A metal-ion electrochemical accumulator, comprising a stack of the following elements: - at least one first and at least one second electrically conductive substrate forming a current collector, each comprising two main faces (21, 22; 31, 32), each supporting at least two electrode active material units (23; 33) defining an electrode (2.1; 3.1), the two units of each of the faces being separated from each other defining a zone (25; 35) free of units, each unit of one face of a substrate being supported in the same zone as a unit of the other face of the same substrate; - at least one electrically insulating separating film (1.1) incorporating an electrolyte, arranged in contact with each of the electrode units supported by one of the faces of the first and of the second substrate; - a flexible packaging arranged to contain the first and the second substrate and the separating film, with leaktightness while at the same time being traversed by a first and a second terminal of the accumulator, which are respectively connected or constituted by a portion of the first or of the second substrate; in which accumulator: - the electrode units of the same substrate have the same polarity; - the electrode units of the second substrate are isometric and superposed with those of the first substrate in the stacking direction; - the electrode units of the second substrate are of polarity opposite to the polarity of those of the first substrate; - the zones free of substrate units are flexible and thus define folding lines of the accumulator; - the substrates have the same cross-section as the units in the zones of its faces supporting them, whereas they are of smaller cross section in the unit-free zones, the unit-free zones including cutouts creating zones without material between two adjacent units on the same face of a substrate. - at least one first and at least one second electrically conductive substrate forming a current collector, each comprising two main faces (21, 22; 31, 32), each supporting at least two electrode active material units (23; 33) defining an electrode (2.1; 3.1), the two units of each of the faces being separated from each other defining a zone (25; 35) free of units, each unit of one face of a substrate being supported in the same zone as a unit of the other face of the same substrate; - at least one electrically insulating separating film (1.1) incorporating an electrolyte, arranged in contact with each of the electrode units supported by one of the faces of the first and of the second substrate; - a flexible packaging arranged to contain the first and the second substrate and the separating film, with leaktightness while at the same time being traversed by a first and a second terminal of the accumulator, which are respectively connected or constituted by a portion of the first or of the second substrate; - the electrode units of the same substrate have the same polarity; - the electrode units of the second substrate are isometric and superposed with those of the first substrate in the stacking direction; - the electrode units of the second substrate are of polarity opposite to the polarity of those of the first substrate; - the zones free of substrate units are flexible and thus define folding lines of the accumulator; - the substrates have the same cross-section as the units in the zones of its faces supporting them, whereas they are of smaller cross section in the unit-free zones, the unit-free zones including cutouts creating zones without material between two adjacent units on the same face of a substrate.