High-Strength Separator

An electricity storage device separator comprising a polyolefin microporous membrane that is used as a separator for an electricity storage device comprising an ultra-high molecular weight polyethylene (UHMWPE),

• wherein a degree of crystallinity (X) of all polyethylenes in the polyolefin microporous membrane is 80 to 99% as measured by X-ray diffraction (XRD), when calculated by the following formula:
• X
• =
• {
• I
• ⁢
• (
• 110
• )
• +
• I
• ⁢
• (
• 200
• )
• }
• /
• {
• I
• ⁢
• (
• 110
• )
• +
• I
• ⁢
• (
• 200
• )
• +
• Iamr
• }
• wherein I (110) is the area of the (110) plane diffraction peak, I (200) is the area of the (200) plane diffraction peak and Iamr is the area of amorphous peak,
• wherein the crystallite size of all polyethylenes in the polyolefin microporous membrane is 14.2 to 40.0 nm as measured by XRD, and
• wherein the cross-sectional crystal orientation of the polyolefin microporous membrane is 0.80 to 0.99.