High-energy-density 0.5Li2MnO3–0.5Li(Mn0.375Ni0.375Co0.25)O2 composite cathodes for lithium rechargeable batteries are synthesized using an auto-combustion method. The electrode charge–discharge properties are studied at 25 and 50 °C in Li+-containing N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP–TFSI) and N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PMP–TFSI) ionic liquid (IL) electrolytes. The IL electrolytes have a high decomposition temperature (∼400 °C) and thus are ideal for high-safety applications. Compared to Li+/BMP–TFSI IL, Li+/PMP–TFSI IL exhibits higher ionic conductivity and lower viscosity. As a result, the composite cathode shows superior electrochemical performance in Li+/PMP–TFSI IL electrolyte. With the increase in cell temperature from 25 to 50 °C, the maximum capacities and rate capabilities of both IL cells improve significantly. Thus at 50 °C, discharge capacities of 304 mAh g−1 (@10 mA g−1) and 223 mAh g−1 (@100 mA g−1) are obtained for the Li+/PMP–TFSI cell. These capacities are superior to those for a control cell made with the same composite cathode and a conventional organic electrolyte. At elevated temperature, the cyclability of the composite cathode in the IL electrolytes is markedly higher than that obtained in a conventional organic electrolyte.