Disclosed herein is a battery cell configured such that an electrode assembly of a cathode/separator/anode stack structure is mounted in a changeable cell case in a state in which the electrode assembly is impregnated with an electrolyte, wherein the electrode assembly and the cell case are curved in the same direction on axial vertical sections thereof in a state in which opposite ends of the electrode assembly and opposite ends of the cell case are directed in the same direction about a middle part of the electrode assembly and a middle part of the cell case.