A microporous polyethylene (PE) film has been developed for use as the separator of a lithium (Li) ion secondary battery (LIB). LIBs are necessary in modern society as a power supply for portable equipment such as cellular phones and notebook computers. The greatest problem with using LIBs has been ensuring safety when using a Li compound and a flammable organic electrolytic solution. The most important point for ensuring the safety of LIBs has been using a separator to prevent contact between the cathode and the anode. The Asahi Kasei Corporation has developed the safety function of the separator and improved the performance of the LIB. The manufacture of battery separators made of microporous PE films was studied for PEsolvent systems (two-component phase-separation systems), as well as for new systems in which an inorganic powder was added to the PE and the solvent in this two-component system to produce three-component phase-separation systems. This method is based on thermally induced phase separation.