Polyolefin microporous film and preparation method thereof

A preparation method of a polyolefin microporous film, characterized by the step of mixing polyolefin resins with viscosity average molecular weights ranging from 3 to 5 million, polyolefin resins with viscosity average molecular weights from 0.4 to 2 million and additives to form a mixture, extruding the mixture, and forming the mixture into a cast sheet at 0° C. or below;

• wherein the polyolefin resins with viscosity average molecular weights ranging from 3 to 5 million comprise polyolefin resins with viscosity average molecular weights of 3.5 million, and polyolefin resins with viscosity average molecular weights from 0.4 to 2 million comprise polyolefin resins with viscosity average molecular weights of 1.5 million and polyolefin resins with viscosity average molecular weights of 0.5 million;
• wherein the polyolefin microporous film has the following characteristics:
• a. porosity above 30% and below 65%;
• b. a median pore size above 10 nm and below 60 nm;
• c. stress-strain (σ-ε) curve integral in Machine Direction (MD) and Transverse Direction (TD) directions simultaneously meet the following definition:
• E=∫0εσ(ε)dε>150 J/m2;
• d. largest pore size, smallest pore size and the median pore size meet the following definition:
• 0.9<P<1.2, wherein P=(the largest pore size−the smallest pore size)/the median pore size.