Haskins Justin B, Bennett William R, Wu James J, Hernández Dionne M, Borodin Oleg, Monk Joshua D, Bauschlicher Charles W, Lawson John W
ERC Inc., Thermal Protection Materials and Systems Branch, ‡Entry Systems and Technology Division, and §Thermal Protection Materials and Systems Branch, NASA Ames Research Center , Moffett Field, California 94035, United States.
J Phys Chem B. 2014 Sep 25;118(38):11295-309. doi: 10.1021/jp5061705. Epub 2014 Sep 15.
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 (τ(Li/-)), 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.
我们采用分子动力学（MD）模拟和实验方法，研究了N-甲基-N-丁基吡咯烷双（三氟甲基磺酰）亚胺（[pyr14][TFSI]）、N-甲基-N-丙基吡咯烷双（氟磺酰）亚胺（[pyr13][FSI]）和1-乙基-3-甲基咪唑四氟硼酸盐（[EMIM][BF4]）的结构、热力学和输运性质，这些性质是锂盐摩尔分数（0.05≤xLi(+)≤0.33）和温度（298 K≤T≤393 K）的函数。在结构上，Li(+)在[pyr14][TFSI]中被三个阴离子溶剂化，在[pyr13][FSI]和[EMIM][BF4]中被四个阴离子溶剂化，并且在所有xLi(+)水平下，我们都发现了锂聚集体的存在。对纯离子液体以及离子液体中0.5摩尔浓度的锂盐溶液进行了脉冲场梯度自旋回波核磁共振扩散测量和离子电导率的电化学阻抗谱测量。通过MD模拟获得了本体离子液体的性质（密度、扩散、粘度和离子电导率），并与实验结果显示出极好的一致性。虽然扩散随着xLi(+)的增加而系统地降低，但Li(+)对离子电导率的贡献增加，直到达到xLi(+) = 0.10的饱和掺杂水平。相比之下，[pyr14][TFSI]的Li(+)电导率比其他液体低一个数量级，其他液体的电导率在0.1至0.3 mS/cm之间。我们的输运结果还表明，在室温下需要进行长时间的MD模拟运行（约200 ns）才能使输运性质收敛。Li(+)输运的差异反映在Li(+)与阴离子的停留时间（τ(Li/-)）上，结果显示[pyr14][TFSI]的停留时间（在最高掺杂水平下高达100 ns）比[EMIM][BF4]或[pyr13][FSI]中的长得多。最后，为了评论每种液体中Li(+)输运的相对动力学，我们发现虽然Li(+)与其溶剂化壳层的净运动（载流子）对所有液体中的净扩散有显著贡献，但在高xLi(+)和含有大阴离子的液体中，通过阴离子交换的输运的重要性增加。
