Enhanced safety and galvanostatic performance of high voltage lithium batteries by using ionic liquids

We demonstrate that the addition of 1-butyl-1-methylpyrrolidinium hexafluorophosphate ([Py14]PF6)to1.0 M LiPF6in ethylene carbonate-dimethyl carbonate (LP30) widens the temperature range, in which theelectrolyte mixtures are ion conductive and safe. Specifically, at the concentrations of [Py14]PF6above50 wt%, the electrolyte mixtures exhibit aflash point higher than room temperature and fulfill the re-quirements of liquids having controlledflammability. In this concentration range, also crystallization ofthe mixtures is completely suppressed, and low temperature ionic conductivity is improved. Withrespect to the electrochemical properties at room temperature, electrochemical stability window iswidened by the addition of [Py14]PF6to LP30. However, it comes at the cost of slightly increased overallimpedance and overpotential in LijLi symmetric cells. We utilize these mixed electrolytes in highvoltage LijLiNi0.5Mn1.5O4cells. In the presence of 30 wt% and 50 wt% of [Py14]PF6in LP30, the cellsexhibit high specific capacity of about 110 mAh g1over 200 cycles and improved coulombic efficiency,suggesting [Py14]PF6is a promising additive for the electrolyte in high-voltage, stable and safe lithiumbatteries.