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 internal space bounded by an internal wall. The mandrel is positioned in the internal space and forms a cavity between a mandrel surface and the internal wall of the container. The sheet of separator material is arranged within the cavity about the mandrel to provide a plurality of discrete separator layers. An electrode is provided between each of the discrete separator layers, the mandrel is compressible, and the shape of the mandrel surface is concentric with the internal wall of the container.