A Comparison among Viscosity, Density, Conductivity, and

Article pubs.acs.org/jced

A Comparison among Viscosity, Density, Conductivity, and Electrochemical Windows of N‑n‑Butyl‑N‑methylpyrrolidinium and Triethyl‑n‑pentylphosphonium Bis(fluorosulfonyl imide) Ionic Liquids and Their Analogues Containing Bis(trifluoromethylsulfonyl) Imide Anion Nédher Sánchez-Ramírez,†,‡ Birhanu Desalegn Assresahegn,‡ Daniel Bélanger,‡ and Roberto M. Torresi*,† †

Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, 05508-000, São Paulo, Brazil ‡ Département de Chimie, Université du Québec à Montréal, Case Postale 8888 succursale Centre-Ville, Montréal, Québec H3C 3P8, Canada S Supporting Information *

ABSTRACT: Ionic liquid consists of an organic cation and generally an inorganic anion. The properties of liquids can be modulated by changing the anion or the cation or both. Two ionic liquids derived from the anion [FSI] were synthesized and characterized: (N-n-butyl-N-methylpyrrolidinium bis(fluorosulfonyl imide), [BMPYR][FSI]) and triethyl-n-pentylphosphonium bis(fluorosulfonyl)imide), [P2225][FSI]). We also report the comparison with [TFSI] based ionic liquids, namely: (N-n-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide), [BMPYR][TFSI]), (N-n-butyl-N-methylpiperidinium bis(trifluoromethylsulfonyl)imide), [BMP][TFSI]), triethyl-npentylphosphonium bis(trifluoromethylsulfonyl)imide), [P2225][TFSI]) and (triethyl(2-methoxyethyl) phosphonium bis(trifluoromethylsulfonyl imide), [P222201][TFSI]. Thermophysical properties, such as viscosity (298.15−373.15 K), density (298.15−373.15 K), conductivity (298.15−361.15 K), electrochemical (cutoff density current within 150 μA·cm−2), and thermal stability (303.15−973.15 K) were experimentally determined. The results showed that [FSI] derivatives exhibit better transport properties and lower thermal and electrochemical stability when compared with their [TFSI] counterparts.

1. INTRODUCTION Ionic liquids (ILs) are liquids consisting exclusively or almost exclusively of ions. They therefore exhibit ionic conductivity; this definition includes liquids that are traditionally known as molten salts or fused salts, which have high melting points.1 Nevertheless, in the last few years, a new definition has appeared: “The term IL refers to compounds consisting entirely of ions and existing in the liquid state below 100 °C”. In many cases the melting point is even below room temperature.2 The scientific and technological importance of ILs today spans a wide range of applications.3 In particular, Li-ion technology has captured the portable electronic market, invaded the power tool equipment market, and is now penetrating the electric vehicles (EVs) segment.4−7 However, lithium-ion batteries must operate within a safe and reliable operating area, which is delimited by temperature and voltage window; both are related to the organic electrolyte typically used as an electrolyte in this technology. Exceeding these restrictions leads to the rapid attenuation of battery performance and even results in safety problems.8 Nowadays, ILs are of great interest for their unique © 2017 American Chemical Society

characteristics, which include: (1) high stability, (2) good transport properties, (3) nonvolatility, and (4) modulation properties.9,10 These unique properties of liquids as electrolytes have the possibility of attenuating the above-mentioned problems concerning lithium batteries. Here, we report a complete characterization of a variety of ILs derived from [TFSI] (bis(trifluoromethylsulfonyl)imide) and [FSI] (bis(fluorosulfonyl) imide) along with various cations derived from phosphorus and nitrogen. The results reveal how these properties are affected when the anion or cation are changed. An adequate selection of the IL made up will allow their application for the lithium-ion technology requirement. Figure 1 displays the cations and anions used in this work. Received: May 19, 2017 Accepted: September 11, 2017 Published: September 25, 2017 3437

