Effects of Room Ionic Liquid as Solute on the Electrolyte Solution Structure and Electrochemical Performances in Ethylene Carbonate-Based Electrolyte

Abstract

Room ionic liquid N-allyl-N-methylpiperidinium bis(trifuoromethansulfonyl)imide (PP13*TFSI) was adopted as a solute to partly substitute LiPF6 in concentrated ethylene carbonate (EC) based electrolyte (1.8 mol L−1 LiPF6/EC). Raman and FTIR spectroscopic studies revealed that the electrolyte solution structure was adjusted in the presence of PP13*TFSI ionic liquid. Solvation number of Li+ cation could be effectively regulated by the competitive coordination of PP13*+ and Li+ with EC molecules. More importantly, the appearance of TFSI− anion brings about a different coordinating environment of PF6− anion in virtue of the stronger association capability of the TFSI− than that of PF6−, which resulted in a content regulation of the SEI components between Li2CO3 and LiF, finally contributing to a remarkable cycle life of graphite/Li cell comprising electrolyte with PP13*TFSI. The optimized molar ratio between LiPF6 salt and PP13*TFSI ionic liquid was obtained to be 3:1. Differential scanning calorimetry (DSC) confirmed significantly widened liquid range in the EC-based electrolytes with PP13*TFSI substitution. This work will provide a new insight and thinking on the application of room ionic liquid.