A secondary battery comprising: an electrode assembly comprising a plurality of radical units in which a first electrode and a second electrode are alternately laminated with a separator therebetween and a separation film surrounding the plurality of radical units to be repeatedly laminated, characterized in that 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, wherein a second protection electrode, a separator, a first protection electrode 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, wherein the inner surface is a surface facing a center of the electrode assembly and the second electrode active material is applied to only one surface of the second protection electrode, and the first protection electrode has a cross-sectional coating structure comprising a first collector and a first electrode active material portion coated with a first electrode active material on an inner surface of the first collector, wherein the inner surface of the first collector is a surface facing the center of the electrode assembly and the first electrode active material is applied to only one surface of the first protection electrode,the second collector has non-coating portion which is not coated with the second electrode active material, is formed on an outer surface of the second collector, and the second collector provided in the second protection electrode has a thickness greater than that of the second collector provided in the second electrode of the radical unit. an electrode assembly comprising a plurality of radical units in which a first electrode and a second electrode are alternately laminated with a separator therebetween and a separation film surrounding the plurality of radical units to be repeatedly laminated, characterized in that 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, wherein a second protection electrode, a separator, a first protection electrode 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, wherein the inner surface is a surface facing a center of the electrode assembly and the second electrode active material is applied to only one surface of the second protection electrode, and the first protection electrode has a cross-sectional coating structure comprising a first collector and a first electrode active material portion coated with a first electrode active material on an inner surface of the first collector, wherein the inner surface of the first collector is a surface facing the center of the electrode assembly and the first electrode active material is applied to only one surface of the first protection electrode,the second collector has non-coating portion which is not coated with the second electrode active material, is formed on an outer surface of the second collector, and the second collector provided in the second protection electrode has a thickness greater than that of the second collector provided in the second electrode of the radical unit.