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. 2021 Oct 18;6(42):27874–27887. doi: 10.1021/acsomega.1c03517

Thermodynamic Properties of Ternary Systems Containing (LiCl and LiBr) + Propylene Carbonate + Ionic Liquid (1-Alkyl-3-methylimidazolium Thiocyanate)

Behrang Golmohammadi †, Hemayat Shekaari †,*, Mohammed Taghi Zafarani-Moattar †
PMCID: PMC8552347  PMID: 34722987

Abstract

graphic file with name ao1c03517_0011.jpg

The development of the Li-ion battery Industry in a green way is crucial for human beings’ future. Ionic liquids (ILs) are green cosolvents that could be applied in Li-ion battery electrolytes. A thermodynamic study has been carried out for a Li-ion electrolyte (propylene carbonate (PC) + LiCl and LiBr) in the presence of IL 1-alkyl-3-methylimidazolium thiocyanate [RMIM][SCN] (R = butyl, hexyl, and octyl). The studied thermodynamic properties were density, speed of sound, apparent molar volume, and compressibility. The effect of ILs in propylene carbonate (PC) has been investigated under atmospheric pressure at T = (288.15–318.15) K. Also, a microscopic approach using scaled particle theory has been implemented. The solvation effect of lithium halides, LiX (X = Cl–, Br–), on the volumetric and compressibility properties of the ILs has been studied at 298.15 K. The results show that [OMIM][SCN] has the strongest interactions with PC in the studied ILs and these interactions are more weakened with the addition of LiBr than LiCl. According to the partial molar compressibility results, the systems containing [OMIM][SCN] could be used under pressure more beneficially than other systems from the thermodynamic aspect of view.

Introduction

Rechargeable world is the aim of recent research studies to reduce excessive energy resource consumption. Lithium capacitors are one of the most important rechargeable energy-storage devices. The lithium capacitors’ industry development led to an increased demand for lithium sources than ever.1 The investigations about extraction, separation, and purification around the systems containing lithium species have been increased.2−5 Also, there is some promising investigation that suggests optimization of the capacitors for a long life span instead of expensive separation methods that may cause environmental damages. Achieving environmentally friendly lithium power sources is the subject of the literature.6−10

Ionic liquids as environmentally friendly and biodegradable chemicals have advantages for industrial uses such as high thermal stability, etc.11,12 Imidazole-based ionic liquids with fluorinated anions have been used effectively to enhance the properties of the lithium batteries.13 However, it found that using fluorinated anion-based ILs may cause environmental damages in long term.14−16 To overcome this problem, cyano-based anions have been suggested recently. One of the best options for replacing fluorinated anions is thiocyanate.17,18 The imidazolium-based ILs with a thiocyanate anion are used effectively in various applications. Based on the literature, the use of these ILs, as well as fluorinated ILs, is promising.19,20

An electrolyte is an important part of a battery that affects ion mobility and consequently generated energy. Also, the electrolyte is the part that would be eliminated after cycles of usage. Accordingly, the electrolyte is related to the life span of the battery rather than other parts. One of the main and convenient species of a lithium capacitor electrolyte is propylene carbonate (PC) that has the best benefits for the developed technologies of Li capacitors.21−23 However, there are limited thermodynamic investigations around these systems.

In the present work, in the continuation of our previous experiences, a modeled lithium capacitor electrolyte has been designed to be investigated with a thermodynamic approach.24,25 The ILs with a thiocyanate anion, [RMIM][SCN] (R = butyl, hexyl, and octyl), have been studied. The density and speed of sound of binary and ternary systems containing (PC + ILs) and (PC + ILs + LiX (X = Cl, Br)), respectively, have been measured. Based on the measured properties, the apparent molar volume and apparent molar isentropic compressibility of the ILs have been evaluated and the standard partial molar volume and partial molar isentropic compressibility have been calculated. Scaled particle theory was used to obtain the different contributions of the standard partial molar volume. The results are used to interpret the interactions between the species.

Results and Discussion

Volumetric Properties

The density data of propylene carbonate are compared in Figure 1 with the literature and good agreement has been achieved.27−34 The error bars have been used in a 0.5% range for our data to compare with literature data, which show less difference than this value. However, in the previous work, the density and speed of sound data for propylene carbonate were different due to different sources of the supplier.25

Figure 1.

Figure 1

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Experimental density of propylene carbonate (PC) versus temperature compared with literature data. Blue multiplication symbol: ref (34), red open square: ref (33), gray plus symbol: ref (32), yellow solid diamond: ref (31), blue solid square: ref (30), green solid triangle: ref (29), blue upward triangle: ref (28), red solid square: ref (27), and blue solid circle: this work.

The density of solutions containing ([RMIM][SCN] + PC) is measured under atmospheric pressure (P = 0.086 MPa) at temperature ranges T = (288.15–318.15) K. These data are given in Table 1, which shows the density data decrease with the addition of IL content. The apparent molar volumes, Vφ, of the ILs in the PC solutions were evaluated using the following equation24

graphic file with name ao1c03517_m001.jpg 1

Table 1. Density (d), Speed of Sound (u), Solvation Number (Sn), Apparent Molar Volume (Vφ), and Apparent Molar Isentropic Compressibility (κφ) Data of ([RMIM][SCN] + PC) at T = (288.15 to 318.15) K under Pressure (P = 0.086 MPa)a.

