A secondary battery having improved safety includes an electrode assembly, a case configured to accommodate the electrode assembly, and a cap assembly coupled to the case. The cap assembly includes a cap plate configured to cover the case and defining a short-circuit opening therein, a short-circuit plate exterior to the cap plate and positioned such that at least a portion of the short-circuit plate is positioned in the short-circuit opening, a first deformable plate inside the cap plate, and a cover exterior to the short-circuit plate extending over the short-circuit plate. A space defined between the cover and the first deformable plate has a negative pressure.