Polyolefin microporous membrane and lithium ion secondary battery using same

摘要

A polyolefin microporous film having a laminated structure comprising at least one layer each comprising a layer A containing polyolefin and a layer B containing polyolefin. The polypropylene contained in the said layer A is 0 mass % or more and less than 3 mass %, and the polypropylene contained in the said B layer is 1 mass % or more and less than 30 mass %. When the ratio of the polypropylene contained in A-layer is PPA (mass %) and the ratio of the polypropylene contained in B-layer is PPB (mass %), PPB>PPA. The polyolefin microporous membrane is MD, and the following formula: load (gf) = 0.01 x puncture strength (gf) of polyolefin microporous membrane x length (mm) of TD of polyolefin microporous membrane (mm) The thermal contraction rate of TD in 120 degreeC measured is 10 % or more and 40 % or less.