DOI: 10.1021/acs.jced.7b00458 J. Chem. Eng. Data 2017, 62, 3437−3444

Journal of Chemical & Engineering Data

Article

3.44 (4H; m); δC (500 MHz; CD3CN; ppm): 13.8; 20.4; 22.3; 26.3; 49.3; 65.1; 65.4. (Figure S1, SI) [P2225][FSI] Data. Elemental analysis data: Found: C; 35.6; H; 7.23; N; 4.08. Calcd for: PC11H26F2NO4S2: C; 35.8; H; 7.09; N; 3.79; δH (500 MHz; CD3CN; ppm): 0.90−0.93 (3H; t; J = 7,5 Hz); 1.15−1.21 (9H; m); 1.33−1.56 (6H; m); 2.03− 2.15 (8H; m); δ13C (500 MHz; CD3CN; ppm): 5.80 (3C); 12.0−12.4 (3C; d; 1JCP = 50 Hz); 14.1; 17.8−18.2 (1C; d; 1JCP = 49 Hz); 21.5; 22.7; 33.4. (Figures S2, SI) [BMPYR][TFSI] Data. Elemental analysis data: Found: C; 31.3; H; 4.74; N; 6.65. Calcd for C11H20N2S2O4F6: C; 31.3; H; 4.77; N; 6.63; δH (500 MHz; CD3CN; ppm): 0.95−0.98 (3H; t; J = 7.5 Hz); 1.33−1.40 (2H; sh; J = 7 Hz); 1.68−1.75 (2H; m); 2.11−2.18 (4H; m); 2.93 (3H; s); 3.20−3.23 (2H; m); 3.35−3.44 (4H; m); δC (500 MHz; CD3CN; ppm): 13.8; 20.4; 22.4; 26.3; 49.3; 65.1; 65.4; 117.2−124.9 (2C; q; 1JCF = 319 Hz) (Figure S3, SI). All of the characterization of [P2225][TFSI] and [P222201][TFSI] can be found elsewhere.10 The NMR spectra and elemental analysis demonstrate the purity of the all compounds. 2.3. Thermal Properties, Density, and Transport Properties. Thermogravimetric analysis was carried out with a thermogravimetric analyzer (TA Instruments TGA (Q500)/ Discovery MS). Samples (typically 2 mg) were placed in a Pt pan and heated from 303.15 to 973.15 K with a temperature ramp of 5 K·min−1, under flowing helium atmosphere. Density and viscosity were measured with a viscometerdensimeter SVM 3000 (Anton Paar) from 298.15 to 373.15 K at atmospheric pressure. The standard uncertainties for the temperature, viscosity, and density are u(T) = 0.01 K, ur(η) = 0.002, and u(ρ) = 0.0003 g·cm−3, respectively. The viscometer/ densimeter was calibrated with mineral oils, namely, S3, N7.5, N26, and N415 according to the manufacturer’s specification. 2.4. Electrochemical Analysis. The electrochemical stabilities of the electrolytes were determined by linear voltammetry using glassy carbon as a working electrode at a scan rate of 0.05 V·s−1 starting from an open circuit potential until the density current reached was within 150 μA·cm−2. The stability was determined by a linear sweep starting at the OCP and moving to the positive or negative potentials. Platinum and silver were used as the counter and pseudoreference electrodes, respectively. Experiments were performed with an Autolab MAC80132 (Metrohm). The standard uncertainty of the applied potential is [0.001 + 0.001] V. The ionic conductivity was determined by electrochemical impedance spectroscopy (EIS) of two parallel Pt electrodes

Figure 1. Schematic diagram of the constituent ions in the ILs studied.

2. EXPERIMENTAL SECTION 2.1. Materials. N-n-Butyl-N-methylpyrrolidinium bromide, lithium bis(fluorosulfonyl)imide, lithium bis(trifluorosulfonyl)imide, triethylphosphine, N-n-butyl-N-methylpiperidinium bis(trifluoromethylsulfonyl)imide, 1-bromopentane, and 2-bromoethylmethyl ether were used as received from the supplier (Table 1). The ILs were dried under vacuum at optimal temperatures for several days until the amount of water was under 0.0001 weight fraction of water; the moisture content of all the electrolytes studied here were determined via Karl Fisher titration (Metrohm). 2.2. Ionic Liquid Synthesis. The ILs, [BMPYR][TFSI], [P2225][TFSI], [P222201][TFSI], [BMPYR][FSI], and [P2225][FSI], were synthesized as described elsewhere.10−12 Briefly, 26 mmol of cations (BMPYRBr or P2225Br or P222201Br) were dissolved in 50 mL of water and mixed with 26 mmol of anion (LiTFSI or LiFSI) which were also dissolved in 50 mL of water. The mixture was stirred for 2 h. The resultant IL is immiscible in water and was separated using dichloromethane. The phase containing the IL was washed five times with water, stirred with activated carbon for 2 h, and subjected to column chromatography (alumina, dichloromethane). [BMPYR][FSI] Data. Elemental analysis data: Found: C; 33.3; H; 6.29; N; 8.71. Calcd for C9H20N2S2O4F2: C; 33.5; H; 6.25; N; 8.69; δH (500 MHz; CD3CN; ppm): 0.95−0.98 (3H; t; J = 7.5 Hz); 1.33−1.41 (2H; sh; J = 8 Hz); 1.68−1.75 (2H; m); 2.13−2.17 (4H; m); 2.93 (3H; s); 3.20−3.23 (2H; m); 3.35− Table 1. Sample Information chemical name N-n-butyl-N-methylpyrrolidinium bromide (≥99.0%) triethylphosphine (≥95%) 1-bromopentane (98%) 2-bromoethyl methyl ether (90%) lithium bis(trifluoromethylsulfonyl)imide (99.9%) N-n-butyl-N-methylpiperidinium bis(trifluoromethylsulfonyl)imide (99%) N-n-butyl-N-methylpyrrolidinium bis(fluorosulfonyl imide (99%) N-n-butyl-N-methylpyrrolidinium bis(trifluorosulfonyl imide (99%) triethyl-n-pentylphosphonium bis(fluorosulfonyl)imide (99%) triethyl-n-pentylphosphonium bis(trifluorosulfonyl)imide (99%) triethyl (2-methoxyethyl) phosphonium bis(trifluoromethylsulfonyl)imide (99%)

abbreviation

CAS No.

