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Vibrational spectroscopic study of the interaction between lithium perchlorate and dimethylsulfoxide
Ekcrrochimica Acfa, Vol. 42, No. 17, pp. 261 l-2617, 1997 Q 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: s0013&86(96)oo44o-9 00134686/97 $17.00 + 0.00
Pergsmon
Vibrational spectroscopic study of the interaction between lithium nerchlorate and dimethylsulfoxike Zhaoxiang Wang, Biying Huang, Sumin Wang, Rongjian Xue, Xuejie Huang and Liquan Chen* Laboratory
for Solid State Ionics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-29, Beijing 100080, People’s Republic of China
(Received 13 May 1996; in revised form 16 October 1996) Abstract-As part of the systematic research on the interactions between the components of the polyacrylonitrile-based electrolytes (plasticizer-lithium salt-polymer), the Raman and IR spectra of pure dimethylsulfoxide (DMSO) and DMSO containing different molar ratios of LiC104 have been studied. By analyzing the Raman and IR spectra of the s---O stretching band, it is found that the introduction of LiC104 to pure DMSO leads to the dissociation of the cyclic dimers. Li+-DMSO associates are formed through the interaction between Li+ and the S=O group of DMSO. The spectral changes of the s---O bending bands and the C-H stretching bands indicate that the interaction between the Li+ ion and the molecule not only breaks down the self-association of the solvent, but also, by way of the induction effect, perturbs the electronic nebula around the C-H bond. It seems that the effect of the ClO; anion on the molecules is too weak to be observed by the vibrational spectroscopy. % 1997 Elsevier Science Ltd Key fjords: Dimethylsulfoxide
(DMSO), lithium perchlorate (LiClO,), Raman spectra, IR spectra, interaction.
INTRODUCTION A number of polymer-salt systems have been found to be suitable solid electrolytes for a variety of applications, such as solid batteries, fuel cells and electrochemical display devices because of their considerable ionic conductivity at room temperature [l-3]. Of the solid electrolytes, polyacrylonitrile (PAN)-based electrolytes seem to be a promising candidate because they exhibit excellent properties in mechanics, ionics and compatibility with the electrodes [4-61. In the PAN-based electrolytes, three components are necessary at least, including the polymer, the plasticizer(s) and the lithium salt. Therefore, it is interesting to find out if there are interactions between the components and how the components interact with each other. These questions are important because they are relevant to the transport mechanism of the lithium ions in the solid electrolytes, which has not been fully understood to *Corresponding EA42.17
B
author.
date, and the synthesis of the gel electrolytes with better properties. Dimethylsulfoxide (DMSO) is commonly used as the plasticizer (or solvent) in PAN-based electrolytes because of its high polarity as well as its high boiling point (189°C) and rather high dielectric constant (48). In PAN-based electrolytes, strong interactions have been observed between lithium ions and the plasticizers such as ethylene carbonate (EC) [7,8], propylene carbonate (PC) [9], and dimethylformamide (DMF) [lo, 111. However, the high self-association of DMSO at room temperature makes the structure of DMSO as a solvent more complicated and brings difficulty to the analysis of the interaction between DMSO and the lithium salt. Ramana and Singh [ 121have studied the ion-molecule interactions of lithium bromide and lithium iodide with DMSO by Raman spectroscopy and observed some interesting interactions between DMSO and the ions. Since the structure of the perchlorate anions are much more complicated than that of the bromide and iodide anions, some different characters may be observed in LiClOd-DMSO systems.
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Zhaoziang Wang
To gain a better insight into the interactions between LiCIOI and DMSO, both the Raman and IR spectroscopy was used in the present study. By recording and analyzing the vibrational spectra of DMSO molecules as well as ClO; anions, more information was obtained.
et al.
shift or band width or intensity. Therefore, in the present investigation, the 1418 cm-’ band is used as the internal intensity standard in discussing the Raman spectra though some other authors have used the CH3 rocking band at ca. 954 cm-’ [12, 131or the envelope of the S==O stretching mode [14] as the standard.
