In this study, LiNi0.5Mn1.5O4 (LNMO) nanoparticles were prepared as a 5 V cathode material via a rheological phase method and annealed at different temperatures: 680 ◦C, 750 ◦C, and 820 ◦C. The sample annealed at 750 ◦C shows the best performance. A room temperature ionic liquid (RTIL) containing 1 M lithium bis(trifluoromethanesulfonyl) imide (LiNTf2) in N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide (C4mpyrNTf2) was used as novel electrolyte in conjunction with the LNMO cathodes and their electrochemical properties have been investigated. The results show that the LNMO using RTIL as electrolyte has better coulombic efficiency and comparable discharge capacities to those of the cells assembled with standard liquid electrolyte (1 M LiPF6 in ethylene carbonate/diethyl carbonate). Electrochemical impedance spectroscopy shows that the RTIL is much more stable as the electrolyte for LiNi0.5Mn1.5O4 than the conventional electrolyte.