A lithium secondary battery comprising: (a) a composite separator comprising a porous substrat, a first porous coating layer coated on one surface of the porous substrate and composed of a mixture comprising inorganic particles and a binder polymers, and a second porous coating layer coated on the other surface of the porous substrate and composed of a mixture comprising inorganic particles and a binder polymer, wherein the first porous coating layer is thicker than the second porous coating layer; wherein the weight ratio of the inorganic particles to the binder polymer in each of the first and second porous coating layers is from 50:50 to 99:1; (b) an anode disposed to face the first porous coating layer; and (c) a cathode disposed to face the second porous coating layer, wherein the thicknesses of the first and second porous coating layers satisfy the following relationship 1: 0.1 ≤ T 1 − T 2 / T tot ≤ 0.70 wherein T1 and T2 represent the thicknesses of the first and second porous coating layers, respectively, and Ttot represents the total thickness of the first and second porous coating layers, and wherein the total thickness or the first and second porous coating layers is from 1.0 to 20.0 μm, characterized in that the binder polymers are each independently selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethyl 1 methacrylate, polybutyl acrylate, polyacrylonltrile, polyvinylpyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cyanoethylpullulan, cyanoethyl polyvinyl alcohol, cyanoethyl sucrose, pullulan, and mixtures of two or more kinds thereof. (a) a composite separator comprising a porous substrat, a first porous coating layer coated on one surface of the porous substrate and composed of a mixture comprising inorganic particles and a binder polymers, and a second porous coating layer coated on the other surface of the porous substrate and composed of a mixture comprising inorganic particles and a binder polymer, wherein the first porous coating layer is thicker than the second porous coating layer; wherein the weight ratio of the inorganic particles to the binder polymer in each of the first and second porous coating layers is from 50:50 to 99:1; (b) an anode disposed to face the first porous coating layer; and (c) a cathode disposed to face the second porous coating layer, wherein the thicknesses of the first and second porous coating layers satisfy the following relationship 1: 0.1 ≤ T 1 − T 2 / T tot ≤ 0.70 wherein T1 and T2 represent the thicknesses of the first and second porous coating layers, respectively, and Ttot represents the total thickness of the first and second porous coating layers, and wherein the total thickness or the first and second porous coating layers is from 1.0 to 20.0 μm, characterized in that the binder polymers are each independently selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethyl 1 methacrylate, polybutyl acrylate, polyacrylonltrile, polyvinylpyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cyanoethylpullulan, cyanoethyl polyvinyl alcohol, cyanoethyl sucrose, pullulan, and mixtures of two or more kinds thereof.