The electrochemical behaviour and electrical performance are investigated for a series of lithium-sulfur (Li-S) cells in which the electrolyte solutions are organic solvent-ionic liquid mixtures that are based on the 1-butyl-1-methylpyrrolidinium (C(4)mpyr) cation with a range of anions. In each case, performance is compared with cells that are based on a standard mixed-ether organic electrolyte. The capacity of cells assembled with electrolytes containing 1-butyl-1-methylpyrrolidinium tris (pentafluoroethyl) trifluorophosphate (C(4)mpyr-FAP), 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate (C(4)mpyr-OTf), or 1-butyl-1-methylpyrrolidinium tricyanomethanide (C(4)mpyr-TCM) decline rapidly due to low conductivity, high polysulfide solubility and side reaction of electrolyte with electrodes, respectively. Our results confirm that polysulfide solubility is strongly controlled by the anion of the ionic liquid and verify that not all ionic liquids decrease polysulfide solubility. In agreement with previous reports, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (C(4)mpyr-TFSI) shows the best compatibility in Li-S batteries and has a higher coulombic efficiency of greater than 99% over 100 cycles. Furthermore, impedance spectroscopy confirms that electrolyte composition influences the SEI layer formed on the lithium anode and its subsequent impedance. (C) 2015 Elsevier Ltd. All rights reserved.