Hybrid Energy Storage Device

A hybrid energy storage device has at least two half cells, wherein each half cell includes an electrode comprising an electrically conductive high surface area material incorporating an electrolyte comprising a dissolved species that can exist in more than two redox states, and at least one separator that separates the at least two half cells and allows transfer of selected charge carriers between the half cells. After an initial charging, a redox pair of one half cell is different from the redox pair of the other half cell. The hybrid energy storage device operates as a battery for low power applications, and as a supercapacitor for high power applications. The hybrid energy storage device may be flexible.