Lithium Solvation and Mobility in Asymmetric Cyano(trifuloromethanesulfonyl)Imide Based Ionic Liquid Electrolyte for Li-Metal Battery

Abstract

The solvation structure and transport properties of Li+ in ionic liquid (IL) electrolytes based on n-methyl-n-butylpyrrolidinium cyano(trifluoromethanesulfonyl)imide [PYR14][CTFSI] and [Li][CTFSI] (0≤ xLi ≤ 0.7) were studied by Raman and Nuclear Magnetic Resonance (NMR) spectroscopy, and molecular dynamics (MD) simulations. Li+-anion coordination is found to be dominated through the cyano group. Coordination through the sulfonyl group is seen only at higher Li-salt concentrations (xLi > 0.3). Such a trend is not observable in the case of the analogous electrolyte composed of the 1:1 mixture of the symmetric anions bis(trifluoromethanesulfonyl)imide ([TFSI]) and dicyanamide ([DCA]) since the Li-salt concentration was limited by its solubility (xLi < 0.05). The calculated ion pair lifetimes of Li+-cyano coordination for [CTFSI] are found to be shorter than that of [DCA] at xLi = 0.05, indicating the competition from the sulfonyl group on [CTFSI] weakens its solvation with Li+. This translated to the higher Li+ transference estimated for the IL electrolyte with [CTFSI] in comparison to [TFSI]:[DCA]. The NMR diffusivity measurements indicated an increase in Li+ diffusivity compared to [CTFSI] as xLi approached 0.7. These findings suggest the Li+ transport mechanism changes in the asymmetric anion at high salt concentrations. In relation to the utility of these electrolytes in energy storage, the Li-LiFePO4 half cells assembled with IL electrolyte were tested at 363 K. The system with xLi=0.7 demonstrated a capacity retention of 61% at 0.1 C-rate and 363 K after 100 cycles; remarkably higher than those of xLi=0.3 and 0.5. This is attributed to the improved electrochemical stability of the IL electrolyte with the asymmetric anion at higher salt concentrations.