Patent ID: 8748020

Claim:
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; cathode and anode inner terminals respectively arranged in the housing to face the cathode and the anode of the electrode unit; cathode and anode outer terminals respectively connected to the cathode and anode inner terminals; and a pressure relief unit for relieving an inner pressure of the housing, wherein a groove is formed in a side of the cathode 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 cathode inner and outer terminals or the anode inner and outer terminals for the communication with the outside, wherein the opening of the inner terminal has a relatively smaller diameter that the opening of the outer terminal, 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.