Secondary Battery

A secondary battery comprises an electrode assembly comprising radical units in which first and second electrodes are alternately laminated with a separator therebetween and a separation film surrounding the radical units to be repeatedly laminated, wherein the electrode assembly comprises a protection unit laminated on an outermost separation film disposed on the uppermost end or lowermost end of the separation film, the protection unit has a structure comprising a second protection electrode, a separator, and a first protection electrode which are successively laminated from the outermost separation film disposed on the uppermost end or lowermost end of the electrode assembly, the second protection electrode has a cross-sectional coating structure comprising a second collector and a second electrode active material portion coated with a second electrode active material on an inner surface of the second collector, and the inner surface is a surface facing a center of the electrode assembly.