source

water content (weight fraction)

BMPYRBr P222 5Br 201Br LiTFSI [BMP][TFSI] [BMPYR][FSI] [BMPYR][TFSI] [P2225][FSI] [P2225][TFSI] [P222201][TFSI]

93457-69-3 554-70-1 110-53-2 6482-24-2 90076-65-6 623580-02-9

SIGMA-Aldrich SIGMA-Aldrich SIGMA-Aldrich SIGMA-Aldrich Solvay Iolitec this work this work this work this work this work

≤0.0001 ≤0.0001 ≤0.0001 ≤0.0001 ≤0.0001 ≤0.0001

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Table 2. Selected Properties of the ILs at 298.15 K at p = 0.1 MPaa LI

η (mPa·s)

ρ (g·cm−3)

σ (mS·cm−·1)

MM (g·mol−1)

EW (V)

[BMPYR][FSI] [P2225][FSI] [BMPYR][TFSI] [P222201][TFSI]10 [P2225][TFSI]10 [BMP[TFSI]

53.24 69.38 77.76 48.14 85.30 188.9

1.307 1.219 1.395 1.378 1.304 1.382

8.66 4.63 3.93 3.84 2.10 1.39

322.4 369.4 422.4 457.4 469.4 436.5

5.07 4.30 5.67 4.11 4.93 4.44

Viscosity, η, density, ρ, electrical conductivity, σ, and electrochemical windows, EW. Standard uncertainties are ur(η) = 0.002, u(ρ) = 0.0003 g·cm−3, u(EW) = [0.001 + 0.001] V, u(T) = 0.01 K, and u(p) = 0.005 MPa for viscosity, density, electrochemical window, temperature, and pressure, respectively. The uncertainties for conductivity are listed in Tables 3−8. a

caused by various factors, such as the percentage weight loss in which the Td is taken, the heating rate, the gas atmosphere, and the pan material used the TGA experiment.14 3.2. Viscosity, Conductivity, and Density Measurement. The viscosity of a fluid arises from the internal friction of the fluid, and it manifests itself externally as the resistance of the fluid to flow.13 This is a very important property since high viscosities limit some electrochemical applications and slow down the rate of diffusion-controlled chemical reactions.20 In general, ILs are 1−2 orders of magnitude more viscous than molecular solvents, and their viscosities at room temperatures typically lie in the range of 10 to over 500 mPa·s.1 Table 2 presents the viscosity at 298.15 K and Figure 2 and Tables 3−8 at different temperatures.

with an Autolab MAC80132 in the frequency range of 0.1− 100 000 Hz in the range of temperature = 298.15−361.15 K. The cell constant (A/l) (a = area and l = distance between the electrodes) was determined with a standard KCl solution. The ionic resistance (R) value was taken from the extrapolation at high frequencies (impedance imaginary part = 0) on the real part of the impedance. So, the conductivity is σ = RxA/l. The temperature was controlled by a thermostat with an uncertainty within 0.02 K.

3. RESULTS AND DISCUSSION 3.1. Thermal Stability of the Electrolytes. ILs are thermally stable but certainly decompose at high temperatures. The decomposition temperature (Td) of ILs can be seen from Figure S4, using the maximum of the derivate of loss weight as a reference; the Td values are about 730, 730, 740, 731, 615, and 644 K for [BMPYR][TFSI], [BMP][TFSI], [P222201][TFSI], [P2225][TFSI], [BMPYR][FSI], and [P2225][FSI], respectively. The ILs under study have no distinguishable vapor pressure; consequently, the first event upon heating these ILs is their thermal decomposition.12 The Td of ILs depends on the component ion structure, similarly to other thermal properties.2 The nature of the ILs containing organic cations generally restricts upper stability temperature, up to 623 K, where pyrolysis occurs if no other lower temperature decomposition pathways are accessible.13 This implies that, in general, Td is more affected by anion structure, being less stable for ILs with strong nucleophilic anions.2,13,14 The decomposition temperatures (Td) for all ILs used in the project were higher than 573 K. The [TFSI]-based ILs are more thermally stable than the [FSI] derivatives presumably due to the higher nucleophilicity of the [FSI] anion. The [TFSI] has two more fluorine atoms in the structure than [FSI]. This provokes a better negative charge delocalization and stabilization by the electron-withdrawing effect of halogen atoms.2 Consequently, [FSI] has a stronger nucleophile effect compared with the [TFSI] anion. It should also be emphasized that the thermal decomposition of the ILs was found to be a time-dependent process. Indeed, the longterm thermal stability of ILs can be more than 50 K lower than the decomposition temperature obtained by step-tangent analysis of rising temperature scanning differential thermal analysis (TGA-onset) experiments13 that are commonly performed at a heating rate of 10 K·min−1. The Td reported in the literature are 698,15 685,16 677,17 653,17 563,18 and 58219 for [BMPYR][TFSI], [BMP][TFSI], [P222201][TFSI], [P2225][TFSI], [BMPYR][FSI], and [P2225][FSI], respectively. These values show the same trend of our data in the sense that [FSI]-based ILs present a lower thermal stability. The discrepancies between measured values could be

