Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14FSI) (1:9 in molar ratio) is successfully tested as electrolyte for the high voltage LiMO2–Li2MnO3 (cathode)/lithium (anode) cells at elevated temperature (40 °C). Compared to conventional electrolytes, such as 1 M LiPF6 solution in the mixed solvent of ethylene and dimethyl carbonate (EC:DMC = 1:1), the use of PYR14FSI–LiTFSI electrolyte results in a net improvement of LiMO2–Li2MnO3 cycling stability while granting comparable initial capacity. In addition, the ionic conductivity of the ionic liquid-based electrolyte at 40 °C is high enough to sustain the excellent rate capability of this cathode material. Li/LiMO2–Li2MnO3 cells delivered initial capacity exceeding 200 mA h g−1 at high current rate (2 C) while retaining 94% of the initial capacity after 100 cycles. Differential capacity versus potential analysis and post-mortem characterization by scanning electron microscope, X-ray diffraction and were carried out to explain the improved performance of LiMO2–Li2MnO3 in the IL-based electrolyte.