EXPERIMENTAL Preparation of the samples The reagents used in the experiment, DMSO and LiClO4, have an analytical grade of purity. On preparing the samples, first LiClO4 is dried in a vacuum oven for ca. 10 h and DMSO is distilled twice. As soon as the reagents are ready to use, they are mixed at the required molar ratios (LiC104:DMSO). Contained in a sealed bottle, the mixture is heated at 100°C in a heating box for 5 h. The spectra are recorded at 20°C. Raman and IR spectrometer and the experimental geometry
The IR and Raman spectra shown in the paper are recorded with an IFS66 + FRA106 type Fourier transform IR-Raman spectrometer (Bruker Instruments Co., Germany). The laser power on the sample is 200 mW with a radiation spot of 0.1 mm in diameter. The excitation line for the Raman spectra is 1.064 pm. The scattered light is collected in the direction of 180” to the incident light (back scattering geometry). Each of the Raman spectra is the result of 200 scans, while that of the IR spectra is the result of 16 scans, at a speed of one scan per 2 s. The resolution of the spectrometer under this experimental setup is 4 cm-‘. A germanium detector of the scattered light is cooled in liquid nitrogen. Optical glass testing tubes are used only once in the Raman measurement to avoid the possible contamination to the following samples. For the IR measurement, a very small drop of the solution is evenly spread on the well-polished KBr crystal. Both the Raman and the IR measurements are carried out in the air. RESULTS
S==O stretching envelope and band fitting Pure DMSO at room temperature is highly self-associated. Associations may occur by means of O.....H bridging [15], S.....H bridging [IS], or S.....O bridging [l&18] of the DMSO molecules. Associations through O.....H and S.‘.‘.H bridgings appear improbable because of the low acidity of the C.....H bond in DMSO, whereas S.....O bridging seems to be more probable because of the strong polar character of the S.....O bond. Due to the self-association in pure DMSO, the S=O stretching band is a wide and structured envelope. It has been generally accepted that the band at ca. 1070 cm-’ belongs to the S=O stretching mode of free DMSO, whereas the two bands observed at ca. 1055 cm-’ and 1040 cm-’ belong to out-of-phase and in-phase S=O
AND DISCUSSION
As will be shown in the following, the intensities of many of the vibrational bands of different samples are changed significantly in the Raman and IR spectra. Therefore, it is necessary to choose a band as the internal intensity standard for comparison. For the accuracy of the comparison and calculation, the band used as the internal standard should be changed as little as possible, both in the vibrational frequency and in the intensity. Moreover, its intensity should not be too weak. It seems from Figs 1 and 2 that throughout the sample concentrations studied in the experiment, the CHj asymmetric deformation mode of DMSO at 1418 cm-’ remains a simple peak and no significant change is observed either in its Raman
I
1500
I
1240
I
1
980 720 Wavenumber(cm-‘)
I
460
I
200
Fig. 1. The Raman spectra of DMSO and DMSO with different mole ratios of LiClO4: (a) pure DMSO, (b) 0.05; (c) 0.1; (d) 0.2; (e) 0.3; (f) 0.4; (g) 0.6 (note: the bands with arrows are modes from Cl0.i).
Interaction
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1150
1100
1050
1000
950
900
Wavenumber(cm-‘) Fig. 3. Band-fitting to the Raman (a) and IR (b) spectra of pure DMSO in the S==O stretching region (legend: solid line, the experimental envelope; circle, the band-fitting contour; short dashed line, the resolved component by band-fitting). I
1
1800
1520
1240
960
680
400
Wavenumber(cm-‘) Fig. 2. The IR spectra of DMSO and DMSO containing different mole ratios of LiClO4: (a) pure DMSO; (b) 0.05; (c) 0.1; (d) 0.2; (e) 0.3; (f) 0.4 (note: the bands with arrows are of the modes of CIO;).
