It is important to note that for many applications, in particular for portable electronics and vehicles, volumetric energy density is more important than gravimetric energy density. In this case, to balance the performance of electrodes in asymmetric cells, similar considerations are applied, but instead volumetric capacitance (Farads per cubic centimetre) and electrode volume are used rather than gravimetric capacitance (Farads per gram) and mass. Volumetric and gravimetric performance are connected through electrode density, and to a large extent are defined by electrode design and porosity20.