The superior suppression of aluminum current collector oxidation by a 1.2 M LiFSI in PYR13FSI ionic liquid electrolyte is demonstrated. Addition of EC:EMC (1:2 wt.) is shown to significantly increase the severity of parasitic aluminum oxidation. Despite leading to increased aluminum oxidation at high voltages (>4.2 V vs. Li/Li+), adding organic solvent to PYR13FSI based ionic liquids greatly enhances important electrochemical properties. The ionic conductivity and lithium ion transference number of the PYR13FSI + 1.2 M LiFSI electrolyte increase with increasing volumetric content of organic co-solvent (EC:EMC), resulting in significant improvements to high rate performance. The electrochemical benefits of organic co-solvent addition and the compatibility of the PYR13FSI + 1.2 M LiFSI electrolyte with Li(Ni(1/3)Mn(1/3)sCo(1/3))O-2 demonstrated in this study substantiate the need to develop strategies to suppress aluminum oxidation during high voltage cycling of lithium-ion batteries in ionic liquid electrolytes. (c) 2014 Elsevier B.V. All rights reserved.