An electrochemical storage cell is disclosed which includes at least one cathode sheet, at least one anode sheet and at least one separator sheet combined to make a core. The core is housed within a rectangular shell with four sides and two ends, sealed with an air-tight seal. The cell further includes a blow out vent in at least one of the two ends of the shell. This blow out vent is adapted to open and release excess pressure above a predetermined level to thereby prevent catastrophic rupture of the shell.