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

摘要

We employ molecular dynamics (MD) simulation and experiment to investigate the structure, thermodynamics, and transport of N-methyl-N-butylpyrroIidinium bis(trifluoromethylsufonyl)imide ([pyrl4][TFSI]), N-ffiethyl-N-propylpyrrohdinium bis(fluorosufonyl)imide ([pyrl3]-[FSI]), and l-ethyl-3-methylimidazolium boron tetrafluoride ([EMTM][BF4]), as a function of Li-salt mole fraction (0.05 ≤ x_(Li~+) ≤0.33) and temperature (298 K ≤ T ≤ 393 K). Structurally, Li~+ is shown to be solyated by three anion neighbors in [pyrl4] [TFSI] and four anion neighbors in both [pyrl3][FSI]. and [EMIM][BF4], and at alllevels 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 [pyrl4] [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 [pyrl3][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.: