Effect of organic solvent addition to PYR 13 FSI þ LiFSI electrolytes on aluminum oxidation and rate performance of Li(Ni 1/3 Mn 1/3 Co 1/3 )O 2 cathodes Tyler Evans a , Jarred Olson b , Vinay Bhat b , Se-Hee Lee a, * a Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309, USA b Boulder Ionics Corporation, Arvada, CO 80007, USA highlights graphical abstract  LiFSI is superior in suppressing Al oxidation in PYR 13 FSI electrolytes.  Addition of EC:EMC to PYR 13 FSI ILs leads to higher t þ, Li and s ionic .  IL-organic solvent mixtures enable high rate performance of Li(Ni 1/3 Mn 1/ 3 Co 1/3 )O 2 . article info Article history: Received 2 January 2014 Received in revised form 25 February 2014 Accepted 28 April 2014 Available online 6 May 2014 Keywords: Ionic liquids PYR 13 FSI LiFSI Aluminum current collector Corrosion abstract The superior suppression of aluminum current collector oxidation by a 1.2 M LiFSI in PYR 13 FSI 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 þ ), adding organic solvent to PYR 13 FSI based ionic liquids greatly enhances important electrochemical properties. The ionic conductivity and lithium ion transference number of the PYR 13 FSI þ 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 bene- fits of organic co-solvent addition and the compatibility of the PYR 13 FSI þ 1.2 M LiFSI electrolyte with Li(Ni 1/3 Mn 1/3 Co 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. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction While lithium-ion batteries (LIBs) offer a potentially game- changing energy storage technology, safety concerns have hin- dered their widespread penetration into the electric vehicle market and have discouraged their consideration in grid level applications. Many of these safety concerns stem from the utilization of volatile organic electrolytes. Currently commercialized LIB technology is based on electrolytes comprised of mixtures of organic carbonate solvents containing lithium hexafluorophosphate (LiPF 6 ) as a salt. These electrolytes are preferred due to their high ionic conductiv- ities and compatibility with commercialized electrode materials, but their flammability and high volatility must be addressed [1,2]. In order to mitigate electrolyte related safety risks without realizing a loss in performance, a new class of non-aqueous elec- trolyte should be developed. Due to their non-flammability, * Corresponding author. Tel.: þ1 303 492 7889; fax: þ1 303 492 3498. E-mail address: sehee.lee@colorado.edu (S.-H. Lee). Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour http://dx.doi.org/10.1016/j.jpowsour.2014.04.138 0378-7753/Ó 2014 Elsevier B.V. All rights reserved. Journal of Power Sources 265 (2014) 132e139