mIL (mol kg–1) d (kg m–3) u (m s–1) Sn 106 Vφ (m3 mol–1) 1014 κφ (m3 mol–1 Pa–1)
[BMIM][SCN]
T = 288.15 K
0.0000 1209.663 1478.33      
0.0129 1209.398 1479.24 76.49 177.13 4.25
0.0165 1209.324 1479.49 76.27 177.16 4.26
0.0192 1209.268 1479.68 76.24 177.19 4.26
0.0218 1209.214 1479.86 76.06 177.22 4.26
0.0235 1209.179 1479.97 75.59 177.23 4.28
0.0272 1209.103 1480.22 75.29 177.26 4.29
0.0289 1209.066 1480.34 75.24 177.30 4.29
0.0331 1208.978 1480.62 74.80 177.36 4.30
T = 298.15 K
0.0000 1199.037 1442.90      
0.0129 1198.772 1443.99 97.46 178.82 3.83
0.0165 1198.697 1444.29 97.19 178.90 3.84
0.0192 1198.641 1444.51 96.61 178.92 3.86
0.0218 1198.587 1444.72 96.12 178.95 3.87
0.0235 1198.551 1444.86 96.03 178.99 3.88
0.0272 1198.475 1445.16 95.74 179.01 3.89
0.0289 1198.437 1445.30 95.50 179.07 3.90
0.0331 1198.349 1445.64 95.20 179.12 3.91
T = 308.15 K
0.0000 1188.430 1407.38      
0.0129 1188.172 1408.61 115.65 180.16 3.42
0.0165 1188.100 1408.95 115.52 180.20 3.42
0.0192 1188.045 1409.20 114.93 180.24 3.45
0.0218 1187.992 1409.44 114.50 180.28 3.46
0.0235 1187.957 1409.59 113.91 180.32 3.48
0.0272 1187.883 1409.93 113.65 180.34 3.49
0.0289 1187.847 1410.08 113.03 180.38 3.51
0.0331 1187.761 1410.48 113.41 180.44 3.50
T = 318.15 K
0.0000 1177.840 1372.65      
0.0129 1177.595 1373.99 131.76 181.19 3.00
0.0165 1177.527 1374.36 131.60 181.20 3.01
0.0192 1177.475 1374.64 131.52 181.24 3.01
0.0218 1177.425 1374.90 130.90 181.27 3.03
0.0235 1177.392 1375.07 130.62 181.30 3.04
0.0272 1177.322 1375.44 130.20 181.32 3.06
0.0289 1177.288 1375.61 129.80 181.35 3.07
0.0331 1177.207 1376.02 129.02 181.40 3.10
[HMIM][SCN]
T = 288.15 K
0.0000 1209.663 1478.33      
0.0119 1209.229 1479.24 71.55 211.23 5.70
0.0147 1209.128 1479.44 70.63 211.30 5.73
0.0174 1209.027 1479.64 69.91 211.32 5.75
0.0205 1208.915 1479.87 69.86 211.35 5.75
0.0215 1208.878 1479.94 69.56 211.38 5.76
0.0241 1208.781 1480.13 69.13 211.43 5.78
0.0265 1208.691 1480.30 68.56 211.49 5.80
0.0299 1208.567 1480.54 68.20 211.57 5.81
T = 298.15 K
0.0000 1199.037 1442.90      
0.0119 1198.608 1443.98 93.42 213.03 5.34
0.0147 1198.508 1444.22 92.55 213.11 5.37
0.0174 1198.407 1444.46 91.78 213.18 5.40
0.0205 1198.295 1444.74 92.07 213.24 5.39
0.0215 1198.256 1444.83 92.03 213.35 5.39
0.0241 1198.157 1445.04 90.42 213.47 5.45
0.0265 1198.066 1445.25 90.11 213.56 5.46
0.0299 1197.941 1445.54 89.80 213.67 5.47
T = 308.15 K
0.0000 1188.430 1407.38      
0.0119 1187.999 1408.62 114.74 215.27 4.95
0.0147 1187.898 1408.90 114.12 215.38 4.97
0.0174 1187.796 1409.18 113.48 215.47 4.99
0.0205 1187.683 1409.49 112.93 215.54 5.01
0.0215 1187.644 1409.59 112.70 215.65 5.03
0.0241 1187.545 1409.85 111.97 215.75 5.05
0.0265 1187.453 1410.09 111.47 215.86 5.07
0.0299 1187.327 1410.43 111.37 215.97 5.08
T = 318.15 K
0.0000 1177.840 1372.65      
0.0119 1177.409 1374.00 131.25 217.44 4.66
0.0147 1177.308 1374.30 130.09 217.55 4.70
0.0174 1177.206 1374.61 129.88 217.64 4.71
0.0205 1177.092 1374.95 129.40 217.75 4.73
0.0215 1177.055 1375.06 129.29 217.79 4.74
0.0241 1176.956 1375.35 128.86 217.90 4.75
0.0265 1176.865 1375.61 128.21 217.99 4.78
0.0299 1176.740 1375.96 127.11 218.08 4.82
[OMIM][SCN]
T = 288.15 K
0.0000 1209.683 1478.39      
0.0088 1209.235 1479.26 89.61 244.32 6.37
0.0115 1209.094 1479.50 86.54 244.69 6.48
0.0133 1208.995 1479.66 84.42 244.92 6.56
0.0161 1208.844 1479.88 80.18 245.20 6.70
0.0183 1208.725 1480.04 76.88 245.42 6.81
0.0206 1208.600 1480.21 74.26 245.58 6.90
0.0229 1208.475 1480.35 70.67 245.84 7.03
0.0252 1208.342 1480.50 67.57 246.05 7.13
T = 298.15 K
0.0000 1199.037 1442.91      
0.0088 1198.601 1443.94 118.19 245.86 5.81
0.0115 1198.465 1444.23 115.34 246.14 5.91
0.0133 1198.370 1444.43 113.87 246.31 5.97
0.0161 1198.223 1444.70 109.26 246.62 6.13
0.0183 1198.108 1444.90 105.85 246.81 6.26
0.0206 1197.985 1445.11 102.91 247.03 6.36
0.0229 1197.864 1445.29 99.18 247.27 6.50
0.0252 1197.734 1445.49 96.31 247.50 6.61
T = 308.15 K
0.0000 1188.430 1407.45      
0.0088 1188.007 1408.60 141.96 247.31 5.31
0.0115 1187.875 1408.93 139.49 247.60 5.41
0.0133 1187.782 1409.15 137.33 247.81 5.50
0.0161 1187.641 1409.48 134.60 248.03 5.60
0.0183 1187.526 1409.71 130.75 248.36 5.75
0.0206 1187.406 1409.95 127.45 248.59 5.88
0.0229 1187.290 1410.18 124.76 248.77 5.98
0.0252 1187.161 1410.41 121.37 249.08 6.12
T = 318.15 K
0.0000 1177.840 1372.72      
0.0088 1177.427 1373.98 164.92 249.03 4.76
0.0115 1177.297 1374.34 161.89 249.38 4.89
0.0133 1177.208 1374.59 160.61 249.49 4.94
0.0161 1177.069 1374.95 157.21 249.77 5.09
0.0183 1176.961 1375.23 155.23 249.92 5.17
0.0206 1176.845 1375.49 151.09 250.13 5.33
0.0229 1176.732 1375.75 148.37 250.32 5.45
0.0252 1176.606 1376.02 145.36 250.63 5.57
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a

Standard uncertainties for molality, temperature, and pressure were u (m) = 0.002 mol kg–1, u (T) = 0.02 K, and u (P) = 10 hPa, respectively, with a 0.68 level of confidence, and the combined standard uncertainties for density and speed of sound were uc (d) = 0.07 kg m–3 and uc (u) = 1.3 m s–1 with a 0.68 level of confidence. The standard uncertainties for the apparent molar volume and apparent molar isentropic compressibility were uc (Vφ) = 5.10–5 m3 mol–1 (level of confidence of 0.68) and uc (κφ) = 3.10–3 m3 mol-1 Pa–1 (level of confidence of 0.68), respectively.

where M is the molar mass of the IL, m is the molality of the IL, and d0 and d are the densities of the solvent (PC) and the solution, respectively. The Vφ values for the studied ILs in binary solutions are given in Table 1, and Figure 2 shows the plot of the Vφ values versus molality of ILs with different cation sizes where the Vφ values increased from butyl to octyl.

