Separator with improved thermal stability and secondary battery comprising the same

A separator, comprising:

• a porous substrate;
• and
• a microcapsule-containing layer coated on at least one surface of the porous substrate,
• wherein the microcapsule consists of a core having a thermal stabilizer, a shell layer surrounding the core, and a spacer layer surrounding the shell layer,
• wherein the spacer layer is softened by the impregnation of an electrolyte solution and ruptures at a temperature higher than 200° C. or a pressure higher than 50 kg/cm2, and
• wherein the spacer layer is made of any one selected from the group consisting of polyvinylidene fluoride, polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethyl methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyacrylic acid, polyvinyl alcohol, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, and a mixture thereof.