A multi-iodine doped strategy for ionic conductivity enhancement of crown ether functionalized ionic liquids

Abstract

A novel crown ether functionalized ionic liquid has been designed, synthesized and characterized in detail. The thermal properties of the ionic liquid are also investigated. When LiI or MgI₂ is introduced into the ionic liquid at the same concentration of the metal cations, the particular trapping ability of the crown ether to metal cations could effectively reduce the electrostatic interaction between the metal cations and iodide anions to release more iodide anions. Electrochemical impedance analysis reveals that ∼35 fold of ionic conductivity enhancement for the multi-iodine doped ionic liquids can be obviously observed at room temperature. For applications, these multi-iodine doped crown ether functionalized ionic liquids can be used as gel electrolytes for dye-sensitized solar cells, displaying a power conversion efficiency of 3.86% at a simulated AM1.5 solar spectrum illumination at 100 mW cm⁻¹. These preliminary results provide us with a feasible strategy to enhance the ionic conductivity and explore new ionic liquid electrolytes for energy storage and conversion devices.

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1. Introduction

Ionic liquids (ILs) are composed of organic cations and organic or inorganic anions.¹ Having non-volatility with a melting temperature below 100 °C, also known as liquid organic, molten or fused salts, they have attracted intensive scientific interest and even become a “star” material in material research. Due to their excellent ability to dissolve an enormous range of organic and inorganic materials,² high chemical and thermal stability,³ ILs have emerged as an attractive “green” alternative to traditional organic solvents in various fields of chemistry: synthesis,⁴ catalysis,^3,5 lubrication,⁶ and separation processes,⁷ etc. Most importantly, in virtue of some unique and favorable properties including low flammability, wide electrochemical window and high ionic conductivity,⁸ ILs are playing an irreplaceable role in utilizing as highly efficient and stable electrolytes for electrochemical and renewable energy applications,⁹ such as lithium batteries,¹⁰ fuel cells¹¹ and dye-sensitized solar cells.¹²

When using ILs as functional electrolytes for high performance devices, several strategies have been proposed to successfully improve their conductivity.^13,14 It is well-known that adding iodine (I₂) to ILs with I⁻ anions could form polyiodide chains such as I₃⁻, I₅⁻, I₇⁻,…, I_2n+1⁻, which promote fast exchange reaction via a Grotthuss exchange mechanism,^15–17 resulting in obvious conductivity enhancement. In addition, introducing certain SiO₂/TiO₂ nanoparticles can greatly enhance the ionic conductivity of employed electrolytes by breaking up ion pairs and/or forming more number of defects at the interfaces.^18–21 In virtue of outstanding electronic transport ability, carbon nanotubes and graphene are also employed into ILs electrolytes to improve their conductivity.^22–25 Recently, by reducing the lamellar structures of a novel ionic conductor with propargyl groups,²⁶ Wang and coworkers find that it is much more beneficial to fast charge transport along the polyiodide chain. Owing to the high conductivity, the solid-state ionic conductor can be used as single-component solid electrolytes for high performance solid-state DSSCs. Thus, these previous research triggers our interest to explore new types of high conductive ILs.

Here, a kind of crown ether functionalized ionic liquid is designed and prepared by combining crown ether with imidazolium structure. The molecular structure and thermal property have been confirmed and characterized by Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) measurements, thermogravimetric analyzer (TGA) and differential scanning calorimetry (DSC) analysis, respectively. Moreover, the influence of metal iodides on the ionic conductivity of resulting multi-iodine doped crown ether functionalized ionic liquids is also investigated in details by electrochemical impedance spectra (EIS). Notably, the particular trapping ability of crown ether to metal cations could effectively reduce the electrostatic interaction between metal cations and iodide anions to release more iodide anions, which hence facilitates enhanced ionic conductivity of the multi-iodine doped ionic liquids. For applications, the resulting ionic liquids are further utilized as gel electrolytes to fabricate dye-sensitized solar cells (DSSCs).

