Xu Yifei, Lin Wendy J, Gliege Marisa, Gunckel Ryan, Zhao Zuofeng, Yu Hongyu, Dai Lenore L
Department of Mechanical and Aerospace Engineering , Hong Kong University of Science and Technology , Clear Water Bay, Kowloon , Hong Kong.
J Phys Chem B. 2018 Dec 20;122(50):12077-12086. doi: 10.1021/acs.jpcb.8b08815. Epub 2018 Nov 29.
Ionic liquids (ILs) show a promising future as electrolytes in electrochemical devices. In particular, IL-based electrolytes bring operations at extreme temperatures to realization that conventional electrolytes fail to accomplish. Although IL electrolytes demonstrate considerable progress in high-temperature applications, their breakthroughs in devices operating at low temperatures are still very limited due to undesirable phase transitions and unsatisfying transport properties. In this study, we present an approach where, by tuning molecular interactions in the system, the designed electrolyte of an IL-based mixture can reach a lower operating temperature with improved transport properties. We have discovered that the incorporation of the IL, ethylammonium nitrate ([EA][N]), can contribute to reforming the molecular interactions within the system, which effectively resolve the crystallization accompanied with the excess of water and retain a low glass transition temperature. The reported liquid electrolyte systems based on a mixture of 1-butyl-3-methylimidazolium iodide ([BMIM][I]), [EA][N], water, and lithium iodide exhibit a glass transition temperature below -105 °C. Furthermore, the optimized electrolyte system shows significant viscosity reduction and ionic conductivity enhancement from 25 to -75 °C. The influence is also noticeable on the increased ionicity, which made the developed electrolyte comparable with other good ILs under the Walden rule. The electrochemical stability of the electrolyte system is revealed by a steady and reproducible profile of iodide/triiodide redox reactions at room temperature over a proper potential window via cyclic voltammetry. The results from this work not only provide a potential solution to applications of the iodide/triiodide redox couple-based electrochemical devices at low temperatures but also show a practical approach to obtain tailored properties of a mixture system via modifying molecular interactions.
离子液体（ILs）作为电化学装置中的电解质展现出了广阔的前景。特别是，基于离子液体的电解质实现了在极端温度下的运行，而传统电解质无法做到这一点。尽管离子液体电解质在高温应用中取得了显著进展，但由于不良的相变和不尽人意的传输性能，它们在低温运行设备中的突破仍然非常有限。在本研究中，我们提出了一种方法，即通过调节系统中的分子相互作用，设计的基于离子液体的混合电解质可以在改善传输性能的同时达到更低的运行温度。我们发现，加入离子液体硝酸乙铵（[EA][N]）有助于重塑系统内的分子相互作用，有效解决因水过量而伴随的结晶问题，并保持较低的玻璃化转变温度。报道的基于1-丁基-3-甲基咪唑碘化物（[BMIM][I]）、[EA][N]、水和碘化锂混合物的液体电解质系统的玻璃化转变温度低于-105°C。此外，优化后的电解质系统在25至-75°C范围内显示出显著的粘度降低和离子电导率增强。这种影响在增加离子性方面也很明显，这使得所开发的电解质在瓦尔登规则下与其他优良的离子液体相当。通过循环伏安法在室温下合适的电位窗口内进行碘化物/三碘化物氧化还原反应的稳定且可重复的曲线，揭示了电解质系统的电化学稳定性。这项工作的结果不仅为基于碘化物/三碘化物氧化还原对的电化学装置在低温下的应用提供了一种潜在的解决方案，还展示了一种通过修饰分子相互作用来获得混合系统定制性能的实用方法。
