Superionicity in Ionic-Liquid-Based Electrolytes Induced by Positive Ion–Ion Correlations

In
ionic-liquid (IL)-based electrolytes, relevant for current energy
storage applications, ion transport is limited by strong ion–ion
correlations, generally yielding inverse Haven ratios (ionicities)
of below 1. In particular, Li is transported in anionic clusters into
the wrong direction of the electric field, requiring compensation
by diffusive anion fluxes. Here, we present a concept to exploit ion–ion
correlations in concentrated IL electrolytes beneficially by designing
organic cations with a Li-coordinating chain. 1H NMR and
Raman spectra show that IL cations with seven or more ether oxygens
in the side chain induce Li coordination to organic cations. An unusual
behavior of an inverse Haven ratio of >1 is found, suggesting an
ionicity
larger than that of an ideal electrolyte with uncorrelated ion motion.
This superionic behavior is consistently demonstrated
in both NMR transport/conductivity measurements and molecular dynamics
(MD) simulations. Key to this achievement is the formation of long-lived
Li–IL cation complexes, which invert the Li drift direction,
yielding positive Li+ ion mobilities for the first time
in a single IL-solvent-based electrolyte. Onsager correlation coefficients
are derived from MD simulations and demonstrate that the main contributions
to the inverse Haven ratio, which induce superionicity, arise from
enhanced Li–IL cation correlations and a sign inversion of
Li-anion correlation coefficients. Thus, the novel concept of coordinating
cations not only corrects the unfortunate anionic drift direction
of Li in ILs but even exploits strong ion correlations in the concentrated
electrolyte toward superionic transport.

在适用于当前储能应用的离子液体（ionic liquid, IL）基电解质中，离子传输过程受较强的离子-离子关联作用制约，通常呈现低于1的哈文比（离子度）。具体而言，锂离子（Li）以阴离子团簇的形式沿电场反方向传输，需要通过扩散阴离子通量进行补偿。本研究提出一种策略，通过设计带有锂离子配位侧链的有机阳离子，将浓离子液体电解质中的离子-离子关联作用加以正向利用。氢核磁共振（1H NMR）与拉曼光谱测试结果表明，侧链含有7个及以上醚氧原子的离子液体阳离子可促使锂离子与有机阳离子形成配位结构。研究团队观测到反哈文比大于1的反常现象，这表明其离子度高于离子运动无关联的理想电解质。这种超离子传输行为在氢核磁共振传输/电导率测试与分子动力学（molecular dynamics, MD）模拟中均得到了一致验证。该成果的核心在于长寿命锂离子-离子液体阳离子配合物的形成，其反转了锂离子的漂移方向，首次在单一离子液体溶剂基电解质中实现了正的Li+离子迁移率。研究从分子动力学模拟中提取了昂萨格关联系数（Onsager correlation coefficients），结果表明，引发超离子行为的反哈文比主要来源于增强的锂离子-离子液体阳离子关联作用，以及锂离子-阴离子关联系数的符号反转。因此，这种配位阳离子的全新策略不仅纠正了离子液体中锂离子沿阴离子团簇漂移的不利方向，更将浓电解质中的强离子-离子关联作用转化为超离子传输的优势。