An energy storage device comprising a container, a mandrel, at least one sheet of separator material, and two or more electrodes. The container comprises an inner surface. The mandrel comprises a mandrel surface, and is positioned within the container so that the mandrel surface is spaced apart from the inner surface to define a cavity within the container. The container has a packing axis that passes through the cavity, the mandrel surface, and the inner surface. The mandrel is compressible in the direction of the packing axis, the at least one sheet of separator material is arranged in the cavity to provide a plurality of separator layers along the packing axis, and an electrode is provided between the separator layers.