2. Experimental

Materials

2-(Hydroxymethyl)-12-crown-4 ([12-C-4][OH]), 5-bromovaleryl chloride were purchased from TCI and used as received. Lithium iodide (LiI), magnesium iodide (MgI₂) and imidazole was purchased from Alfa Aesar and used as received. Triethylamine (TA), dichloromethane (DCM), potassium hydroxide (KOH), acetonitrile (CH₃CN), iodomethane (CH₃I), iodine (I₂) and methanol (CH₃OH) were of analytical-reagent grade and purchased from China National Pharmaceutical Group Corporation. TiCl₄, H₂PtCl₆ and organic dye Z907 were purchased from Aldrich. All the chemical reagents were used without further treatment. Fluorine-doped tin oxide (FTO) glass electrodes (8 Ω sq⁻¹), and slurries containing 20 nm-sized mesoporous and 200 nm-diameter light-scattering TiO₂ colloidal were purchased from Dalian Hepat Chroma Solar Tech. Co., Ltd (China).

Synthesis of [12-C-4][Br]

[12-C-4][OH] (5 g) was mixed with 1.1 equiv. of TA (2.7 g) in 50 ml DCM. The solution was magnetically stirred under ice-water bath. 1.1 equiv. of 5-bromovaleryl chloride was added dropwise. The color of the solution turned epinephelos gradually. The mixture was stirred for 1 h at 4 °C and overnight at room temperature. After reaction, the precipitate was filtered off and the filtrate was concentrated under vacuum to gain raw product. The pure product was received by chromatographic separation on neutral alumina using acetone/DCM/CH₃OH (200 : 450 : 1, v/v/v) as eluent. The pure product was viscous transparent oils, showing as single spots on TLC. The chemical structure was further confirmed by ¹HNMR (d-CDCl₃, 400 MHz): 3.75–3.81 (t, 1H), 3.40–3.50 (m, 1H), 3.94–4.00 (m, 1H), 3.50–3.72 (m, 15H), 3.51–3.60 (m, 2H), 2.32–2.40 (t, 2H), 1.82–1.94 (m, 2H), 1.58–1.70 (m, 2H).

Synthesis of [12-C-4][Im]

Imidazole (0.95 g) was mixed with CH₃CN (25 ml) and stirred at room temperature for a few minutes. 1.2 equiv. of KOH was added into the mixture after trituration. Equiv. of synthesized [12-C-4][Br] was added in the solution dropwise and reacted for 4 h. After reaction, the precipitate was filtered off and the filtrate was concentrated under vacuum to gain raw product. The pure product was received by chromatographic separation on neutral alumina using acetone/DCM/CH₃OH (200 : 450 : 1, v/v/v) as eluent. The chemical structure was further confirmed by ¹HNMR (d₆-DMSO, 400 MHz): 7.60 (s, 1H), 7.14 (s, 1H), 6.87 (s, 1H), 4.02–4.06 (t, 1H), 3.40–3.50 (m, 1H), 3.92–3.98 (m, 3H), 3.74–3.79 (m, 1H), 3.62–3.68 (m, 4H), 3.60 (s, 1H), 3.47–3.58 (m, 15H), 3.40–3.46 (m, 2H), 2.32–2.36 (t, 2H), 1.68–1.74 (m, 2H), 1.41–1.48 (m, 2H).

Synthesis of [12-C-4][Im][I]

[12-C-4][Im] (0.86 g) was mixed with CH₃OH (25 ml) and stirred under ice-water bath. 1.5 equiv. of CH₃I was added into the mixture dropwise. The mixture was stirred for 1 h at 4 °C and then overnight at 50 °C. After evaporation of the solvent, the crude product was washed with ethyl acetate and diethyl ether three times, respectively, and dried under vacuum for 24 h to provide 8.17 g [12-C-4][Im][I] (yield: 83.5%, viscous milky white gel). The chemical structure was further confirmed by ¹HNMR (d₆-DMSO, 400 MHz): 9.16 (s, 1H), 7.78 (t, 1H), 7.72 (t, 1H), 4.16–4.22 (t, 2H), 4.04–4.08 (t, 1H), 3.93–3.97 (t, 1H), 3.85 (s, 3H), 3.75–3.80 (m, 1H), 3.47–3.68 (m, 15H), 3.40–3.46 (m, 2H), 3.40–3.46 (m, 2H), 2.35–2.40 (t, 2H), 1.76–1.84 (m, 2H), 1.46–1.52 (m, 2H).

