Ion correlation and negative lithium transference in polyelectrolyte solutions

Polyelectrolyte solutions (PESs) recently have been proposed as high conductivity, high lithium transference number (t+) electrolytes where the majority of the ionic current is carried by the electrochemically active Li-ion. While PESs are intuitively appealing because anchoring the anion to a polymer backbone selectively slows down anionic motion and therefore increases t+, increasing the anion charge will act as a competing effect, decreasing t+. In this work, we directly measure ion mobilities in a model non-aqueous polyelectrolyte solution using electrophoretic Nuclear Magnetic Resonance Spectroscopy (eNMR) to probe these competing effects. While previous studies that rely on ideal assumptions predict that PESs will have higher t+ than monomeric solutions, we demonstrate that below the entanglement limit, both conductivity and t+ decrease with increasing degree of polymerization. For polyanions of 10 or more repeat units, at 0.5m Li+ we directly observe Li+ move in the “wrong direction” in an electric field, evidence of a negative transference number due to correlated motion through ion clustering. This is the first experimental observation of negative transference in a non-aqueous polyelectrolyte solution. We also demonstrate that t+ increases with increasing Li+ concentration. Using Onsager transport coefficients calculated from experimental data, and insights from previously published molecular dynamics studies we demonstrate that despite selectively slowing anion motion using polyanions, distinct anion-anion correlation through the polymer backbone and cation-anion correlation through ion aggregates reduce the t+ in non-entangled PESs. This leads us to conclude that short-chained polyelectrolyte solutions are not viable high transference number electrolytes. These results emphasize the importance of understanding the effects of ion-correlations when designing new concentrated electrolytes for improved battery performance.

聚电解质溶液（Polyelectrolyte Solutions, PESs）近年来被提出作为兼具高电导率与锂离子迁移数（lithium transference number, t+）的电解质体系，其绝大多数离子电流由具有电化学活性的锂离子承载。尽管聚电解质溶液凭借将阴离子锚定在聚合物主链上，可选择性减慢阴离子运动从而提升t+，在直觉上颇具吸引力，但提高阴离子电荷会产生相反效应，导致t+降低。本研究中，我们借助电泳核磁共振波谱法（Electrophoretic Nuclear Magnetic Resonance Spectroscopy, eNMR），直接测定模型非水聚电解质溶液中的离子迁移率，以探究这些相互竞争的效应。尽管以往依赖理想假设的研究预测聚电解质溶液的t+高于单体溶液，但我们的研究表明，在缠结极限以下，电导率与t+均会随聚合度升高而降低。对于拥有10个及以上重复单元的聚阴离子，在0.5 mol/L锂离子体系中，我们直接观测到锂离子在电场中向"错误方向"移动，这一现象证明了因离子团簇引发的关联运动所导致的负迁移数。这是非水聚电解质溶液中负迁移数的首次实验观测结果。我们还证实，t+会随锂离子浓度升高而增大。我们借助从实验数据中计算得到的昂萨格输运系数（Onsager transport coefficients），结合已发表的分子动力学研究所得见解，证明尽管聚阴离子可选择性减慢阴离子运动，但通过聚合物主链形成的显著阴离子-阴离子关联，以及通过离子聚集体形成的阳离子-阴离子关联，会降低非缠结聚电解质溶液的t+。据此我们得出结论：短链聚电解质溶液并非具备实用价值的高迁移数电解质。本研究结果凸显了在设计用于提升电池性能的新型浓电解质时，阐明离子关联效应的重要性。