Figure 2. Arrhenius plots of the viscosity of neat ILs. [BMP][TFSI] (red *), [P2225][TFSI] (light green □), [BMPYR][TFSI] (light blue ○), [P2225][FSI] (dark green ■), [BMPYR][FSI] (dark blue ●), [P222201][TFSI] (△). The lines, with the same color nomenclature, represent the best fits of the VTF equation.

The full lines in Figure 1 represent the best fits of the VTF (Vogel−Tammann−Fulcher) equation:21 ⎛ B ⎞ η = ηo exp⎜ ⎟ ⎝ T − T0 ⎠

(1)

where η0, B, and T0 are adjustable parameters whose values can be seen in Table S1. T0 is the temperature at which the viscosity (η) goes to zero. The relationship B/T0 can be related to the fragility of the liquid or, in other words, how the transport properties vary with the temperature change.21,22 The transport properties of strong liquids (high B/T0) suffer less change with the temperature than do weak liquids (low B/T0). Table S1 show the VTF parameters for viscosity. For both FSI-based ILs, B/T0 is higher compared with TFSI derivative, indicating that 3439



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the latter are more fragile, bringing about the “funnel shape” curve observed in Figure 1. It is worth noting, however, that fragility analyses are usually made near the glass transition temperature, and in this work, we are considering the strong/ fragile classification only in the temperature range studied here. A different η(T) profile must be considered at a different range of temperatures.9 The viscosity of RT ILs is, generally, affected by the nature of both the cations and anions. It is noticeable from Table 2 that all ILs containing [FSI] ([BMPYR][FSI] and [P2225][FSI]) are 19 and 31% less viscous than their [TFSI] analogues, which is ascribable to the smaller size of the [FSI] anion (van der Waals force) and the weaker electrostatic interactions caused by the smaller interaction energies for the [FSI] based ILs.23−25 Furthermore, the same result was found when [FSI]-Li+ and [TFSI]-Li+ interactions were compared.24,26 The lower viscosity observed in [P222201][TFSI] compared with [P2225][TFSI] is attributed to a weaker electrostatic cation−anion interaction provoked by the alkyl ether chain. It is also suggested that the methoxy group can decrease the positive charge in the central atom. Additionally, the alkyl ether group increases the chain flexibility, which increases the ionic mobility.10,17 The high viscosity of of [BMP][TFSI] was explained by DFT calculations which clearly showed the higher number of hydrogen bonding present in piperidinium based ILs compared to imidazolium.27 This tendency of piperidium cation to form viscous ILs was also observed using a tetracyanoborate anion and a pyrrolodinium cation as a comparison.9 The viscosity values for these ILs at 298 K agree with those reported in the literature: 53.2,28 70,19 76,16 44,17 88,17 and 18216 mPa·s for [BMPYR][FSI], [P2225][ FSI], [BMPYR] [TFSI], [P222201][TFSI], [P2225][TFSI], and [BMP][FSI] respectively. The divergences between measured values can be ascribing to the issues with the instrument or differences in purity of the ILs. The ionic conductivity as an electrochemical window (EW) of a IL are of critical importance in its selection for an electrochemical application.13 Unlike aqueous electrolytes, ILs display intrinsic ionic conductivity because; by definition they consist of ions. Table 2 show the ionic conductivity at 298.15 K and Figure 3 and Tables 3−8 at different temperatures.

Table 3. Experimental Values of Density, ρ, Viscosity, η, and Electrical Conductivity, σ, of [BMPYR][FSI] at Several Temperatures and p = 0.1 MPaa T (K)

density (g·cm−3)

viscosity (mPa·s)

conductivity (mS·cm−1)

298.15 300.15 306.15 312.15 318.15 324.15 330.15 336.15 341.15 348.15 355.15 361.15 373.15

1.307 1.306 1.301 1.296 1.292 1.287 1.283 1.278 1.274 1.269 1.264 1.259 1.251

53.24 49.74 40.74 33.83 28.42 24.15 20.73 17.94 16.01 13.77 11.95 10.65 8.610

8.66 9.17 10.9 12.7 14.7 16.8 19.1 21.6 26.9 33.1

Standard uncertainties are ur(η) = 0.002, u(ρ) = 0.0003 g·cm−3, uc(σ) = 0.05σ, u(T) = 0.01 K, and u(p) = 0.005 MPa for viscosity, density, conductivity, temperature, and pressure, respectively. a