stretching of the cyclic dimer. The low-frequency band at cu. 1025 cm-’ is assigned to the linear dimer and/or higher polymers. We indeed identified four components both in the Raman and IR spectra of pure DMSO by band-fitting technique (ORIGIN 3.5 of Microsoft Co.). However, as is shown in Fig. 3, only the latter three reported components were discriminated in the present study. No trace was observed about the 1070 cm-’ component, meaning that there are too few free molecules at room temperature to be detected. These band-fitting results in the Raman and IR spectra agree to each other and also coincide with the results of Rintoul and Shurvell [14] both in the frequency and the relative intensity. The component at ca. 1015 cm-’ under this envelope will be discussed in the third section. Figures 4 and 5 show the Raman and IR spectra of the S=O stretching mode of DMSO at different concentrations of LiClOd. It can be seen that the change of the contour of the spectra is more apparent in the Raman than in the IR. At low LiClO4 content in the solution, the intensity of the ca. 1025 cm-’ band is dominant. When LiClO4 is added into DMSO, the top of the contour of the S==G stretching mode moves towards the low frequency side. That is, the relative intensity of the 1025 cm-’ component
increases further, while the relative intensities of the modes related to the cyclic dimers decrease until the molar ratio of LiClO4 in the solution reaches 0.3 [Fig. 4(d)]. In the meantime, the 1070cm-’ band,
1180
1130 1080 Wavenumber(
1030 cm-*)
980
Fig. 4. The Raman spectra of the S=O mode of DMSO at different mole ratios of LiC104: (a) 0.05; (b) 0.1; (c) 0.2; (d) 0.3; (e) 0.4; (f) 0.6.
Zhaoziang Wang et
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I
1300
I
I
1210 1120 1030 Wavenumber(cm-‘)
I
940
1
850
Fig. 5. The IR spectra of the S=O stretching mode of DMSO at different mole ratios of LiClO4: (a) 0.05; (b) 0.1; (c) 0.2; (d) 0.3; (e) 0.4.
which represents the existence of free DMSO molecules, appears and a wide band with even higher frequency (the fifth band) shows up at ca. 1090 cm-’ [Fig. 4(b)]. This new band is assigned to the Li+-associated DMSO molecules (see the following). This means that the addition of LiC104 into DMSO has perturbed the DMSO molecules self-associated in the form of cyclic dimers. However, the dissociation of the cyclic dimers does not promote the increase of free DMSO molecules in the solution. Instead, the new wide band at 1090 cm-’ grows in intensity with the increasing content of LiC104 and moves towards the high frequency side of the spectra. At the molar ratio of 0.3, the new band has moved from 1072 cm-’ of free DMSO molecules [Fig. 4(a)] to 1089 cm-’ [Fig. 4(b)] and then to 1101 cm-’ [Fig. 4(d)]. At the molar ratio of 0.3 [Fig. 4(d)], the spectral intensities of the cyclic dimers and the linear dimers and/or higher polymers become comparable. However, when LiClO4 is further added into DMSO, the 1040 and 1055 cm-’ components decrease so much that they are almost negligible in intensity while that of the 1027 cm-’ component increases significantly [compare Fig. 4(d) with Figs 4(e) and (f)]. This implies that the molar ratio of ca. 0.3 is the critical concentration for the avalanche-like dissociation of the loosely self-associated cyclic dimers. The LiClOdperturbed cyclic dimers become free DMSO molecules and then associates with LiC104. Similar to the phenomena observed in Figs 4(a) and (b), as the concentration of the solution increases, the wide band
al.
due to the association of LiC104 and DMSO continues to move towards the high frequency side of the Raman spectra. At a molar ratio of ca. 0.6, this band moves to 1123 cm-‘. In the meantime, its relative intensity increases significantly. By the band-fitting technique, Ramana and Singh [12] identified a band at ca. lOOOcm-’ in the LiBrand LiI-DMSO solutions and they assigned this new component to the m stretching mode of Li+-DMSO complex species. However, as has been described above, no such 1000 cm-’ component was identified either in the Raman or IR spectra in the present investigation. It seems that the S=O stretching mode of Li+-DMSO is not located at ca. 1000 cm-’ in the LiCIOrDMSO solutions. In previous studies on the association of Li+ ions with EC [7, 81, PC [9], and DMF [ll], it was found that the Li+ ions associated with the oxygen atoms of EC, PC and DMF and with the nitrogen atom of DMF. The associations make the related Raman and IR bands move towards the high frequency side of the spectrum. This is reasonable. When Li+ ions associate with the 0 or N atoms of the solvents, the effective masses of the associates such as [Li:O=C]+ and/or [Li:O-Cl+ in EC and PC or [Li:N-C=O]+ and [Li:O=C-N]+ in DMF become much less than the original O-C or N-C bond though the association makes the C-O or C-N bonding less tight. Therefore, the total effect of the association makes the vibrational frequency of the bond increase because the vibrational frequency is proportional to fi, where p is the effective mass of the associates and K is proportional to the strength of the chemical bond. In the light of this viewpoint, we assign the wide band at ca. 1100 cm-’ in the LiClOpDMSO system to the S=O stretching mode of Li+-DMSO complex species in the solution. Although the Li+-DMSO association occurs mainly between Li+ and free DMSO molecules, the possibility cannot be ruled out that there may be interaction between Li+ ions and the self-associated DMSO molecules, especially the linear dimers and/or higher polymers. As a result, the environment around the Li+ ions is very complicated. Therefore, the width of the 1100 cm-’ band becomes larger when the solution becomes more concentrated and the species in the solution becomes more complicated. Raman spectra in the S=O bending region Figure 6 shows the Raman spectra in the S=O bending region of solutions of LiC104 in DMSO. In pure DMSO, three bands are observed at 383, 335 and 306 cm-‘, which have been assigned to the in-plane and the out-of-plane bending of the M bond and the in-plane CSC bending of DMSO, respectively. With the increase of LiC104 content, the positions of these three bands remain unchanged. These results agree quite well with those of Ramana and Singh [12] in the LiBr- and LiI-DMSO systems.
Interaction
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between lithium perchlorate and dimethylsulfoxide
Nevertheless, the relative intensities of the two S=O bending modes decrease significantly when LiClO4 is added into DMSO, especially the 383 cm-’ band. Since only the bands related to the S=O bond changes significantly in the intensity, while the CSC bond-related band almost remains unchanged, it is possible that the intensity changes of the two bands are due to the interaction between Li+ ions and the S=O group of DMSO. However, Ramana and Singh [ 191 have pointed out that even in the system of tetrabutylammonia bromide (TBAB)-DMSO-d6 solution, such a phenomenon is observed. Therefore, it seems that the cation-molecule interaction alone are not responsible for this effect. Considering that the DMSO molecules are self-associated by the S. .. ..O bridging and the addition of LiClOd breaks down the self-association to some extent, it seems possible that the changes in the intensity of these bands might be related to the breaking down of the complex species by LiClOd. That is, both the cation and anion in the solution are responsible to the intensity changes of these two bands. This question will be further discussed in the following section. Raman and IR spectra related to the CHJ groups
Besides all the three components in the s---O stretching envelope discussed in the first section,
2615
f
e
4
I
3100
d ,1
I
3050 3000 2950 Wavenumber(cm-‘)
1
2900
2850
Fig. 7. The Raman spectra of the C-H symmetric stretching and asymmetric modes of DMSO at various contents of LiCIO4: (a) pure DMSO; (b) 0.05; (c) 0.1; (d) 0.2; (e) 0.3; (f) 0.4.
I
I
520
475
430 385 340 Wavenumber(cm-I)
295
1
250
Fig. 6. The Raman spectra of the S=O bending region of DMSO with different contents of LiC104: (a) pure DMSO; (b) 0.05; (c) 0.1; (d) 0.2; (e) 0.3; (f) 0.4; (g) 0.6.