Figure 2.

Figure 2

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Apparent molar volumes Vφ of [RMIM][SCN] in PC solution versus its molality m at T = 298.15 K. (●) [BMIM][SCN], (■) [HMIM][SCN], and (⧫) [OMIM][SCN], and solid lines represent the Redlich–Mayer model.

Also, the increasing Vφ values with temperature and molality are shown in Figure 3. The standard partial molar volumes, Vφ0, have been calculated with the Redlich–Mayer equation24

graphic file with name ao1c03517_m002.jpg 2

Figure 3.

Figure 3

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Apparent molar volumes Vφ of [BMIM][SCN] in PC versus its molality m at different temperatures. (●) T = 288.15 K, (■) T = 298.15 K, (⧫) T = 308.15 K, and (▲) T = 318.15 K, and solid lines represent the corresponding Redlich–Mayer model.

where Vφ0, Sv, and Bv are given in Table 2, for the binary solutions. The Vφ values are criteria of solute–solvent interaction, while the Sv values are criteria of solute–solute interactions, and Bv is an adjustable parameter. The Vφ0 values of the studied ILs are increased by the alkyl chain length and increasing temperature in the binary solutions.

Table 2. Standard Partial Molar Volume Vφ0, Empirical Parameters of eq 2, SV and BV, the Standard Deviation of the Apparent Molar Volume σ(Vφ ), Standard Apparent Molar Expansibility Eφ, Isobaric Thermal Expansion (α), and the Constant of eq12 (∂2Vφ0/∂T2)p for the Solutions of [RMIM][SCN] in PC at Different Temperatures under Pressure (P = 0.086 MPa)a.

T (K) 106 Vφ0 (m3 mol–1) 106 SV (m3 mol–1 kg–1/2) 106 BV (m3 mol–1 kg–1) 106 σ(Vφ) 106 Eφ0 (m3 mol–1 K–1) 104 α (K–1) 106 (∂2 Vφ0/∂T2) (m3 mol–1 K–1)
[BMIM][SCN]
288.15 176.70 ± 0.28 3.81 ± 0.04 –1.92 ± 0.12 0.013 0.1721 9.738 –0.0025
298.15 178.31 ± 0.32 4.67 ± 0.04 –1.71 ± 0.14. 0.015 0.1472 8.253  
308.15 179.64 ± 0.25 4.63 ± 0.03 –1.75 ± 0.11 0.012 0.1222 6.805  
318.15 180.75 ± 0.26 3.87 ± 0.04 –1.99 ± 0.12 0.012 0.0973 5.384  
[HMIM][SCN]
288.15 211.57 ± 0.32 –8.12 ± 0.05 46.53 ± 0.16 0.014 0.2313 10.9318 –0.0048
298.15 213.52 ± 0.60 –13.97 ± 0.09 86.42 ± 0.31 0.026 0.1837 8.6029  
308.15 215.31 ± 0.48 –7.54 ± 0.07 66.22 ± 0.24 0.021 0.1361 6.3207  
318.15 216.31 ± 0.17 10.3 ± 0.02 –0.55 ± 0.09 0.014 0.0885 4.0911  
[OMIM][SCN]
288.15 241.95 ± 0.14 25.13 ± 0.02 3.35 ± 0.09 0.022 0.2918 12.06 –0.0059
298.15 244.84 ± 0.24 2.44 ± 0.04 90.02 ± 0.15 0.009 0.2327 9.503  
308.15 246.47 ± 0.22 –1.58 ± 0.04 112.58 ± 0.14 0.030 0.1735 7.041  
318.15 248.18 ± 0.26 1.33 ± 0.04 86.74 ± 0.16 0.040 0.1144 4.610  
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a

Standard uncertainties for temperature and pressure were u (T) = 0.02K and u (P) = 10 hPa, respectively, with a 0.68 level of confidence.

Scaled particle theory (SPT), as a microscopic viewpoint, was used to determine different contributions of the partial molar volume, namely, the cavity volume (Vcav), the interactional volume (Vint), and the state transitional volume (κTRT) changes due to components’ isothermal transition from a vapor to liquid phase, and κT is isothermal compressibility of the solvent. The corresponding equation is24,26

graphic file with name ao1c03517_m003.jpg 3

where R is the universal gas constant and T is the absolute temperature. The cavity volume was calculated using equations

graphic file with name ao1c03517_m004.jpg 4
graphic file with name ao1c03517_m005.jpg 5
graphic file with name ao1c03517_m006.jpg 6

In eqs 4–6, NA is the Avogadro constant, V is the molar volume of the solvent, and σ1 and σ2 are the diameters of the solvent (PC) and solute (IL), respectively, which are obtained by a procedure defined by Abraham35 using the Bondi36 method for atomic Van der Waals volumes. The symbol z is the ratio of the solute to solvent diameters. The κT values for PC were calculated by the following equation

graphic file with name ao1c03517_m007.jpg 7

where κs is isentropic compressibility, CP is the isobaric heat capacity of the solvent (PC) that is taken from the literature, α is thermal expansion, and V is the molar volume of the solvent. The calculated κT values are in good agreement with the literature.37,38 The calculated values of Vcav and Vint are given in Table 3. As can be seen, Vcav increases and Vint decreases with an increase in the alkyl chain length. A more negative value of Vint demonstrates stronger solute–solvent interactions between PC and [OMIM][SCN]. However, increasing temperature led to decreased interactions.

Table 3. Isothermal Compressibility (κT), Isothermal Volume Transition Contribution (κTRT), and Interactional and Cavity Volumes of the Standard Partial Molar Volume of [RMIM][SCN] in Propylene Carbonate with SPT at T = (288.15–318.15) K under Pressure (P = 0.086 MPa)a.

T (K) 1010 κT (Pa–1) 106 κT RT (m3 mol–1) 106 Vcav 106 Vint
[BMIM][SCN]
288.15 5.22 1.25 1563.23 –1391.64
298.15 5.08 1.26 1465.88 –1292.68
308.15 5.00 1.28 1376.51 –1201.98
318.15 4.99 1.32 1294.33 –1118.69
[HMIM][SCN]
288.15 5.60 1.34 1760.58 –1567.32
298.15 5.17 1.28 1651.17 –1456.08
308.15 4.90 1.26 1550.73 –1353.96
318.15 4.79 1.24 1458.38 –1260.68
[OMIM][SCN]
288.15 5.99 1.44 2015.52 –1778.93
298.15 5.43 1.35 1890.17 –1650.69
308.15 5.05 1.30 1775.11 –1534.00
318.15 4.86 1.28 1669.32 –1426.50
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a

Standard uncertainties for temperature and pressure were u (T) = 0.02K and u (P) = 10 hPa, respectively, with a 0.68 level of confidence.

