Patent ID: 7785735

Claim:
A lithium secondary battery separator formed by a microporous composite membrane comprising a microporous polyethylene membrane and a polypropylene coating layer formed on at least one surface of said microporous polyethylene membrane, wherein said polypropylene has a mass-average molecular weight within a range of 5,000-500,000, a racemic diad fraction [r] within a range of 0.16-0.84, and solubility of 0.5 g or more in 100 g of toluene at a temperature of 25° C., wherein 0.1-5 g of said coating layer is formed on one or both surfaces of said microporous polyethylene membrane per 1 m 2 , and wherein said microporous composite membrane has the following properties (1)-(6): (1) an air permeability (converted to the value at 25-μm thickness) of 100-3,000 seconds/100 cc, (2) a porosity of 25-95%, (3) a pin puncture strength of 5,000 mN/25 m or more, (4) a shutdown temperature of 120-140° C., (5) a meltdown temperature of 155° C. or higher, and (6) a battery capacity recovery ratio after storing at 80° C. for 30 days [(battery capacity after storing/initial battery capacity)×100] of 70% or more in a lithium secondary battery.