Casalegno Mosè, Raos Guido, Appetecchi Giovanni Battista, Passerini Stefano, Castiglione Franca, Mele Andrea
Department of Chemistry, Materials and Chemical Engineering "G. Natta", Politecnico di Milano , Piazza L. Da Vinci, 32, 20133 Milano, Italy.
ENEA, Italian National Agency for New Technology, Energy and Sustainable Economic Development, Materials and Physicochemical Processes Laboratory , Via Anguillarese 301, 00196 Rome, Italy.
J Phys Chem Lett. 2017 Oct 19;8(20):5196-5202. doi: 10.1021/acs.jpclett.7b02431. Epub 2017 Oct 10.
Knowledge of the ion motion in room temperature ionic liquids (RTILs) is critical for their applications in a number of fields, from lithium batteries to dye-sensitized solar cells. Experiments on a limited number of RTILs have shown that on macroscopic time scales the ions typically undergo conventional, Gaussian diffusion. On shorter time scales, however, non-Gaussian behavior has been observed, similar to supercooled fluids, concentrated colloidal suspensions, and more complex systems. Here we characterize the diffusive motion of ionic liquids based on the N-butyl-N-methylpyrrolidinium (PYR) cation and bis(trifluoro methanesulfonyl)imide (TFSI) or bis(fluorosulfonyl)imide (FSI) anions. A combination of pulsed gradient spin-echo (PGSE) NMR experiments and molecular dynamics (MD) simulations demonstrates a crossover from subdiffusive behavior to conventional Gaussian diffusion at ∼10 ns. The deconvolution of molecular displacements into a continuous spectrum of diffusivities shows that the short-time behavior is related to the effects of molecular caging. For PYRFSI, we identify the change of short-range ion-counterion associations as one possible mechanism triggering long-range displacements.
了解室温离子液体（RTILs）中的离子运动对于它们在从锂电池到染料敏化太阳能电池等众多领域的应用至关重要。对有限数量的RTILs进行的实验表明，在宏观时间尺度上，离子通常经历常规的高斯扩散。然而，在更短的时间尺度上，已经观察到非高斯行为，类似于过冷流体、浓胶体悬浮液和更复杂的系统。在这里，我们基于N-丁基-N-甲基吡咯烷鎓（PYR）阳离子和双（三氟甲磺酰）亚胺（TFSI）或双（氟磺酰）亚胺（FSI）阴离子来表征离子液体的扩散运动。脉冲梯度自旋回波（PGSE）核磁共振实验和分子动力学（MD）模拟相结合表明，在约10纳秒时从亚扩散行为转变为常规高斯扩散。将分子位移解卷积为连续的扩散率谱表明，短时间行为与分子笼效应有关。对于PYRFSI，我们确定短程离子-反离子缔合的变化是触发长程位移的一种可能机制。
