TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Composition: The mole ratio of the first lithium salt to the second lithium salt is 1:0.25 to 1:0.43. The first lithium salt is present at 0.5-1.3 M. The second lithium salt is present at 0.05-0.5 M. Preferred Components: The first lithium salt comprises LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO4, LiAlCl4, and/or LiSO3CF3 (preferably LiPF6). EXAMPLE - A negative active material slurry was prepared by mixing modified natural graphite negative active material, styrene butadiene rubber binder, and carboxylmethyl cellulose thickener in water at weight ratio of 98:1:1. A negative electrode was fabricated by coating Cu-foil current collector with the negative active material slurry, drying coated current collector, and compressing. A positive active material slurry was prepared by mixing positive active material including mixture of LiNi0.8Co0.15Al0.05O2 and LiNi0.5Co0.2Mn0.3O2, carbon black conductive material, and polyvinylidene fluoride binder in N-methyl pyrrolidone solvent at weight ratio of 92:4:4. A positive electrode was fabricated by coating Al-foil current collector with the positive active material slurry, drying coated current collector, and compressing. A rechargeable lithium battery cell of 18650 size was manufactured by using negative electrode, positive electrode and electrolyte comprising mixed solvent of ethylene carbonate, methyl propionate, and ethyl propionate (2:4:4 volume ratio) including 0.3 M LiPF6 and 0.7 M lithium bisfluorosulfonyl imide (LiFSI) dissolved in it. Measurement of viscosity and ion conductivity of lithium salt electrolytes were prepared by, respectively, adding 1 M LiPF6, 1 M LiFSI, and 1 M LiTFSI to mixed solvent of ethylene carbonate and ethylmethyl carbonate (1:1 volume ratio). The viscosity and ion conductivity of electrolytes were measured at 25, 0, -20 degrees C for LiPF6, LiFSI and LiTFSI, and were found to be 7.5, 6.3 and 8.1, 21, 13 and 20, and 60, 35 and 54; and 8.1, 8.8 and 6.3, 4.1, 4.9 and 3.3, and 1.7, 2.7 and 1.4, respectively. Thus viscosities of LiFSI and LiPF6 were similar at room temperature, which was 25 degrees C, but as temperature went down, increase in viscosity of LiFSI was not greater than that of LiPF6. In consequence, since viscosity of LiFSI at low temperature (-25 degrees C) was lower than viscosity of LiPF6. Also, when LiFSI was used for electrolyte ion mobility increased at low temperature. Since improvement in ion conductivity decreased cell internal resistance, output improvement effect was expected. Further, LiTFSI showed little higher viscosity than that of LiFSI at room temperature of 25 degrees C, but viscosity drastically increased to much higher than that of LiFSI at low temperature of -20 degrees C. As a result, effect of increasing ion mobility at low temperature was not much better, when LiTFSI was used for electrolyte than when LiFSI was used.