Electric vehicles are coming, ready or not. And one of the enabling technologies that is making them more driver friendly is the humble battery, particularly lithium-ion versions which can store enough energy to give these cars a reasonable range for city driving.

Of course, car makers are always searching for ways to improve the efficiency, and therefore the range, of these vehicles. And one way to do this is to recover and reuse the energy that would normally be wasted when the brakes slow down a vehicle.

There is a problem doing this with conventional batteries, however. Braking occurs over timescales measured in seconds but that’s much too fast for batteries which generally take many hours to charge. So car makers have to find other ways to store this energy.