We demonstrate that the addition of 1-butyl-1-methylpyrrolidinium hexafluorophosphate ([Py14]PF6) to 1.0 M LiPF6 in ethylene carbonate-dimethyl carbonate (LP30) widens the temperature range, in which the electrolyte mixtures are ion conductive and safe. Specifically, at the concentrations of [Py14]PF6 above 50 wt%, the electrolyte mixtures exhibit a flash point higher than room temperature and fulfill the requirements of liquids having controlled flammability. In this concentration range, also crystallization of the mixtures is completely suppressed, and low temperature ionic conductivity is improved. With respect to the electrochemical properties at room temperature, electrochemical stability window is widened by the addition of [Py14]PF6 to LP30. However, it comes at the cost of slightly increased overall impedance and overpotential in Li | Li symmetric cells. We utilize these mixed electrolytes in high voltage Li | LiNi0.5Mn1.5O4 cells. In the presence of 30 wt% and 50 wt% of [Py14]PF6 in LP30, the cells exhibit high specific capacity of about 110 mAh g−1 over 200 cycles and improved coulombic efficiency, suggesting [Py14]PF6 is a promising additive for the electrolyte in high-voltage, stable and safe lithium batteries.