CHAPTER-4 PMMA Based Polymer Gel Electrolytes Containing Ionic Liquid 2,3-dimethyl-1-hexylimidazolium bis(trifluoromethanesulfonyl)imide 4.1 Introduction Polymer gel electrolytes belong to a salt-solvent-polymer hybrid system in which the salt solution is immobilized with the addition of a suitable polymer matrix [Feuillade and Perche (1975), Tsushida et al. (1983)]. Gel electrolytes or quasi-solid state electrolytes eliminate the problem of leakage and offer good electrode electrolyte contact, wide range of composition and hence wider control of properties, mechanical stability and flexibility etc. Owing to the unique and advantageous physicochemical properties of ionic liquids, viz. high ionic conductivity, thermal stability, inflammability, wide electrochemical window etc., polymer gel electrolytes containing ionic liquids are gaining considerable attention [Fuller et al. (1997, 1998); Tiyapiboonchaiya et al. (2002); Ohno et al. (2003); Susan et al. (2005); Morita et al. (2005); Tamada et al. (2008); Egashira et al. (2008); Pandey and Hashmi (2009)]. Ionic liquids, besides providing the ions for conduction, also improves the segmental motion of polymer, act as plasticizer and expand the temperature range over which polymer electrolytes can be used. They also improve the thermal stability and reduce the glass transition temperature [Scott et al. (2002, 2003)]. The solvent retained in these electrolytes helps in the conduction process. The present chapter deals with the preparation and characterisation of polymer gel electrolytes containing ionic liquid 2,3-dimethyl-1-hexylimidazolium bis(trifluoromethanesulfonyl)imide (DMHxImTFSI). Ionic liquid with a trisubstituted imidazolium cation has been used, as substituting the ring hydrogens with methyl groups adds to the stability of cation while the TFSI is a non-coordinating anion that leads to thermally and electrochemically stable ionic liquids with low melting points due to anion s structure, delocalized electron cloud and inability to hydrogen bond [Hagiwara and Ito (2000); Bonhote et al (1996)]. The formation of ionic liquid has been confirmed by 1 H and 13 C NMR spectroscopy. The variation of conductivity and viscosity of ionic liquid and polymer gels as a function of concentration and temperature has been studied. FTIR spectroscopy has been carried out to check the presence of ion aggregates and their dissociation with the 50
addition of polymer. Thermal stability of electrolytes has been studied by simultaneous differential scanning calorimetry (DSC) and themogravimetric analysis (TGA). 1 H and 19 F solid state NMR spectroscopy technique has been used for the determination of ion transport parameters. Polymer gel electrolytes containing ionic liquid DMHxImTFSI and polymers polymethylmethacrylate (PMMA), polyvinylidenefluoride-co-hexafluoropropylene (PVdF-HFP) have been studied for following set of systems: PC+DMHxImTFSI+PMMA DMHxImTFSI+PMMA PC+DMHxImTFSI+PVdF-HFP 4.2 Characterisation techniques Ionic liquid and polymer gel electrolytes containing ionic liquid have been investigated by using various experimental techniques as given below: 1 H & 13 C Nuclear Magnetic Resonance (NMR) Spectroscopy Ionic Conductivity Viscosity Density Melting point Fourier Transform Infrared (FTIR) Spectroscopy Differential Scanning Calorimetry/ Thermogravimetric Analysis (DSC/TGA) 1 H and 19 F Solid State Nuclear Magnetic Resonance (NMR) Spectroscopy 4.3 Results and discussion 4.3.1 1 H and 13 C NMR spectroscopy Ionic liquid 2,3-dimethyl-1-hexylimidazolium bis(trifluoromethanesulfonyl)imide (DMHxImTFSI) has been prepared by the anion exchange reaction of 2,3-dimethyl-1- hexylimidazolium bromide with LiN(CF 3 SO 2 ) 2. The formation of ionic liquid has been studied by 1 H and 13 C NMR spectroscopy. 1 H and 13 C NMR spectra of ionic liquid were recorded at room temperature with JEOL AL-300 MHz NMR spectrometer at 300MHz and 75MHz respectively in CDCl 3 with trimethylsilane (TMS) as internal standard. Chemical shifts were expressed as δ 51
