NOVELTY - The method involves extruding resin composition (BB). The resin composition comprises polyolefin and plasticizer. The extruded resin composition is extended to obtain a polyolefin film (CC). The plasticizer is extracted from the obtained polyolefin film to obtain a porous polyolefin film (DD). The porous polyolefin film is coated with slurry through a heat-setting process to obtain a composite membrane (EE). A porous coating layer is formed on the composite membrane. The polyolefin is polypentene, polybutylene, polypropylene, polyethylene, polyhexene or polyoctene.    USE - Method for manufacturing a separation film of an electrochemical device i.e. secondary lithium battery (all claimed), that is utilized as an energy source of an electric vehicle and an electronic device e.g. mobile phone, camcorder and notebook computer.    ADVANTAGE - The method enables extruding the resin composition, and extending the extruded resin composition so as to easily obtain the polyolefin film without performing heat-setting/winding process, improve physical, mechanical and thermal properties, permeability and quality of the composite membrane and reduce production cost and processing cost of a separation film, thus improving productivity and space utilization efficiency of the separation film.    DETAILED DESCRIPTION - The polyolefin comprises copolymer that is ethylene, propylene, butene, pentene, 4-methylpentene, hexene or octene. The plasticizer comprises fatty acid esters, fatty acid alcoholic with carbon number 10-20, fatty acids of paraffinic oil, mineral oil, wax, soybean oil, phthalic ester class or aromatic ether class with carbon number 10. The slurry comprises inorganic particle, detritus, binder polymer and solvent. The binder polymer is polyvinyl fluorife-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene, polymethyl methacrylate, polybutylacrylate, polyacrylonitrile, polyvinyl pyrrolidone, polyvinylacetate, ethylene-vinyl acetate copolymer, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl polyvinylalcohol, cyanoethylcellulose, cyanoethyl sucrose, carboxyl methyl cellulose, acrylonitrile styrene butadiene copolymer or polyimide. The detritus is copolymer of polystyrene, polyethylene, melamine resin, phenolic resin, cellulose, cellulose denaturant, polypropylene, polyester, polyphenylenesulfide, polyaramide, polyamideimide, polyimide, butyl acrylate or ethylmethacrylate. INDEPENDENT CLAIMS are also included for the following:    (1) a separation film    (2) an electrochemical device.    DESCRIPTION OF DRAWING(S) - The drawing shows a process flow diagram illustrating a method for manufacturing a separation film of an electrochemical device.'(Drawing includes non-English language text)'    Step for extruding resin composition (BB)    Step for extending extruded resin composition to obtain a polyolefin film (CC)    Step for extracting plasticizer from an obtained polyolefin film to obtain a porous polyolefin film (DD)    Step for coating slurry on a porous polyolefin film through a heat-setting process to obtain a composite membrane (EE)    Step for packing a composite membrane (GG)