Bergstrom Helen K, Fong Kara D, Halat David M, Karouta Carl A, Celik Hasan C, Reimer Jeffrey A, McCloskey Bryan D
Department of Chemical & Biomolecular Engineering, University of California Berkeley CA 94720 USA
Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory Berkeley CA 94720 USA.
Chem Sci. 2023 May 16;14(24):6546-6557. doi: 10.1039/d3sc01224g. eCollection 2023 Jun 21.
Polyelectrolyte solutions (PESs) recently have been proposed as high conductivity, high lithium transference number () 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 , increasing the anion charge will act as a competing effect, decreasing . 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 than monomeric solutions, we demonstrate that below the entanglement limit, both conductivity and decrease with increasing degree of polymerization. For polyanions of 10 or more repeat units, at 0.5 m 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 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 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(PESs)最近被提议作为高电导率、高锂迁移数((t_{Li^+}))的电解质,其中大部分离子电流由具有电化学活性的锂离子携带。虽然聚电解质溶液直观上很有吸引力,因为将阴离子锚定到聚合物主链上会选择性地减缓阴离子运动,从而提高(t_{Li^+}),但增加阴离子电荷会产生竞争效应,降低(t_{Li^+})。在这项工作中,我们使用电泳核磁共振光谱(eNMR)直接测量模型非水聚电解质溶液中的离子迁移率,以探究这些竞争效应。虽然之前依赖理想假设的研究预测聚电解质溶液的(t_{Li^+})将高于单体溶液,但我们证明在缠结极限以下,电导率和(t_{Li^+})都会随着聚合度的增加而降低。对于具有10个或更多重复单元的聚阴离子,在0.5 m Li浓度下,我们直接观察到Li在电场中向“错误方向”移动,这是由于离子簇的相关运动导致负迁移数的证据。这是在非水聚电解质溶液中首次观察到负迁移现象。我们还证明(t_{Li^+})会随着Li浓度的增加而增加。利用从实验数据计算得到的昂萨格传输系数,以及先前发表的分子动力学研究的见解,我们证明尽管使用聚阴离子选择性地减缓了阴离子运动,但通过聚合物主链的不同阴离子 - 阴离子相关性以及通过离子聚集体的阳离子 - 阴离子相关性会降低非缠结聚电解质溶液中的(t_{Li^+})。这使我们得出结论,短链聚电解质溶液不是可行的高迁移数电解质。这些结果强调了在设计用于改善电池性能的新型浓电解质时,理解离子相关性影响的重要性。