Microporous membrane, the membrane preparation and the use as a battery separator membrane

The present invention relates to a microporous film having a high melting temperature and low shutdown temperature, and heat shrinkage at a high temperature. The film was prepared as follows: stretching a sheet containing polymethylpentene, polyethylene and the diluent, and then removing the diluent. The film may be used, for example, a battery separator of a lithium ion battery.