LITHIUM ION BATTERY SEPARATOR AND LITHIUM ION BATTERY

It is an object of the present invention to provide a lithium ion battery separator which can be made thin, is excellent in tensile strength and cuttability and has low internal resistance, a low incidence of internal short circuit failure and high cycle characteristics and a lithium ion battery including the separator. In a lithium ion battery separator containing heat-resistant fibers and synthetic resin short fibers, fibrillated heat-resistant fibers having a modified freeness of not more than 300 ml are contained as the heat-resistant fibers, and the content of the fibrillated heat-resistant fibers having a modified freeness of not more than 300 ml is not less than 1.0 mass % to less than 5.0 mass % based on the total of all the fiber components contained in the separator. Modified freeness: This is a value measured in accordance with JIS P8121-2:2012 except that an 80-mesh wire net having a wire diameter of 0.14 mm and an opening of 0.18 mm is used as a screening plate and the concentration of a sample is 0.1%.