Table 4. Experimental Values of Density, ρ, Viscosity, η, and Electrical Conductivity, σ, of [BMPYR][TFSI] at Several Temperatures and p = 0.1 MPaa T (K)

density (g·cm−3)

viscosity (mPa·s)


298.15 300.15 306.15 312.15 318.15 324.15 330.15 336.15 341.15 348.15 355.15 361.15 373.15

1.395 1.393 1.388 1.382 1.377 1.372 1.367 1.361 1.357 1.351 1.345 1.340 1.330

77.76 71.11 54.97 43.41 34.92 28.56 23.69 19.91 17.37 14.53 12.31 10.78 8.272

3.93 4.23 5.20 6.38 7.72 9.24 10.8 12.6 16.4 21.1

Standard uncertainties are ur(η) = 0.002, u(ρ) = 0.0003 g·cm−3, uc(σ) = 0.05σ, u(T) = 0.01 K, and u(p) = 0.005 MPa for viscosity, density, conductivity, temperature, and pressure, respectively. a

As well as viscosity, the conductivity is often best described using the empirical Vogel−Tammann−Fulcher (VTF) equation: ⎛ − B′ ⎞ σ = σo exp⎜ ⎟ ⎝ T − T0′ ⎠

(2)

The parameters can be seen in Table S2. The B′/T′0 quotients for all of the systems are consistent with those obtained in the viscosity analyses. The ionic conductivities measured for the [FSI] based ILs were found (at RT) to be approximately 50% greater than their [TFSI] analogues. This inverse trend with respect to viscosity is expected taking into account that, in general, the mobility of charge carriers correlates with their rate of diffusion in the electrolyte. The diffusion coefficient is also inversely proportional to the viscosity of the electrolyte (Stokes−Einstein equation).1 Nevertheless, these facts indicate that the decrease of the ionic conductivity in ILs containing [TFSI] cannot be

Figure 3. Arrhenius plot of ILs’ temperature-dependent conductivity data. [BMP][TFSI] (red *), [P2225][TFSI] (light green □), [BMPYR][TFSI] (light blue ○), [P2225][FSI] (dark green ■), [BMPYR][FSI] (dark blue ●), [P222201][TFSI] (△). The lines, with the same color nomenclature, represent the best fits of the VTF equation. 3440



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Table 5. Experimental Values of Density, ρ, Viscosity, η, and Electrical Conductivity, σ, of [P2225][FSI] at Several Temperatures and p = 0.1 MPaa

Table 7. Experimental Values of Density, ρ, Viscosity, η, and Electrical Conductivity, σ, of [P2225][TFSI] at Several Temperatures at p = 0.1 MPaa,10

T (K)

density (g·cm−3)

viscosity (mPa·s)


T (K)

density (g·cm−3)

viscosity (mPa·s)


298.15 300.15 306.15 312.15 318.15 324.15 330.15 336.15 341.15 348.15 355.15 361.15 373.15

1.219 1.217 1.213 1.208 1.204 1.199 1.195 1.191 1.187 1.182 1.177 1.173 1.165

69.38 64.02 50.72 40.86 33.40 27.65 23.17 19.66 17.25 14.52 12.37 10.86 8.543

4.63 4.95 6.02 7.23 8.61 10.2 11.9 13.7

25 27 33 39 45 51 57 63 68 75 82 88 101

1.304 1.302 1.297 1.292 1.287 1.282 1.277 1.272 1.268 1.262 1.256 1.251 1.241

85.30 77.43 58.68 45.42 35.82 28.79 23.50 19.46 16.79 13.86 11.58 10.02 7.531

2.10 2.23 2.86 3.58 4.35 5.21 6.12 7.11

17.6 22.3

9.17 11.7

a

Standard uncertainties are ur(η) = 0.002, u(ρ) = 0.0003 g·cm−3, uc(σ) = 0.06σ, u(T) = 0.01 K, and u(p) = 0.005 MPa for viscosity, density, conductivity, temperature, and pressure, respectively.


Table 6. Experimental Values of Density, ρ, Viscosity, η, and Electrical Conductivity, σ, of [BMP][TFSI] at Several Temperatures and p = 0.1 MPaa

Table 8. Experimental Values of Density, ρ, Viscosity, η, and Electrical Conductivity, σ, of [P222201][TFSI] at Several Temperatures and p = 0.1 MPaa,10

a

T (K)

density (g·cm−3)

viscosity (mPa·s)


T (K)

density (g·cm−3)

viscosity (mPa·s)


