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Computational and Experimental Investigation of Li-Doped Ionic Liquid Electrolytes: [pyr14][TFSI], [pyr13][FSI], and [EMIM][BF<sub>4</sub>]
We employ molecular dynamics (MD)
simulation and experiment to
investigate the structure, thermodynamics, and transport of <i>N</i>-methyl-<i>N</i>-butylpyrrolidinium bis(trifluoromethylsufonyl)imide
([pyr14][TFSI]), <i>N</i>-methyl-<i>N</i>-propylpyrrolidinium
bis(fluorosufonyl)imide ([pyr13][FSI]), and 1-ethyl-3-methylimidazolium
boron tetrafluoride ([EMIM][BF<sub>4</sub>]), as a function of Li-salt
mole fraction (0.05 ≤ <i>x</i><sub>Li<sup>+</sup></sub> ≤ 0.33) and temperature (298 K ≤ <i>T</i> ≤ 393 K). Structurally, Li<sup>+</sup> is shown to be solvated
by three anion neighbors in [pyr14][TFSI] and four anion neighbors
in both [pyr13][FSI] and [EMIM][BF<sub>4</sub>], and at all levels
of <i>x</i><sub>Li<sup>+</sup></sub> 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 <i>x</i><sub>Li<sup>+</sup></sub>, the contribution
of Li<sup>+</sup> to ionic conductivity increases until reaching a
saturation doping level of <i>x</i><sub>Li<sup>+</sup></sub> = 0.10. Comparatively, the Li<sup>+</sup> 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<sup>+</sup> transport are reflected in the residence times of Li<sup>+</sup> with the anions (τ<sup>Li/–</sup>), which are
revealed to be much larger for [pyr14][TFSI] (up to 100 ns at the
highest doping levels) than in either [EMIM][BF<sub>4</sub>] or [pyr13][FSI].
Finally, to comment on the relative kinetics of Li<sup>+</sup> transport
in each liquid, we find that while the net motion of Li<sup>+</sup> with its solvation shell (vehicular) significantly contributes to
net diffusion in all liquids, the importance of transport through
anion exchange increases at high <i>x</i><sub>Li<sup>+</sup></sub> and in liquids with large anions