Patent ID: 7892672

Claim:
A nonaqueous secondary battery separator comprising a polyolefin microporous membrane base, and a heat resistant porous layer formed on at least one surface of the membrane base and made of a heat resistant resin having a melting point of 200° C. or more, wherein the membrane base has (A) a thickness of 5 to 20 μm, (B) a porosity of 35 to 50%, (C) an air permeability per unit thickness (JIS P8117) of 10 to 30 sec/100 cc·μm and a total air permeability of the membrane (JIS P8117) of 400 sec/100 cc or less, and (D) Y/X of 1×10 −3 to 1×10 −2 ohm·cm 2 /(sec/100 cc), wherein the air permeability (JIS P8117) is represented by X sec/100 cc, and the membrane resistance upon impregnating the polyolefin microporous membrane with an electrolytic solution is represented by Y ohm·cm 2 .