SECONDARY BATTERY

A secondary battery includes: an electrode assembly including a negative electrode tab at a first end and a positive electrode tab at a second end; a case including a pair of first side portions facing each of a pair of long side portions of the electrode assembly, and a pair of second side portions facing each of a pair of short side portions of the electrode assembly, the case being open in directions of the negative electrode tab and the positive electrode tab; and a retainer between at least one of the second side portions and the electrode assembly and including a concave-convex structure.