The Vφ0 values of temperature dependency are fitted with a second-degree polynomial equation24,26

graphic file with name ao1c03517_m008.jpg 8

where A, B, and C are the empirical parameters of the equation. The standard apparent molar expansibility at constant pressure Eφ0 was calculated using the following equation24

graphic file with name ao1c03517_m009.jpg 9

The Eφ0 values are given in Table 2. These values are positive and increased with increasing IL cation size. Also, this variable decreased with increasing temperature. The isobaric thermal expansion was evaluated as a function of Vφ and Eφ0 by the following equation24

graphic file with name ao1c03517_m010.jpg 10

The calculated values of α for ([RMIM][SCN] + PC) are given in Table 2. The α value is increased with increasing cation size and decreases with increasing temperature. The value of α is a criterion for the response of the volume of a system to increasing temperature. The large value of this factor gets more sensitive in the system volume with temperature change. The observed trend for α and Eφ0 is similar to the cavity volume. The pressure would also break the solvent structure and the same reason suggests that the heat capacity decreases. Hepler et al.’s39 determined relation for structure making or breaking behavior of a solute in a solution is given by the following equations

graphic file with name ao1c03517_m011.jpg 11
graphic file with name ao1c03517_m012.jpg 12

where (∂2Vφ0/∂T2) is the constant for the ILs, as given in Table 2. As can be seen, this parameter decreases with increasing cation size. Negative values of this parameter mean the ILs have structure-breaking behavior in PC, and this behavior intensity order is octyl > hexyl > butyl.The measured density data of ternary solutions containing solute [RMIM][SCN], in the solvent consisting of (PC + LiCl or LiBr), and the corresponding Vφ values that have been calculated with eq 1 are given in Table 4.

Table 4. Density (d), Speed of Sound (u), Solvation Number (Sn), Apparent Molar Volume (Vφ), and Apparent Molar Compressibility (κφ) Data of [RMIM][SCN] in (PC + LiX (X = Cl– and Br–)) Solutions at T = 298.15 K under Pressure (P = 0.086 MPa)a.

