Dong Dengpan, Bedrov Dmitry
Department of Materials Science & Engineering , University of Utah , 122 S. Central Campus Drive , Room 304, Salt Lake City , Utah 84112 , United States.
J Phys Chem B. 2018 Nov 1;122(43):9994-10004. doi: 10.1021/acs.jpcb.8b06913. Epub 2018 Oct 18.
Molecular dynamics simulations using fully atomistic polarizable force field have been performed on solvate ionic liquids (SILs), comprised of tetraglyme (G4) solvent molecules, Li cations, and bis(trifluoromethane) sulfonimide (TFSI) anions, [Li(G4)][TFSI]. The SILs with equimolar salt:G4 composition were investigated in the 303-373 K temperature range, whereas several systems with lower salt concentrations were investigated at 373 K. The simulations using polarizable force field demonstrate very good consistency of structural and dynamic properties with experimental data. The ability to accurately sample the ion transport mechanisms is particularly encouraging, taking into account that previous simulations employing nonpolarizable models had challenges in sampling dynamics in these systems. Here, we correlate Li ion local environment and glyme conformations with dynamic characteristics, such as residence time of species around Li, self-diffusion coefficients, transference number, and conductivity. The analysis of contributions to Li mobility due to changing its local environment (i.e., moving from one glyme/anion to another) and from translational motion of Li with its' coordination environment showed significant dominance of the latter. The contributions of cross-ion dynamic correlations to the total conductivity have been quantified, showing strongly positive contribution from the cation-anion anticorrelation. Despite the high degree of Li-TFSI dissociation and positive contribution of the cation-anion anticorrelated motion to conductivity, the Li transference numbers for equimolar SILs are very low under the anion blocking conditions.
使用全原子可极化力场对溶剂化离子液体（SILs）进行了分子动力学模拟，该离子液体由四甘醇二甲醚（G4）溶剂分子、锂离子和双（三氟甲烷）磺酰亚胺（TFSI）阴离子组成，即[Li(G4)][TFSI]。研究了盐与G4等摩尔组成的SILs在303 - 373 K温度范围内的情况，而几个较低盐浓度的体系在373 K下进行了研究。使用可极化力场的模拟结果表明，其结构和动力学性质与实验数据具有很好的一致性。考虑到之前使用非极化模型的模拟在这些体系的动力学采样方面存在挑战，能够准确采样离子传输机制尤其令人鼓舞。在这里，我们将锂离子的局部环境和甘醇二甲醚构象与动态特性相关联，例如物种在锂周围的停留时间、自扩散系数、迁移数和电导率。分析了由于锂离子局部环境变化（即从一个甘醇二甲醚/阴离子转移到另一个）以及锂离子与其配位环境的平移运动对锂迁移率的贡献，结果表明后者占主导地位。对交叉离子动态相关性对总电导率的贡献进行了量化，结果表明阳离子 - 阴离子反相关具有很强的正向贡献。尽管Li-TFSI高度解离且阳离子 - 阴离子反相关运动对电导率有正向贡献，但在阴离子阻挡条件下，等摩尔SILs的锂迁移数非常低。
