The heart of future hybrid automobiles, which will run on both gasoline and electric power, will be the lithium-ion batteries that store the electricity to power these vehicles. These batteries will have to be efficient, inexpensive and safe. Plastics will play a vital role in such batteries, particularly in their use as separator membranes between the anodes and cathodes of these devices. To meet the performance criteria of advanced lithium-ion vehicle batteries, separator membranes must be highly porous, so that ions can pass through them easily when the battery is being charged or discharged. Yet the membranes must lose that porosity, thus shutting off the flow of current, if the battery starts to overheat. Such overheating can occur if a battery is overcharged, or if manufacturing defects cause a short circuit.