Ionic Liquid in Li Salt Electrolyte: Modifying the Li+ Transport Mechanism by Coordination to an Asymmetric Anion

Abstract

In binary ionic liquid/Li salt electrolytes for lithium‐ion batteries the use of asymmetric anions reduces the tendency for crystallization and enables liquid systems with high Li concentration. The ionic liquid composed of the N‐(methoxyethyl)‐N‐methylpyrrolidinium (Pyr12O1) cation and the (fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTFSI) asymmetric anion at molar Li FTFSI fractions up to 0.6 are investigated by 19F and 7Li chemical shifts, 1H, 19F, and 7Li electrophoretic nuclear magnetic resonance (NMR) and density functional theory (DFT) calculations. Thereby, the local coordination environment of Li is elucidated and correlated with the Li+ ion transport properties. At low Li salt fraction, the preferred Li+ coordination is to the trifluoromethanesulfonyl side of the anion, resulting in vehicular Li+ transport in stable, net negatively charged Li‐anion clusters causing negative Li transference numbers. The Li coordination is, however, shifting to the fluorosulfonyl group at salt fractions >0.4, as consistently evidenced by DFT and 19F NMR. Herein, Li+ mobilities give evidence of an increasing relevance of structural Li+ ion transport, which is key toward developing efficient ionic liquid–based batteries. This knowledge will serve further tailored design of cations and anions, which reduces crystallization and promote structural transport in ionic liquids for safe and high‐power batteries.