Provided are a non-aqueous electrolyte solution, which includes (i) a first lithium salt, (ii) lithium bis(fluorosulfonyl)imide as a second lithium salt, (iii) a phosphazene-based compound as a first additive, and (iv) a non-aqueous organic solvent, and a lithium secondary battery including the non-aqueous electrolyte solution.With respect to a lithium secondary battery including the non-aqueous electrolyte solution of the present invention, since a robust solid electrolyte interface (SEI) may be formed on the surface of a negative electrode during initial charge and flame retardancy in a high-temperature environment may be provided to prevent the decomposition of the surface of a positive electrode and an oxidation reaction of the electrolyte solution, output characteristics and capacity characteristics after high-temperature storage as well as output characteristics may be improved.