Introduction

Nanostructured liquid crystals have attracted attention for applications in the transport of ions [1,2,3,4,5,6,7,8], electrons [9,10,11,12,13], and molecules [14,15,16,17,18] owing to the anisotropic properties of these materials. The transport of ions in 1D [19, 20], 2D [21,22,23,24], and 3D [25,26,27,28,29] nanochannels of columnar, smectic, and bicontinuous cubic structures has been achieved for these nanostructured liquid-crystalline (LC) materials. Among these properties, 2D transport in layered nanostructured liquid crystals has been applied to Li-ion batteries (LIBs) [30, 31]. A cyclic carbonate-based LC electrolyte with layered ionic pathways showed a relatively high ionic conductivity and electrochemical stability suitable for operation in LIBs, resulting in reversible charge-discharge processes for both the positive and negative electrodes [30]. Other LC electrolytes containing poly- or oligo (ethylene oxide) [21, 23], imidazolium [20, 23], ammonium [25], phosphonium [25], and zwitterionic [24, 26, 27] moieties have also been developed for potential application in LIBs.

In general, the mechanism of Li ion conduction in poly(ethylene oxide) (PEO) occurs as follows [32,33,34]: ether oxygens coordinate to Li cations, leading to the dissociation of Li salts. The dissociated Li cations can be transported through segmental motion of the polymer chains. The coordination structures of the ether oxygens with Li cations is relatively stable. Because of these polymer-coordinated structures of Li cations, the diffusion of the cations is usually slower than that of the anions. In contrast, the coordination of the ether oxygens contained in a perfluorinated oligoether to metal ions is weaker than the coordination of those in a non-fluorinated oligoether; perfluorinated ethers do not coordinate metal ions at all [35]. This difference is due to the weaker Lewis basicity of the ether oxygens resulting from the inductive electron-withdrawing effect of the fluoro substituents. Instead, these compounds preferentially coordinate anions over cations [36].

Recently, the application of perfluoropolyether (PFPE)-based electrolytes in LIBs was proposed by DeSimone and Balsara et al [37,38,39,40,41,42,43]. The researchers developed low-molecular-weight PFPEs complexed with lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) (Fig. 1). The electrochemical performance of these electrolytes in Li batteries was evaluated through cycling tests of a cell composed of Li metal and LiNi1/3Co1/3Mn1/3O2 as the anode and cathode, respectively, and the electrolytes exhibited potential for application in LIBs. LIBs with these electrolytes are expected to show enhanced safety and durability because of their nonflammability, thermal stability, and electrochemical stability. These electrolytes also showed an excellent lithium transference number close to unity (>0.9). The researchers explained that this ion-conducting behavior results from (1) solvation of the perfluorinated anions in the PFPEs and (2) weakened coordination of the carbonyl groups to Li cations caused by the fluoro substituents in the polymers. The researchers also developed hybrid electrolytes with inorganic sulfide glass particles that exhibited single ion-conducting behavior and limited the dissolution of lithium polysulfides. Thus, these electrolytes are potentially applicable to lithium-sulfur batteries [38].

Fig. 1
figure 1

Molecular structures of PFPE-based electrolytes reported by Wong et al [37,38,39,40].

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The self-assembly of nanosegregated structures is often driven by molecules having perfluorinated moieties through the immiscible nature of the perfluorinated and non-fluorinated parts [44,45,46,47]. For example, block-structured semifluorinated alkanes exhibit LC properties that originate from the immiscibility of the block segments and the rigidity of the fluorocarbons [48,49,50,51,52,53]. We previously developed LC Li ion conductors bearing perfluoroalkyl chains at the termini of the mesogenic parts [54]. The introduction of a perfluorinated moiety resulted in an improvement in the thermal stability of the LC phases, which led to enhanced ionic conductivities in a wider temperature range compared to those of the analogous non-fluorinated molecules. We expected that the nanosegregation of the immiscible perfluorinated sections and hydrocarbon groups can drive the formation of LC structures. Our intention here is to examine the effects of a perfluorinated oligo (ethylene oxide) moiety on the LC properties and ionic conductivity.