25 27 33 39 45 51 57 63 68 75 82 88 100

1.382 1.380 1.375 1.370 1.365 1.360 1.355 1.349 1.345 1.339 1.333 1.329 1.319

188.9 168.0 120.2 88.67 67.13 52.01 41.12 33.12 27.98 22.51 18.36 15.62 11.62

1.39 1.53 2.02 2.64 3.37 4.24 5.19 6.26

25 27 33 39 45 51 57 63 68 75 82 88 101

1.378 1.376 1.370 1.365 1.360 1.354 1.349 1.344 1.339 1.333 1.327 1.322 1.311

48.14 44.28 34.95 28.10 22.97 19.04 16.01 13.62 11.98 10.15 8.692 7.672 5.990

3.84 4.11 4.98 6.08 7.19 8.48 9.87 11.3

8.76 11.9


14.5 18.4


a

a

completely attributed to the detrimental effect of the viscosity because the viscosity of ILs based on [FSI] is 19−30% lower than their corresponding analogues containing [TFSI]. In other words, the increase of the ionic conductivity of the ILs containing [FSI] cannot be attributed merely to the resistance effect of the ions that is associated with the viscosity. Therefore, to explain this result, one must consider the actual concentration of the charge carriers in both types of ILs. To evaluate the ionicity of the reported electrolytes, that is, state of ionization of each IL, a classical Walden rule diagram was used. This approach allows evaluating the state of ionization of ILs and classifying them according to the position in the Walden plot.29,30 The Walden rule correlates the logarithm of molar conductivity (Λ) and the logarithm of fluidity (1/η) through a temperature-dependent constant (k), as reported in Figure 4a and b. Plotting the molar conductivity (Λ) instead of the absolute conductivity (κ), to an extent, normalizes the effects of molar concentration and density on the conductivity and, thus, gives a

Figure 4. Walden plot of neat ILs: (a) log−log, (b) linear scale. [BMP][TFSI] (red *), [P2225][TFSI] (light green □), [BMPYR][TFSI] (light blue ○), [P2225][FSI] (dark green ■), [BMPYR][FSI] (dark blue ●), [P222201][TFSI] (△).

better indication of the number of mobile charge carriers in an IL.13 3441



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Figure 4a presents the Walden plot, the logarithm of molar ionic conductivity as a function of the logarithm of fluidity (inverse of viscosity). The dotted line indicates the ideal Walden line (dilute aqueous KCl solution). Deviations from this line is related to the IL ionicity. As it can be observed from Figure 4a, the [FSI]-based ILs, namely, [BMPYR][FSI] and [P2225][FSI], are closer to the ideal line; the rest of the substances lie even below, indicating that the former have higher number of charge carriers lessening the aggregate formation between cations and anions. The distance from the line and the position of the set of point on the graph suggests that both ILs, according to the Walden representation, are closer to the “good” IL zone.29,30 Another way to present the Walden plot is shown in Figure 4b, with a larger slope signifying a larger ionicity change with respect to change in viscosity. The slopes of these lines are 85, 73, 66, 60, 47, and 41 S·cm2·mol−1·(mPa·s)−1 for [BMPYR][FSI], [P2225][FSI], [BMPYR][TFSI], [BMP][FSI][P222201][TFSI], and [P2225][TFSI], respectively. This indicates that both ILs have a higher increase in molar ionic conductivity related to an equal decrease in viscosity or increase in fluidity compared to [TFSI] derivates. The conductivities reported for these ILs at 298 K are 6.9,31 3.0,19 3.0,32 3.58,17 1.73,17 and 1.116 for [BMPYR][FSI], [P2225][FSI], [BMPYR][TFSI], [P222201][TFSI], [P2225][TFSI], and [BMP][FSI], respectively. Unlike the viscosity the comparison of the obtained conductivities with the literature seems to have higher differences which are assigned to the more sensitivity of EIS to systematic error. For instance, for [BMPYR][TFSI], the following conductivity values can be found: 4.8,18 6.2,28 6.9,31 and so on. In Figure 4 and Tables 3−8, the densities of all of the ILs are shown as a function of temperature. All ILs are denser than water. Unlike the viscosity and conductivity, it is observed that the densities decrease slightly with an increase in temperature. In addition, all of the ILs studied exhibited a similar degree of volume expansion with temperature. From Figure 4 it is possible to infer that the density of the ILs increases with the increasing molecular weight of the anion. [BMPYR][TFSI] and [P2225][TFSI] have a higher density with their [FSI] counterparts. The molecular weights of [TFSI] and [FSI] are 280.1 and 180.1 g·mol−1, respectively. This trend is most likely due to the nature of the anions studied. Since none of the anions have a particularly long or steric chain, it is possible that they all can occupy favorable, relatively close-approach positions around the cation, resulting in high densities.33 The effect of having different substituents on the cation is also shown in Figure 5. It is noticeable that the pentyl chain [P2225] present a more irregular and less flexible structure compared to [P222201] that provokes less dense ILs. In general, the density decreases as the alkyl-substituted chain length on the cations or anions increases. Adding CH2 groups to the alkyl chain on the cyclic cation decreases the density since CH2 is less dense than the cyclic cation ring.20,33 In addition, the long chain could enable a more irregular shape in the ion hindering and packing. At 298 K, the densities in Table 2 matches literature values, namely, 1.307,28 1.24,19 1.393,16 1.39,17 1.32,17 and 1.37916 for [BMPYR][FSI], [P2225][FSI], [BMPYR][TFSI], [P222201][TFSI], [P2225][TFSI], and [BMP][FSI], respectively. Unsurprisingly, the impact of impurities are less for density than in viscosity.13 3.3. Electrochemical Analysis. Working Potential Window of the Ionic Liquids. Figure 6 illustrates the linear

