Dereka Bogdan, Lewis Nicholas H C, Zhang Yong, Hahn Nathan T, Keim Jonathan H, Snyder Scott A, Maginn Edward J, Tokmakoff Andrei
James Franck Institute, The University of Chicago, Chicago, Illinois 60637, United States.
Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, United States.
J Am Chem Soc. 2022 May 18;144(19):8591-8604. doi: 10.1021/jacs.2c00154. Epub 2022 Apr 26.
Understanding the mechanisms of charge transport in batteries is important for the rational design of new electrolyte formulations. Persistent questions about ion transport mechanisms in battery electrolytes are often framed in terms of vehicular diffusion by persistent ion-solvent complexes versus structural diffusion through the breaking and reformation of ion-solvent contacts, i.e., solvent exchange events. Ultrafast two-dimensional (2D) IR spectroscopy can probe exchange processes directly via the evolution of the cross-peaks on picosecond time scales. However, vibrational energy transfer in the absence of solvent exchange gives rise to the same spectral signatures, hiding the desired processes. We employ 2D IR on solvent resonances of a mixture of acetonitrile isotopologues to differentiate chemical exchange and energy-transfer dynamics in a comprehensive series of Li, Mg, Zn, Ca, and Ba bis(trifluoromethylsulfonyl)imide electrolytes from the dilute to the superconcentrated regime. No exchange phenomena occur within at least 100 ps, regardless of the ion identity, salt concentration, and presence of water. All of the observed spectral dynamics originate from the intermolecular energy transfer. These results place the lower experimental boundary on the ion-solvent residence times to several hundred picoseconds, much slower than previously suggested. With the help of MD simulations and conductivity measurements on the Li and Zn systems, we discuss these results as a continuum of vehicular and structural modalities that vary with concentration and emphasize the importance of collective electrolyte motions to ion transport. These results hold broadly applicable to many battery-relevant ions and solvents.
了解电池中的电荷传输机制对于合理设计新的电解质配方至关重要。关于电池电解质中离子传输机制的一些持久问题，通常围绕着由持久的离子 - 溶剂络合物进行的载体扩散，与通过离子 - 溶剂接触的断裂和重新形成（即溶剂交换事件）进行的结构扩散展开。超快二维（2D）红外光谱可以通过皮秒时间尺度上交叉峰的演化直接探测交换过程。然而，在没有溶剂交换的情况下的振动能量转移会产生相同的光谱特征，从而掩盖了所需的过程。我们利用二维红外光谱对乙腈同位素混合物的溶剂共振进行研究，以区分一系列从稀溶液到超浓溶液体系中锂、镁、锌、钙和钡双（三氟甲基磺酰）亚胺电解质中的化学交换和能量转移动力学。无论离子种类、盐浓度和水的存在情况如何，至少在100皮秒内都不会发生交换现象。所有观察到的光谱动力学都源于分子间的能量转移。这些结果将离子 - 溶剂停留时间的实验下限设定为几百皮秒，比之前认为的要慢得多。借助对锂和锌体系的分子动力学模拟和电导率测量，我们将这些结果讨论为随浓度变化的载体和结构模式的连续统一体，并强调集体电解质运动对离子传输的重要性。这些结果广泛适用于许多与电池相关的离子和溶剂。