In the present study, fluorinated block-structured molecules containing a perfluorinated oligo (ethylene oxide) moiety and an alkyl group (1a and 2a) have been developed (Fig. 2). For comparison, non-fluorinated molecules (1b and 2b) have also been prepared, and their properties have been examined.

Fig. 2
figure 2

Molecular structures of fluorinated block-structured molecules and analogous non-fluorinated molecules with a terminal methyl carbonate (1a and 1b) or a terminal hydroxyl (2a and 2b) group

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Experimental procedures

General procedures

NMR spectra were recorded on a JEOL JNM-ECX400 instrument at 400 MHz for 1H NMR spectroscopy, 100 MHz for 13C NMR spectroscopy, and 376 MHz for 19F NMR spectroscopy. The chemical shifts of the 1H, 13C, and 19F NMR signals were referenced to the Me4Si internal standard (δ = 0.00), residual CDCl3 (δ = 77.16), and C6F6 (δ = −164.90), respectively, and described in terms of the chemical shift in ppm (δ), multiplicity, coupling constant (Hz), and relative intensity. Matrix-assisted laser desorption ionization time-of-flight mass spectra (MALDI-TOF MS) were obtained using a Bruker Daltonics Autoflex Speed instrument using trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene] malononitrile as the matrix. The thermal properties of the materials were examined by differential scanning calorimetry (DSC) using a Netzsch DSC204 Phoenix system. The heating and cooling rates were both 10 K min−1. The transition temperatures were taken at the onsets of the transition peaks on first cooling. An Olympus BX53 polarizing optical microscope (POM) equipped with a Linkam LTS350 hot stage was used to study the LC properties. X-ray diffraction (XRD) patterns were taken on a Rigaku RINT-2500 diffractometer with CuKα radiation. Molecular mechanics calculations were carried out by the Gaussian 09 W software package.

Materials

2,2,4,4,5,5,7,7,8,8,10,10-Dodecafluoro-3,6,9-trioxaundecane-1,11-diol was supplied by Asahi Glass Co. Ltd., and all other reagents and solvents used for the synthesis were purchased from commercially available sources and used as received without further purification.

Synthesis of 2,2,4,4,5,5,7,7,8,8,10,10-dodecafluoro-3,6,9,12-tetraoxatriacontan-1-ol (2a)

1-Bromooctadecane (726 mg, 2.18 mmol), 2,2,4,4,5,5,7,7,8,8,10,10-Dodecafluoro-3,6,9-trioxaundecane-1,11-diol (2.052 g, 5.00 mmol), and K2CO3 (440 mg, 3.18 mmol) in DMF (15 mL) were stirred at 80 °C for 2 h. A saturated aqueous NH4Cl solution was added to the solution. The reaction mixture was extracted with hexane and dried over MgSO4. After the solvent was evaporated, purification of the residue by silica gel chromatography (eluent: 7 vol% ethyl acetate in hexane) afforded the title compound as a white solid (786 mg, 1.19 mmol, 54%).

1H NMR (400 MHz, CDCl3, δ): 3.94 (m, 2 H; CH2), 3.81 (t, J = 9.6 Hz, 2 H; CH2), 3.60 (t, J = 6.8 Hz, 2 H; CH2), 2.19 (t, J = 7.8 Hz, 1 H; OH), 1.62–1.55 (m, 2 H; CH2), 1.37–1.25 (m, 30 H; CH2), 0.88 (t, J = 6.8 Hz, 3 H; CH3); 13C NMR (100 MHz, CDCl3) δ: 73.40, 32.09, 29.85, 29.73, 29.68, 29.61, 29.52, 29.49, 25.87, 22.85, and 14.28 (only carbons not coupled to fluorine); 19F NMR (376 MHz, CDCl3) δ: −77.49 (2 F; CF2), −80.37 (2 F; CF2), −88.60 (4 F; CF2), −88.80 (4 F; CF2).

