Case assembly, prismatic secondary battery, and fabrication method thereof

A prismatic secondary battery comprising:

• an electrode assembly comprising a first electrode plate, a second electrode plate, and a separator between the first and second electrode plates;
• a case formed of a first material, wherein an electrode assembly accommodation cavity is formed in the case to accommodate the electrode assembly, and a plurality of walls that define an opening through which the electrode assembly is to be inserted;
• and
• a cap plate sealing the opening, the cap plate having an electrolyte inlet configured to receive an electrolyte prior to a pre-charging operation,
• wherein a gas discharge hole is formed in one of the walls of the case comprising a passage configured to discharge gas generated during the pre-charging operation, and
• the gas discharge hole is sealed with a gas seal formed of a second material that is less rigid than the first material, the gas seal configured to be broken before the case is broken,
• wherein the second material is a flexible pouch-type material comprising:
• a thin metal layer;
• and
• a bonding layer formed of an insulation material,
• wherein the bonding layer thermally fuses the thin metal layer to the case.