Figure 5. Density at different temperatures for neat ILs. [BMP][TFSI] (red *), [P2225][TFSI] (light green □), [BMPYR][TFSI] (light blue ○), [P2225][FSI] (dark green ■), [BMPYR][FSI] (dark blue ●), [P222201][TFSI] (△).

Figure 6. Combination of two linear voltammograms recorded by starting from open circuit potential and cycling to either more positive or negative potentials. Scan rate of 50 mV·s−1 and ±150 μA·cm−2 of current cutoff. Working electrode: glassy carbon; counter electrode: Pt; pseudoreference electrode: Ag; [BMP][TFSI] (red line), [P2225][TFSI] (light green line), [BMPYR][TFSI] (light blue line), [P2225][FSI] (dark green line), [BMPYR][FSI] (dark blue line), [P222201][TFSI] (black line).

sweep voltammograms measured in the neat ILs on GC electrode. The overall values of the EW are listed in Table 2. By definition, the EW of the IL is the voltage range in which the substance is neither oxidized or reduced at an electrode.1,34 Therefore, for a specific IL, the oxidation potential is determined by the voltage of the anion oxidation (HOMO energy), and the reduction potential is determined by the voltage of the cation reduction (LUMO energy).34 Nevertheless, Ong et al.35 has demonstrated that both cations and anions can play a role in the overall electrochemical stability of the ILs. The values of the potential window are at least 4.0 V for all ILs investigated in this work. ILs derivative from [TFSI]−[P2225][TFSI] and [BMPYR][TFSI] registered an electrochemical window of 4.9 and 5.7 V, respectively. These values are slightly higher than those presented by the [FSI]based ILs, whose values were 4.3 and 5.1 V for [P2225][FSI] and [BMPYR][FSI], respectively. The side chain that contains ether in [P222201][TFSI] causes a decrease of the electrochemical window, as previously reported.36 It was also found that the pyrrolidynium cation seems to increase the electrochemical 3442