Preparation of multi-iodine doped crown ether functionalized ionic liquids electrolytes

[12-C-4][Im][I], LiI doped [12-C-4][Im][I] and MgI₂ doped [12-C-4][Im][I] are selected to prepare three multi-iodine doped ionic liquids Electrolytes I, II and III for DSSCs (Devices A-C). For further optimization, I₂ is added to form the I₃⁻ ions to enhance the ionic conductivity for the electron relay at the counter electrode. Thus, addition of 10 wt% iodine to Electrolyte III can form Electrolyte IV for the fabrication of Device D. The easy synthesis process is depicted as follows: [12-C-4][Im][I] (0.5 g) was mixed with equiv. of LiI (Electrolyte I) or MgI₂ (Electrolyte II) and stirred at 40 °C for 5 h. Then, the mixtures were moved to dry in vacuum oven overnight before the characterization and fabrication of the DSSCs.

Device fabrication

The fabrication of DSSCs is present as follows. The FTO glass was cleaned by acetone and ethanol successively. Cleaned FTO glass was immersed into 40 mM aqueous TiCl₄ solution at 70 °C for 30 min, and washed with water and ethanol. Firstly, a 10 μm-thick film of 20 nm-sized TiO₂ particles was coated onto the FTO glass electrode by the doctor-blade technique and then dried at 125 °C for 5 min. Secondly, a 5 μm-thick layer of 200 nm light-scattering TiO₂ particles was deposited on the first TiO₂ layer. The two layers of TiO₂ particles were used as photoanode. The resulting TiO₂ films were annealed at 500 °C for 30 min and gradually cooled to 90 °C. Then, the TiO₂ electrode was soaked in the Z907 solution (0.5 mM) in acetonitrile/tert-butyl alcohol (1 : 1, v/v) for 24 h. Afterward, the dyed TiO₂ electrode was washed with anhydrous ethanol and dried with a N₂ stream. To prepare the Pt counter electrode, one drop of 5 mM H₂PtCl₆ in ethanol was dipped onto the cleaned FTO substrate, followed by dried and annealed at 400 °C for 15 min. DSSCs were fabricated by sandwiching the heated gel electrolytes between a dye-sensitized TiO₂ electrode and a platinized counter electrode. The electrodes and Pt conducting glass were separated by using a 25 μm thick Surlyn hot-melt ring (DuPont) and sealed up by heating. The gel electrolytes were injected into sandwiched cells by a vacuum back filling system. The hole for electrolyte injection on the Pt counter electrode was finally sealed with a Surlyn sheet and a thin glass by heating.

Characterization and measurement

¹HNMR spectra were recorded on a Varian 400 MHz spectrometer. Fourier transform infrared (FTIR) spectra of the synthesized compounds were recorded on a Varian CP-3800 spectrometer in the range of 4000–400 cm⁻¹. The viscosity measurement was carried out on viscometer (Haake® Rheo Stress 6000, Germany). Thermal analysis was carried out on Universal Analysis 2000 thermogravimetric analyzer (TGA). Sample was heated from 50 to 500 °C at a heating rate of 10 °C min⁻¹ under a N₂ flow. Differential scanning calorimetry (DSC) measurement was performed under a N₂ atmosphere with a heating rate of 10 °C min⁻¹ in a temperature range of −50 to 250 °C on DSC-Q200. The conductivity of the electrolytes was characterized in an ordinary cell composed of Teflon tube and two identical stainless steel electrodes (diameter of 1 cm) on a CHI660c electrochemical workstation, using the AC impedance method over the frequency range 0.1 to 10⁵ Hz and the amplitude is 10 mV. The conductivity can be calculated by the following equation:²⁵

σ = l/(RS) (1)

where σ is the conductivity in S cm⁻¹, R is the ohmic resistance of the electrolyte, l is the distance between two electrodes, and S is the active area of the electrodes. The photocurrent density–voltage (J–V) curves of the assembled DSSCs shielded by an Al foil mask with an aperture area of ∼0.25 cm⁻² were measured with a digital source meter (Keithley, model 2612) under simulated air mass (AM) 1.5 solar spectrum illumination at 100 mW cm⁻². The photoelectrochemical parameters, such as fill factor (FF) and power conversion efficiency (PCE) were calculated according to the previous reports.^13–16

