1 . A lithium-sulfur secondary battery comprising: a positive electrode; a negative electrode; a separator; and an electrolyte solution comprising a lithium salt and a solvent, wherein a concentration of the lithium salt is 1.5 M (mol/L) to 2.5 M (mol/L), wherein the solvent comprises a nitrile-based solvent, a fluorinated ether-based solvent, and a disulfide-based solvent, wherein the nitrile-based solvent is at least one selected from the group consisting of acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentane carbonitrile, cyclohexane carbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile, wherein the fluorinated ether-based solvent is at least one selected from the group consisting of 2,2,2-trifluoroethylmethylether(CF 3 CH 2 OCH 3 ), 2,2,2trifluoroethyldifluoromethylether(CF 3 CH 2 OCHF 2 ), 2,2,3,3,3-pentafluoropropylmethylether(CF 3 CF 2 CH 2 OCH 3 ), 2,2,3,3,3-pentafluoropropyldifluoromethylether(CF 3 CF 2 CH 2 OCHF 2 ), 2,2,3,3,3-pentafluoropropyl-1,1,2,2-tetrafluoroethylether(CF 3 CF 2 CH 2 OCF 2 CF 2 H), 1,1,2,2-tetrafluoroethylmethylether(HCF 2 CF 2 OCH 3 ), 1,1,2,2-tetrafluoroethylethylether(HCF 2 CF 2 OCH 2 CH 3 ), 1,1,2,2-tetrafluoroethylpropylether(HCF 2 CF 2 OC 3 H 7 ), 1,1,2,2-tetrafluoroethylbutylether(HCF 2 CF 2 OC 4 H 9 ), 2,2,3,3-tetrafluoroethyldifluoromethylether(H(CF 2 ) 2 CH 2 O(CF 2 )H), 1,1,2,2-tetrafluoroethylisobutylether(HCF 2 CF 2 OCH 2 CH(CH 3 ) 2 ), 1,1,2,2-tetrafluoroethylisopentylether(HCF 2 CF 2 OCH 2 C(CH 3 ) 3 ), 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethylether(HCF 2 CF 2 OCH 2 CF 3 ), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether(HCF 2 CF 2 OCH 2 CF 2 CF 2 H), hexafluoroisopropylmethylether((CF 3 ) 2 CHOCH 3 ), 1,1,3,3,3-pentafluoro-2-trifluoromethylpropylmethylether((CF 3 ) 2 CHCF 2 OCH 3 ), 1,1,2,3,3,3-hexafluoropropylmethylether(CF 3 CHFCF 2 OCH 3 ), 1,1,2,3,3,3-hexafluoropropylethylether(CF 3 CHFCF 2 OCH 2 CH 3 ), and 2,2,3,4,4,4-hexafluorobutyldifluoromethylether(CF 3 CHFCF 2 CH 2 OCHF 2 ), wherein the disulfide-based solvent is dimethyl disulfide, wherein the nitrile-based solvent is present in the solvent in an amount of 25 to 35 vol. % based on the total volume of the solvent, wherein the fluorinated ether-based solvent is present in the solvent in an amount of 30 to 50 vol. % of the total volume of the solvent, wherein the disulfide-based solvent is present in the solvent in an amount of 20 to 35 vol. % based on the total volume of the solvent, wherein the positive electrode has an SC factor value of 0.45 or more, expressed by Equation 1 below [Equation 1] SC factor = L p [ Equation 1 ] wherein P is the porosity (%) of a positive electrode active material layer in the positive electrode, L is the mass of sulfur per unit area (mg/cm 2 ) of the positive electrode active material layer in the positive electrode, and is 10.
2 . The lithium-sulfur secondary battery according to claim 1 , wherein the lithium salt is at least one selected from the group consisting of LiCl, LiBr, LiI, LiClO 4 , LiBF 4 , LiB 10 Cl 10 , LiB(Ph) 4 , LiC 4 BO 8 , LiPF 6 , LiCF 3 SO 3 , LiCF 3 CO 2 , LiAsF 6 , LiSbF 6 , LiAlCl 4 , LiSO 3 CH 3 , LiSO 3 CF 3 , LiSCN, LiC(CF 3 SO 2 ) 3 , LiN (CF 3 SO 2 ) 2 , LiN(C 2 F 5 SO 2 ) 2 , LiN(SO 2 F) 2 , lithium chloroborane, lithium lower aliphatic carboxylate, and lithium imide.
3 . The lithium-sulfur secondary battery according to claim 1 , wherein a volume ratio of the fluorinated ether-based solvent and the disulfide-based solvent is 1:0.75 to 1:1.