(ppm) downfield from TMS and the abbreviations used are s-singlet, d-doublet, t-triplet, m-multiplet, J-coupling constant. 1 H NMR (CDCl 3, 300MHz) δ (ppm): 0.86-0.90 (t, 3H, -CH 3, J=6.9 Hz), 1.30-1.35 (m, 6H, 3x-CH 2 ), 1.76-1.80 (t, 2H, -CH 2 ), 2.54-2.62 (t, 3H, -CH 3, J=11.7 Hz), 3.74-3.82 (t, 3H, -CH 3, J=12.3 Hz), 4.00-4.05 (t, 2H, -CH 2, J=7.5 Hz), 7.17-7.20 (d, 2H, J=6.6 Hz, ring proton). 13 C NMR (CDCl3, 75Hz) δ (ppm): 9.41 (CH 3 ), 13.7 (CH 3 ), 22.21 (CH 2 ), 25.7 (CH 2 ), 29.3 (CH 2 ), 30.9 (CH 2 ), 35.1 (CH 2 ), 48.7 (CH 3 ), 120 (CF 3 ), 122 (CF 3 ). 1 H and 13 C NMR spectra is shown in Fig. 4.1 The chemical structure of the ionic liquid is given below DMHxImTFSI: 4.3.2 Density In the present work density of the ionic liquid was measured by taking a known volume of ionic liquid using precise micropipette and then measured its mass by weighing machine having least count 0.0001 g. Density of ionic liquid (DMHxImTFSI) has been found to be 1.3 g cm -3. 4.3.3 Melting point The melting point of the ionic liquid was measured by a simple physical observation. A small amount (200-300 mg) of ionic liquid was taken in a glass tube having diameter 5 mm, which was then immersed in liquid nitrogen to freeze the ionic liquid. After that, melting point was observed by physical observation. The temperature of the sample was measured by thermocouple equipped with digital temperature controller. The melting point of the ionic liquid is -35 o C. 52
4.3.4 Ionic conductivity and viscosity The ionic conductivity of the electrolytes has been measured by a.c. impedance spectroscopy with a computer interfaced HIOKI-3532-50 LCR Hi-Tester using a cell with platinum electrodes. Viscosity of the electrolytes has been measured by Fungilab rotating viscometer (Visco Basic L) using a small sample adapter assembly. The temperature of the sample was controlled within ± 0.1 o C with a Julabo water circulator (F-12 EC). 53
4.3.4.1 Ionic liquid Ionic liquids consist of ions (cations and anions) and are in molten form at ambient temperatures, owing to which they show high value of ionic conductivity at room temperature. The conductivity and viscosity of the ionic liquid (DMHxImTFSI) was measured in the temperature range 30-90 o C. The conductivity of the ionic liquid has been found to increase from 2.47x10-3 S cm -1 at 30 o C to 1.52x10-2 S cm -1 at 90 o C. The viscosity of the ionic liquid decreases from 109 mpa s at 30 o C to 16.5 mpa s at 90 o C. The variation of conductivity and viscosity with temperature has been shown in Fig. 4.2. Conductivity depends on the ion mobility which is related to the viscosity by the relation: μ=q/6πηr (where μ is mobility, η is viscosity, q is the charge and r is the radius of ion). Figure 4.2 Variation of conductivity ( ) and viscosity ( ) as a function of temperature for DMHxImTFSI With an increase in temperature, the viscosity of the ionic liquid decreases. As viscosity and mobility are inversely related, mobility of the ions increases contributing to the increase in conductivity. Some important physical properties of the ionic liquid have been given in Table 4.1. 54
Table 4.1 Some physical properties of ionic liquid DMHxImTFSI density melting point conductivity viscosity (g cm -3 ) ( o C) (at 30 o C) (S cm -1 ) (at 30 o C) (mpa s) 1.3-35 2.47x10-3 109 4.3.4.2 Polymer gel electrolytes containing ionic liquids In the present system the variation of ionic conductivity and viscosity as a function of concentration of ionic liquid, polymer and temperature has been studied. 4.3.4.2 (a) Ionic conductivity vs. concentration of ionic liquid Liquid electrolytes have been prepared by dissolving ionic liquid DMHxImTFSI in different concentrations in PC. The conductivity of liquid electrolytes has been measured as a function of molar concentration of ionic liquid and is given in Fig. 4.3. In the liquid electrolytes (PC+ xm DMHxImTFSI), the conductivity of the solvent (PC~ 10-6 S cm -1 ) increases by three orders of magnitude (~ 10-3 S cm -1 ) with the addition of even a small amount of ionic liquid and attains maximum value for the composition PC+ 0.5M DMHxImTFSI which has been further used to prepare polymer gel electrolytes. As a function of concentration of ionic liquid in PC, the increase in ionic conductivity is very sharp at low concentrations (up to 0.1M DMHxImTFSI), but does not show much change at higher concentrations of ionic liquid and reaches a saturation value. At high concentrations of ionic liquid/salt in the solvent, there is probability of the formation of ion aggregates/ ion pairs due to strong columbic interactions and increased probability of hydrogen bonding between the ions. These ion aggregates are neutral and do not contribute to ionic conductivity and as result conductivity does not increase at the same rate as observed at low concentrations of ionic liquid [Battisti et al. (1993), Deepa et al. (2004)]. 55
