Battery separator, and battery separator manufacturing method

1. A battery separator comprising: a porous membrane A comprising a polyethylene resin, and a porous membrane B laminated thereon comprising a heat resistant resin and inorganic particles or cross-linked polymer particles,

wherein the porous membrane A satisfies expressions (a) to (c) below, the battery separator satisfies (d) to (f), and the heat resistant resin comprises a fluororesin:

t ( A )<10 μm  (a)

wherein t(A) is a thickness of the porous membrane A;

0.01 μm≦ R ( A )≦1.0 μm  (b)

wherein R(A) is an average pore size of the porous membrane A;

30%≦ V ( A )≦70%  (c)

wherein V(A) is a porosity of the porous membrane A:

t ( T )≦13 μm  (d)

wherein t(T) is a thickness of the battery separator;

F ( A/B )≧1.0 N/25 mm  (e)

wherein F(A/B) is a peeling strength at an interface between the porous membrane A and the porous membrane B; and

20≦ Y−X≦ 100  (f)

wherein X is an air resistance (sec/100 cc Air) of the porous membrane A, and

Y is an air resistance (sec/100 cc Air) of the battery separator.

2. A process of producing the battery separator according to claim 1 , comprising:

(i): applying a varnish comprising a heat resistant resin which comprises a fluororesin and inorganic particles to a porous membrane A comprising a polyethylene resin, and then passing the porous membrane A through a low humidity zone at an absolute humidity of not less than 0.5 g/m 3 and less than 6 g/m 3 and a high humidity zone at an absolute humidity of not less than 7.0 g/m 3 and less than 25.0 g/m 3 to form a heat resistant resin membrane on the porous membrane A; and

(ii): immersing the composite membrane obtained in (i), in which the heat resistant resin membrane is laminated, in a coagulation bath to convert the heat resistant resin membrane into a porous membrane B, and washing and drying the porous membrane B to obtain the battery separator.