another component identified is located at cu. 1015 cm-‘. Different authors assign this component differently. Ramana and Singh [12] assigned it to the polymeric species of DMSO while Rintoul and Shurvell [14] assigned it to the asymmetric CH3 rocking frequency because of its greater intensity in the IR spectrum and its higher frequency. However, it seems more reasonable to rule out the attribution of this component to the CHJ rocking mode of DMSO, according to the IR analysis of the deuterated DMSO [19]. From Figs 4 and 5, it can be seen that the relative intensity of this component increases with the LiClO4 content. Figure 7 shows the Raman spectra of the symmetric (at cu. 2910 cm-‘) and asymmetric (at cu. 3000 cm-‘) C-H stretching bands in DMSO with different LiClO4 concentrations, while Fig. 8 shows the LiClOd-concentration dependence of the Raman shifts of these two modes. It can be seen from these two figures that the Raman shifts of the symmetric and asymmetric C-H stretching bands at 2996 and 2912 cm-’ increase with increasing LiC104 molar ratio and the relationships of the Raman shift vs the LiClO4 concentration are almost linear, but the positions of their counterparts in the IR spectra (not shown here) seem to be constant, although the concentration of the solution changes greatly For the sake of clarity, the
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Zhaoziang Wang et al.
concentration dependences of the Raman shifts of these two bands are shown in Fig. 8. These observations cannot be due to the experimental error since there are many bands in the Raman spectra whose positions remain constant throughout the concentrations investigated here and therefore, their positions can be used as the internal frequency standards, such as the bands at 1418, 951 (CHj rocking) and 931 cm-’ (the vI mode of CIO; anion) in the Raman spectra. The IR and Raman spectra of solutions of TBAB in DMSO-& [ 19, 201 and alkali metals and tetrabutylammonia halides in acetonitrile [ 12,21-241 have
3020
1
3017 3014 -3011 ; g 3006 V < 3005 .d A m 3002 9 g 2999 d
p: 2996
-
2993
-
2990
’
’
-0.10.0
’
’
Molar 2g30
’
’
’
’
0.1 0.2 0.3 0.4 0.5 0.6 0.7
ratio
I
2924 t
shown the possible interactions between the CHJ group of the solvents and the halides anions. These interactions result in the anion-molecular complex species in the solution and, therefore, the vibrational frequencies of the C-H stretching bands move towards the low-frequency side in the spectra. However, as Yearger et al. [25] have pointed out, the solvation effect of ClO; anions in the nonaqueous solution is very weak, therefore, the influence of ClO; on DMSO is too weak to be detected. Even if considering the perturbation of the solvent to the ClO; anions, such as the splittings of the v4 mode (at 624cm-‘) and the VI mode (at 931 cm-‘) in the Raman spectra and the activation of the vI mode in the IR spectra, which will be discussed elsewhere, it is still difficult to expect that the ClO;-CHJ group interaction will lead to the frequency increase of the C-H stretching modes. Instead, such an interaction will make the C-H stretching frequency decrease. This phenomenon also rules out the possibility that the intensity changes of the 383 and 335 cm-’ bands come from the anion-molecular interactions. The vibrational frequencies of the C-S symmetric stretching modes of DMSO at 699 and 669 cm-‘, respectively, also move towards the high frequency side in the Raman spectra (Fig. 1), where the v4mode of the ClO; anion at 624cm-’ can be used as the internal frequency standard). The relationship between the position of this band with the concentration of LiClO4 in the solution has been plotted in Fig. 9. These changes are continuous in the Raman shift with the increase of the LiC104 contents and during this process, no new components are discriminated from the original bands. This changing process indicates that there is no change in the symmetry of the CH3 structure or the CH+S-CH, structure during the addition of LiC104 into DMSO. What has been changed is the gradual and continuous increase of the strength of the chemical bond or the electronic nebula. Therefore, it is proposed that the increase of the Li+ concentration in the solution not only results in the formation of Li+-DMSO associates, but also makes the electronic nebula of the C-H bond increase by way of the induction effect transferred along the molecular chain. This mechanism of interaction can also explain the change features in the frequency of the C-S stretching modes at ca. 699 and 669 cm-’ as well as the C-H stretching modes. CONCLUSIONS
-0.10.0
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Molar
ratio
Fig. 8. The LiCIOd-content dependence of the Raman shifts of the symmetric (A) and asymmetric (B) CH3 stretching modes of DMSO.
