Diaphragm and battery cell

The diaphragm is used for being wound together with a pole piece to form the battery cell, the diaphragm comprises a first section, a second section and a third section which are sequentially arranged in the length direction of the diaphragm, the width of the first section is smaller than that of the second section, and/or the width of the third section is smaller than that of the second section. The diaphragm is divided into areas with different widths, the width of the area at the head or the tail of the diaphragm is smaller than that of the middle position of the diaphragm, and the width of the head of the diaphragm is narrowed, so that the contact area of the diaphragm and the winding needle can be reduced, the friction force between the surface of the diaphragm and the winding needle is reduced, and the diaphragm is prevented from being pulled out when the winding needle is pulled out; the problem of diaphragm core pulling is solved; the width of the tail part of the diaphragm is narrowed, that is, the w