3. Results and discussion

Scheme 1 illustrates the general procedures to synthesize the ionic liquid [12-C-4][Im][I]. The synthesis processes are feasible and easy to operate. The high purity and exact chemical structure of the resultant products are also confirmed by ¹HNMR (see Fig. S1, ESI†). The FTIR spectra of [12-C-4][OH], [12-C-4][Br], [12-C-4][Im] and [12-C-4][Im][I] between 400 and 4000 cm⁻¹ at room temperature are further presented in Fig. 1. In Fig. 1a, the broad band around 3385 cm⁻¹ is usually assigned to the O–H bond, which is not observed in Fig. 1b. Peaks between 2800 and 3000 cm⁻¹ are ascribed to –CH₃ and –CH₂ stretching vibration.²⁶ Peaks lower 1000 cm⁻¹ are due to bending vibration of C–H. In Fig. 1b, characteristic absorption bands of ester group at 1736 cm⁻¹ derives its formation. A peak at 559 cm⁻¹ is also found for C–Br bond. There are two intensive peaks at 1454 cm⁻¹ and 1508 cm⁻¹ for C=C and C=N, and a peak at 3109 cm⁻¹ for C–N appearing in Fig. 1c. Moreover, in Fig. 1c and d, obvious broad band with the maximum at 3420 cm⁻¹ arises from the N–H stretching vibrations,²⁷ which confirms the existence of imidazole circle. When CH₃I is introduced in Fig. 1d, the interaction of imidazole circle weakens, thus the intensity of peak at 1454 cm⁻¹ and 1508 cm⁻¹ decreases. From the above discussion, it can be confirmed that the novel crown ether functionalized ionic liquid has been synthesized successfully.

Scheme 1 Synthetic procedures for the ionic liquid [12-C-4][Im][I].

Fig. 1 FTIR spectra of (a) [12-C-4][OH]; (b) [12-C-4][Br]; (c) [12-C-4][Im]; (d) [12-C-4][Im][I], respectively.

Typical 3D molecular structures and photographs of the products are further shown in Fig. 2. The intermediate compounds [12-C-4][Br] (a) and [12-C-4][Im] (b) exhibit yellow liquid state, while the ionic liquid [12-C-4][Im][I] (c) displays milky white gel state with a viscosity value of 6850 cP. Moreover, after introducing metal iodides to [12-C-4][Im][I], the viscosity value of LiI doped [12-C-4][Im][I] (d) increase up to 8345 cP with slight color change. Interestingly, combing MgI₂ with [12-C-4][Im][I] (e) leads to obvious color change from milky white to orange. Actually, the viscosity value of MgI₂ doped [12-C-4][Im][I] is 9720 cP. Therefore, the electrostatic interaction between crown ether and metal cations could enhance the van der Waals forces²⁸ of metal iodides doped ionic liquids, leading to higher viscosity value.

Fig. 2 Typical 3D molecular structures and photographs of compounds (a) [12-C-4][Br], (b) [12-C-4][Im], (c) [12-C-4][Im][I], (d) LiI doped [12-C-4][Im][I], and (e) MgI₂ doped [12-C-4][Im][I], respectively.

The effect of LiI and MgI₂ on the thermal property of ionic liquids [12-C-4][Im][I], LiI doped [12-C-4][Im][I] and MgI₂ doped [12-C-4][Im][I] are also studied by TGA curves. As shown in Fig. 3a, typical TGA curves depicts the decomposition processes of three ionic liquids. It can be noted that the decomposition of [12-C-4][Im][I] (black line) is one-step process starting at ∼266 °C. When introducing LiI, the decomposition temperature lowers to ∼243 °C (red line). The main weight loss corresponds to the whole decomposition of LiI doped [12-C-4][Im][I], probably because of the inducing effect²⁹ of Li cations to the crown ring of [12-C-4][Im][I]. In addition, when MgI₂ is employed, there are more decomposition processes and the initial decomposition temperature greatly lowers to ∼150 °C. The decomposition at lower temperature represents more obvious inducing effect of divalent cation Mg²⁺ to the crown ring of [12-C-4][Im][I]. However, it is clearly found that all the three ionic liquids display good thermal stability upon 150 °C. Therefore, it is still well acceptable for a wide range of electrolyte applications.³⁰

Fig. 3 (a) TGA and (b) DSC curves of [12-C-4][Im][I], LiI doped [12-C-4][Im][I], and MgI₂ doped [12-C-4][Im][I] under a N₂ atmosphere at a heating rate of 10 °C min⁻¹.

