BASE FOR LITHIUM ION SECONDARY BATTERY SEPARATORS, METHOD FOR PRODUCING BASE FOR LITHIUM ION SECONDARY BATTERY SEPARATORS, AND LITHIUM ION SECONDARY BATTERY SEPARATOR

A base material for a lithium ion secondary battery separator provided by the present invention comprises a polyethylene terephthalate fiber, in which an average fiber diameter of the polyethylene terephthalate fiber is 9.0 µm or less, a specific X-ray diffraction intensity derived from the polyethylene terephthalate fiber is 300 cps/ (g/m 2 ) or more, and a coefficient of variation of a specific X-ray diffraction intensity is 12.0% or less, and is excellent in the workability during production and excellent in mechanical strength, uniformity and handling in a subsequent treatment step.