An energy storage device comprising at least one negative electrode, wherein each negative electrode is individually selected from (i) an electrode comprising negative battery electrode material; (ii)an electrode comprising capacitor electrode material; (iii) a mixed electrode comprising either - a mixture of battery and capacitor electrode material or - a region of battery electrode material anda region of capacitor electrode material, or - a combination thereof, and wherein the energy storage device either comprises at least one electrode of type (iii), or comprises at least one electrodeof each of types (i) and (ii), - at least one positive electrode, wherein the positive electrode comprises positive battery electrode material and a charging ability-increasing additive, such as one or a mixture of: (a) carbon nanomaterial, vapour grown carbon fibre, fullerene, or a mixture thereof, and (b) tin dioxide conductive materials.