Pyrrolidinium-Based Ionic Liquids Doped with Lithium Salts: How Does Li+ Coordination Affect Its Diffusivity?

We present the characterization of LiX-doped room-temperature ionic liquids (ILs) based on the N-butyl-N-methyl pyrrolidinium (PYR14) cation w i t h t w o fl u o r i n a t e d a n i o n s : ( t r i fl u o r o m e t h a n e s u l f o n y l ) - (nonafluorobutanesulfonyl)imide (XIM14) and bis(pentafluoroethanesulfonyl)- imide (XBETI). The new data are also compared with previous results on PYR14TFSI (bis(trifluoromethanesulfonyl)imide). Their local organization has been investigated via NMR nuclear Overhauser effect (NOE) experiments for {1H−19F} and {1H−7Li} that give us details on PYR14 +/X− and PYR14 +/Li+ contacts. We confirm the presence of [Li(X)2]− coordinated species in all systems. The longrange, intermolecular NOEs have been detected and provide information on the ions’ organization beyond the first solvation sphere. The ionic conductivity, viscosity and self-diffusion coefficients of the ionic mixtures have also been measured. The activation energies for the diffusion of the individual ions and for the fluidity are compared with those for the pure ILs. Finally, density functional calculations on [Li(BETI)2]−, [Li(IM14)2]−, and [Li(TFSI)2]− complexes demonstrate that the minimum energy structures for all systems correspond to a tetrahedral coordination of the Li-ion by four oxygen atoms of the anions. Assuming very simple key steps for the Li+ diffusion process (i.e., the concerted breaking and formation of Li−O bonds or the rearrangement around a tetrahedrally coordinated Li+), we calculate activation barriers that agree well with the experimental results (approximately 46 kJ/mol, in all systems). 1. INTRODUCTION Room-temperature ionic liquids (ILs) are being incorporated as electrolyte materials in a wide variety of electrochemical devices including solar cells, fuel cells, and electrochemical (super or ultra) capacitors because of their favorable properties, such as a wide electrochemical window, high chemical and thermal stability, and negligible vapor pressure.1−7 In particular, ILs represent a greener and safer alternative to volatile organic liquids as solvents in lithium batteries.8−10 Several studies have reported that the addition of a lithium salt to an IL leads to an increase in viscosity and often does not produce the desired performance in terms of Li-ion mobility.11,12 To improve lithium batteries, particularly in terms of current density, it is thus important to clarify the transport properties of IL solutions. Important structural information on Li-doped ILs has been obtained by nuclear magnetic resonance (NMR) and vibrational spectroscopies.13−15 Molecular dynamics simulations and ab initio