Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, USA.
J Chem Phys. 2020 Oct 28;153(16):164502. doi: 10.1063/5.0024486.
Lithium ion solutions in organic solvents have become ubiquitous because of their use in energy storage technologies. The widespread use of lithium salts has prompted a large scientific interest in elucidating the molecular mechanisms, giving rise to their macroscopic properties. Due to the complexity of these molecular systems, only few studies have been able to unravel the molecular motions and underlying mechanisms of the lithium ion (Li) solvation shell. Lately, the atomistic motions of these systems have become somewhat available via experiments using ultrafast laser spectroscopies, such as two-dimensional infrared spectroscopy. However, the molecular mechanism behind the experimentally observed dynamics is still unknown. To close this knowledge gap, this work investigated solutions of a highly dissociated salt [LiTFSI: lithium bis(trifluoromethanesulfonyl)imide] and a highly associated salt (LiSCN: lithium thiocyanate) in acetonitrile (ACN) using both experimental and theoretical methods. Linear and non-linear infrared spectroscopies showed that Li is found as free ions and contact ion pairs in ACN/LiTFSI and ACN/LiSCN systems, respectively. In addition, it was also observed from the non-linear spectroscopy experiments that the dynamics of the ACN molecules in the Li first solvation shell has a characteristic time of ∼1.6 ps irrespective of the ionic speciation of the cation. A similar characteristic time was deducted from ab initio molecular dynamics simulations and density functional theory computations. Moreover, the theoretical calculations showed that molecular mechanism is directly related to fluctuations in the angle between Li and the coordinated ACN molecule (Li⋯N≡C), while other structural changes such as the change in the distance between the cation and the solvent molecule (Li⋯N) play a minor role. Overall, this work uncovers the time scale of the solvent motions in the Li solvation shell and the underlying molecular mechanisms via a combination of experimental and theoretical tools.
有机溶剂中的锂离子溶液因其在储能技术中的应用而变得无处不在。由于锂盐的广泛使用，人们对阐明其分子机制产生了浓厚的兴趣，从而揭示了它们的宏观性质。由于这些分子体系的复杂性，只有少数研究能够揭示锂离子（Li）溶剂化壳的分子运动和潜在机制。最近，通过使用超快激光光谱学（如二维红外光谱学）等实验，这些体系的原子运动变得有些可用。然而，实验观察到的动力学背后的分子机制仍然未知。为了填补这一知识空白，本工作使用实验和理论方法研究了高度离解的盐[LiTFSI：双（三氟甲烷磺酰基）亚胺锂]和高度缔合的盐（LiSCN：硫氰酸锂）在乙腈（ACN）中的溶液。线性和非线性红外光谱表明，Li 在 ACN/LiTFSI 和 ACN/LiSCN 体系中分别以游离离子和接触离子对的形式存在。此外，从非线性光谱实验中还观察到，Li 第一溶剂化壳中 ACN 分子的动力学具有约 1.6 ps 的特征时间，与阳离子的离子态无关。从从头算分子动力学模拟和密度泛函理论计算中也得出了类似的特征时间。此外，理论计算表明，分子机制与 Li 和配位的 ACN 分子（Li⋯N≡C）之间角度的波动直接相关，而其他结构变化（如阳离子和溶剂分子之间的距离变化（Li⋯N）则起次要作用。总的来说，本工作通过实验和理论工具的结合，揭示了 Li 溶剂化壳中溶剂运动的时间尺度和潜在的分子机制。
