Electrochemistry The Electrochemical Society of Japan Communication
Received: April 16, 2015 Accepted: June 4, 2015 Published: October 5, 2015 http://dx.doi.org/10.5796/electrochemistry.83.824
Electrochemistry, 83(10), 824–827 (2015)
Ionic Conductivity and Viscosity of Solvate Ionic Liquids Composed of Glymes and Excess Lithium Bis(Trifluoromethylsulfonyl)Amide Hirotsugu HIRAYAMA,a Naoki TACHIKAWA,a Kazuki YOSHII,a Masayoshi WATANABE,b and Yasushi KATAYAMAa,* a
b
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku, Yokohama, Kanagawa 223-8522, Japan Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
* Corresponding author:
[email protected] ABSTRACT Ionic conductivity and viscosity of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA)-glyme solvate ionic liquids with the mole fraction of LiTFSA from 50.0 to 54.5 mol% were investigated at 298–323 K. The ionic conductivity of the solvate ionic liquids was found to be inversely proportional to the viscosity, as expected from Walden’s rule. The ionic conductivity of the solvate ionic liquid decreased with increasing the mole fraction of LiTFSA, probably due to formation of a bulky lithium species, [Li(TFSA)2]−, which forms at the compositions of more than 50 mol% of LiTFSA. © The Electrochemical Society of Japan, All rights reserved.
Keywords : Ionic Liquid, Lithium Batteries, Concentrated Electrolytes, Mass Transport 1. Introduction The performance of rechargeable lithium batteries is strongly dependent on the transport properties, such as ionic conductivity and viscosity, which are important parameters of liquid electrolytes. The concentration of lithium salts in ordinary organic electrolyte solutions is usually ca. 1 mol dm¹3 since the ionic conductivity reaches at a maximum around this concentration.1 Recently, the electrolytes containing lithium salts at high concentrations have attracted attention due to the distinct characteristics emerged from unique coordination environment of lithium ion and/or unusually low activity of free solvent molecules.2–12 Some lithium salts are able to be dissolved in glymes (Gn, CH3O(CH2CH2O)nCH3), such as triglyme (G3, n = 3) and tetraglyme (G4, n = 4), at extremely high concentrations (> 3 mol dm¹3).13,14 It has been known that stoichiometric LiTFSAglyme mixtures (TFSA¹ = bis(trifluoromethylsulfonyl)amide), of which the mole fractions of LiTFSA, XLiTFSA, are 50 mol%, are able to be regarded as solvate ionic liquids since the mixtures consist of a complex cation of lithium ion and glyme, [Li(glyme)]+, and TFSA¹ anion according to the following equilibrium.15 LiTFSA þ glyme ½LiðglymeÞþ þ TFSA
ð1Þ
Very recently, it has been reported that the percentage of free glymes is less than 3% in the solvate ionic liquid at XLiTFSA = 50 mol%.15 These solvate ionic liquids where the amount of free glymes is very small are compatible with graphite anode,12 4 V-class cathode7 and Li-S battery,9,16 while the stable charge-discharge operation of these batteries cannot be achieved in the mixtures at XLiTFSA < 50 mol%. In addition, slight improvement in the Li-S battery performance with the solvate ionic liquids at XLiTFSA = 54.5 mol% has been investigated compared to the solvate ionic liquids at XLiTFSA = 50.0 mol% in terms of a coulombic efficiency and discharge capacity retention rate.17 Thus, tuning the composition of solvate ionic liquids is a key issue to apply these solvate ionic liquids to battery applications. On the other hand, there is some possibility that free glymes are generated dynamically during electrochemical reaction even in the LiTFSA-glyme solvent ionic liquids at XLiTFSA = 50 mol%. In the case of deposition of Li, the reduction of [Li(glyme)]+ leads to 824
liberation of glymes near the electrode surface at higher current densities.18 To optimize the solvate ionic liquids for battery application, it is needed to investigate the properties of the LiTFSA-glyme solvate ionic liquids with a wide composition range. Although the properties of the solvate ionic liquids at XLiTFSA ¯ 50 mol% have been reported so far,13 those at XLiTFSA > 50 mol% have not been studied yet. In the present study, such fundamental transport properties as the viscosity and ionic conductivity of the LiTFSA-glyme solvate ionic liquids at XLiTFSA from 50.0 to 54.5 mol% were investigated in the temperature range from 298 to 323 K. 2. Experimental G3 and G4 (Nippon Nyukazai, water content < 50 ppm) and LiTFSA (Solvay) were used as received. LiTFSA-glyme