Microporous polyolefin membrane, its production method, battery separator, and battery

#This application is a National Stage of International Application No. PCT/JP2007/058215 filed Apr. 9, 2007, claiming priority based on Japanese Patent Application No. 2006-106175, filed Apr. 7, 2006, the contents of all of which are incorporated herein by reference in their entirety.

#The present invention relates to a microporous polyolefin membrane having suitable permeability, mechanical strength, heat shrinkage resistance, compression resistance and electrolytic solution absorption, a method for producing such a microporous polyolefin membrane, a battery separator formed by such a microporous polyolefin membrane, and a battery comprising such a separator.

#Microporous polyolefin membranes are useful as separators for primary batteries and secondary batteries such as lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver-zinc secondary batteries, etc. When the microporous polyolefin membrane is used as a battery separator, particularly lithium ion battery separator, the membrane's performance significantly affects the battery's properties, productivity, and safety. Accordingly, the microporous polyolefin membrane should have appropriate permeability, mechanical properties, heat resistance, dimensional stability, shutdown properties, meltdown properties, etc. As is known, it is desirable for the batteries to have a relatively low shutdown temperature and a relatively high meltdown temperature for improved battery safety properties, particularly for batteries exposed to high temperatures during under operating conditions. High separator permeability is desirable for high capacity of batteries. A separator with high mechanical strength is desirable for improved battery assembly and fabrication properties.

#FIG. 1 is a graph showing a typical pore size distribution curve.

#FIG. 2 is a graph showing the pore size distribution curve of the microporous membrane of Example 3.

#FIG. 3 is a graph showing the pore size distribution curve of the microporous membrane of Example 7.

#[1] Composition of Microporous Polyolefin Membrane

#(1) Polyolefin Composition

#In an embodiment, the polyolefin composition comprises (a) a polyethylene resin, which contains a polyethylene composition comprising ultra-high-molecular-weight polyethylene having a weight-average molecular weight “Mw” of 1×106 or more and a polyethylene having a Mw less than that of the ultra-high molecular weight polyethylene; the percentage of the ultra-high-molecular-weight polyethylene in the polyethylene composition being 7% or less by mass based on the mass of the polyethylene composition. Alternatively, the polyethylene composition comprises the polyethylene having a lower molecular weight (also called the “lower molecular weight polyethylene”) without the ultra-high-molecular-weight polyethylene. In another embodiment, the polyolefin composition comprises (b) a combination of the polyethylene resin and 25% or less by mass of a polypropylene resin based on the total mass of the polyethylene resin and polypropylene resin.