Hybrid devices constitute a special case of asymmetric cells. In hybrid devices, different charge storage mechanisms are implemented in the positive and negative electrode materials (Fig. 3a–c). For example, one electrode utilizes the double-layer storage mechanism (that is, porous carbon) while the other stores charge by means of faradaic reactions (that is, as occurs in transition metal oxides)2. The concept of asymmetric cells is very compelling, allowing us to produce an ‘ideal' combination of positive and negative electrode materials and to perform ‘smart' balancing of their weight/volume, so that overall device performance can be improved significantly in terms of voltage window, capacitance (and therefore energy density) and longevity in comparison with their symmetric counterparts21.