mIL (mol kg–1) d (kg m–3) u (m s–1) Sn 106 Vφ (m3 mol–1) 1014 κφ (m3 mol–1 Pa–1)
[BMIm]SCN + PC + LiBr
mLiBr = 0.0035 mol kg–1
0.0000 1199.180 1442.83      
0.0141 1198.884 1443.78 74.26 179.16 4.64
0.0177 1198.808 1444.04 75.53 179.19 4.59
0.0206 1198.748 1444.25 76.50 179.20 4.56
0.0250 1198.656 1444.59 78.46 179.20 4.49
0.0281 1198.591 1444.83 79.53 179.22 4.45
0.0317 1198.515 1445.11 80.39 179.22 4.42
0.0340 1198.467 1445.29 81.02 179.23 4.40
0.0383 1198.375 1445.62 81.54 179.26 4.38
mLiBr = 0.0062 mol kg–1
0.0000 1199.383 1442.70      
0.0142 1199.040 1443.60 66.34 181.34 5.00
0.0167 1198.979 1443.77 67.27 181.38 4.97
0.0197 1198.905 1443.98 68.28 181.41 4.93
0.0235 1198.813 1444.24 69.18 181.46 4.90
0.0290 1198.679 1444.63 70.50 181.49 4.85
0.0326 1198.590 1444.90 71.66 181.53 4.81
0.0334 1198.571 1444.98 72.82 181.54 4.77
0.0385 1198.444 1445.35 73.40 181.59 4.75
mLiBr = 0.0097 mol kg–1
0.0000 1199.688 1442.63      
0.0131 1199.333 1443.40 57.91 183.40 5.37
0.0172 1199.220 1443.66 59.10 183.42 5.33
0.0189 1199.174 1443.77 59.83 183.46 5.30
0.0242 1199.028 1444.10 60.14 183.48 5.29
0.0282 1198.920 1444.36 61.04 183.49 5.26
0.0325 1198.802 1444.65 62.07 183.52 5.22
0.0358 1198.712 1444.89 63.43 183.55 5.18
0.0392 1198.618 1445.15 64.93 183.58 5.12
[BMIm]SCN + PC + LiCl
mLiCl = 0.0031 mol kg–1
0.0000 1199.109 1442.84      
0.0138 1198.829 1443.85 83.11 178.70 4.33
0.0171 1198.761 1444.10 83.46 178.71 4.31
0.0211 1198.681 1444.40 84.12 178.72 4.29
0.0241 1198.619 1444.63 84.39 178.74 4.28
0.0295 1198.510 1445.04 84.97 178.77 4.26
0.0332 1198.434 1445.32 85.00 178.78 4.25
0.0359 1198.377 1445.53 85.14 178.82 4.25
0.0395 1198.302 1445.80 85.13 178.86 4.25
mLiCl = 0.0059 mol kg–1
0.0000 1199.241 1442.73      
0.0130 1198.960 1443.63 77.12 179.63 4.58
0.0174 1198.864 1443.94 77.31 179.65 4.57
0.0204 1198.798 1444.16 77.88 179.67 4.55
0.0240 1198.721 1444.41 77.96 179.68 4.55
0.0290 1198.611 1444.77 78.22 179.71 4.54
0.0331 1198.523 1445.07 78.75 179.71 4.52
0.0356 1198.467 1445.26 79.09 179.73 4.50
0.0400 1198.372 1445.58 79.41 179.75 4.49
mLiCl = 0.0100 mol kg–1
0.0000 1199.447 1442.65      
0.0131 1199.142 1443.47 66.79 180.70 4.99
0.0174 1199.042 1443.74 66.94 180.74 4.98
0.0198 1198.987 1443.89 67.04 180.74 4.98
0.0234 1198.901 1444.13 67.56 180.77 4.96
0.0280 1198.796 1444.42 67.78 180.78 4.95
0.0318 1198.706 1444.66 67.68 180.82 4.95
0.0355 1198.620 1444.91 68.29 180.83 4.93
0.0388 1198.543 1445.12 68.27 180.84 4.93
[HMIm]SCN + PC + LiBr
mLiBr = 0.0035 mol kg–1
0.0000 1199.208 1442.87      
0.0111 1198.787 1443.58 56.07 214.47 6.67
0.0143 1198.662 1443.80 56.94 214.49 6.64
0.0197 1198.458 1444.15 57.06 214.51 6.64
0.0236 1198.311 1444.41 57.58 214.55 6.62
0.0278 1198.149 1444.70 58.11 214.56 6.60
0.0358 1197.847 1445.25 59.09 214.59 6.56
0.0390 1197.726 1445.46 59.05 214.63 6.56
0.0447 1197.508 1445.85 59.31 214.67 6.55
mLiBr = 0.0058 mol kg–1
0.0000 1199.497 1442.76      
0.0103 1199.076 1443.38 48.23 216.27 7.02
0.0159 1198.848 1443.72 48.56 216.31 7.00
0.0203 1198.672 1443.99 49.16 216.32 6.98
0.0251 1198.477 1444.29 49.62 216.34 6.96
0.0282 1198.348 1444.49 49.89 216.35 6.95
0.0336 1198.130 1444.82 49.90 216.36 6.95
0.0387 1197.926 1445.13 49.97 216.40 6.95
0.0426 1197.767 1445.38 50.28 216.43 6.94
mLiBr = 0.0095 mol kg–1
0.0000 1199.743 1442.58      
0.0104 1199.284 1443.17 41.01 218.55 7.35
0.0149 1199.085 1443.43 41.35 218.57 7.34
0.0188 1198.916 1443.65 41.47 218.61 7.34
0.0243 1198.673 1443.97 41.72 218.62 7.33
0.0279 1198.515 1444.18 41.94 218.64 7.32
0.0331 1198.287 1444.48 42.02 218.67 7.32
0.0386 1198.046 1444.80 42.17 218.69 7.31
0.0417 1197.911 1444.99 42.58 218.71 7.30
[HMIm]SCN + PC + LiCl
mLiCl = 0.0033 mol kg–1
0.0000 1199.225 1442.85      
0.0114 1198.793 1443.63 62.35 214.41 6.47
0.0142 1198.686 1443.83 62.96 214.42 6.44
0.0191 1198.501 1444.17 63.16 214.43 6.44
0.0248 1198.285 1444.57 63.42 214.45 6.42
0.0299 1198.093 1444.92 63.32 214.48 6.43
0.0328 1197.980 1445.13 63.45 214.50 6.42
0.0381 1197.781 1445.51 63.93 214.52 6.40
0.0401 1197.705 1445.66 64.31 214.55 6.39
mLiCl = 0.0061 mol kg–1
0.0000 1199.368 1442.75      
0.0103 1198.958 1443.40 53.41 215.54 6.82
0.0146 1198.789 1443.68 54.52 215.57 6.78
0.0185 1198.636 1443.93 54.81 215.59 6.77
0.0229 1198.460 1444.22 55.14 215.61 6.76
0.0292 1198.209 1444.63 55.27 215.65 6.76
0.0331 1198.055 1444.89 55.67 215.67 6.74
0.0375 1197.881 1445.17 55.50 215.69 6.75
0.0426 1197.681 1445.51 55.93 215.72 6.73
mLiCl = 0.0104 mol kg–1
0.0000 1199.467 1442.62      
0.0111 1199.014 1443.23 42.04 216.43 7.25
0.0143 1198.880 1443.42 42.92 216.44 7.22
0.0197 1198.661 1443.73 43.66 216.45 7.20
0.0236 1198.503 1443.95 43.80 216.48 7.19
0.0278 1198.329 1444.19 43.76 216.50 7.19
0.0358 1198.005 1444.65 44.18 216.52 7.18
0.0390 1197.876 1444.84 44.55 216.54 7.16
0.0447 1197.644 1445.17 44.68 216.56 7.16
[OMIm]SCN + PC + LiBr
mLiBr = 0.0038 mol kg–1
0.0000 1199.227 1442.80      
0.0096 1198.733 1443.41 44.26 247.20 8.39
0.0133 1198.543 1443.66 45.84 247.26 8.34
0.0155 1198.428 1443.81 46.29 247.27 8.32
0.0184 1198.280 1444.01 47.22 247.30 8.29
0.0237 1198.010 1444.37 48.03 247.33 8.26
0.0266 1197.856 1444.58 48.60 247.37 8.24
0.0300 1197.686 1444.81 48.97 247.37 8.23
0.0322 1197.570 1444.98 49.74 247.39 8.20
mLiBr = 0.0060 mol kg–1
0.0000 1199.422 1442.68      
0.0099 1198.897 1443.28 39.14 248.38 8.61
0.0127 1198.743 1443.47 40.65 248.44 8.56
0.0162 1198.562 1443.69 41.38 248.43 8.54
0.0194 1198.389 1443.90 41.82 248.44 8.52
0.0227 1198.217 1444.11 42.20 248.47 8.51
0.0263 1198.027 1444.34 42.41 248.49 8.50
0.0299 1197.832 1444.58 42.76 248.53 8.49
0.0322 1197.709 1444.74 43.32 248.58 8.47
mLiBr = 0.0098 mol kg–1
0.0000 1199.584 1442.52      
0.0097 1199.043 1443.09 34.22 250.03 8.85
0.0132 1198.850 1443.31 35.92 250.07 8.79
0.0168 1198.648 1443.54 36.95 250.13 8.76
0.0194 1198.504 1443.70 37.13 250.14 8.75
0.0244 1198.230 1444.02 38.23 250.16 8.71
0.0267 1198.100 1444.18 39.03 250.19 8.68
0.0302 1197.904 1444.40 39.04 250.22 8.68
0.0330 1197.749 1444.59 39.71 250.25 8.66
[OMIm]SCN + PC + LiCl
mLiCl = 0.0032 mol kg–1
0.0000 1199.177 1442.90      
0.0110 1198.620 1443.65 51.88 246.98 8.13
0.0140 1198.445 1443.89 52.32 247.04 8.11
0.0180 1198.237 1444.18 52.92 247.07 8.09
0.0200 1198.161 1444.29 53.41 247.12 8.07
0.0240 1197.926 1444.62 53.81 247.13 8.06
0.0290 1197.715 1444.91 53.77 247.17 8.06
0.0300 1197.646 1445.01 54.00 247.18 8.05
0.0340 1197.459 1445.29 54.86 247.21 8.02
mLiCl = 0.0064 mol kg–1
0.0000 1199.276 1442.74      
0.0100 1198.724 1443.41 44.61 248.42 8.44
0.0150 1198.479 1443.72 45.77 248.49 8.40
0.0170 1198.394 1443.83 46.22 248.51 8.39
0.0200 1198.233 1444.04 46.99 248.55 8.36
0.0220 1198.124 1444.18 47.24 248.58 8.35
0.0260 1197.869 1444.51 47.76 248.60 8.33
0.0300 1197.669 1444.77 48.10 248.62 8.32
0.0320 1197.556 1444.92 48.38 248.63 8.31
mLiCl = 0.0100 mol kg–1
0.0000 1199.413 1442.63      
0.0095 1198.887 1443.17 32.51 250.01 8.92
0.0133 1198.675 1443.40 33.78 250.05 8.88
0.0155 1198.551 1443.54 34.70 250.07 8.85
0.0184 1198.391 1443.72 35.53 250.11 8.82
0.0237 1198.100 1444.04 36.04 250.13 8.80
0.0266 1197.934 1444.23 36.63 250.16 8.78
0.0300 1197.750 1444.45 37.56 250.19 8.75
0.0322 1197.625 1444.60 38.11 250.21 8.73
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a