solvate ionic liquids were prepared by mixing LiTFSA and glymes in the XLiTFSA range from 50.0 to 54.5 mol%. All the hygroscopic materials were handled in an Ar filled glove box with a continuous gas purification apparatus (Miwa MFG, DBO-1KP-K02). The concentrations of H2O and O2 were kept below 800 ppb and 1 ppm, respectively. The ionic conductivity was measured with an airtight four-probe conductivity cell consisting of two inner platinum electrodes (0.3 mmT) for monitoring the potential difference and two outer platinum electrodes (1.0 mmT) for feeding the alternating current amplitude of 10 µA root-mean-square with a computer-controlled potentiostat/galvanostat (Prinston Applied Research, PARSTAT 2263 or 2273) in the frequency range from 10 kHz to 100 Hz. The airtight conductivity cell was sealed with vacuum fittings in the Ar glove box. The viscosity was measured by a vibronic viscometer (Sekonic, VM-10A). The samples for the viscosity measurement were injected into glass cell under N2 atmosphere without exposure to air. The temperature of the sample cells for the ionic conductivity and viscosity was controlled with a liquid bath (3MTM FluorinertTM) using a Peltier type temperature controller (AS ONE, Hot & Cool Bath) at the range of temperature from 298 to 323 K. The density of the solvate ionic liquid was measured using Ostwald pycnometer (SANSYO) at 298 K.
Electrochemistry, 83(10), 824–827 (2015) Table 1. The density and optimized parameters for the ionic conductivity and viscosity of the LiTFSA-G3 and LiTFSA-G4 solvate ionic liquids with various compositions. Solvate ionic liquids LiTFSA-G3
LiTFSA-G4
XLiTFSA /mol%
µ /g cm¹3
10¹5 ¬0 /mS cm¹1
Ea(¬) /kJ mol¹1
104 ©0 /mPa s
Ea(©) /kJ mol¹1
50.0 51.2 52.4 53.5 54.5 50.0 51.2 52.4 53.5 54.5
1.44 1.45 1.46 1.47 1.48 1.42 1.42 1.43 1.44 1.45
2.1 2.5 2.6 14 17 7.5 18 46 65 110
30 31 32 37 38 33 35 38 40 42
7.0 2.8 1.2 1.9 0.39 3.0 1.0 1.7 0.78 0.30
31 34 37 36 41 32 35 34 37 40
3. Results and Discussion The amount of free glyme in the LiTFSA-glyme solvate ionic liquid at XLiTFSA = 50 mol% has been estimated to be less than 3% by Raman spectroscopy and potentiometric titration, suggesting that almost all the glyme molecules coordinate Li+ to form [Li(glyme)]+ based on Eq. (1).15 Consequently, this LiTFSAglyme solvate ionic liquid at XLiTFSA = 50 mol% is considered similar to the typical TFSA¹-based ionic liquids with organic cations. In the case of the TFSA¹-based ionic liquids, such as DEMETFSA (DEME+ = N,N-Diethyl-N-methyl-N-(2-methoxyethyl)ammonium), addition of LiTFSA leads to formation of [Li(TFSA)2]¹.19 Similarly, the previous research regarding LiTFSAglyme solvate ionic liquid at XLiTFSA = 50 mol% suggested a small amount of [Li(TFSA)2]¹ forms through the ligand exchange equilibrium as follows;12 ½LiðglymeÞþ þ 2TFSA ½LiðTFSAÞ2 þ glyme
ð2Þ
Table 1 lists the density (µ) of LiTFSA-glyme solvate ionic liquids at 298 K. The density increased with an increase in XLiTFSA, as reported for LiTFSA/DEMETFSA ionic liquid.20 Assuming formation of [Li(glyme)]+ and [Li(TFSA)2]¹ in the LiTFSA-glyme solvate ionic liquids at XLiTFSA > 50 mol%, the mole fraction of each ion is able to be calculated for different XLiTFSA. Accordingly, the molar concentrations of [Li(TFSA)2]¹ were estimated for the LiTFSA-glyme solvate ionic liquids with different XLiTFSA using the density. Figure 1 shows the temperature dependence of the ionic conductivity of LiTFSA-G3 and LiTFSA-G4 solvate ionic liquids with various XLiTFSA. The ionic conductivity of LiTFSA-G4 solvate ionic liquid was higher than that of LiTFSA-G3 one at the same XLiTFSA and the same temperature. The ionic conductivity was fitted to the following Arrhenius equation within the temperature range in this study; Ea ð¬Þ ð3Þ ¬ ¼ ¬0 exp RT where ¬0 (mS cm¹1) is the pre-exponential factor of the ionic conductivity, Ea(¬) (kJ mol¹1) is the activation energy for the ionic conductivity, R (J mol¹1 K¹1) is the gas constant and T (K) is the absolute temperature. The optimized parameters are also listed in Table 1. Figure 2 shows the temperature dependence of the viscosity of LiTFSA-G3 and LiTFSA-G4 solvate ionic liquids. The viscosity increased with an increase in XLiTFSA for both solvate ionic liquids. The viscosity of LiTFSA-G3 solvate ionic liquid was higher than that of LiTFSA-G4 one at the same temperature and the
Figure 1. Temperature dependence of the ionic conductivities of (a) LiTFSA-G3 and (b) LiTFSA-G4 solvate ionic liquids.
