An electrode-composite separator assembly for a flexible lithium battery, the electrode-composite separator assembly comprising: an electrode (12a,13a); and a composite separator, wherein the composite separator comprises a copolymer, and at least one selected from an inorganic particle and an organic-inorganic particle, characterised in that: the copolymer comprises a first repeating unit represented by Formula 1, a second repeating unit represented by Formula 2, and a third repeating unit represented by Formula 3; the composite separator further comprises at least one polymer selected from a fluorinated polymer and a heat-resistant polymer; the heat-resistant polymer is at least one selected from a polyimide, polyethersulfone, polyvinylidene fluoride, polyamide, poly(meta-phenylene isophthalamide), polysulfone, polyetherketone, polyethylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, a polyphosphazene, poly (diphenoxy phosphazene), poly{bis[2-(2-methoxyethoxy)phosphazene]}, polyurethane copolymers, polyurethane, polyether urethane, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, polyester sulfone, polyetherimide, polycarbonate, polyphenylene sulfide, poly(C1-8)acrylate, and polytetrafluoroethylene; and the copolymer comprising the first repeating unit represented by Formula 1, the second repeating unit represented by Formula 2, and the third repeating unit represented by Formula 3 is a block copolymer having a weight average molecular weight of 400,000 to 1,000,000 Daltons; wherein Formulas 1 to 3 are wherein, in Formula 1, R 1 is a hydrogen or a C1-C5 alkyl group, and R 2 is a C2-C20 alkyl group, wherein in Formula 2, R 5 is a hydrogen or a C1-C5 alkyl group, and R 6 is a methyl group, and wherein in Formula 3, R 3 is a hydrogen or a C1-C5 alkyl group, and R 4 is a C1-C10 alkyl group. an electrode (12a,13a); and a composite separator, wherein the composite separator comprises a copolymer, and at least one selected from an inorganic particle and an organic-inorganic particle, characterised in that: the copolymer comprises a first repeating unit represented by Formula 1, a second repeating unit represented by Formula 2, and a third repeating unit represented by Formula 3; the composite separator further comprises at least one polymer selected from a fluorinated polymer and a heat-resistant polymer; the heat-resistant polymer is at least one selected from a polyimide, polyethersulfone, polyvinylidene fluoride, polyamide, poly(meta-phenylene isophthalamide), polysulfone, polyetherketone, polyethylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, a polyphosphazene, poly (diphenoxy phosphazene), poly{bis[2-(2-methoxyethoxy)phosphazene]}, polyurethane copolymers, polyurethane, polyether urethane, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, polyester sulfone, polyetherimide, polycarbonate, polyphenylene sulfide, poly(C1-8)acrylate, and polytetrafluoroethylene; and the copolymer comprising the first repeating unit represented by Formula 1, the second repeating unit represented by Formula 2, and the third repeating unit represented by Formula 3 is a block copolymer having a weight average molecular weight of 400,000 to 1,000,000 Daltons; the copolymer comprises a first repeating unit represented by Formula 1, a second repeating unit represented by Formula 2, and a third repeating unit represented by Formula 3; the composite separator further comprises at least one polymer selected from a fluorinated polymer and a heat-resistant polymer; the heat-resistant polymer is at least one selected from a polyimide, polyethersulfone, polyvinylidene fluoride, polyamide, poly(meta-phenylene isophthalamide), polysulfone, polyetherketone, polyethylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, a polyphosphazene, poly (diphenoxy phosphazene), poly{bis[2-(2-methoxyethoxy)phosphazene]}, polyurethane copolymers, polyurethane, polyether urethane, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, polyester sulfone, polyetherimide, polycarbonate, polyphenylene sulfide, poly(C1-8)acrylate, and polytetrafluoroethylene; and the copolymer comprising the first repeating unit represented by Formula 1, the second repeating unit represented by Formula 2, and the third repeating unit represented by Formula 3 is a block copolymer having a weight average molecular weight of 400,000 to 1,000,000 Daltons; wherein Formulas 1 to 3 are wherein, in Formula 1, R 1 is a hydrogen or a C1-C5 alkyl group, and R 2 is a C2-C20 alkyl group, wherein in Formula 2, R 5 is a hydrogen or a C1-C5 alkyl group, and R 6 is a methyl group, and wherein in Formula 3, R 3 is a hydrogen or a C1-C5 alkyl group, and R 4 is a C1-C10 alkyl group.