A plate-shaped battery cell comprising an electrode assembly (210; 300; 400; 500; 600; 700), which comprises a positive electrode, a negative electrode, and a separator, and a cell case, in which the electrode assembly is mounted, sealed outer edges (222a, 222b, 224, 226, 228) of the cell case being sealed by thermal bonding, characterized in that the electrode assembly (210; 300; 400; 500; 600; 700) is configured such that each electrode plate having the same polarity is partitioned into at least two electrode parts (214a, 214b; 250, 260; 350, 360; 450, 460, 470, 480; 550, 560a, 560b, 570; 650, 660; 750, 760) having different planar shapes and sizes forming a step due to a differences in size between the electrode parts (214a, 214b; 250, 260; 350, 360; 450, 460, 470, 480; 550, 560a, 560b, 570; 650, 660; 750, 760), the at least two electrode parts (214a, 214b; 250, 260; 350, 360; 450, 460, 470, 480; 550, 560a, 560b, 570; 650, 660; 750, 760) having different planar shapes and sizes are partitioned from each other by a boundary line and an outside corner (230; 330) is the intersection between the boundary line and outer edges (211a, 211b, 251a, 251b) of the electrode parts (214a, 214b; 250, 260; 350, 360; 450, 460, 470, 480; 550, 560a, 560b, 570; 650, 660; 750, 760), the outside corner (230; 330) has an angle of 30 to 150 degrees, an outside recess (212; 312; 420, 430; 512, 513, 514) is formed in a region of each of the electrode plates and the separator at the outside corner (230; 330) such that the outside recess (212; 312; 420, 430; 512, 513, 514) is formed so as to be recessed inward, the cell case is further sealed at a position thereof corresponding to the outside recess (212; 312; 420, 430; 512, 513, 514) by thermal bonding so as to have an area corresponding to an area of the outside recess (212; 312; 420, 430; 512, 513, 514), and the cell case comprises: the sealed outer edges (222a, 222b, 224, 226, 228) sealed along the outer edges (211a, 211b, 251a, 251b) of the electrode assembly in parallel therewith by thermal bonding, excluding the outside recess (212; 312; 420, 430; 512, 513, 514); and a sealed extension sealed by thermal bonding at a position thereof corresponding to the outside recess (212; 312; 420, 430; 512, 513, 514). the electrode assembly (210; 300; 400; 500; 600; 700) is configured such that each electrode plate having the same polarity is partitioned into at least two electrode parts (214a, 214b; 250, 260; 350, 360; 450, 460, 470, 480; 550, 560a, 560b, 570; 650, 660; 750, 760) having different planar shapes and sizes forming a step due to a differences in size between the electrode parts (214a, 214b; 250, 260; 350, 360; 450, 460, 470, 480; 550, 560a, 560b, 570; 650, 660; 750, 760), the at least two electrode parts (214a, 214b; 250, 260; 350, 360; 450, 460, 470, 480; 550, 560a, 560b, 570; 650, 660; 750, 760) having different planar shapes and sizes are partitioned from each other by a boundary line and an outside corner (230; 330) is the intersection between the boundary line and outer edges (211a, 211b, 251a, 251b) of the electrode parts (214a, 214b; 250, 260; 350, 360; 450, 460, 470, 480; 550, 560a, 560b, 570; 650, 660; 750, 760), the outside corner (230; 330) has an angle of 30 to 150 degrees, an outside recess (212; 312; 420, 430; 512, 513, 514) is formed in a region of each of the electrode plates and the separator at the outside corner (230; 330) such that the outside recess (212; 312; 420, 430; 512, 513, 514) is formed so as to be recessed inward, the cell case is further sealed at a position thereof corresponding to the outside recess (212; 312; 420, 430; 512, 513, 514) by thermal bonding so as to have an area corresponding to an area of the outside recess (212; 312; 420, 430; 512, 513, 514), and the cell case comprises: the sealed outer edges (222a, 222b, 224, 226, 228) sealed along the outer edges (211a, 211b, 251a, 251b) of the electrode assembly in parallel therewith by thermal bonding, excluding the outside recess (212; 312; 420, 430; 512, 513, 514); and a sealed extension sealed by thermal bonding at a position thereof corresponding to the outside recess (212; 312; 420, 430; 512, 513, 514). the sealed outer edges (222a, 222b, 224, 226, 228) sealed along the outer edges (211a, 211b, 251a, 251b) of the electrode assembly in parallel therewith by thermal bonding, excluding the outside recess (212; 312; 420, 430; 512, 513, 514); and a sealed extension sealed by thermal bonding at a position thereof corresponding to the outside recess (212; 312; 420, 430; 512, 513, 514).