Apart from positive and negative electrodes, each energy storage cell/device contains electrolyte and a separator (to prevent short circuit between electrodes), current collectors and casing (Fig. 3d). Therefore, for many state-of-the-art energy storage devices, especially small ones, the weight of the overall device is 5–10 times the total weight of the positive and negative electrodes due to combined contributions of all device components20. Thus, to minimize the weight/volume of the inactive materials, several approaches can be applied: