A flexible secondary battery comprising: an electrode stack structure comprising: a first electrode layer (10, 100), a second electrode layer (11, 101), and a separator between the first electrode layer and the second electrode layer, characterized by: the first electrode layer comprising a first metal current collector, the second electrode layer comprising a second metal current collector, the flexible secondary battery further comprising: connection tabs (10a, 11 a) respectively extended from an end portion of the first metal current collector at a first end portion of the first electrode layer and an end portion of the second metal current collector at a first end portion of the second electrode layer; and a fixing element (13, 13a, 13b, 13c, 13d) which fixes the end portions of the first and second metal current collectors only at a first end portion of the electrode stack structure, wherein second end portions of the first and second electrode layers opposite to the first end portions thereof are movable. an electrode stack structure comprising: a first electrode layer (10, 100), a second electrode layer (11, 101), and a separator between the first electrode layer and the second electrode layer, a first electrode layer (10, 100), a second electrode layer (11, 101), and a separator between the first electrode layer and the second electrode layer, characterized by: the first electrode layer comprising a first metal current collector, the second electrode layer comprising a second metal current collector, the flexible secondary battery further comprising: connection tabs (10a, 11 a) respectively extended from an end portion of the first metal current collector at a first end portion of the first electrode layer and an end portion of the second metal current collector at a first end portion of the second electrode layer; and a fixing element (13, 13a, 13b, 13c, 13d) which fixes the end portions of the first and second metal current collectors only at a first end portion of the electrode stack structure, wherein second end portions of the first and second electrode layers opposite to the first end portions thereof are movable. the first electrode layer comprising a first metal current collector, the second electrode layer comprising a second metal current collector, the flexible secondary battery further comprising: connection tabs (10a, 11 a) respectively extended from an end portion of the first metal current collector at a first end portion of the first electrode layer and an end portion of the second metal current collector at a first end portion of the second electrode layer; and a fixing element (13, 13a, 13b, 13c, 13d) which fixes the end portions of the first and second metal current collectors only at a first end portion of the electrode stack structure, wherein second end portions of the first and second electrode layers opposite to the first end portions thereof are movable. connection tabs (10a, 11 a) respectively extended from an end portion of the first metal current collector at a first end portion of the first electrode layer and an end portion of the second metal current collector at a first end portion of the second electrode layer; and a fixing element (13, 13a, 13b, 13c, 13d) which fixes the end portions of the first and second metal current collectors only at a first end portion of the electrode stack structure, wherein second end portions of the first and second electrode layers opposite to the first end portions thereof are movable.