Electrochemical Behavior of Li2FeSiO4 with Ionic Liquids at Elevated Temperature

Ionic liquids, 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide with 0.7 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) or 0.5 M LiPF6, were successfully tested as electrolytes for Li2FeSiO4 cathodes operating at elevated temperatures of 60 degrees C. The electrolytes based on ionic liquids show good ionic conductivity (from 3.3 x 10(-3) to 4.5 x 10(-3) S cm(-1)) together with good electrochemical stability up to 5 V vs Li/Li+. The electrochemical stability of electrolytes based on ionic liquids against an aluminum current collector was comparable to a diethyl carbonate: ethylene carbonate (DEC:EC) 1 M LiPF6 electrolyte and even better when the electrolytes were tested with blank electrodes (only aluminum current collector, carbon black, and binder). In the first cycle the electrochemical testing of Li2FeSiO4 showed a slightly lower reversibility in ionic liquids when compared to a DEC:EC 1 M LiPF6 electrolyte; at higher cycles the reversibility and the obtained capacity were comparable to the one obtained in a DEC:EC 1 M LiPF6 electrolyte. All electrochemical results show that LiTFSI can be used as a salt in ionic liquid-based electrolytes. These properties allow their potential application in large-scale lithium-ion batteries with improved safety. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3133183] All rights reserved.