Synthesis of 2,2,4,4,5,5,7,7,8,8,10,10-dodecafluoro-3,6,9,12-tetraoxatriacontan-1-yl methyl carbonate (1a)

Compound 2a (762 mg, 1.15 mmol) and pyridine (200 µL) in CH2Cl2 (15 mL) were stirred at 0 °C. Methyl chloroformate (163 mg, 1.73 mmol) was added dropwise to the solution. The mixture was warmed to room temperature and stirred overnight. Evaporation of the solvent and subsequent purification of the residue by silica gel chromatography (eluent: hexane/CH2Cl2 = 2/1) afforded the title compound as a white solid (722 mg, 1.00 mmol, 87%).

1H NMR (400 MHz, CDCl3, δ): 4.52 (t, J = 9.2 Hz, 2 H; CH2), 3.86 (s, 3 H; CH3), 3.81 (t, J = 10.0 Hz, 2 H; CH2), 3.59 (t, J = 6.6 Hz, 2 H; CH2), 1.62–1.55 (m, 2 H; CH2), 1.37–1.25 (m, 30 H; CH2), 0.88 (t, J = 6.8 Hz, 3 H; CH3); 13C NMR (100 MHz, CDCl3) δ: 154.78, 112.02 (td, J = 281, 170 Hz), 114.54 (tm, J = 286 Hz), 73.26, 69.82 (t, J = 31 Hz), 64.74 (t, J = 34 Hz), 55.57, 32.19, 29.98, 29.85, 29.80, 29.76, 29.64, 29.60, 26.01, 22.90, and 14.07; 19F NMR (376 MHz, CDCl3) δ: −77.61 (2 F; CF2), −77.71 (2 F; CF2), −88.52 (4 F; CF2), −88.74 (4 F; CF2); MS (MALDI-TOF) m/z: [M + Na]+ calcd for C28H44F12NaO7, 743.28; found, 743.20.

Synthesis of 3,6,9,12-tetraoxatriacontan-1-ol (2b)

A mixture of 1-bromooctadecane (3.90 g, 11.7 mmol), tetraethylene glycol (10.43 g, 53.7 mmol), and NaOH (2.09 g, 52.3 mmol) in water (5 mL) was stirred under reflux for 20 h. The reaction mixture was extracted with hexane, washed with a saturated NH4Cl solution and water, and dried over MgSO4. After the solvent was evaporated, purification of the residue by silica gel chromatography (eluent: ethyl acetate) yielded the title compound as a white solid (1.58 g, 3.54 mmol, 30%).

1H NMR (400 MHz, CDCl3, δ): 3.75–3.71 (m, 2 H; CH2), 3.70–3.57 (m, 14 H; CH2), 3.45 (t, J = 7.0 Hz, 2 H; CH2), 2.73 (t, J = 6.2 Hz, 1 H; OH), 1.61–1.54 (m, 2 H; CH2), 1.35–1.23 (m, 30 H; CH2), 0.88 (t, J = 7.0 Hz, 3 H; CH3); 13C NMR (100 MHz, CDCl3) δ: 72.67, 71.70, 70.70, 70.44, 70.11, 61,82, 32.03, 29.80, 29.72, 29.67, 29.47, 26.17, 22.79, and 14.22; MS (MALDI-TOF) m/z: [M + Na]+ calcd for C26H54NaO5, 469.39; found, 469.38.

Synthesis of 3,6,9,12-tetraoxatriacontan-1-yl methyl carbonate (1b)

Compound 2b (1.54 g, 3.06 mmol) and pyridine (2.5 mL) in CH2Cl2 (30 mL) were stirred at 0 °C. Methyl chloroformate (2.89 g, 30.6 mmol) was added dropwise to the solution. The mixture was warmed to room temperature and stirred overnight. Evaporation of the solvent and subsequent purification of the residue by silica gel chromatography (eluent: ethyl acetate) gave the title compound as a white solid (1.58 g, 1.00 mmol, 91%).

