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

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posted on 25.09.2014, 00:00 by Justin B. Haskins, William R. Bennett, James J. Wu, Dionne M. Hernández, Oleg Borodin, Joshua D. Monk, Charles W. Bauschlicher, John W. Lawson
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.

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