Figure 4.3 Ionic conductivity of liquid electrolyte (PC+ xm DMHxImTFSI) as a function of concentration of ionic liquid in PC Figure 4.4 Log conductivity vs. log concentration of ionic liquid for liquid electrolyte (PC+ xm DMHxImTFSI) The presence of ion aggregates has also been studied qualitatively by mass action considerations [Ratner (1987)]. According to this model the plot between log and log C (where C is the concentration ionic liquid/salt) should be a straight line if all the ions are present as free ions whereas a deviation from a straight line behavior indicates the presence of neutral ion aggregates which do not contribute to conductivity. 56
The variation of log conductivity (log with log concentration (log C) for liquid electrolyte (PC+ xm DMHxImTFSI) has been shown in Fig. 4.4. The plot shows a straight line behavior at low concentrations of IL in PC (upto 0.1M); however, a deviation from the straight line is observed as the concentration increases beyond 0.1M IL. This deviation suggests the presence of ion aggregates at high concentrations of ionic liquid and this is in agreement with the conductivity results discussed above. 4.3.4.2 (b) Conductivity and viscosity vs. concentration of polymer Polymer gel electrolytes have been prepared by the addition of polymer (PMMA) in different concentrations (expressed as wt% of liquid electrolytes) to the liquid electrolytes [Chandra et al. (2000), Sekhon et al. (2003)] and their conductivity and viscosity behavior have been studied. The variation of ionic conductivity and viscosity of electrolytes as a function of concentration of polymer has been given in Fig. 4.5. With the addition of PMMA to liquid electrolyte PC+ 0.5M DMHxImTFSI, the ionic conductivity was expected to decrease due to an increase in the viscosity. But, contrary to this, the conductivity values show an increase at low concentrations of PMMA (0-0.2 wt %), reach a maximum conductivity of 9.3x10-3 S cm -1 for 0.2 wt% PMMA in PC+0.5M DMHxImTFSI and then decrease as the concentration of PMMA is further increased. The small increase in the ionic conductivity of electrolytes with the addition of polymer has been explained to be due to the dissociation of ion aggregates. Figure 4.5 Variation of ionic conductivity ( ) and viscosity (Δ) of PC+ 0.5M DMHxImTFSI+ x wt% PMMA with concentration of PMMA 57
Polymer gel consists of free ions, ion aggregates and polymer chains dispersed in gel matrix. Polymer breathes while folding/unfolding of its chains causing the fluctuations in density/pressure at the microscopic level. These localized variations assist in the dissociation of ion pairs/aggregates resulting in an increase in mobile charge carriers and thus explaining the increase in conductivity at low concentrations of polymer [Bruce (1991); Sekhon (2003); Chandra et al. (2000)]. At higher concentration the effect of increased viscosity becomes dominant leading to decrease in conductivity. As shown in Fig. 4.5, the viscosity values show an exponential increase as the concentration of polymer increases. From above, it appears that both free ion concentration as well as viscosity affects the conductivity behavior of polymer gel electrolytes. The free ion concentration plays a dominant role at low concentrations of PMMA, whereas viscosity plays a dominant role at higher concentrations of PMMA. The formation and dissociation of ion aggregates has also been studied by FTIR spectroscopy (explained in the next section). In order to study the role of ion aggregates in modifying the conductivity behavior, polymer gel electrolyte has also been prepared by the addition of PMMA to neat ionic liquid. In this case the conductivity gradually decreases by small amount as the concentration of polymer increases (Fig. 4.6). In case of IL, ions are present as free ions due to the self-dissociating nature of ionic liquid. The addition of polymer increases the viscosity exponentially and hence conductivity decreases due to reduced mobility. The polymer gel electrolyte (DMHxImTFSI+ 15 wt% PMMA) possesses ionic conductivity of 3.06x10-4 S cm -1 and viscosity of 16705 mpa s at 30 o C Figure 4.6 Variation of ionic conductivity ( ) and viscosity (Δ) of DMHxImTFSI+ x wt% PMMA with concentration of PMMA 58