1H NMR (400 MHz, CDCl3, δ): 4.30–4.28 (m, 2 H; CH2), 3.79 (s, 3 H; CH3), 3.74–3.71 (m, 2 H; CH2), 3.68–3.62 (m, 10 H; CH2), 3.59–3.56 (m, 2 H; CH2), 3.44 (t, J = 7.2 Hz, 2 H; CH2), 1.61–1.54 (m, 2 H; CH2), 1.35–1.23 (m, 30 H; CH2), 0.88 (t, J = 6.8 Hz, 3 H; CH3); 13C NMR (100 MHz, CDCl3) δ: 155.84, 71.65, 70.73, 70.68, 70.14, 69.01, 54.87, 32.01, 29.79, 29.72, 29.59, 29.45, 26.18, 22.78, and 14.21; MS (MALDI-TOF) m/z: [M + Na]+ calcd for C28H56NaO7, 527.39; found, 527.68.

Preparation of the complexes of 1a and 1b with LiTFSI

The complexes of 1a and 1b with LiTFSI were prepared through slow evaporation of tetrahydrofuran (THF) solutions containing the requisite amounts of the compounds and the Li salt followed by drying under reduced pressure at 80 °C.

Measurement of ionic conductivities

The ionic conductivities were measured as a function of the temperature by an alternating current impedance method using a Hioki 3532–80 chemical impedance meter (frequency range: 100 Hz–1 MHz, applied voltage: 0.3 V) equipped with a Linkam LTS 350 hot stage, a Linkam TMS 94 temperature controller, and a Linkam LNP 9412 hot stage cooler (the cooling rate was fixed at 2 K min−1). The ionic conductivities were calculated as the product of the formula 1/Rb (Ω−1) (Rb: bulk resistance of the sample) and the cell constant (cm−1) for comb-shaped gold electrodes and were calibrated with the conductivity of a KCl standard solution (HI7033L, 84 µS cm−1) from Hanna Instruments. The impedance data were modeled as a parallel resistor–capacitor circuit in series.

Results and discussion

Material design and synthesis

Oligo (ethylene oxide) monoalkyl ethers have been previously studied for application as nonionic surfactants and detergents [55,56,57,58,59,60,61,62,63]. The phase behavior and ionic conductivity of lyotropic LC systems of hydrated Li salts and surfactants were also examined [64, 65]. For polymeric systems, poly (methacrylate) comb polymers having alkyl end-capped oligo (ethylene oxide) side chains were developed as nanostructured polymer electrolytes [66]. Perfluorinated ethylene oxide moieties have attracted attention for their fluorophilic self-assembly processes, self-lubricating properties, and good solubility in several polar organic solvents [67]. For example, block-structured ligands consisting of perfluorinated and non-fluorinated oligo (ethylene oxide) segments were used as structure-directing agents for the preparation of size-controlled gold nanoparticles [67,68,69]. Block copolymers consisting of PFPE and PEO segments were developed, and their electrochemical properties were studied [40, 70]. It should be noted that all the PFPE-based electrolytes previously reported [37, 39,40,41,42] form isotropic liquid states except for those electrolytes hybridized with inorganic solids [38] and those cured by thermally or UV activated crosslinking reactions [43]. To the best of our knowledge, the LC behavior and ion-conducting properties of perfluorinated oligo (ethylene oxide) monoalkyl ethers have not yet been studied.

In the present study, we designed and synthesized block-structured molecules with a terminal methyl carbonate 1a with the expectation of achieving a relatively high ionic conductivity and electrochemical stability for potential application in LIBs (Fig. 2). A dodecyl group was introduced at one end of the perfluorinated oligo (ethylene oxide) moiety. We anticipated that a sufficiently long alkyl chain may introduce LC properties in the block-structured molecules through the packing forces of the chain and the nanosegregation that occurs between fluorous and alkyl moieties. Compared to perfluoroalkyl moieties, perfluorinated oligo (ethylene oxide) compounds are conformationally more flexible owing to the existence of ether oxygens, which may favor ionic conduction in LC structures. The effects of fluoro substituents on the LC properties were also examined for molecule 2a, which has a hydroxyl group.

