| abstract |
Provided is a battery separator which has extremely little variation in air permeation resistance and does not exhibit large-scale elevation in air permeation resistance, even in the case of a battery separator having comparatively large width such as is required when the size of the battery is increased. A composite porous film which is employed as a battery separator, wherein the composite porous film is formed by laminating a porous film (B) containing heat-resistant resin onto a porous film (A) comprising polyolefin resin, the surface of the porous film (B) on the side that does not face the porous film (A) has a three-dimensional mesh structure having nodes, and the separation interface on the porous film (B) side when the porous film (A) and the porous film (B) are separated is of a film form having at least 100 pores of pore diameter 50 to 500 nm per 10 μm². |