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(3) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-Liquid Materials for the Electrochemical Challenges of the Future. Nat. Mater. 2009, 8, 621−629. (4) Tarascon, J. M. Key Challenges in Future Li-Battery Research. Philos. Trans. R. Soc., A 2010, 368, 3227−3241. (5) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Batteries. J. Power Sources 2011, 196, 6688−6694. (6) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587−603. (7) Damen, L. Advanced Lithium and Lithium-Ion Rechargeable Batteries for Automotive Applications; Università di Bologna, 2011. (8) Lu, L.; Han, X.; Li, J.; Hua, J.; Ouyang, M. A Review on the Key Issues for Lithium-Ion Battery Management in Electric Vehicles. J. Power Sources 2013, 226, 272−288. (9) Sanchez-Ramirez, N.; Martins, V. L.; Ando, R.; Camilo, F.; Urahata, S.; Ribeiro, M. C. C.; Torresi, R. M. Physicochemical Properties of Three Ionic Liquids Containing Tetracyanoborate Anion and Their Lithium Salt Mixtures. J. Phys. Chem. B 2014, 118, 8772− 8781. (10) Martins, V. L.; Sanchez-Ramirez, N.; Ribeiro, M. C. C.; Torresi, R. M. Two Phosphonium Ionic Liquids with High Li+ Transport Number. Phys. Chem. Chem. Phys. 2015, 17, 23041−23051. (11) Rennie, A. J. R.; Sanchez-Ramirez, N.; Torresi, R. M.; Hall, P. J. Ether-Bond-Containing Ionic Liquids as Supercapacitor Electrolytes. J. Phys. Chem. Lett. 2013, 4, 2970−2974. (12) Bazito, F. F. C.; Kawano, Y.; Torresi, R. M. Synthesis and Characterization of Two Ionic Liquids with Emphasis on Their Chemical Stability towards Metallic Lithium. Electrochim. Acta 2007, 52, 6427−6437. (13) Wasserscheid, P.; Welton, T. Ionic Liquid in Synthesis; WileyVCH: Weinheim, 2008. (14) Ngo, H. L.; LeCompte, K.; Hargens, L.; McEwen, A. B. Thermal Properties of Imidazolium Ionic Liquids. Thermochim. Acta 2000, 357−358, 97−102. (15) Nádherná, M.; Reiter, J.; Moškon, J.; Dominko, R. Lithium Bis(fluorosulfonyl)imide-PYR14TFSI Ionic Liquid Electrolyte Compatible with Graphite. J. Power Sources 2011, 196, 7700−7706. (16) Zhou, Z. B.; Matsumoto, H.; Tatsumi, K. Cyclic Quaternary Ammonium Ionic Liquids with Perfluoroalkyltrifluoroborates: Synthesis, Characterization, and Properties. Chem. - Eur. J. 2006, 12, 2196−2212. (17) Tsunashima, K.; Sugiya, M. Physical and Electrochemical Properties of Low-Viscosity Phosphonium Ionic Liquids as Potential Electrolytes. Electrochem. Commun. 2007, 9, 2353−2358. (18) Zhou, Q.; Henderson, W. A.; Appetecchi, G. B.; Montanino, M.; Passerini, S. Physical and Electrochemical Properties of N-Alkyl-NMethylpyrrolidinium Bis (Fluorosulfonyl) Imide Ionic Liquids: PY13FSI and PY14FSI. J. Phys. Chem. B 2008, 112, 13577−13580. (19) Tsunashima, K.; Kawabata, A.; Matsumiya, M.; Kodama, S.; Enomoto, R.; Sugiya, M.; Kunugi, Y. Low Viscous and Highly Conductive Phosphonium Ionic Liquids Based on Bis(fluorosulfonyl)amide Anion as Potential Electrolytes. Electrochem. Commun. 2011, 13, 178−181. (20) O’Mahony, A. M.; Silvester, D. S.; Aldous, L.; Hardacre, C.; Compton, R. G. Effect of Water on the Electrochemical Window and Potential Limits of Room-Temperature Ionic Liquids. J. Chem. Eng. Data 2008, 53, 2884−2891. (21) Schreiner, C.; Zugmann, S.; Hartl, R.; Gores, H. J. Temperature Dependence of Viscosity and Specific Conductivity of FluoroborateBased Ionic Liquids in Light of the Fractional Walden Rule and Angell’s Fragility Concept. J. Chem. Eng. Data 2010, 55, 4372−4377. (22) Xu, W.; Cooper, E. I.; Angell, C. A. Ionic Liquids: Ion Mobilities, Glass Temperatures, and Fragilities. J. Phys. Chem. B 2003, 107, 6170−6178. (23) Borodin, O.; Gorecki, W.; Smith, G. D.; Armand, M. Molecular Dynamics Simulation and Pulsed-Field Gradient NMR Studies of Bis(fluorosulfonyl)imide (FSI) and Bis[(trifluoromethyl)sulfonyl]imide (TFSI)-Based Ionic Liquids. J. Phys. Chem. B 2010, 114, 6786−6798.

stability compared with the other cations. However, more research needs to be carried out to know what happens in each application, for instance, in the lithium ion battery, during lithium intercalation taking into account that the EW was measured using glassy carbon as the working electrode. We have avoided comparing the EW with those reported in the literature. Since the electrode material, sweep rate, and the cutoff current are arbitrary, the comparison is almost impossible. The reader is referred to corresponding bibliography, for BMPYRTFSI37 BMPTFSI,38 P2225FSI,19 P2225TFSI,17 P222201TFSI,17 and BMPYRFSI.18

4. CONCLUSION The properties for two ILs derived from [FSI] were measured; four [TFSI]-based ILs were used by comparison. It was found that [FSI] ILs have a 50% higher conductivity and at least 19% lower viscosity values. The Walden plot showed higher ionicity for [FSI]-based ILs, indicating a larger change in the mobility of the ions for a change in viscosity. In spite of [FSI] ILs presented an electrochemical window higher than 4 V; those values were slightly lower in comparison with the [TFSI] counterpart. The main disadvantage of [FSI] based IL compared to [TFSI] derivatives is their inferior thermal stability. Further studies are needed to evaluate this parameter especially when lithium salt is added, as can be the case for lithium-ion battery application.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00458. NMR of [BMPYR][FSI], [P2225][FSI], and [BMPYR][TFSI] (Figures S1−S3). Thermogravimetric analyses (Figure S4). VTF parameters for viscosity and conductivity for the ILs (Tables S1−S2). Graphical comparison of thermophysical properties with the literature (Figures S5−S10) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +54 1130912350. E-mail: [email protected]. ORCID

Roberto M. Torresi: 0000-0003-4414-5431 Funding

The authors acknowledge FAPESP (15/26308-7) for funding. N.S.R. thanks FAPESP (2014/01987-6 and 2015/11164-0) for scholarship support. This work was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) by a Discovery Grant to D.B. The research infrastructure of NanoQAM was used during this work. Notes

The authors declare no competing financial interest. Author e-mail: Daniel Belanger, [email protected]; Nedher Sanchez-Ramirez, [email protected]; Birhanu Desalegn Assresahegn, [email protected].

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REFERENCES

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A Comparison among Viscosity, Density, Conductivity, and

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