McEldrew Michael, Goodwin Zachary A H, Molinari Nicola, Kozinsky Boris, Kornyshev Alexei A, Bazant Martin Z
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
Department of Chemistry, Molecular Sciences Research Hub, Imperial College of London, White City Campus, Wood Lane, London W12 0BZ, U.K.
J Phys Chem B. 2021 Dec 23;125(50):13752-13766. doi: 10.1021/acs.jpcb.1c05546. Epub 2021 Dec 13.
Salt-in-ionic liquid electrolytes have attracted significant attention as potential electrolytes for next generation batteries largely due to their safety enhancements over typical organic electrolytes. However, recent experimental and computational studies have shown that under certain conditions alkali cations can migrate in electric fields as if they carried a net negative effective charge. In particular, alkali cations were observed to have negative transference numbers at small mole fractions of alkali-metal salt that revert to the expected net positive transference numbers at large mole fractions. Simulations have provided some insights into these observations, where the formation of asymmetric ionic clusters, as well as a percolating ion network, could largely explain the anomalous transport of alkali cations. However, a thermodynamic theory that captures such phenomena has not been developed, as ionic associations were typically treated via the formation of ion pairs. The theory presented herein, based on the classical polymer theories, describes thermoreversible associations between alkali cations and anions, where the formation of large, asymmetric ionic clusters and a percolating ionic network are a natural result of the theory. Furthermore, we present several general methods to calculate the effective charge of alkali cations in ionic liquids. We note that the negative effective charge is a robust prediction with respect to the parameters of the theory and that the formation of a percolating ionic network leads to the restoration of net positive charges of the cations at large mole fractions of alkali metal salt. Overall, we find excellent qualitative agreement between our theory and molecular simulations in terms of ionic cluster statistics and the effective charges of the alkali cations.
离子液体中的盐电解质作为下一代电池的潜在电解质已引起了广泛关注，这主要是因为它们相对于典型的有机电解质具有更高的安全性。然而，最近的实验和计算研究表明，在某些条件下，碱金属阳离子在电场中迁移时，其行为就好像它们带有净负有效电荷一样。特别是，在碱金属盐的摩尔分数较小时，观察到碱金属阳离子的迁移数为负，而在摩尔分数较大时，迁移数又恢复为预期的净正迁移数。模拟研究为这些观察结果提供了一些见解，其中不对称离子簇以及渗流离子网络的形成可以很大程度上解释碱金属阳离子的异常迁移。然而，由于离子缔合通常通过离子对的形成来处理，尚未建立能够解释此类现象的热力学理论。本文提出的理论基于经典聚合物理论，描述了碱金属阳离子与阴离子之间的热可逆缔合，其中大的不对称离子簇和渗流离子网络的形成是该理论的自然结果。此外，我们还提出了几种计算离子液体中碱金属阳离子有效电荷的通用方法。我们注意到，负有效电荷对于该理论的参数而言是一个可靠的预测，并且渗流离子网络的形成会导致在碱金属盐的摩尔分数较大时阳离子的净正电荷得以恢复。总体而言，我们发现我们的理论与分子模拟在离子簇统计和碱金属阳离子的有效电荷方面具有出色的定性一致性。