In the above studied polymer gel electrolytes PMMA was utilised to provide matrix, which is a hydrophilic polymer. To study the effect of polymer, electrolytes using a hydrophobic polymer polyvinylidenefluoride-co-hexafluoropropylene (PVdF- HFP) have also been examined. In case of these electrolytes also, with the addition of PVdF- HFP, conductivity values first show an increase at low concentrations varying from 0-0.6 wt%, attains a maximum value of 9.62x10-3 S cm -1 and then shows a continuous decrease upto 10 wt% of polymer due to an increase in viscosity (Fig. 4.7). The polymer gel electrolyte PC+0.5M DMHxImTFSI+PVdF-HFP (10 wt%) possesses ionic conductivity of 5.70x10-3 S cm -1 at 30 o C This suggests that in case of presently investigated gel electrolytes both the polymers PMMA and PVdF-HFP, inspite of their different properties show a similar kind of interaction with the ionic liquid and solvent. Figure 4.7 Variation of ionic conductivity of PC+ 0.5M DMHxImTFSI+ x wt% PVdF-HFP) with concentration of PVdF-HFP 4.3.4.2 (c) Ionic conductivity and viscosity vs. temperature The dependence of conductivity and viscosity on temperature has been studied for polymer gel electrolytes. Variation of log conductivity with reciprocal temperature for polymer gel electrolyte (PC+ 0.5M DMHxImTFSI + 20 wt% PMMA) is given in Fig 4.8. On increasing the temperature of polymer gel electrolyte, the ionic conductivity follows Arrhenius behavior and increases from 2.73x10-3 S cm -1 (30 o C) to 1.29x10-2 S cm -1 (130 o C). Correspondingly the viscosity of the electrolyte decreases from 8575.1 59
mpa s (30 o C) to 328.1 mpa s (90 o C). The decrease in the viscosity enhances the mobility which in turn increases the conductivity of the electrolyte. Figure 4.8 Variation of log conductivity and log viscosity with reciprocal temperature for polymer gel electrolyte (PC+ 0.5M DMHxImTFSI+ 20 wt% PMMA) For comparison, the variation of log conductivity with reciprocal temperature has been shown for ionic liquid, liquid electrolyte (PC+ 0.5M DMHxImTFSI) and polymer gel electrolytes (PC+ 0.5M DMHxImTFSI+ 20 wt% PMMA) in Fig. 4.9 Figure 4.9 Variation of ionic conductivity as a function of temperature for DMHxImTFSI ( ), liquid electrolyte (PC+ 0.5M DMHxImTFSI (Δ)) and polymer gel electrolyte (PC+ 0.5M DMHxImTFSI+ 20 wt% PMMA ( )) 60
Transmission % T The conductivity of the neat ionic liquid has been observed to increase with temperature, which is due to a decrease in viscosity, which enhances ionic mobility and conductivity. At room temperature the addition of PC to the ionic liquid results in an increase in conductivity and electrolytes having composition PC + 0.5M DMHxImTFSI show higher conductivity than for the ionic liquid at all temperatures. The increase in conductivity with the addition of PC is due to its lower viscosity as compared with IL and higher dielectric constant and both these factors contribute to higher conductivity. Polymer gel electrolytes containing 20 wt% PMMA show lower conductivity as compared to the liquid electrolytes at all temperatures which is due to an increase in viscosity with the addition of polymer. However, the ionic conductivity of polymer gel electrolyte is only slightly lower than the liquid electrolyte and also as a function of temperature the conductivity for both liquid and gel electrolytes show similar variation suggesting liquid like behavior of gel electrolytes which is desirable for their application in devices. 4.3.5 FTIR FTIR spectroscopy has been used to study the interactions of the ionic liquid with polymer and solvent and to check the formation and dissociation of ion aggregates. The FTIR spectrum of the ionic liquid is shown in Fig. 4.10 and the position of different peaks and their assignment are listed in Table 4.2. [Rey et al. (1998), Katsyuba et al. (2004), Kim et al. (2007)]. Wavenumber (cm -1 ) Figure 4.10 FTIR spectrum of DMHxImTFSI 61
