A separator for a bobbin-style electrochemical cell is inserted into an interior opening within a ring-shaped cathode in an electrochemical cell can. An expansion force is then applied to an interior surface of the separator to press the separator against the interior walls of the cathode. A tool may then remove various creases and/or wrinkles in the separator and/or may then heat seal at least a portion of the tubular walls of the separator to minimize the void space between the separator and active material (e.g., cathode and/or anode) within the electrochemical cell.