SECONDARY BATTERY

A secondary battery having weldability between a current collector plate and an electrode uncoated portion by increasing an absorption rate of a laser beam irradiated onto the current collector plate. The secondary battery includes an electrode assembly including a first electrode plate having a first electrode-uncoated portion protruding to one side, a second electrode plate having a second electrode-uncoated portion protruding to an opposite side, and a separator between the first and second electrode plates; a case accommodating the electrode assembly and having a bottom surface and long and short side surfaces extending upward from the bottom surface; and a first current collector plate having a flat portion, parallel to a short side surface, that is laser-welded to the first electrode-uncoated portion, and a protrusion portion protruding from the flat portion toward the short side surface. A surface treatment part is on the flat portion.