Enhancement of Functional Properties of Liquid Electrolytes for Lithium‐Ion Batteries by Addition of Pyrrolidinium‐Based Ionic Liquids with Long Alkyl‐Chains

Abstract Three ionic liquid belonging to the N ‐alkyl‐ N ‐methylpyrrolidinium bis(trifluoromethanesulfonyl) imides (Pyr 1 , n TFSI with n =4,5,8) have been added as co‐solvent to two commonly used electrolytes for Li‐ion cells: (a) 1 M lithium hexafluorophosphate (LiPF 6 ) in a mixture of ethylene carbonate (EC) and linear like dimethyl carbonate (DMC) in 1 : 1 v/v and (b) 1 M lithium bis‐(trifluoromethanesulfonyl)imide (LiTFSI) in EC : DMC 1 : 1 v/v. These electrolyte formulations (classified as P and T series containing LiPF 6 or LiTFSI salts, respectively) have been analyzed by comparing ionic conductivities, transport numbers, viscosities, electrochemical stability as well as vibrational properties. In the case of the Pyr 1,5 TFSI and Pyr 1,8 TFSI blended formulations, this is the first ever reported detailed study of their functional properties in Li‐ion cells electrolytes. Overall, P‐electrolytes demonstrate enhanced properties compared to the T‐ones. Among the various P electrolytes those containing Pyr 1,4 TFSI and Pyr 1,5 TFSI limit the accumulation of irreversible capacity upon cycling with satisfactory performance in lithium cells.