Galle Kankanamge Susith R, Kuroda Daniel G
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States.
J Phys Chem B. 2020 Mar 12;124(10):1965-1977. doi: 10.1021/acs.jpcb.9b10795. Epub 2020 Feb 27.
High concentration lithium electrolytes have been found to be good candidates for high energy density and high voltage lithium batteries. Recent studies have shown that limiting the free solvent molecules in the electrolytes prevents the degradation of the battery electrodes. However, the molecular level knowledge of the structure and dynamics of such an electrolyte system is limited, especially for electrolytes based on typical organic carbonates. In this article, the interactions and motions involved in lithium bis(trifluoromethanesulfonyl)imide in carbonyl-containing solvents are investigated using linear and time-resolved vibrational spectroscopies and computational methods. Our results suggest that the overall structure and the speciation of the three high concentration electrolytes are similar. However, the cyclic carbonate-based electrolyte presents an additional interaction as a result of dimer formation. Time-resolved studies reveal similar and fast dynamics for the structural motions of solvent molecules in electrolytes composed of linear molecules, while the electrolyte made of cyclic solvent molecules shows slower structural changes as a result of the dimer formation. Additionally, a picosecond time scale process is observed and assigned to the coordination and decoordination of solvent molecules from a lithium-ion solvation shell. This process of solvent exchange is found to be directly correlated to the making and breaking of structures between the lithium-ion and the anion and, consequently, to the conduction mechanism. Overall, our data show that the molecular structure of the solvent does not significantly affect the speciation and distribution of the lithium-ion solvation shells. However, the presence of dimerization between solvent molecules of two neighboring lithium-ions appears to produce a microscopic ordering that it is manifested macroscopically in properties of the electrolyte, such as its viscosity.
高浓度锂电解质已被发现是高能量密度和高电压锂电池的良好候选材料。最近的研究表明，限制电解质中的游离溶剂分子可防止电池电极降解。然而，对于这种电解质体系的结构和动力学的分子水平认识有限，特别是对于基于典型有机碳酸盐的电解质。在本文中，使用线性和时间分辨振动光谱以及计算方法研究了双（三氟甲磺酰）亚胺锂在含羰基溶剂中的相互作用和运动。我们的结果表明，三种高浓度电解质的整体结构和形态相似。然而，基于环状碳酸酯的电解质由于二聚体的形成而呈现出额外的相互作用。时间分辨研究表明，由线性分子组成的电解质中溶剂分子的结构运动具有相似且快速的动力学，而由环状溶剂分子制成的电解质由于二聚体的形成而显示出较慢的结构变化。此外，观察到一个皮秒时间尺度的过程，并将其归因于溶剂分子从锂离子溶剂化壳层的配位和解配位。发现这种溶剂交换过程与锂离子和阴离子之间结构的形成和破坏直接相关，因此与传导机制相关。总体而言，我们的数据表明溶剂的分子结构不会显著影响锂离子溶剂化壳层的形态和分布。然而，两个相邻锂离子的溶剂分子之间二聚化的存在似乎产生了一种微观有序性，这种有序性在宏观上表现为电解质的性质，如粘度。
