Disclosed herein is a battery cell configured to have a structure in which an electrode assembly including a separator disposed between a cathode and an anode is mounted in a battery case, wherein the battery case includes an upper case and a lower case, the upper case and/or the lower case being provided with a receiving part, in which the electrode assembly is mounted, the electrode assembly includes a plurality of electrodes or unit cells stacked in a height direction on the basis of a plane, two or more of the electrodes or the unit cells having different planar sizes, and the receiving part of the battery case is provided with stair-like steps corresponding to an external appearance of the electrode assembly.