ion batteries typically contain two electrodes and an electrolyte. Shorting or overcharging the battery makes it generate heat. If the temperature reaches about 300 degrees Fahrenheit (150 degrees Celsius), the electrolyte can catch fire and explode.
There have been several attempts to make safer lithium-ion cells, but often these safety measures render them unusable after overheating. Stanford University researchers have a new method to protect from overheating cells that uses–what else–nanotechnology graphene. The trick is a thin film of polyethylene that contains tiny nickel spikes coated with graphene (see electron micrograph to the right).
The film conducts electricity from one electrode because the nickel spikes touch each other at normal temperatures. If the cell overheats, the polyethylene film expands, and the spikes no longer touch each other, breaking the circuit. Once the cell cools, the film contracts, the nickel spikes make contact again, and the cell resumes normal operation.