McDaniel Jesse G, Son Chang Yun
School of Chemistry and Biochemistry , Georgia Institute of Technology , Atlanta , Georgia 30332-0400 , United States.
Division of Chemistry and Chemical Engineering , California Institute of Technology , Pasadena , California 91125 , United States.
J Phys Chem B. 2018 Jul 19;122(28):7154-7169. doi: 10.1021/acs.jpcb.8b04886. Epub 2018 Jul 6.
Quantifying ion association and collective dynamical processes in organic electrolytes is essential for fundamental property interpretation and optimization for electrochemical applications. The extent of ion correlation depends on both the ion concentration and dielectric strength of the solvent; ions may be largely uncorrelated in sufficiently high-dielectric solvents at low concentration, but properties of concentrated electrolytes are dictated by correlated and collective ion processes. In this work, we utilize molecular dynamics simulations to characterize ion association and collective ion dynamics in organic electrolytes composed of binary mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF] and 1,2-dichloroethane, acetone, acetonitrile, and water solvents. We illustrate different physical regimes of characteristically distinct ion correlations for the systematic range of electrolyte concentrations and solvent dielectric strengths. Dilute electrolytes composed of low-dielectric solvents exhibit significant counterion correlation in the form of ion pairing and clustering driven by both weak screening and relatively low solvation energies. This regime is characterized by enhanced ion coordination numbers and near equality of cation and anion diffusion coefficients, despite the significantly different ion sizes. In contrast, ion correlation in highly concentrated electrolytes is dominated by the anti-correlated motion of both like-charge and opposite-charge ions, approaching neat ionic liquid behavior. We show that the cross-over of these correlation regimes is clearly illuminated by quantifying the fractional self and distinct contributions to the net ionic conductivity. For organic electrolytes composed of low-dielectric solvents, we conclude that significant ion correlation exists at all concentrations but the nature of the correlation changes markedly from the dilute electrolyte to the pure ionic liquid limit.
量化有机电解质中的离子缔合和集体动力学过程对于基本性质的解释以及电化学应用的优化至关重要。离子相关性的程度取决于离子浓度和溶剂的介电强度；在低浓度的高介电溶剂中，离子可能在很大程度上不相关，但浓电解质的性质由相关的集体离子过程决定。在这项工作中，我们利用分子动力学模拟来表征由1-丁基-3-甲基咪唑四氟硼酸盐[BMIM][BF]与1,2-二氯乙烷、丙酮、乙腈和水溶剂的二元混合物组成的有机电解质中的离子缔合和集体离子动力学。我们展示了在电解质浓度和溶剂介电强度的系统范围内，特征明显不同的离子相关性的不同物理状态。由低介电溶剂组成的稀电解质表现出显著的反离子相关性，其形式为离子对和簇集，这是由弱屏蔽和相对较低的溶剂化能驱动的。尽管离子尺寸有显著差异，但该状态的特征是离子配位数增加，阳离子和阴离子扩散系数接近相等。相比之下，高浓度电解质中的离子相关性主要由同性电荷和异性电荷离子的反相关运动主导，接近纯离子液体行为。我们表明，通过量化对净离子电导率的分数自贡献和独特贡献，可以清楚地揭示这些相关状态的转变。对于由低介电溶剂组成的有机电解质，我们得出结论，在所有浓度下都存在显著的离子相关性，但相关性的性质从稀电解质到纯离子液体极限有明显变化。
