JPH0551143B

PURPOSE:To improve safety by using an insulating porous film of which break elongation is small in the direction in which break strength is large, when the break strength and break elongation of a separator are isotropic or non- isotropic. CONSTITUTION:When the break strength and break elongation of a separator are isotropic or non-isotropic an insulating porous film is used of which break elongation is small in the direction in which break strength is large. When a mechanical external force is applied to separators, they are almost simultaneously broken without being limited to a special direction such as the direction parallel or perpendicular to the winding direction. Therefore the area of breakage becomes large and the short-circuit current is not concentrated to a partial area, which represses abnormal temperature rise. It results in the improvement of safety.