Hybridizing Organic and Solid State Single-Ion Conductors: Thermotropic Ionic Liquid Crystals for Lithium Battery Electrolytes

The continuously increasing importance of lithium-ion batteries for the transportation sector does and will require also substantial enhancements of their energy and power density as well as their safety. As the latter aspect is mainly determined by the currently utilized liquid organic electrolytes, comprising LiPF 6 as conducting salt, extensive research efforts are directed towards the development and optimization of alternative (solid) electrolytes. 1–3 In addition to a suitable ionic conductivity, these new electrolyte systems have to fulfill a series of other requirements, most importantly, a sufficient electrochemical stability, beneficial interfacial properties, enhanced thermal stability, and, preferably, flexibility as well as single-ion conductivity to avoid the necessity of using LiPF 6 and detrimental cell polarization. The two electrolyte systems, which are currently considered most promising to realize safer lithium-based batteries, are basically solid ceramic and polymer-based electrolytes. 1–3 However, while the former frequently suffer electrode/electrolyte interfacial instability and thus rapid capacity fading (apart from its intrinsic rigidity), the latter commonly face challenges regarding their thermal stability and large cell polarization due to the necessity of adding a conducting salt to the polymer matrix. In an attempt to combine the beneficial characteristics of these two systems, we designed and synthesized a completely new electrolyte system: single-ion conducting thermotropic liquid crystals (TILCs, Figure 1). These TILCs reveal self-assembling, homeotropically aligned, lamellar nanostructures, providing confined ionic nanochannels for reversible lithium ion transport across suitable electrode/electrolyte interfaces. Herein, we will report their extended characterization as lithium battery electrolytes with a particular focus on the structure to transport interplay, highlighting the importance of large-scale facility experiments (structure and dynamics) to finally unveil and understand the ion transport mechanism on the molecular and sub-molecular level. Figure 1. General chemical formula of TILCs. References 1. B. Scrosati and J. Garche, J. Power Sources , 195 , 2419–2430 (2010). 2. J. Kalhoff, G. G. Eshetu, D. Bresser, and S. Passerini, ChemSusChem , 8 , 2154–2175 (2015). 3. Y. Wang and W.-H. Zhong, ChemElectroChem , 2 , 22–36 (2015). Figure 1