Physical and electrochemical properties of electrolytes prepared from ionic liquids in combination with carbonate solvents were measured. Cation, anion, and substituent chain length effects on electrical conductivity were observed, where higher conductivities were observed with imidazolium cations, tetrafluoroborate (BF4-) anions, and shorter chain length substituents, while lower conductivities were observed with pyridinium cations, bis(trifluoromethanesulfonyl) imide (TFSI-) anions, and longer chain length substituents. High voltage stability was demonstrated by the ionic liquid based mixtures, where the majority of the mixtures demonstrated voltage stability at >= 5 V, and some demonstrated stability at >= 6 V. The development and optimization of new ionic liquid battery electrolyte systems have the potential to improve realizable energy density and safety of lithium ion battery systems. (C) 2013 The Electrochemical Society.