Ionic liquids described here have potential for cost efficient, high energy density electrolytes for redox flowbattery (RFB) applications. Iron chloride based electrolytes containing up to 6.3 M iron have been synthesized and the effects of iron chloride, choline chloride, and ethylene glycol ratios on electrochemical and rheological properties were measured. Itwas found that electrolyte conductivity and viscosity as well as the nature of the electroplated iron are greatly influenced by electrolyte composition. An electrolyte containing iron chloride, choline chloride, and ethylene glycol in a 1:1:4 molar ratio was favored due to its higher plating efficiencies and fast kinetics. (C) The Author(s) 2016 Published by ECS. All rights reserved.