Abstract
Relying on redox reactions, most batteries are limited in their ability to operate at
very low or very high temperatures. While performance of electrochemical capacitors is less
dependent on the temperature, present-day devices still cannot cover the entire range needed for
automotive and electronics applications under a variety of environmental conditions. We show
that the right combination of the exohedral nanostructured carbon (nanotubes and onions)
electrode and a eutectic mixture of ionic liquids can dramatically extend the temperature range of
electrical energy storage, thus defying the conventional wisdom that ionic liquids can only be
used as electrolytes above room temperature. We demonstrate electrical double layer capacitors
able to operate from 50 to 100 C over a wide voltage window (up to 3.7 V) and at very high
charge/discharge rates of up to 20 V/s.