Table 4.2 Important peaks in the FTIR spectrum of ionic liquid DMHxImTFSI position of peaks assignment (cm -1 ) 407 SO 2 wagging mode of SO 2 512 a CF 3 asymmetric CF 3 bending mode 571 a CF 3 asymmetric CF 3 bending mode 615 a SO 2 asymmetric SO 2 bending mode 662 SNS free imide ions 739 s CF 3 symmetric CF 3 bending mode 761 s SNS S-N symmetric stretching mode 1056 a SNS SNS asymmetric stretching mode 1122 s SO 2 symmetric stretching mode of SO 2 1348 a SO 2 asymmetric stretching mode ofso 2 2960 CH C-H stretching As it is shown in Fig. 4.11(a), in the spectrum of PC, a doublet appears at 1800 cm -1 (due to C=O stretching vibration) and 1896 cm -1 (overtone of PC) [Battisti et al. (1993), Deepa et al. (2004)]. Addition of IL (DMHxImTFSI) to PC causes the imidazolium cation to interact with the electronegative oxygen of C=O group of PC, which results in the shifting of peak from 1800 cm -1 to1790 cm -1. Further with the addition of PMMA, broadening has been observed in the 1790 cm -1 peak which is due to its overlapping with C=O stretching peak of PMMA at 1731 cm -1 [Hummel (1966)]. 62
Relative Intensity a b c Wavenumber (cm -1 ) Figure 4.11(a).FTIR spectra for PC (a), PC+ 0.1M DMHxImTFSI (b) and PC+ 0.5M DMHxImTFSI+ 0.1 wt% PMMA (c) in 1700-2000 cm -1 wavenumber range Figure 4.11(b).FTIR spectra of DMHxImTFSI (a), PC+ 0.1M DMHxImTFSI (b), PC+0.5 M DMHxImTFSI (c), PC+ 0.5M DMHxImTFSI + 0.1 wt% PMMA)(d) and PC+ 0.5M DMHxImTFSI + 20 wt% PMMA (e) in the 630-680 cm -1 wavenumber range Figure 4.11(c) FTIR spectra of DMHxImTFSI (a), PC+ 0.1M DMHxImTFSI (b), PC+0.5 M DMHxImTFSI (c), PC+ 0.5M DMHxImTFSI + 0.1 wt% PMMA(d) and PC+ 0.5M DMHxImTFSI + 20 wt% PMMA (e) in the 1000-1170 cm -1 wavenumber range The FTIR spectra of different liquid and polymer gel electrolytes also reveal the formation and dissociation of ion aggregates. In the selected wavenumber region of 63
630-2000 cm -1, ionic liquid contributes peaks due to free ions at 662 cm -1 (δ SNS), 1056 cm -1 (δ SNS), 1122 cm -1 (SO 2 ) [Rey et al. (1998), Katsyuba et al. (2004), Kim et al. (2007)]. These peaks are also present in the spectrum of liquid electrolyte (PC+ 0.1M DMHxImTFSI). However, when the concentration of ionic liquid in the liquid electrolyte is increased to 0.5 M, following changes have been observed: (a) a peak due to free imide ions at 662 cm -1 (δ SNS) present in the spectra of IL and liquid electrolyte (PC+ 0.1M DMHxImTFSI) splits into two components at 653 cm -1 and 669 cm -1 for the liquid electrolyte (PC+ 0.5M DMHxImTFSI) (Fig 4.11(c)) (b) appearance of a shoulder at 1070 cm -1 (for PC+ 0.5M DMHxImTFSI) adjacent to a peak due to free imide ions at 1056 cm -1 (δ SNS) (Fig 4.11(b)). (c) appearance of a new peak at 1148 cm -1 (for PC+ 0.5M DMHxImTFSI) in the vicinity of a peak due to free ions at 1122 cm -1 (SO 2 ). Splitting of the 662 cm -1 peak and the appearance of a shoulder and a peak at 1070 cm -1 and 1148 cm -1 respectively in the FTIR spectrum of liquid electrolyte PC+ 0.5M DMHxImTFSI have been explained to be due to the formation of ion aggregates [Wang et al. (1999); Deepa et al. (2004)] at higher concentration of ionic liquid in the solvent. However, the 1070 cm -1 shoulder and 1148 cm -1 peak disappear with the addition of a small concentration of PMMA (0.1 wt%) to liquid electrolyte showing that the PMMA interacts with the electrolyte and helps in dissociating the ion aggregates. FTIR analysis has also been done for the polymer gel electrolytes containing PVdF-HFP (Fig. 4.12). In the FTIR spectrum of (PC+ 0.1M DMHxImTFSI) small peaks due to free ions contributed by the ionic liquid are observed at 739 (δ s CF 3 ) and 761(ν s SNS) cm -1. On further increasing the concentration of IL in PC to 0.5M, an additional shoulder is observed at 757 cm -1 suggesting the presence of ion aggregates. The spectrum of PC+ 0.1M DMHxImTFSI has a peak at 1056 cm -1 (ν a SNS). As mentioned above also, adjacent to this peak, a shoulder appears in the spectrum for sample having composition PC+ 0.5M DMHxImTFSI. However, the addition of PVdF- HFP in small concentrations (0.1wt %), results in the disappearance of the shoulders at 757 cm -1 and 1070 cm -1, indicating the dissociation of ion aggregates. The results obtained above from the FTIR spectroscopy are consistent with the ionic conductivity results for these electrolytes i.e. high concentrations of IL in PC leads to 64
