We have incorporated polymer additives such as poly(ethylene glycol) dimethyl ether (PEGDME) and tetra(ethylene glycol) dimethyl ether (TEGDME) into N-methyl-N-butylpyrrolidinium bis(trifluoromethane sulfonyl)imide (PYR14TFSI)–LiTFSI mixtures. The resulting PYR14TFSI+LiTFSI+polymer additive ternary electrolyte exhibited relatively high ionic conductivity as well as remarkably low viscosity over a wide temperature range compared to the PYR14TFSI+LiTFSI binary electrolytes. The charge/discharge cyclability of Li/LiFePO4 cells containing the ternary electrolytes was investigated. We found that Li/PYR14TFSI+LiTFSI+PEGDME (or TEGDME)/LiFePO4 cells containing the two different polymer additives showed very similar discharge capacity behavior, with very stable cyclability at room temperature (RT). Li/PYR14TFSI+LiTFSI+TEGDME/LiFePO4 cells can deliver about 127mAh/g of LiFePO4 (74.7% of theoretical capacity) at 0.054mA/cm2 (0.2C rate) at RT and about 108mAh/g of LiFePO4 (63.4% of theoretical capacity) at 0.023mA/cm2 (0.1C rate) at −1°C for the first discharge. The cell exhibited a capacity fading rate of approximately 0.09–0.15% per cycle over 50 cycles at RT. Consequently, the PYR14TFSI+LiTFSI+polymer additive ternary mixture is a promising electrolyte for cells using lithium metal electrodes such as the Li/LiFePO4 cell reported here. These cells showed the capability of operating over a significant temperature range (∼0–∼30°C).