Induction of Liquid-Crystalline Bicontinuous Cubic Phases into Zwitterions by Addition of Lithium Salts

Liquid crystals have great potential for the development of functional nanostructured materials because they form dynamic and ordered states. They have been applied to a variety of fields including ion and electron transporting materials, separation membranes, and nanostructured catalysts. In particular, their application to electrolytes for energy devices such as lithium ion batteries, dyesensitized solar cells and fuel cells is of interest. To date, a number of liquid-crystalline (LC) ion conductive materials have been reported. For example, the use of smectic (Sm) and columnar (Col) LC structures is advantageous for the development of low-dimensional ion conductors. 2 These LC materials forming anisotropic structures show nanostructure-dependent functions and properties only when they form aligned monodomain states. However, it is generally difficult to align LC materials into monodomain states. To avoid the difficulty, the use of bicontinuous cubic (Cubbi) LC structures is expected to be a potential approach. Cubbi phases are a kind of nanosegregated LC phases consisting of threedimensionally interconnected nanochannel networks and a surrounding sheath domain. These materials have potential to function as transporting materials without the orientation of LC domains because of their threedimensional ordered structures. We have reported on the development of a new class of electrolyte for lithium ion batteries by using zwitterions. Zwitterions are organic salts, in which both cation and anion are tethered with covalent bond. Previously, we have reported that the equimolar mixtures of imidazolium-based zwitterions and lithium bis(trifluoromethanesulfonyl)imide (LiTf2N) form homogeneous liquids. They acted as novel electrolytes transporting lithium cations selectively. Our intention here is to introduce Cubbi LC properties into zwitterions. In the present study, we designed and synthesized an amphiphilic zwitterion (ZI) consisting of an ionophilic zwitterionic part and an ionophobic long alkyl chain part (Figure 1, top). This zwitterion was mixed with LiTf2N in various ratios, and the thermotropic LC properties of mixtures were examined by polarizing optical microscopic observation, differential scanning calorimetry and wide-angle X-ray diffraction measurements. ZI shows a Sm phase approximately ranging from 120 to 200 °C (Figure 1, bottom left). It is of interest that the mixtures of ZI and LiTf2N (mixtures ZI/LiTf2N) form a variety of nanostructures that are significantly different from those of ZI alone. For example, mixture ZI/LiTf2N in a 2:3 molar ratio exhibits a Cubbi phase (Figure 1, bottom middle). The characterization of the Cubbi phase was performed by polarizing optical microscopic observation and wide-angle X-ray diffraction measurements. Under cross-Nicol condition, the mixture shows no birefringence at room temperature (Figure 2a). The wide-angle X-ray diffraction pattern of the mixture at 60 °C shows two intense peaks at 38.3 and 33.6 A in a small-angle region. The reciprocal d-spacing ratio of the two peaks is √6:√8, which can be assigned to the (211) and (220) reflections of Cubbi structures with the Ia3d symmetry. It is also of interest that further addition of LiTf2N into ZI induces the exhibition of Col phases excluding the formation of Cubbi LC structures. For example, mixture ZI/LiTf2N in a 5:9 molar ratio exhibits a Col phase (Figure 1, bottom right) ranging from room temperature to 90 °C on heating. A typical texture of the mixture in the Col phase is shown in Figure 2b. The wideangle X-ray diffraction pattern of the mixture at 80 °C shows two intense peaks at 34.4 and 16.4 A in a smallangle region. The reciprocal d-spacing ratio of the two peaks is 1:2, which can be assigned to the (100) and (200) reflections of a Col structure. In summary of the LC behavior of mixtures ZI/LiTf2N, it has been revealed that they form Sm, Cubbi, and Col phases in this order with increasing the molar fraction of LiTf2N. These results suggest that these mixtures form normal-type of Cubbi and Col phases in which ionophobic alkyl chains form channel domains and ionophilic zwitterionic parts form surrounding sheath domains. Schematic illustration of the self-assembled nanostructures is shown in Figure 1, bottom. Ionic conductivity measurements on these materials are underway.