Relative Intensity the formation of ion aggregates which is reflected from the splitting of 662 cm -1 peak and the presence of a new shoulder and small peak at 1070 cm -1 and 1148 cm -1 Wavenumber (cm -1 ) Figure 4.12. FTIR spectra of PC+0.1 M DMHxImTFSI (a), PC+ 0.5M DMHxImTFSI (b) and PC+ 0.5M DMHxImTFSI + 0.1 wt% PVdF- HFP(c), in the 760-780 cm -1 and 1000-1100 cm -1 wavenumber range respectively in the FTIR spectrum of PC+ 0.5M DMHxImTFSI. These peaks were not observable in the FTIR spectrum of composition PC+ 0.1M DMHxImTFSI. Mass action considerations also show that the ion aggregates from when the concentration of IL is increased beyond 0.1 M. With the addition of polymer to PC+ 0.5M DMHxImTFSI in small concentrations, ion aggregates dissociate, indicated by the absence of 1070 and 1148 cm -1 peak in the spectra of PC+ 0.5M DMHxImTFSI+ 0.1 wt% PMMA and a corresponding increase in the conductivity values due to increase in the number of free ions. 65
4.3.6 DSC/TGA In the present study, thermal stability of ionic liquid and polymer gel electrolytes has been studied by simultaneous DSC/TGA measurement in 25 550 o C temperature range at a heating rate of 10 o C/min under nitrogen atmosphere and the results are given in Fig. 4.13 The TGA thermogram of ionic liquid shows no significant weight loss upto 400 o C indicating high thermal stability of ionic liquid. A weight loss of less than 9% has been observed upto 425 o C. Above this temperature the weight loss takes place in a single step process. This weight loss is closely related to an endothermic peak at 434 o C in the DSC plot, indicating the decomposition of ionic liquid. In the TGA plot of the liquid electrolyte (PC+ 0.5M DMHxImTFSI), there is no significant weight loss upto 100 o C. Weight loss of 12.71% has been observed upto 149 o C. Above this temperature there is a sharp increase in the rate of weight loss. A major reduction of 57% by weight occurs as the temperature increases from 150 o C to 200 o C. DSC thermogram also shows an endothermic peak at 197 o C. This peak was not observed in the plot for ionic liquid, suggesting that this peak is due to the loss of PC. As PC forms a major component (83.87 wt%) of the electrolyte solution, higher weight loss occurs at this temperature. The second transition in the TGA curve at 425 o C corresponds to the onset of decomposition of ionic liquid. As the quantity of ionic liquid in the electrolyte is only 16.13 wt % (0.5M solution of IL in PC), this transition involves only a small weight loss. In polymer gel electrolytes having composition PC+ 0.5M DMHxImTFSI+ 20 wt% PMMA, weight loss of less than 2% has been observed upto 100 o C. Above 100 o C, the degradation of polymer gel electrolyte occurs through four weight loss processes. The first weight loss process corresponds to a peak at 174 o C in the DSC thermogram, which is due to the loss of PC. The endothermic peaks at 282 o C and 388 o C respectively are due to the degradation of PMMA, which involves two reaction processes. The first one at lower temperature involves a small weight loss and is due to the degradation of polymer s unsaturated end groups. While, the peak at 388 o C is due to the random bond scission produced by monomer volatilization. [Grillone et al. (1999)]. The next weight loss step at 428 o C is due to the decomposition of ionic liquid. 66
Temperature (Celsius) Temperature (Celsius) Temperature (Celsius) Figure 4.13. DSC/TGA/DTG plots for DMHxImTFSI (a), PC+ 0.5M DMHxImTFSI (b) PC+ 0.5M DMHxImTFSI+ 20 wt% PMMA(c). 67
4.3.7 1 H and 19 F NMR spectroscopy Ionic liquid DMHxImTFSI, which is the source of ions in the presently studied electrolytes, contains 1 H and 19 F nuclei which are NMR sensitive, therefore, 1 H and 19 F NMR investigations of ionic liquid and polymer gel electrolytes have been carried out to study the ion dynamics and its relation to the conductivity of these materials. 1 H and 19 F NMR spectra were recorded over 100-300 K temperature range by using a pulsed NMR spectrometer at 6.4 T with corresponding Larmor frequency of 271.25 and 255.20 MHz respectively. The spectra were obtained by a single pulse sequence followed by a Fourier transformation and the typical dead time of the spectrometer is 2 microseconds. 