Method for preparing composite separator including controlling amount of non-solvent supplied during hot-air drying, composite separator including porous substrate with fluorine-based resin porous layer with more small-diameter pores than large-diameter pores, and lithium battery comprising composite separator

An electrode assembly comprising:

• a positive electrode;
• a negative electrode;
• and
• a composite separator interposed between the positive electrode and the negative electrode,
• wherein the positive electrode, the negative electrode, and the composite separator are wound in a jelly roll shape,
• wherein the electrode assembly is configured to have a bending strength of 460 N or more, the bending strength being an amount of force needed to bend the jelly roll shaped electrode in a downward direction, and
• wherein the composite separator comprises:
• a porous substrate;
• and
• a porous layer formed on one side or both sides of the porous substrate,
• wherein a coating amount of the porous layer is in a range from 3.5 g/m2 to 4.5 g/m2,
• wherein the porous layer comprises a fluorine-based binder having a polar functional group and inorganic particles,
• wherein the porous layer has a surface morphology in which a plurality of pores are discontinuously arranged in the form of island on a polymer film,
• wherein a ratio (a/b) of the number of pores having a diameter of 500 nm to 1000 nm per 1 μm2 of the porous layer (a) to the number of pores having a diameter less than 500 mm per 1 μm2 of the porous layer (b) is from 0.25 to 0.85, and
• wherein the porous layer has a peel strength of 0.3 N/m or more with respect to the porous substrate.