The present invention provides a battery cell comprising an electrode assembly, which comprises one or more unit cells, and which is contained in a variable cell case while being impregnated with an electrolyte, the battery cell being characterized in that at least one of the unit cells comprises a flexible electrode that can be bent or curved, the flexible electrode has an electrode collector comprising a first surface, to which an electrode active material is applied, and a second surface, which faces the first surface, and which has no electrode active material applied thereto, and the second surface has a mesh formed thereon for the sake of flexibility of the electrode.