The Raman and IR spectra of pure DMSO and DMSO containing different amounts of LiClOd have been investigated. With the addition of LiClO4 into the solvent, the self-associates of DMSO molecules are disassembled. Both the Raman and the IR spectra indicate that with the increase of LiClO4 content in the solvent, the loosely associated cyclic dimers in the solvent are dissociated. The interaction between the
Interaction
between lithium perchlorate and dimethylsulfoxide
725
2617
ACKNOWLEDGEMENTS
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%
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,
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This work was supported by Ford & NSFC, National Science Foundation of China (Contract No. 09412303). Z.-X.Wang is also thankful to Dr T. Wang of Bruker Instruments Co., Germany (Beijing Office) for his kind help in recording the spectra and Professor Y.-L. Liu of the Raman Laboratory, Institute of Physics, CAS, for his helpful discussion. REFERENCES 1. P. V. Wright, Br. Polym. .I. 7, 319 (1975). 2. P. V. Wright, J. Polym. Sci. Polym. Phys., Ed. 14, 955 (1976). 3. M. Armand, J. M. Chabagno and M. J. Duclot, 2nd International Conference on Solid Electrolytes, St Andrews, paper, 6, 5 (1978). 4. B.-Y. Huang, R.-J. Xue, L.-Q. Chen and F.-S. Wang, Ph.D Thesis (Institute of Physics, Chinese Academy of Sciences) 1995. 5. K. M. Abraham and M. Alamgir, J. Electrochem. Sot.
ratio
137, 1657 (1990).
H.-Huang, L.-Q. Chen and R.-J. Xue, in Solid Stare ionics: Materials and Applications (Edited by B. V. R. Chowdari, S. Chandra, Shri Singh and P. C. Srivastava), p. 199. World Scientific Pub. Co.. Singapore (1992). 7. S. Hyodo and K. Akabayashi, Electrochim. Acia 34,
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B.-Y. Huang, H. Huang, R.-J. Xue, L.-Q. Chen and F.-S. Wang, J. Electrochem. Sot. 143, 1510 (1996).
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10. Z.-X. Wang, B.-Y. Huang, H. Huang, L.-Q. Chen and R.-J. Xue, Solid State Ionics, M-88, 313 (1996). Il. Z.-X. Wang, B.-Y. Huang, Z.-H. Lu, H.-W. Yan, H. Huang, R.-J. Xue and L.-Q. Chen, Solid State Ionics. in press. 12. K. V. Ramana and S. Singh, J. Raman Spectrosc. 20,
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15, 80
14. L. Rintoul and H. F. Shurvell, J. Raman Spectrosc. 21, 501 (1990).
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Molar
M. I. Sastry and S. Singh, J. (19X41. \-- - ./-
r3---0.10.0
D. Battisti, G. A. Nazri, B. Klassen and R. Aroca, J. Phys. Chem. 97, 5826 (1993).
ratio
Fig. 9. The LiCIOa-content dependence of the Raman shifts symmetric (A) and asymmetric (B) modes of
of the C-S DMSO.
Li+ ions and the s---O groups of the molecules result in the formation of Lit-DMSO associates. Different from the result of the simple halide anions, it seems that the effect of the ClO; anions on the molecules is too weak to be observed. Further spectroscopic study on the C-H stretching modes and the S-C stretching modes indicates that the interaction of the Li+ ions and the s--O groups of the molecules can also have an induction effect on the C-S bonding and the C-H bonding and makes the latter bonding stronger.
15. H. L. Schlafer and W. Schaffernicht, Angew. Chem. 72, 618 (1960).
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Sot. 85, 715 (1963). 19. K. Burger, Solvation, Ionic and Complex Formation Reactions in Non-Aqueous Solvents, p. 118, Elsevier, Amsterdam (1983). 20. K. V. Ramana and S. Singh, J. Mol. Struci. 194, 73 (1989). 21. H. Michel and E. Lippert, in Organic Liquids:Structure, Dynamics, and Chemical Properties
(Edited by A. D.
Buckingham, E. Lippert and S. Bratos) p. 293. Willey, New York (1978). 22. M. I. Sastry and S. Singh, Curr. Sci. 55, 1157 (1986). 23. K. V. Ramana and S. Singh, Spectrochim. Acta A 44, 277 (1988).
