A battery pack includes an electrode assembly having a cathode, an anode, and a separator interposed therebetween, a first pack case having the electrode assembly and an electrolyte disposed therein, a second pack case with the first pack case disposed therein and a portion of the interior surface separated from the first pack case exterior surface, a fire extinguishing agent disposed between the exterior surface of the first pack case and the interior surface of the second pack case and a cap assembly that simultaneously seals an open surface of the first and second pack cases when the first pack case is disposed in the second pack case. A first venting portion is configured to open to enable the fire extinguishing agent to flow into the interior of the first pack case when the pressure within the first pack case is equal to or greater than a critical pressure.