To improve the safety of the electrolyte used for lithium secondary batteries, binary mixed solvent electrolytes containing trifluoropropylene carbonate (TFPC) as cosolvent have been studied. Chloroethylene carbonate (ClEC), ethylene carbonate, and propylene carbonate were chosen as the other component of the binary mixed solvent for the electrolytes. The solution properties of these electrolytes were characterized using conductivity and nuclear magnetic resonance (NMR) spectroscopy. The chemical shift of ClEC and TFPC did not vary with the mixing ratio due to their similar enthalpies of solvation as derived by molecular orbital simulation. The ClEC/TFPC electrolyte showed higher discharge capacities with lower irreversible capacity loss in both a graphite/Li cell and Li1+xMn2O4/Li cell than other electrolyte systems. Electrochemical impedance spectroscopy measurements were made for cells composed of each electrolyte. The surface of the graphite anode was analyzed using X-ray photoelectron spectroscopy, infrared spectroscopy, and solid Li-7-NMR spectroscopy.