In order to further investigate the thermal property of [12-C-4][Im][I], LiI doped [12-C-4][Im][I] and MgI₂ doped [12-C-4][Im][I], DSC curves are also tested, as shown in Fig. 3b. For [12-C-4][Im][I], there is a glass transition temperature at −5.2 °C. When LiI or MgI₂ is added, no obvious melting point can be found, indicating the state is still a gel for LiI or MgI₂ doped [12-C-4][Im][I]. In addition, for LiI doped [12-C-4][Im][I], the glass transition temperature disappears. It also demonstrates the strong inducing effect between monovalent Li⁺ cations and crown ether rings. Furthermore, for MgI₂ doped [12-C-4][Im][I], an endothermic peak with slight decomposition appears when the temperature is higher than 120 °C, which indicates more strong inducting effect between divalent Mg²⁺ to crown ether rings. These DSC analysis is also in accordance with the previous results of TGA curves.

To gain more insight into the influence of metal iodide with different valence state on the ionic conductivity of the ionic liquids at room temperature, [12-C-4][Im][I], LiI and MgI₂ doped [12-C-4][Im][I] are employed to form Electrolytes I–III. The electrochemical impedance spectra are further carried out to investigate the charge transfer ability.^16,22,23 As shown in Fig. 4, a typical impedance curve is composed of two depressed semicircles.³¹ The first semicircle in high-frequency region represents charge transfer resistance between the Pt electrode and the electrolyte. While, the second semicircle in intermediate-frequency region indicates the resistance of Nernst diffusion in the electrolyte. It can be seen that adding LiI to [12-C-4][I] to form Electrolyte II can obviously decrease the resistance value of Electrolyte I. Additionally, when MgI₂ are introduced into [12-C-4][I], formed Electrolyte III displays much smaller resistance value than Electrolyte II. From the enlarged drawing, the diminution can be clearly found. Therefore, for Electrolyte I–III, the ionic conductivities are 1.04 × 10⁻⁷, 3.73 × 10⁻⁷ and 3.67 × 10⁻⁶ S cm⁻¹ at room temperature, respectively. Therefore, for Electrolytes I–III, ∼35 folds of ionic conductivity enhancement can be obviously observed at room temperature. It is well known that the cavity radius of 12-crown-4 is between 0.060 and 0.075 nm, while the ionic radius of Li⁺ and Mg²⁺ are 0.060 and 0.065 nm.^31,32 The particular trapping and dissociating ability of crown ether rings to metal cations (Li⁺ and Mg²⁺) could effectively reduce the electrostatic interaction between metal cations and iodide anions to release more iodide anions,³³ which should be responsible for the enhanced ionic conductivity of the multi-iodine doped electrolytes. The multi-iodine doped strategy to enhance the ionic conductivity of ionic liquids triggers our interest in further investigating the temperature effect on the ionic conductivity.

Fig. 4 Nyquist plots of EIS spectra of Electrolyte I with [12-C-4][Im][I] (black line), Electrolyte II with LiI doped [12-C-4][Im][I] (red line), Electrolyte III with MgI₂ doped [12-C-4][Im][I] (green line) at room temperature, respectively. Inset shows the curve at the high-frequency range.

The ionic conductivity of ILs is an important parameter related to their potential applications in electrochemical devices. The ionic conductivity of the medium involves the number of the carrier ions as well as the degree of correlation in ionic motion. To better understand the relationship of the AC impedance with temperatures, EIS spectra and ionic conductivity are studied as shown in Fig. 5a–c. It is obvious that higher temperature will lead to lower impedance values for all the three Electrolytes I–III. According to previous thermal measurements, each electrolyte performs a liquid/gel state at high temperature and the viscosity of them decreases greatly, resulting in obvious increase in ionic mobility. Consequently, the ionic conductivity enhances obviously, as shown in Fig. 5d. Therefore, the ionic conductivity is 6.49 × 10⁻⁶, 3.88 × 10⁻⁵ and 2.12 × 10⁻⁴ S cm⁻¹ at 85 °C, which are much higher than their corresponding values at room temperature, displaying a potential application as novel ionic liquids electrolytes. Besides, the ionic conductivity presents a relatively linear dependence on the temperature, indicating typical Arrhenius behavior.¹⁹ The thermally activated process can be expressed as:

σ = σ_o exp(−E_a/κT) (2)

where σ_o represents the pre-exponential factor, E_a represents the activation energy, κ represents the Boltzmann constant and T represents the temperature in Kelvin.³⁴ The calculated E_a values of Electrolytes I–III are 39.84 kJ mol⁻¹, 36.67 kJ mol⁻¹ and 31.01 kJ mol⁻¹, respectively. It is clearly found that the activation energy of Electrolyte II or Electrolyte III reduces when LiI or MgI₂ is added into [12-C-4][Im][I] (Electrolyte I). Briefly, it can be concluded that the decrease in the activation energy is mainly due to the multi-iodine doping of I⁻, thus leading to more favorable ionic transport in the electrolyte system.

