Lytle T K, Yethiraj A
Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
J Chem Phys. 2022 Mar 14;156(10):104901. doi: 10.1063/5.0083414.
Polyelectrolyte solutions have been proposed as a method to improve the efficiency of lithium-ion batteries by increasing the cation transference number because the polymer self-diffusion coefficient is much lower than that of the counterion. However, this is not necessarily true for the polymer mobility. In some cases, negative transference numbers have been reported, which implies that the lithium ions are transporting to the same electrode as the anion, behavior that is often attributed to a binding of counterions to the polyion. We use a simple model where we bind some counterions to the polymer via harmonic springs to investigate this phenomenon. We find that both the number of bound counterions and the strength of their binding alter the transference number, and, in some cases, the transference number is negative. We also investigate how the transference number depends on the Manning parameter, the ratio of the Bjerrum length to charge separation along the chain. By altering the Manning parameter, the transference number can almost be doubled, which suggests that charge spacing could be a way to increase the transference number of polyelectrolyte solutions.
聚电解质溶液已被提议作为一种通过提高阳离子迁移数来提高锂离子电池效率的方法，因为聚合物的自扩散系数远低于抗衡离子的自扩散系数。然而，聚合物的迁移率情况并非一定如此。在某些情况下，已报道了负迁移数，这意味着锂离子与阴离子传输到同一电极，这种行为通常归因于抗衡离子与聚离子的结合。我们使用一个简单模型，通过谐振子将一些抗衡离子与聚合物结合，来研究这一现象。我们发现，结合的抗衡离子数量及其结合强度都会改变迁移数，并且在某些情况下，迁移数为负。我们还研究了迁移数如何取决于曼宁参数，即比耶鲁姆长度与沿链电荷间距的比值。通过改变曼宁参数，迁移数几乎可以翻倍，这表明电荷间距可能是增加聚电解质溶液迁移数的一种方法。
