NOVELTY - The energy storage device comprises a first sheet (110) of a conductive material, a sheet (120) of a substance disposed on the first sheet of the conductive material, and a second sheet (130) of same or different conductive material disposed on the sheet of the substance. The substance is able to conduct free ions. The first and/or second sheets comprise a metal foil layer and a carbon nanotube layer, which is arranged to face the sheet of the substance, and is directly grown on the metal foil layer. The first and second sheets and the sheet of the substance form a multi-layered stack.    USE - The energy storage device is useful as portable electronic devices such as lithium ion batteries, lithium metal batteries and supercapacitors.    ADVANTAGE - The energy storage device can be economically manufactured with light weight, good durability, high surface area, improved power and energy density, good flexibility, enhanced charge storage capacity and good mechanical strength, electrical conductivity and thermal conductivity without any physical damage.    DETAILED DESCRIPTION - The energy storage device comprises a first sheet (110) of a conductive material, a sheet (120) of a substance disposed on the first sheet of the conductive material, and a second sheet (130) of same or different conductive material disposed on the sheet of the substance. The substance is able to conduct free ions. The first and/or second sheets comprise a metal foil layer and a carbon nanotube layer, which is arranged to face the sheet of the substance, and is directly grown on the metal foil layer. The first and second sheets and the sheet of the substance form a multi-layered stack. The device further comprises a first insulating sheet (140) and a second insulating sheet (150) disposed on outer surfaces of the multi-layered stack respectively. The device has larger width and length, and is rolled up or folded and then hermetically sealed to form an energy storage unit, which is a rechargeable battery or a capacitor. The first and the second conductive sheets are configured to be engaged with an external energy source or drain. The metal foil has a thickness of 5-100 microns. The carbon nanotube layer is directly grown on the metal foil by coating a catalyst on a surface of the metal foil by low temperature evaporation of the catalyst, annealing the catalyst coated metal foil in ammonia gas at a first temperature, and growing the carbon nanotubes directly on the catalyst coated surface of the metal foil in a hydrocarbon gas atmosphere at a second temperature. The first temperature is lower than the second temperature, which is not more than 550 degrees C. The sheet of the substance comprises a sheet of microperforated plastic film and a room temperature ionic liquid electrolyte disposed on surfaces of the plastic film. The carbon nanotubes in the carbon nanotube layer are partially aligned in a direction nearly perpendicular to the surface of the metal foil. INDEPENDENT CLAIMS are included for:    (1) a process for forming a layer of carbon nanotubes on a flexible metal foil; and    (2) a method of forming energy storage device.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a thin film energy storage device structure.    First sheet (110)    Sheet of a free ion conductive electrolyte/separator (120)    Second sheet (130)    First insulator sheet (140)    Second insulator sheet. (150)