Fig. 5 Nyquist plots of EIS spectra of Electrolyte I (a), Electrolyte II (b), Electrolyte III (c), and ionic conductivity (d) as a function of temperatures, respectively. Inset shows the curves at the high-frequency range.

As one of most important components in DSSCs, the electrolyte is responsible for the inner charge carrier transport between photoanodes and counter electrodes, and continuously regenerates the dye and itself during the device operation.^13,14 Taking the above analysis into account, [12-C-4][Im][I], LiI doped [12-C-4][Im][I], MgI₂ doped [12-C-4][Im][I], and MgI₂ doped [12-C-4][Im][I] with 10 wt% I₂ are chosen as four Electrolytes I–IV for the fabrication of DSSCs (Devices A–D). The J–V curves are shown in Fig. 6, and the performance parameters are summarized in Table 1. It can be seen that Device A with [12-C-4][Im][I] only exhibits a low PCE of 0.3%. Moreover, adding LiI into [12-C-4][Im][I], higher PCE for Devices B with Electrolyte II is observed to be 1.23%, which is consistent with the conductivity results in Fig. 4 and 5. Furthermore, higher ionic conductivity for Electrolyte III with MgI₂ doped [12-C-4][Im][I] can lead to higher PCE ∼2.09% for Device C. In virtue of the function of I₂ to form I₃⁻ to greatly enhance the ionic conductivity for more effective dye regeneration and electron relay, I₂ is applied to further improve the PCE of Device D. As shown in Table 1, a much higher PCE of 3.86% can be achieved, corresponding to a J_sc of 8.75 mA cm⁻², a V_oc of 0.61 V and a FF of 0.724. Unfortunately, high viscosity values of Electrolytes I–IV from 6850 to 10 030 cP, greatly restrain the mass transport of ion species and lead to relatively low ionic conductivity, which should be responsible for the photovoltaic performance.

Fig. 6 J–V curves for Devices A–D containing Electrolyte I, Electrolyte II, Electrolyte III, and Electrolyte IV, respectively.

Table 1 Photovoltaic performance parameters of Devices A–D with Electrolytes I–IV, respectively

Device	Conductivity^a (10^−7 S cm^−1)	Viscosity^b (cP)	Jsc (mA cm^−2)	Voc (mV)	FF	PCE (%)
a The state of electrolytes is present at 25 °C.b Viscosity tested at 25 °C.
A	1.04	6850	1.12	579	0.458	0.30
B	3.73	8745	3.23	595	0.637	1.23
C	36.7	9720	5.23	587	0.680	2.09
D	1260	10 030	8.75	610	0.724	3.86

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4. Conclusions

In summary, a novel crown ether functionalized ionic liquid has been successfully designed, synthesized and characterized. The influence of metal iodine and temperature on the thermal properties and ionic conductivities is also investigated in details. Compared with the ionic liquid [12-C-4][Im][I] without any metal iodine, introducing LiI or MgI₂ can greatly enhance the ionic conductivity of multi-iodine doped ionic liquids. Using these ionic liquids as gel electrolytes, the resulting DSSCs displays a power conversion efficiency of 3.86%. There is still large space to improve the electrochemical property of these ionic liquids electrolytes by employing other metal cations with higher valents (e.g. AlI₃ and SnI₄), which can facilitate more free iodide anions for better ionic conductivity at the same concentration of metal cations. In addition, the viscosity of crown ether functionalized ionic liquids can be obviously reduced by exchanging the counteranions of imidazolium structure to be other anions such as BF₄⁻ and TFSI⁻, etc. However, these preliminary results provide us a feasible method to develop higher conductive ionic liquids electrolytes for energy conversion devices. More research is still under way in our laboratory.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21504061 and grant #U1401248), the Natural Science Foundation of Jiangsu Province (no. BK20140311), University Science Research Project of Jiangsu Province (no. 13KJB150033), “973 Program” Special Funds for the Chief Young Scientist (no. 2015CB358600), the Excellent Young Scholar Fund from National Natural Science Foundation of China (21422103), the Jiangsu Fund for Distinguished Young Scientist (BK20140010), the Open Foundation of Jiangsu Key Laboratory of Thin Films and the Project Funded by the PAPD of Jiangsu Higher Education Institutions.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23229e

‡ Authors with equal contributions.

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