Method for preparing composite separator, composite separator, and lithium battery comprising composite separator

A method of preparing a composite separator, the method comprising:

• coating a binder composition on one side or both sides of a porous substrate;
• hot-air drying the porous substrate coated with the binder composition;
• and
• supplying a non-solvent during the hot-air drying,
• wherein the composite separator comprises a porous layer on the one side or both sides of the porous substrate,
• the binder composition comprises a good-solvent, a binder, and inorganic particles,
• an amount of the non-solvent supplied during the hot-air drying is from 12 g/m3 to 17 g/m3, and
• a ratio (e) represented by Equation 1 is from 2.2 to 5.0:
• Ratio (e) of the hot-air supply speed to the moving speed of the porous substrate per unit transit time [s−1]=[hot-air supply speed/moving speed of porous substrate]/transit time of porous substrate in the hot-air dryer. Equation 1