Disclosed herein is a battery cell configured to have a structure in which an electrode assembly, including positive electrodes, negative electrodes, and separators disposed respectively between the positive electrodes and the negative electrodes, is mounted in a battery case, wherein the electrode assembly includes two or more electrodes or unit cells stacked in a height direction when viewed in a plan view, the battery case is provided with a receiving unit, in which electrode assembly is mounted, the electrode assembly is configured to have a quadrangular shape having side (a), side (b), side (c), and side (d) when viewed in a plan view, and inclined such that an interior angle between side (a) of each of the electrode plates or the unit cells, at which an electrode terminal is located, and side (b) adjacent to side (a) is less than 90 degrees, and the receiving unit of the battery case has an internal shape corresponding to a planer shape of the electrode assembly.