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

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 Li₂ CO₃ 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.