A novel vapor induced demixing method is introduced to prepare poly (ether ether ketone) separator for high temperature lithium ion batteries. This method takes advantages of the hygroscopic property of the solvent, sulfuric acid and methanesulfonic acid. Gradual increase in the crystallinity was observed following the reduction in the content of sulfuric acid, leading to control pore size and porosity. Overall, benefit from the unique microstructure, poly (ether ether ketone) membranes show excellent electrolyte uptake, high porosity, high ionic conductivity (1.68 mS cm(-1)) and good electrochemical stability up to 5.5 V. The membrane has excellent high temperature resistance with no shrink at 200 degrees C, ensuring lithium ion battery's high temperature security. The half-cell shows excellent performance even after treated at 150 degrees C and better discharge capacity of 154.3 mA h g(-1) after the 150th cycle than the cell based on the Celgard membrane. The results show that vapor induced demixing approach is an alternative for preparing promising high capacity, high temperature resistance separators for lithium ion batteries.