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

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• 作者：Justin B. Haskins ; William R. Bennett ; James J. Wu ; Dionne M. Hern谩ndez ; Oleg Borodin ; Joshua D. Monk ; Charles W. Bauschlicher ; Jr. ; John W. Lawson
• 刊名：Journal of Physical Chemistry B
• 出版年：2014
• 出版时间：September 25, 2014
• 年：2014
• 卷：118
• 期：38
• 页码：11295-11309
• 全文大小：855K
• ISSN：1520-5207

文摘

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 鈮?x_Li⁺ 鈮?0.33) and temperature (298 K 鈮?T 鈮?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 x_Li⁺ 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 x_Li⁺, the contribution of Li⁺ to ionic conductivity increases until reaching a saturation doping level of x_Li⁺ = 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 (鈭?00 ns) to converge transport properties at room temperature. The differences in Li⁺ transport are reflected in the residence times of Li⁺ with the anions (蟿^{Li/鈥?/sup>), 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 x_Li+ and in liquids with large anions.}