Compounds 1a and 2a were synthesized according to Scheme 1. Williamson ether synthesis by the reaction of fluorinated tetraethylene glycol with octadecyl bromide in a suspension of potassium carbonate in DMF yielded compound 2a. Compound 2a was converted into methyl carbonate-terminated compound 1a by an esterification reaction with methyl chloroformate in the presence of pyridine as an acid scavenger. Compounds 1b and 2b were synthesized by similar procedures.

Scheme 1
scheme 1

Synthetic scheme for the fluorinated block-structured molecules 1a and 2a

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LC properties of the block-structured molecules

The phase transition properties of block-structured molecules 1a, 1b, 2a, and 2b were examined by DSC, POM, and XRD. Table 1 compares the phase transition properties of 1a, 1b, 2a, and 2b on cooling. We denote the unidentified ordered smectic phases as Sm1, Sm2, and Sm3. Figure 3 shows the POM image of 1a in the Sm3 phase at 20 °C. A needle-like morphology was observed for 1a when the sample was placed between glass substrates and cooled from the isotropic phase (Fig. 3a). After applying a mechanical shear force to the sample, its optical texture changed owing to the fluidity of the phase, indicating that the sample formed a soft anisotropic phase (Fig. 3b). The LC structures of 1a and 1b in the Sm3 phase at 20 °C were examined by wide-angle XRD (Fig. 4). The XRD pattern of 1a in the Sm3 phase showed five sharp peaks with d values of 48.5, 24.8, 16.4, 12.2, and 9.7 Å (Fig. 4a). The reciprocal d-spacing ratio of these values was 1:2:3:4:5, corresponding to the reflections of d001, d002, d003, d004, and d005, respectively. The layer spacing was calculated to be 49 Å from the XRD measurement. The layer spacing was 7 Å larger than the molecular length of 1a in the fully extended conformation, which was estimated to be 42 Å by molecular mechanics calculations. These results indicate that the Sm3 phase has a highly interdigitated and tilted bilayer structure (Fig. 5). We also estimated the volume ratio of the perfluorinated moiety combined with a terminal methyl carbonate moiety. The combined section occupied nearly half the volume (≃47%) of the entire 1a molecule, as evaluated from space-filling models. This property could affect the formation of a layered nanosegregated structure by 1a. In the case of 1b, the layer spacing of the Sm3 phase was calculated to be 43 Å (Fig. 4b), which was 1 Å larger than the estimated molecular length of 1b (42 Å). The layer spacing of 1a in the Sm3 phase was 6 Å larger than that of 1b, although the molecular length of 1a was nearly the same as that of 1b. The DSC thermograms of compounds 1a and 1b are shown in Fig. 6. All of the block-structured molecules showed smectic phases near room temperature. For example, compounds 1a and 1b exhibited transitions from the isotropic phase to the smectic phase at 23 and 34 °C, respectively, while compounds 2a and 2b showed the same transitions at 32 and 40 °C during the cooling process. The enthalpy changes of the transition between the isotropic and the smectic phases on cooling were much larger (41–49 kJ mol−1) than those of the smectic–smectic phase transitions (0.5–3.8 kJ mol−1). These results suggest that the smectic phases have a rather high order for all of these block-structured molecules. From comparison of the phase transition temperatures of 1a and 1b with those of 2a and 2b (block-structured molecules with different end functional groups), 1a and 1b showed lower isotropization temperatures than 2a and 2b, respectively. The terminal hydroxyl groups of 2a and 2b induced stronger thermal stabilization effects in the smectic phases than the terminal carbonate groups of 1a and 1b. The effects of the fluoro substituents were examined through comparison of the transition temperatures. It is noteworthy that the isotropization temperatures of fluorinated compounds 1a and 2a were lower than those of the analogous non-fluorinated compounds 1b and 2b, respectively. This observation differs from the general trend, [44,45,46,47, 54] in which perfluoro substituents on the terminal alkyl chains of LC molecules promote stabilized nanosegregation, thus leading to the formation of thermally more stable LC structures. The fluoro substituents in the oligo (ethylene oxide) moieties may destabilize the packing structures of the highly ordered smectic phases, which is most likely due to the steric hindrance of the fluorine atoms. The effects of the fluoro substituents led to lower isotropization temperatures for the fluorinated block-structured molecules (1a and 2a) than for the non-fluorinated molecules (1b and 2b). This explanation is supported by the physical properties of PFPEs recently determined from molecular simulations combined with experiments in a report by Black et al. [71]; this study suggests that PFPEs have a reduced cohesive energy than PEOs due to their lower overall polarity and weaker intermolecular interactions.