Standard uncertainties for molality, temperature, and pressure were u (m) = 0.002 mol kg–1, u (T) = 0.02 K, and u (P) = 10 hPa, respectively, with a 0.68 level of confidence, and the combined standard uncertainties for density and speed of sound were uc (d) = 0.07 kg m–3 and uc (u) = 1.3 m s–1 with a 0.68 of level of confidence. The standard uncertainties of the apparent molar volume and apparent molar isentropic compressibility were uc (Vφ) = 5.10–5 m3 mol–1 (level of confidence of 0.68) and uc (κφ) = 3.10–3 m3 mol-1 Pa–1 (level of confidence of 0.68), respectively.

The effect of LiX (X = Cl, Br) on the Vφ values of the [HMIM][SCN] is shown in Figure 4. This figure demonstrates that the addition of the LiBr content increases the Vφ values of [HMIM][SCN].

Figure 4.

Figure 4

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Apparent molar volumes Vφ of [HMIM]SCN against its molality m in a (PC + LiBr) solution at T = 298.15 K at different concentrations of LiBr. (●) 0.0036 mol kg–1, (■) 0.0064 mol kg–1, and (⧫) 0.0099 mol kg–1, and solid lines represent the corresponding Redlich–Mayer model.

In Figure 5, the effect of anion size (Cl– and Br–) on the Vφ values of [HMIM][SCN] has been shown. It is clear that a Br– anion has a stronger effect rather than a Cl– anion.

Figure 5.

Figure 5

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Apparent molar volumes Vφ of [HMIM][SCN] in PC versus its molality m in the presence of about 0.01 mol kg–1 of LiX salts at T = 298.15 K. (■) LiCl and (●) LiBr, and solid lines represent the calculated Redlich–Mayer values.

The standard partial molar volumes Vφ0 of the ILs in (LiX + PC) solutions at different concentrations of LiX are given in Table 5. Results show that a Br– anion has a stronger effect than a Cl– anion on the Vφ values. The partial molar volumes of transfer ΔtrVφ0 of the ionic liquids have been obtained for [RMIM][SCN] from PC to (LiX + PC) solutions.

graphic file with name ao1c03517_m013.jpg 13

Table 5. Standard Partial Molar Volume (Vφ0), Empirical Parameters of eq 2, SV and BV, the Partial Molar Volume of Transfer (ΔtrVφ0), and the Standard Deviation of the Apparent Molar Volume σ(Vφ) for the Ternary Solutions Containing (IL + LiX + PC) at Different Concentrations of LiX at T = 298.15 K under Pressure (P = 0.086 MPa)a.

mLiX (mol kg–1) 106 Vφ0(m3 mol–1) 106 SV (m3 mol–1 kg–1/2) 106 BV (m3 mol–1 kg–1) 106 Δtr Vφ0 (m3 mol–1) 106 σ(Vφ0)
[BMIM][SCN] + PC + LiBr
0.0000 178.31 ± 0.32 4.67 ± 0.04 –1.71 ± 0.15   0.015
0.0035 178.96 ± 0.13 2.02 ± 0.02 –2.96 ± 0.05 0.65 0.008
0.0056 180.93 ± 0.16 3.81 ± 0.02 –2.44 ± 0.06 2.62 0.008
0.0095 183.10 ± 0.17 2.89 ± 0.02 –2.74 ± 0.07 4.79 0.011
[BMIM][SCN] + PC + LiCl
0.0000 178.31 ± 0.32 4.67 ± 0.04 –1.71 ± 0.15   0.015
0.0028 178.98 ± 0.12 –4.92 ± 0.02 21.37 ± 0.05 0.67 0.007
0.0057 179.40 ± 0.08 2.3 ± 0.01 –2.88 ± 0.03 1.09 0.006
0.0100 180.45 ± 0.12 2.55 ± 0.02 –2.79 ± 0.05 2.13 0.008
[HMIM][SCN] + PC+ LiBr
0.0000 213.52 ± 0.60 –13.97 ± 0.09 86.42 ± 0.31   0.026
0.0036 214.46 ± 0.08 –0.66 ± 0.01 7.79 ± 0.3 0.94 0.007
0.0064 216.29 ± 0.07 –0.84 ± 0.01 7.08 ± 0.03 2.77 0.007
0.0099 218.46 ± 0.08 0.57 ± 001 3.20 ± 0.02 4.94 0.005
[HMIM][SCN] + PC+ LiCl
0.0000 213.52 ± 0.60 –13.97 ± 0.09 86.42 ± 0.31   0.026
0.0036 214.53 ± 0.07 –2.46 ± 0.01 12.52 ± 0.03 1.01 0.005
0.0065 215.47 ± 0.02 0.20 ± 0.00 4.97 ± 0.01 1.95 0.002
0.0104 216.39 ± 0.07 –0.07 ± 0.01 4.18 ± 0.03 2.87 0.006
[OMIM][SCN] + PC+ LiBr
0.0000 244.84 ± 0.24 2.44 ± 0.04 90.02 ± 0.15   0.009
0.0038 246.94 ± 0.07 3.09 ± 0.01 –3.07 ± 0.04 2.10 0.005
0.0060 248.58 ± 0.22 –3.86 ± 0.03 21.02 ± 0.12 3.74 0.016
0.0092 249.73 ± 0.14 3.37 ± 0.02 –3.09 ± 0.07 4.89 0.010
[OMIM][SCN] + PC+ LiCl
0.0000 244.84 ± 0.24 2.44 ± 0.04 90.02 ± 0.15   0.009
0.0032 246.64 ± 0.16 3.67 ± 0.02 –3.20 ± 0.08 1.80 0.010
0.0060 248.10 ± 0.16 3.60 ± 0.02 –3.16 ± 0.08 3.26 0.010
0.0100 249.73 ± 0.08 3.18 ± 0.01 –3.07 ± 0.04 4.89 0.006
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a

Standard uncertainties for molality, temperature, and pressure were (m) = 0.002 mol kg–1, u (T) = 0.02K, and u (P) = 10 hPa, respectively, with a 0.68 level of confidence.