4.3.7 (a) 1 H and 19 F NMR spectra and linewidth The variation of the linewidth with temperature in 1 H and 19 F NMR spectra reveals the information about the mobility of corresponding ions. As observed in Fig. 4.14, NMR spectra of all the electrolytes show broad line at low temperatures (100-160 K). The broad NMR lines at low temperature are due to the dipolar and quadrupolar interactions. As the temperature increases, these interactions average out leading to line narrowing. As the temperature increases following observations have been made from the NMR data: (a) for ionic liquid, unnarrowed linewidth of 30-36 khz in 1 H NMR spectrum in the temperature range of 100-180 K (Fig 4.14). Line narrowing begins above 200 K and a residual linewidth of 1 khz observed above 250 K (b) in 19 F NMR spectrum, line narrowing takes place through two steps. One line narrowing down at 200 K, whereas the second line at 260 K. Above 260 K a single line with linewidth of 3.5 khz has been observed. (c) for liquid electrolyte (PC+ 0.5M DMHxImTFSI), the line narrowing of 1 H and 19 F NMR line occurs at 170-180 K. Linewidth of 19 F NMR line in the low temperature region is considerably lower than the linewidth for IL and also it has lower value than 1 H NMR line of liquid electrolyte for corresponding temperatures. (d) in polymer gel electrolyte (PC+ 0.5M DMHxImTFSI+ 20 wt% PMMA), line narrowing takes place at 180-190 K for both 1 H and 19 F NMR lines, with a slightly lower value of residual linewidth for 19 F (1 khz) than for 1 H (2 khz). 68
a 1 H a 19 F b b c c Figure 4.14. 1 H NMR and 19 F NMR spectra of DMHxImTFSI (a), PC+ 0.5M DMHxImTFSI (b) and (PC+ 0.5M DMHxImTFSI+ 20 wt% PMMA)( c). 69
a b c Figure 4.15. Variation of line width of 1 H NMR line ( ), 19 F NMR line ( ) and ionic conductivity ( ) for DMHxImTFSI (a), PC+0.5M DMHxImTFSI (b) and PC+ 0.5M DMHxImTFSI+ 20 wt% PMMA(c)) with temperature. 70
Motional narrowing commences when the rate of fluctuations of the local dipolar field is comparable to their rigid lattice linewidths. [Chung et al. (1991)]. Narrow linewidths indicate high mobility of the nuclei i.e. the system transforms from a rigid lattice state to ion diffusing state for the temperatures at which motional narrowing takes place. At high temperatures, increased diffusivity of the ions should also contribute to the ionic conductivity. To compare and correlate, the variation of ionic conductivity has been studied in the temperature range 100-400 K. From the Fig 4.15 it is clear that the electrolytes possess low conductivity, of the order of 10-5 S cm -1 in the low temperature region. For the ionic liquid, the ionic conductivity starts increasing above 230 K. This also corresponds with the melting point of the ionic liquid at 238 K.The order of magnitude higher conductivity has been observed above 260 K and it increases to the order of 10-2 S cm -1 at 340 K. Whereas, in case of liquid electrolyte (PC+0.5M DMHxImTFSI) and polymer gel electrolyte (PC+ 0.5M DMHxImTFSI+ 20wt % PMMA), the increase in conductivity is observed at the temperature165 K, which is quite lower than observed for IL. At high temperatures the value of conductivity for IL and liquid electrolyte is approximately same, though the liquid electrolyte shows the order of 10-2 S cm -1 conductivity at a temperature lower (300 K) than for IL (340 K). However, the value of conductivity for polymer gel electrolyte is marginally lower than for liquid electrolyte at all temperatures, which is due to its high viscosity. Upon comparison of the results obtained from NMR and ionic conductivity data, it is inferred that the temperature for onset of long range translational motion, also marked by the line narrowing of 1 H and 19 F NMR linewidths, is closely related to the temperature at which large increase in the conductivity is observed for all the presently investigated electrolytes. Different ion transport parameters viz jump frequency ( c ), diffusion coefficient (D), conductivity ( R) have been calculated from NMR results and their variation with temperature has been studied. The results obtained have been discussed below: 4.3.7 (b) Jump frequency Probability for ion to jump from one site to other in a given direction in unit time is known as jump frequency ( ). It depends upon the potential barrier seen by ions. The angular jump frequency was calculated [Abrgam (1961); Bolembergen (1948); Gutowsky and Pake (1950)] at different temperatures by using the relation: 71