Table 1 Thermal properties of block-structured molecules 1a, 1b, 2a, and 2b
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Fig. 3
figure 3

Polarizing optical micrograph of 1a at 20 °C in the Sm3 phase a before and b after applying a mechanical shear force

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Fig. 4
figure 4

Wide-angle XRD patterns of a 1a and b 1b in the Sm3 phase at 20 °C

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Fig. 5
figure 5

Proposed assembled structure of 1a in the Sm3 phase

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Fig. 6
figure 6

DSC thermograms of block-structured molecules 1a (a) and 1b (b) obtained at a scanning rate of 10 K min−1

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LC behavior and ionic conductivities of complexes formed from the block-structured molecules and LiTFSI

The ion-conducting properties of the complexes formed from 1a and 1b with LiTFSI as LC electrolytes were examined. The complexes were prepared by slow evaporation of THF solutions containing the requisite amount of the compounds and the Li salt. Compound 1a formed homogeneous mixtures with up to 60 mol% LiTFSI. The complexes formed from 1a and 1b with LiTFSI also exhibited smectic phases. The complex of 1a with 60 mol% LiTFSI showed a transition from the isotropic phase to the smectic phase at 34 °C, which was 11 °C higher than the transition of 1a alone. In contrast, the complex of 1b with 60 mol% LiTFSI exhibited an isotropization transition at 33 °C, which was 1 °C lower than that of 1b alone. The different trends in the transition temperatures of 1a and 1b when complexed with LiTFSI can possibly be attributed to the different coordination states of 1a and 1b with Li cations. In the case of 1a, the terminal carbonate moieties coordinate to Li cations, resulting in strong intermolecular interactions through ion–dipole interactions of Li cations and the carbonate moieties. The Li cations and TFSI anions can be located at the extremities of 1a. These effects lead to a more stabilized nanostructure and a higher isotropization temperature for the complex of 1a with LiTFSI than for 1a alone, as is often observed in other carbonate-based LC electrolyte systems [19, 22, 30]. On the other hand, in the complex of 1b with LiTFSI, in addition to the carbonate moieties, the oligoether moieties can also coordinate to Li cations. Although the addition of LiTFSI could promote nanosegregation of the molecules, the incorporation of Li cations and TFSI anions within the smectic layers of the oligo (ethylene oxide) moieties may destabilize the relatively highly ordered packing structure of 1b due to the steric bulk of the ion pairs.

The ionic conductivities of these complexes were measured as a function of temperature by an alternating current impedance method. The measurement was conducted using comb-shaped gold electrodes deposited on a glass substrate during the cooling process. Figure 7 presents the ionic conductivities of 1a and 1b complexed with 60 mol% LiTFSI. For comparison, the ionic conductivities of a low-molecular-weight liquid PFPE-based electrolyte (PFPE-DMC in Fig. 1) [37] and a poly(ethylene glycol) electrolyte [39] are also given in Fig. 7. The complex of 1a exhibited ionic conductivities on the order of 10−6 and 10−5 S cm−1 in the ordered smectic phase at approximately 30 °C and in the isotropic temperature range of 40–100 °C, respectively. For example, the complex of 1a showed conductivities of 3.6 × 10−6 and 1.7 × 10−5 S cm−1 in the smectic phase at 30 °C and in the isotropic phase at 60 °C, respectively. These values were similar to those of the PFPE-based electrolyte. [37] The ionic conductivities of the complex of 1b were close to those of the poly (ethylene glycol) electrolyte [39]. The values were approximately one order of magnitude higher than those of the complex of 1a. The lower conductivities of 1a compared to those of 1b can be explained as follows: perfluorinated oligo (ethylene oxide) moieties do not coordinate to Li cations. The dissociation and transport of Li ions are affected by the terminal methyl carbonate moiety present in 1a. On the other hand, the oligo (ethylene oxide) moiety of 1b can contribute to the dissociation and transport of Li cations in addition to the contribution of the carbonate moiety. These effects led to higher ionic conductivities for 1b than for 1a. A sharp decrease in the ionic conductivity on cooling was observed at the phase transitions from the isotropic phase to the smectic phase that occurred at 34 and 33 °C for the complexes of 1a and 1b, respectively. This behavior is due to the formation of unaligned polydomains and the higher viscosity of the ordered smectic phase, in which the mobility of the molecules and ions is lower than that in the isotropic phase.