The ΔtrVφ0 values are reported in Table 5. These values are positive and increase with increasing LiX concentration. The ΔtrVφ value is a measure of interaction between solute1 (IL) and solute2 (LiX). The possible interactions for the studied solutions due to the different functional groups of the components are polar–ionic, polar–polar, polar–nonpolar, and nonpolar–nonpolar interactions.40,41 According to the cosphere overlap model, the positive ΔtrVφ0 value indicates strong ion–ion and ion–polar interactions between [RMIM][SCN] and LiX.24,26 On the other hand, LiBr has stronger interactions than LiCl with ionic liquids, as shown in Table 5.

Compressibility Properties

The measured speeds of sound (u) data for the binary (IL + PC) and ternary (IL + PC +LiX) solutions are given in Tables 1 and 4, respectively. These data were used to calculate the isentropic compressibility, κs, with help of Laplace–Newton’s relation.24

graphic file with name ao1c03517_m014.jpg 14

This quantity can be considered as the bulk modulus behavior of the solution. The solvation numbers were calculated from the κs values by the Pasynski equation.42

graphic file with name ao1c03517_m015.jpg 15

where n1 and n2 are numbers of moles of the solvent and the solute, respectively, and κs and κs0 are isentropic compressibility of the solution and the solvent [PC or (PC + LiX)], respectively. The calculated values of Sn are given in Tables 1 and 4 for the investigated binary and ternary solutions. The Sn values were increased with increasing temperature. These values were decreased with the addition of the LiX salt. This may be related to the coordination of PC molecules to Li+, as confirmed by the Raman spectroscopy study of the lithium salts in PC.40 Consequently, IL solvation numbers were decreased. It means that the coordination of PC to Li+ is more favorable rather than the IL solvation.

The apparent molar isentropic compressibility, κφ, of the ILs has been determined in the solution with the following relation24

graphic file with name ao1c03517_m016.jpg 16

The κφ values of the ILs in the studied solutions are given in Tables 1 and 4 for the corresponding binary and ternary solutions, respectively. These values increase by the addition of the IL in the studied binary solutions (IL + PC). The κφ values are higher for a longer alkyl chain length of the ILs [OMIM][SCN], as shown in Figure 6.

Figure 6.

Figure 6

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Apparent molar isentropic compressibility κφ values of [RMIM][SCN] versus its molality m at T = 298.15 K. (●) [BMIM][SCN], (■) [HMIM][SCN], and (⧫) [OMIM][SCN], and solid lines show the corresponding Redlich–Mayer model.

The results indicate that the κφ values of the ILs, [OMIM][SCN], increase at a higher concentration of LiBr (Figure 7).

Figure 7.

Figure 7

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Apparent molar isentropic compressibility values, κφ, of [OMIM][SCN] in PC + LiBr {(●) 0.0038 mol kg–1, (■) 0.0060 mol kg–1, and (⧫) 0.0092 mol kg–1}, and solid lines show the Redlich model at T = 298.15 K.

Also, it found that the LiBr solution κφ values are larger than LiCl solutions, as given in Table 4. The influence of temperature on the κφ values of [BMIM][SCN] at (288.15–318.15) K is plotted in Figure 8, which represents a decreasing trend at a higher temperature.

Figure 8.

Figure 8

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Apparent molar isentropic compressibility κφ values of [BMIM][SCN] in PC versus its molality m at different temperatures. (●) T = 288.15 K, (■) T = 298.15 K, (⧫) T = 308.15 K, and (▲) T = 318.15 K, and solid lines represent the Redlich–Mayer model.

The obtained κφ values of the studied ILs of binary (IL + PC) and ternary (IL + LiX + PC) solutions were fitted to the following equation24

graphic file with name ao1c03517_m017.jpg 17

where κφ0 is the partial molar isentropic compressibility and Sk and Bk are the empirical parameters of the equation. The obtained parameters for the investigated solutions are listed in Tables 6 and 7 for the studied solutions. The κφ values increase with the alkyl chain length of the ILs.

Table 6. Partial Molar Isentropic Compressibility κφ0, Empirical Parameters of eq 17, Sκ and Bκ, and Standard Deviation of Apparent Molar Isentropic Compressibility σ(κφ) of [RMIM][SCN] in PC T = (288.15–318.15) K under Pressure (P = 0.086 MPa)a.

T (K) 1014 κφ0 (m3 mol–1 Pa–1) 1014 Sκ (m3 mol–3/2 kg1/2 Pa–1) 1014 Bκ (m3 mol–2 kg Pa–1) 1014 σ (κφ0)
[BMIM][SCN]
288.15 4.17 ± 0.07 0.56 ± 0.01 0.81 ± 0.03 0.004
298.15 3.72 ± 0.07 0.89 ± 0.01 0.91 ± 0.03 0.004
308.15 3.27 ± 0.22 1.14 ± 0.03 0.99 ± 0.10 0.01
318.15 2.84 ± 0.09 1.15 ± 0.01 1.02 ± 0.04 0.009
[HMIM][SCN]
288.15 5.55 ± 0.13 1.35 ± 0.02 1.01 ± 0.06 0.046
298.15 5.13 ± 0.31 1.81 ± 0.04 1.15 ± 0.16 0.014
308.15 4.72 ± 0.13 1.93 ± 0.02 1.19 ± 0.06 0.006
318.15 4.43 ± 0.20 1.99 ± 0.03 1.22 ± 0.10 0.010
[OMIM][SCN]
288.15 6.12 ± 0.14 –2.83 ± 0.02 57.96 ± 0.09 0.008
298.15 5.83 ± 0.24 –7.97 ± 0.04 81.41 ± 0.15 0.013
308.15 5.51 ± 0.22 –10.83 ± 0.04 92.51 ± 0.14 0.013
318.15 5.02 ± 0.26 –11.47 ± 0.04 94.37 ± 0.16 0.014
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a

Standard uncertainties for temperature and pressure were u (T) = 0.02K and u (P) = 10 hPa, respectively, with a 0.68 level of confidence.

