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. When the battery cell is mounted in an electronic device the external shape of which is curved or in an electronic device configured such that a battery mounting region thereof is curved, the tight contact between the battery cell and the electronic device is achieved, thereby maximizing space utilization and thus providing high efficiency. Also, electronic devices having aesthetically pleasing appearance and various designs satisfying liking of consumers are developed using the battery cell.