In this paper, we report on the physicochemical properties of mixed electrolytes based on an ionic liquid N-propyl-N-methylpyrrolidiniumbis (trifluoromethanesulfonyl) imide (PYR13TFSI), organic additives, and lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) for high safety lithium-ion batteries. The proposed optimal content of ionic liquid in the mixed electrolyte is 65 vol%, which results in non- flammability, high thermal stability, a wide electrochemical window of 4.8V, low viscosity, low bulk resistance and the lowest interface resistance to lithium anode. The effects of the concentration of LiTFSI in the above electrolyte are critical to the rate performance of the LiFePO4-based battery. We have found the suitable LiTFSI concentration (0.3M) for good capacity retention and rate capability.