Device for storing electrical energy

An energy device having high input-output performance, in particular being excellent in low temperature performance. An energy device characterized by storing and discharging electric energy by both a faradaic reaction mechanism wherein mainly the oxidation state of an active material changes and electric charge transfers inside said active material and a non-faradaic reaction mechanism wherein mainly ions are physically adsorbed and desorbed on the surface of an active material and resultantly electric charge is accumulated and discharged. Further, output performance at a low temperature is improved by providing an energy device characterized by storing and discharging electric energy by at least two kinds of reaction mechanisms that show low and high reaction rates respectively in faradaic reaction wherein mainly the oxidation state of an active material changes and electric charge transfers to said active material through an electrode interface. The different reaction mechanisms take place on different areas of the electrodes.