Fig. 7
figure 7

Ionic conductivities of block-structured molecules 1a (closed circle: ●) and 1b (open circle: ○) complexed with 60 mol% LiTFSI during cooling. For comparison, the ionic conductivities of the methyl carbonate-terminated PFPE electrolyte [37] (PFPE-DMC in Fig. 1) (gray diamond: ♦) and the poly(ethylene glycol) electrolyte [39] (open diamond: ♢) are also given

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We compared the ionic conductivities of the complexes of 1a and 1b with those of other LC electrolytes (Figs. 8 and 9). Figure 8 shows the molecular structures of the components of a cyclic carbonate-based LC electrolyte complexed with LiTFSI [30] (Fig. 8a) and a PEO-based LC electrolyte bearing perfluoroalkyl groups complexed with lithium triflate [54] (Fig. 8b). These LC electrolytes aligned homeotropically when the samples were sandwiched between glass substrates. The aligned LC electrolytes showed effective ion conduction in the direction parallel to the smectic layers (squares and inverted triangles plotted in Fig. 9). On the other hand, both the complexes of 1a and 1b formed unaligned polydomains in the smectic phases, resulting in a decrease in the conductivity at the transition from the isotropic phase to the smectic phase. However, it should be noted that the ionic conductivities of the complex of 1a in the temperature range of the smectic phase (~30 °C) are comparable to those of the carbonate-based LC electrolyte and the PEO-based LC electrolyte in the aligned monodomain state.

Fig. 8
figure 8

Molecular structures of the cyclic carbonate-based LC electrolyte [30] (a) and the LC compound having perfluoroalkyl-terminated mesogens at both ends of a PEO chain complexed with lithium triflate [54] (b)

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Fig. 9
figure 9

Ionic conductivities of the LC electrolytes of 1a (closed circle: ●) and 1b (open circle: ○) complexed with 60 mol% LiTFSI, the cyclic carbonate-based LC compound complexed with 10 mol% LiTFSI [30] (Fig. 8a) (square: □), and the LC compound having a perfluoroalkyl-terminated mesogen at both ends of a PEO chain complexed with lithium triflate (Li+/CH2CH2O = 0.05) [54] (Fig. 8b) (conductivities parallel to the smectic layers of the aligned monodomain sample (inverted triangle: ▽) and the unaligned polydomain sample (triangle: △))

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Conclusions

Perfluorinated oligo (ethylene oxide) monoalkyl ethers containing a terminal methyl carbonate or a hydroxyl moiety were developed for potential application as quasi-solid-state electrolytes for LIBs. Both the fluorinated and non-fluorinated molecules formed highly ordered smectic phases near room temperature. The introduction of fluoro substituents in the oligo (ethylene oxide) moieties lowered the isotropization temperatures of the block-structured molecules. The complexes formed between the fluorinated block-structured molecule and the Li salt showed ionic conductivities on the order of 10−6 S cm−1 and 10−5 S cm−1 in the smectic and the isotropic phases, respectively. The ionic conductivities of the complexes formed from the fluorinated block-structured molecule were lower than those formed from the non-fluorinated molecule.

Nanostructured molecular assemblies containing perfluorinated moieties have potential for application as transport materials.