The definition of an asymmetric energy storage cell is very broad as it refers to every combination of positive and negative electrodes whenever there is any difference between the two electrodes (weight, thickness, material and so on). Thus, in designing supercapacitors, it is essential to recognize that a symmetric device (when both electrodes are identical, that is made from the same material and have same mass, thickness and so on) might not be optimal. For example, it is possible to expand the operating potential window, V, by using negative and positive electrodes made of different materials, as in a battery (Fig. 3a). The energy density of a device can be described by the well-known equation: , where Ctot is the capacitance of the device and V is the operating potential window of the cell. Other important considerations for charge storage are that (i) 1/Ctot=1/C++ 1/C−, where C+ and C− are corresponding capacitances (in Farads) of the positive and negative electrodes, (ii) Q+=Q− for the cells with ∼100% coulombic efficiency, where Q+=C+V+ and Q−=C−V− are charges stored on the positive and negative electrodes, respectively and (iii) V+ and V− are the respective potential windows at positive and negative electrodes that are limited by irreversible processes, such as decomposition of electrolyte and over-reduction or over-oxidation of the electrode material, also V=V+−V−, as shown in Fig. 3a18. These fundamental equations provide the basis for characterizing asymmetric cell configurations.