NOVELTY - The battery (100) has an external casing (140) i.e. moisture-proof multi-layered film, sealing a cell stack. The cell stack comprises a set of unit cells (110-130) comprising an electrolyte layer (115) placed between a negative electrode (111) and a positive electrode (113). A polymer film (116) surrounds the electrodes and the electrolyte layer. The negative electrode comprises a negative active material layer placed on a surface of a negative current collector. The positive electrode comprises a positive active material layer placed on a surface of a positive current collector.    USE - Battery i.e. lithium secondary battery, for use as a power source for a flexible electronic device (all claimed) such as smart device, electronic book, body-attached electronic device and medical device. Can also be used as a lithium ion battery and a jelly-roll type battery.    ADVANTAGE - The battery is designed such that elements can be bonded using heat when the battery is prepared, so that deterioration of the elements of the battery caused by operations at high temperature can be prevented. The polymer film is in sliding contact with another polymer film when normal force is applied to the battery so as to provide flexibility to the battery, thus preventing breaking of the battery. The polymer film easily slides on the surfaces of the latter polymer film when the former polymer film contacts with the latter polymer film due to low friction coefficient of contact surfaces between the polymer films. The polymer film allows the negative electrode and the positive electrode, which are placed at sides of the electrolyte layer to closely contact each other when the battery is repeatedly folded and unfolded, so that relative positions of the negative electrode and the positive electrode can be stably maintained to realize stable battery performance.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of a flexible battery.    Battery (100)    Unit cells (110-130)    Negative electrodes (111, 112, 121, 122, 131, 132)    Positive electrodes (113, 114, 123, 124, 133, 134)    Electrolyte layers (115, 125, 135)    Polymer films (116, 126, 136)    External casing (140)