There is disclosed a core for an electrochemical cell that comprises an anode sheet, a cathode sheet, and a separator sheet disposed between the anode sheet and cathode sheet. The anode sheet, cathode sheet, and separator sheet are wound to form a flattened coil structure. The flattened coil structure has opposed flattened sides and opposed arcuate sides. The anode and cathode sheets terminate at the same or different arcuate sides. Further, the separator sheet may terminate at one of the arcuate sides.