Computational and Experimental Investigation of Li-Doped Ionic Liquid Electrolytes: [pyr14][TFSI], [pyr13][FSI], and [EMIM][BF4]

We employ molecular dynamics (MD) simulation and experiment to investigate the structure, thermodynamics, and transport of N-methyl-N-butylpyrrolidinium bis­(trifluoromethylsufonyl)­imide ([pyr14]­[TFSI]), N-methyl-N-propylpyrrolidinium bis­(fluorosufonyl)­imide ([pyr13]­[FSI]), and 1-ethyl-3-methylimidazolium boron tetrafluoride ([EMIM]­[BF4]), as a function of Li-salt mole fraction (0.05 ≤ xLi+ ≤ 0.33) and temperature (298 K ≤ T ≤ 393 K). Structurally, Li+ is shown to be solvated by three anion neighbors in [pyr14]­[TFSI] and four anion neighbors in both [pyr13]­[FSI] and [EMIM]­[BF4], and at all levels of xLi+ we find the presence of lithium aggregates. Pulsed field gradient spin-echo NMR measurements of diffusion and electrochemical impedance spectroscopy measurements of ionic conductivity are made for the neat ionic liquids as well as 0.5 molal solutions of Li-salt in the ionic liquids. Bulk ionic liquid properties (density, diffusion, viscosity, and ionic conductivity) are obtained with MD simulations and show excellent agreement with experiment. While the diffusion exhibits a systematic decrease with increasing xLi+, the contribution of Li+ to ionic conductivity increases until reaching a saturation doping level of xLi+ = 0.10. Comparatively, the Li+ conductivity of [pyr14]­[TFSI] is an order of magnitude lower than that of the other liquids, which range between 0.1 and 0.3 mS/cm. Our transport results also demonstrate the necessity of long MD simulation runs (∼200 ns) to converge transport properties at room temperature. The differences in Li+ transport are reflected in the residence times of Li+ with the anions (τLi/–), which are revealed to be much larger for [pyr14]­[TFSI] (up to 100 ns at the highest doping levels) than in either [EMIM]­[BF4] or [pyr13]­[FSI]. Finally, to comment on the relative kinetics of Li+ transport in each liquid, we find that while the net motion of Li+ with its solvation shell (vehicular) significantly contributes to net diffusion in all liquids, the importance of transport through anion exchange increases at high xLi+ and in liquids with large anions.