c (angular frequency) = / tan 2 ( 2 B 2 ) (A 2 -B 2 ) where =1, is 2.52x10 4 sec -1 G -1 for 19 F and 2.67x10 4 sec -1 G -1 for 1 H. is the linewidth at temperature T, B is the fully narrowed linewidth and A is the unnarrowed linewidth and the units of c,, A and B are of angular frequency. As shown in the Fig. 4.16, value of the jump frequency is low, of the order of 10 6 rad sec -1, for IL (100-200 K) as well as PC+ 0.5M DMHxImTFSI+ 20 wt% PMMA (100-170 K) in the given temperature ranges. These are the temperature regions where the electrolytes are in static lattice state and the motion of the ions is seized. At high temperatures when quadrupolar interactions average out, the probability for ion to jump from one site to another increases and the jump frequency has been observed to increase to the order of 10 9-10 10 rad sec -1. a b Figure 4.16 Variation of jump frequency with temperature for 1 H NMR line ( ) and 19 F NMR line (Δ) for DMHxImTFSI (a) and PC+ 0.5M DMHxImTFSI+ 20 wt% PMMA 4.3.7 (c) Diffusion coefficient Diffusion coefficient was calculated from NMR data by using the Einstein relation: D = (1/6) L 2 where D is the diffusion coefficient, c is the jump frequency and L is the H--H (1.747x10-8 cm) or F--F (2.204x10-8 cm) interatomic distance in DMHxImTFSI. Diffusion coefficient is directly proportional to the jump frequency, so the variation of diffusion coefficient as a function of temperature follows similar variation as observed c 72
for jump frequency. The value of the diffusion coefficient increases from 10-10 to 10-6 cm 2 sec -1 ( 1 H NMR line) and 10-10 to 10-7 cm 2 sec -1 ( 19 F NMR line) for ionic liquid. For PC+ 0.5M DMHxImTFSI+ 20 wt% PMMA the diffusion coeffiecient varies from 10-10 to 10-6 cm 2 sec -1 ( 1 H NMR line) and 10-10 to 10-8 cm 2 sec -1 ( 19 F NMR line). 4.3.7 (d) Conductivity ( NMR ): The electrical conductivity ( NMR ) was also evaluated from NMR data by using the relation: NMR = ndq 2 / kt where n is the carrier concentration (taken as 1 10 21 cm -3 ), q is the charge on the mobile ions (1.6x 10-19 C), D is diffusion coefficient, k is Boltzmann constant and T is the absolute temperature. The values of the conductivity obtained from NMR results ( NMR ) are consistent with the values of ionic conductivity measured experimentally ( exp ). The comparison of NMR and exp have been given in Table 4.3 Table 4.3 Comparison of conductivity values measured experimentally and conductivity values calculated from NMR data at different temperatures Sample Temperature Range (K) Measured Experimentally (σ exp ) (S cm -1 ) Ionic Conductivity Calculated from NMR data (σ NMR ) (S cm -1 ) 1 H NMR 19 F NMR Broad Component Narrow Component 100-220 10-5 10-6 10-6 DMHxImTFSI 220-250 10-5 10-5 10-6 10-4 250-300 10-4 -10-3 10-2 10-4 -10-3 100-180 10-5 10-6 10-6 PC + 0.5M DMHxImTFSI 180-200 10-5 10-5 -10-4 10-5 -10-4 200-300 10-4 -10-3 10-3 -10-2 10-3 100-180 10-5 10-6 10-6 PC + 0.5M DMHxImTFSI + 20 wt% PMMA 180-200 10-4 10-5 -10-4 10-5 200-300 10-4 -10-3 10-2 10-4 73
4.4 Conclusion On the basis of the above studies on ionic liquid and polymer gel electrolytes containing ionic liquid, following conclusions have been drawn: (i) Ionic liquid 2,3-dimethyl-1-hexylimidazolium bis(trifluoromethanesulfonyl) imide (DMHxImTFSI) shows high value of ionic conductivity increasing from 2.47x10-3 S cm -1 at 30 o C to 1.52x10-2 S cm -1 at 90 o C. (ii) Variation of ionic conductivity corresponds well with the decrease in viscosity from 109 mpa s at 30 o C to 16.5 mpa s at 90 o C. (iii) Addition of polymers PMMA and PVdF-HFP to the liquid electrolyte results in an initial increase in the conductivity values leading to maxima, hence, polymer gel electrolytes having conductivity higher than the corresponding liquid electrolytes have been obtained. (iv) Ionic liquid is thermally stable upto 400 o C, whereas polymer gel electrolytes containing solvent PC is stable upto 100 o C. (v) Both cations and anions show mobility in the 1 H and 19 F solid state NMR spectra of the ionic liquid and polymer gel electrolytes. (vi) In ionic liquid, the temperature for onset of ion diffusional motion corresponds well with the melting point of ionic liquid. (vii) Presence of PC affects the motional narrowing by lowering the temperatures for onset of ion diffusional motion. 74