same XLiTFSA. The activation energy for the viscosity was calculated by fitting to Andrade equation as follows. Ea ð©Þ © ¼ ©0 exp ð4Þ RT where ©0 (mPa s) is the pre-exponential factor of the viscosity and Ea(©) (kJ mol¹1) is the activation energy for the viscosity. The optimized parameters are also listed in Table 1. Ea(©) increased with an increase in XLiTFSA, as reported in LiTFSA/DEMETFSA ionic liquid.21 In addition, Ea(©) is close to Ea(¬) for each composition, indicating the ionic conductivity is determined dominantly by the viscosity of the LiTFSA-glyme solvate ionic liquids as expected from the Walden rule. 825
Electrochemistry, 83(10), 824–827 (2015)
Figure 2. Temperature dependence of the viscosities of (a) LiTFSA-G3 and (b) LiTFSA-G4 solvate ionic liquids.
Figure 3. Walden plots of (a) LiTFSA-G3 and (b) LiTFSA-G4 solvate ionic liquids.
It has been reported that the viscosities of LiTFSA/DEMETFSA and LiTFSA/MPPTFSA (MPP+ = 1-methyl-1-propylpyrrolidinium) containing 0.64 mol dm¹3 [Li(TFSA)2]¹ are 2.0 and 2.6 times larger, respectively, than those of the ionic liquids without LiTFSA.20,22 In the case of the solvate ionic liquids, the viscosities of LiTFSA-G3 and LiTFSA-G4 solvate ionic liquids at XLiTFSA = 54.5 mol% (0.5–0.6 mol dm¹3 [Li(TFSA)2]¹) were 2.6 and 2.7 times larger, respectively, than those at XLiTFSA = 50.0 mol% (0 mol dm¹3 [Li(TFSA)2]¹), suggesting the relative change of the viscosity is related to the molar concentration of [Li(TFSA)2]¹ in both solvate ionic liquids and conventional ionic liquids. In the case of LiTFSAglyme solution (XLiTFSA < 50.0 mol%), Broullette and co-workers have pointed out that the dependence of the ionic conductivity of the electrolyte on the concentration of LiTFSA is able to be explained by the empirical Casteel-Amis equation in the XLiTFSA range from 0 to 40 mol%.23 However, the ionic conductivity of the solvate ionic liquids at XLiTFSA ² 50.0 mol% deviated from the reported relation, presumably reflecting the difference in the Li(I) species in the electrolyte solution and the solvate ionic liquids. The molar conductivity, $ (S cm2 mol¹1), of the solvate ionic liquids was estimated with the following equation, assuming the solvate ionic liquids are the mixtures of [Li(glyme)]TFSA and [Li(glyme)][Li(TFSA)2] at XLiTFSA ² 50.0 mol%; XLiTFSA ¬ FWglyme þ FWLiTFSA 1 XLiTFSA ð5Þ ¼ µ
4. Summary
where FWglyme and FWLiTFSA (g mol¹1) are the formula weight of glyme and LiTFSA, respectively, µ (g cm¹3) is the density. Figure 3 shows the Walden plots of LiTFSA-glyme solvate ionic liquids. The molar conductivity is approximately proportional to the inverse of the viscosity, suggesting that the mobility of the ions in the solvate ionic liquids is mainly determined by the fluidity of the media. 826
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