N-Alkyl-N-ethylpyrrolidinium cation-based ionic liquid electrolytes for safer lithium battery systems

The performance of ionic liquid (IL) electrolytes based on N-alkyl-N-ethylpyrrolidinium cations combined with different lithium salts has been used for the design of safer lithium battery systems. These mixtures of ILs, synthesized through an eco-friendly procedure route, and salts were studied and their performance was compared on the basis of anion species. The N-butyl-N-ethylpyrrolidinium bis(fluorosulfonyl)imide/lithium bis(fluorosulfonyl)imide (PYR24FSI-LiFSI) mixture was found to exhibit superior characteristics in terms of ion transport properties even at low temperatures in combination with wide electrochemical stability, feasibility of reversibly plating lithium metal without any reduction of electrolyte and good compatibility with Li anode. These are considered to be due to good film-forming ability of the FSI anion. Conversely, lower thermal stability was also observed. Preliminary tests in lithium/lithium iron phosphate, Li/LiFePO4, half-cells using PYR24FSI-LiFSI as the electrolyte evidenced cycling performance approaching that in organic electrolytes with high capacity retention and efficiency values.