AbstractThe ionic liquid (IL)‐based electrolyte comprising 1.2 M lithium bis(fluorosulfonyl)imide (LiFSI) in N‐propyl‐N‐methylpyrrolidinium bis(fluorosulfonyl)imide (PYR13FSI) (ILE) has been evaluated as a suitable system for the high‐voltage cathode material LiNi0.5−xMn1.5+xO4 (LNMO) when cycled vs. graphite anodes. The oxidative stability of the ILE was evaluated by linear sweep voltammetry (LSV) and synthetic charge‐discharge profile voltammetry (SCPV) and was found to exceed that of state‐of‐the‐art 1 M LiPF6 in 1 : 1 ethylene carbonate (EC) : diethylcarbonate (DEC) (LP40). Improved cycling performance both at 20 °C and 45 °C was found for LNMO||graphite full cells with the IL electrolyte. X‐ray photoelectron spectroscopy (XPS) analysis showed that robust and predominantly inorganic surface layers were formed on the LNMO cathode using the ILE, which stabilized the electrode. Although the high viscosity of the ILE limits the rate performance at 20 °C, this ILE is a promising alternative electrolyte for use in lithium‐ion batteries (LiBs) with high‐voltage cathodes such as LNMO, especially for use at elevated temperatures.