Table 7. Partial Molar Isentropic Compressibility κφ0, Empirical Parameters of eq 17, Sκ and Bκ, and Standard Deviation of Apparent Molar Isentropic Compressibility σ(κφ) of [RMIM][SCN] in (PC + LiX) at Different Concentrations of LiX at T = 298.15 K under Pressure (P = 0.086 MPa)a.

mLiX (mol kg–1) 1014 κφ0 (m3 mol–1 Pa–1) 1014 Sκ (m3 mol–3/2 kg1/2 Pa–1) 1014 Bκ (m3 mol–2 kg Pa–1) 1014 σ (κφ0)
[BMIM][SCN] + PC + LiBr
0.0000 3.72 ± 0.07 0.89 ± 0.01 0.91 ± 0.03 0.004
0.0035 5.05 ± 0.15 –3.44 ± 0.02 –0.49 ± 0.06 0.009
0.0056 5.37 ± 0.22 –3.09 ± 0.03 –0.40 ± 0.09 0.011
0.0095 5.76 ± 0.39 –3.03 ± 0.05 –0.38 ± 0.16 0.023
[BMIM][SCN] + PC + LiCl
0.0000 3.72 ± 0.07 0.89 ± 0.01 0.91 ± 0.04 0.004
0.0028 4.45 ± 0.05 –1.11 ± 0.01 0.26 ± 0.02 0.006
0.0057 4.71 ± 0.06 –1.11 ± 0.01 0.24 ± 0.02 0.005
0.0100 5.08 ± 0.08 –0.85 ± 0.01 0.32 ± 0.03 0.005
[HMIM][SCN] + PC+ LiBr
0.0000 5.13 ± 0.31 1.81 ± 0.04 1.15 ± 0.16 0.014
0.0036 6.80 ± 0.07 –1.20 ± 0.01 0.16 ± 0.03 0.007
0.0064 7.10 ± 0.05 –0.85 ± 0.01 0.29 ± 0.02 0.006
0.0099 7.42 ± 0.03 –0.63 ± 0.00 0.35 ± 0.01 0.003
[HMIM][SCN] + PC+ LiCl
0.0000 5.13 ± 0.31 1.81 ± 0.04 1.15 ± 0.16 0.014
0.0036 6.54 ± 0.09 –0.76 ± 0.01 0.33 ± 0.04 0.007
0.0065 6.89 ± 0.06 –0.87 ± 0.01 0.30 ± 0.02 0.008
0.0104 7.33 ± 0.06 –0.90 ± 0.01 0.26 ± 0.03 0.008
[OMIM][SCN] + PC+ LiBr
0.0000 5.83 ± 0.24 –7.97 ± 0.04 81.41 ± 0.15 0.013
0.0038 8.59 ± 0.07 –2.17 ± 0.01 –0.03 ± 0.04 0.006
0.0060 8.75 ± 0.09 –1.6 ± 0.01 0.14 ± 0.05 0.011
0.0092 9.05 ± 0.09 –2.14 ± 0.01 –0.03 ± 0.05 0.009
[OMIM][SCN] + PC+ LiCl
0.0000 5.83 ± 0.24 –7.97 ± 0.04 81.41 ± 0.15 0.013
0.0032 8.26 ± 0.11 –1.27 ± 0.02 0.20 ± 0.05 0.007
0.0060 8.60 ± 0.04 –1.66 ± 0.01 0.10 ± 0.02 0.006
0.0100 9.12 ± 0.10 –2.16 ± 0.02 –0.04 ± 0.05 0.008
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a

Standard uncertainties for molality, temperature, and pressure were u (m) = 0.002 mol kg–1, u (T) = 0.02K, and u (P) = 10 hPa, respectively, with a 0.68 level of confidence.

The κφ0 values for the ILs in PC and in the presence of LiX salts are positive, which increase with increasing LiX content, and also, it is found that the value κφ in the presence of LiBr is higher than LiCl. This trend shows that bulk propylene carbonate is more compressible rather than electrostrictive PC molecules (solvated PC molecules); upon addition of LiX, electrostriction interactions between IL and PC become weaker due to PC molecules’ coordination on Li+, as previously mentioned. On the other hand, according to the SPT results, it is seen that interactional and cavity volumes are increased for a longer alkyl chain length of the ILs. The highly available cavity volume is the main reason for the high compressibility value of [OMIM][SCN] in the PC solution. The intermolecular interaction between the cation of the IL and PC is the dominant factor of this phenomenon but there is intramolecular negative ion interaction, which is another factor that is negligible in the dilute region.

Conclusions

This study is a thermodynamic approach to a model of Li-ion battery electrolytes. The volumetric and compressibility properties of the ILs, [RMIM][SCN], in PC in the presence of LiCl and LiBr have been investigated to understand the existing interactions in these systems. The ILs’ interaction with PC increased with the alkyl chain length of the imidazolium cation from butyl to octyl. However, these interactions were weakened at a higher temperature. The studied ILs, [RMIM][SCN], show structure-breaking behavior in propylene carbonate with the following trend: [OMIM][SCN] > [HMIM][SCN] > [BMIM][SCN]. Although ILs with larger cations are more compressible, this feature decreases with increasing temperature. The ion–polar interactions are dominant rather than other interactions in the ternary systems. Also, interactions between LiX and ILs increase with the lithium halide content, and LiBr has a stronger effect rather than LiCl. Although the addition of the lithium halide salts leads to an increase in the compressibility of the ILs, LiBr has a stronger effect rather than LiCl. To summarize, [OMIM][SCN] is an appropriate cosolvent for Li-ion battery electrolytes that can enhance the mechanical and thermal stability of the batteries.

Experimental Section

Chemicals

All of the reagents used in this work are listed in Table 8. Also, the purification methods, supplier company names, and CAS numbers are given. The water content of all components was determined with Karl-Fisher titration (Titrino GPD 751, electrode: Metrohm Pt—6.0338.100).

Table 8. Summary of the Chemicals and their Characteristics That Were Used in This Work.

graphic file with name ao1c03517_0010.jpg

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Ionic Liquids [RMIM][SCN] and their Properties

The ionic liquid synthesis procedure is given in our previous publications with the corresponding density and speed of sound data at different temperatures.24,26 Also, brief information about the synthesized ionic liquids is given in Table 8.

Apparatus and Procedure

The solutions were prepared using an analytical balance (Shimadzu AW-220) with a precision of ± 1 × 10–4 g in a molal-based concentration. The density and speed of sound were measured with a digital densitometer (Anton Paar DSA5000). The instrument was calibrated with air pressure and distilled water. The frequency for the speed of sound measurement was 3 MHz.

Acknowledgments

The authors wish to thank the financial support from the Graduate Council of the University of Tabriz.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c03517.

  • HNMR spectra of [BMIM][SCN], FT-IR spectra of [BMIM][SCN], HNMR spectra of [HMIM][SCN], FT-IR spectra of [HMIM][SCN], HNMR spectra of [OMIM][SCN], and FT-IR spectra of [OMIM][SCN] (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c03517_si_001.pdf (494.8KB, pdf)

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Supplementary Materials

ao1c03517_si_001.pdf (494.8KB, pdf)

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