A method of preparing a lithium secondary battery including a multi-phase separator having a gelled polymer uniformly impregnated therein, includes surface-treating a porous separator to provide a coated weight of a gelled polymer less than or equal to 20 mg/cm.sup.2, by impregnating the porous separator into a gelled polymer solution containing less than or equal to 5 wt % of the gelled polymer based on a weight of an organic solvent. The separator is interposed between a cathode and an anode to prepare an electrode assembly. The electrode assembly is placed into a housing, an electrolytic solution is injected into the housing, followed by sealing and curing a resultant structure.