Energy storage device

An energy storage device, comprising:

• an electrode unit in which a cathode having a cathode lead, an anode having an anode lead, and a separator located between the cathode and the anode to separate the cathode and the anode from each other are rolled together;
• a housing receiving the electrode unit;
• an electrolyte filled in the housing;
• an inner terminal arranged in the housing to face the electrode unit;
• an outer terminal connected to the inner terminal;
• and
• a pressure relief unit for relieving an inner pressure of the housing,
• wherein a groove is formed in a side of the inner terminal, and a side protrusion is formed on an inner wall of the housing at a location corresponding to the groove,
• wherein openings are respectively formed in the inner terminal and the outer terminal for the communication with the outside, and
• wherein the pressure relief unit is installed in the openings and selectively opens the openings in association with the change of the inner pressure of the housing when the inner pressure is increased over an allowable level.