Allyl-functionalized ionic liquids as electrolytes for electric double-layer capacitors
Introduction
Ionic liquids (ILs) have received a lot of interest recently as the ion conductive materials as well as the reaction media because of the unusual properties of ILs such as a wide liquid range, high ionic conductivities, a wide voltage window, non-volatility, and non-flammability [1], [2], [3], [4], [5]. In addition, the designability of ILs makes themselves attractive alternatives to the conventional organic electrolytes and solvent systems. The common ILs are imidazolium and pyridinium derivatives [6], [7]. Phosphonium and tetraalkylammonium compounds can be also used for the same purpose [8], [9].
Recently some studies have been reported to improve the high-temperature safety and durability of electrochemical devices such as lithium rechargeable batteries [10], electric double-layer capacitors (EDLCs) [11], [12], [13], [14], [15], and titanium oxide dye-sensitized solar cells [16].
For EDLC electrolytes, various solvents and salts are available, offering specific advantages such as a high capacitance and low temperature performances. Generally, the organic electrolyte, which is the solid quaternary ammonium salt dissolved in the high dielectric constant solvent, has been used for high voltage EDLCs of 2 or higher than 2 V. Here, the salt is N,N,N,N-tetraethylammonium–BF4 (TEA–BF4) or N,N,N-triethyl-N-methylammonium–BF4 (TEMA–BF4) and the solvent is propylene carbonate (PC). EDLCs store electricity physically, not chemically, in contrast with the rechargeable batteries [17].
EDLCs have attracted much attention recently because of the power delivery performance that perfectly fills the gap between dielectric capacitors and traditional batteries. Recently, various salts for EDLCs such as salts of asymmetric ammonium, pyrrolidinium and piperidinium have been reported [18], [19], [20]. In addition, imidazolium-type ILs, such as 1-ethyl-3-methylimidazolium (EMIm), have been intensely researched because of the low viscosities, high ionic conductivities and low melting points.
Various ionic liquids of imidazolium cations having alkene or allyl groups were synthesized and the properties and the strong supercooling nature were reported [21], [22], [23], [24], [25], [26]. However, the previously reported application was limited to the solvent for cellulose [27], [28], [29], the synthetic solvents [30], the electrolytes for the dye-sensitized solar cell [31], and the additive for lithium secondary batteries [32]. There has been no study on the application as salts for EDLC electrolytes of ILs with allyl groups.
Of special interest is to evaluate the performance of EDLC cells of ionic liquids and ammonium salts with saturated alkyl groups or olefinic substituents, including allyl, vinyl, and butenyl groups. In this paper we report on the ionic conductivities, the dynamic viscosities of EDLC electrolytes containing cations with allyl groups, the initial performances and the life tests of cells at a wide range of temperature. We also report on the effect of combined anions and solvents.
Section snippets
Preparation of various salts
The structures of salts studied in this study are shown in Table 1. The preparation of salts was carried out according to the standard procedures [21], [22]. For example, we synthesized the 1-allyl-3-methylimidazolium (AMIm)–TFSA by the alkylation of 1-allylimidazole with iodomethane in acetonitrile, followed by the exchange reaction with LiTFSA in water. The high-grade salt of AMIm–TFSA was obtained after washing with excess purified water repeatedly for cleaning up, followed by the evacuation
Physicochemical properties of salts
At first we evaluated TFSA-based electrolytes, because we were able to synthesize high-grade salts repeatedly by a simple method in the case of TFSA-based salts. The total amount of impurities in each TFSA-based salt was less than 150 ppm, and the amount of water in each salt was less than 100 ppm. Afterwards the effect of anions was examined. Table 1 shows the viscosities and ionic conductivities of the salts of various cations. The salts of all imidazolium–TFSA showed the properties of the
Conclusions
We synthesized the various TFSA-based ionic liquids of imidazolium and ammonium cations with the saturated alkyl, vinyl, allyl and 1-butenyl groups and examined the properties and EDLC performances of the electrolytes. It was confirmed that the substitution of allyl groups causes the low viscosity compared to saturated alkyl, vinyl and butenyl groups. The low viscosity may come from the plasticizing effect of allyl groups on the N-position, although the effect was not confirmed in the case of BF
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