An energy storage device for a reversible storage of energy has a reversibly designed metal/metal oxide storage unit for indirectly storing energy in form of a fluid material and a reversibly designed electrolysis device for providing and using the fluid material in an electrolysis reaction. The metal/metal oxide storage unit is disposed spatially separated from the electrolysis device. A fluid exchanging unit is provided for exchanging the fluid material between the reversibly designed metal/metal oxide storage unit and the electrolysis device, and a heat exchanging unit is provided for exchanging thermal energy between the metal/metal oxide storage unit and the electrolysis device. Further, a method for a reversible storage of energy is provided.