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Pergsmon
Vibrational spectroscopic study of the interaction between lithium nerchlorate and dimethylsulfoxike Zhaoxiang Wang, Biying Huang, Sumin Wang, Rongjian Xue, Xuejie Huang and Liquan Chen* Laboratory
for Solid State Ionics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-29, Beijing 100080, People’s Republic of China
(Received 13 May 1996; in revised form 16 October 1996) Abstract-As part of the systematic research on the interactions between the components of the polyacrylonitrile-based electrolytes (plasticizer-lithium salt-polymer), the Raman and IR spectra of pure dimethylsulfoxide (DMSO) and DMSO containing different molar ratios of LiC104 have been studied. By analyzing the Raman and IR spectra of the s---O stretching band, it is found that the introduction of LiC104 to pure DMSO leads to the dissociation of the cyclic dimers. Li+-DMSO associates are formed through the interaction between Li+ and the S=O group of DMSO. The spectral changes of the s---O bending bands and the C-H stretching bands indicate that the interaction between the Li+ ion and the molecule not only breaks down the self-association of the solvent, but also, by way of the induction effect, perturbs the electronic nebula around the C-H bond. It seems that the effect of the ClO; anion on the molecules is too weak to be observed by the vibrational spectroscopy. % 1997 Elsevier Science Ltd Key fjords: Dimethylsulfoxide
(DMSO), lithium perchlorate (LiClO,), Raman spectra, IR spectra, interaction.
INTRODUCTION A number of polymer-salt systems have been found to be suitable solid electrolytes for a variety of applications, such as solid batteries, fuel cells and electrochemical display devices because of their considerable ionic conductivity at room temperature [l-3]. Of the solid electrolytes, polyacrylonitrile (PAN)-based electrolytes seem to be a promising candidate because they exhibit excellent properties in mechanics, ionics and compatibility with the electrodes [4-61. In the PAN-based electrolytes, three components are necessary at least, including the polymer, the plasticizer(s) and the lithium salt. Therefore, it is interesting to find out if there are interactions between the components and how the components interact with each other. These questions are important because they are relevant to the transport mechanism of the lithium ions in the solid electrolytes, which has not been fully understood to *Corresponding EA42.17
B
author.
date, and the synthesis of the gel electrolytes with better properties. Dimethylsulfoxide (DMSO) is commonly used as the plasticizer (or solvent) in PAN-based electrolytes because of its high polarity as well as its high boiling point (189°C) and rather high dielectric constant (48). In PAN-based electrolytes, strong interactions have been observed between lithium ions and the plasticizers such as ethylene carbonate (EC) [7,8], propylene carbonate (PC) [9], and dimethylformamide (DMF) [lo, 111. However, the high self-association of DMSO at room temperature makes the structure of DMSO as a solvent more complicated and brings difficulty to the analysis of the interaction between DMSO and the lithium salt. Ramana and Singh [ 121have studied the ion-molecule interactions of lithium bromide and lithium iodide with DMSO by Raman spectroscopy and observed some interesting interactions between DMSO and the ions. Since the structure of the perchlorate anions are much more complicated than that of the bromide and iodide anions, some different characters may be observed in LiClOd-DMSO systems.
2611
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Zhaoziang Wang
To gain a better insight into the interactions between LiCIOI and DMSO, both the Raman and IR spectroscopy was used in the present study. By recording and analyzing the vibrational spectra of DMSO molecules as well as ClO; anions, more information was obtained.
et al.
shift or band width or intensity. Therefore, in the present investigation, the 1418 cm-’ band is used as the internal intensity standard in discussing the Raman spectra though some other authors have used the CH3 rocking band at ca. 954 cm-’ [12, 131or the envelope of the S==O stretching mode [14] as the standard.
EXPERIMENTAL Preparation of the samples The reagents used in the experiment, DMSO and LiClO4, have an analytical grade of purity. On preparing the samples, first LiClO4 is dried in a vacuum oven for ca. 10 h and DMSO is distilled twice. As soon as the reagents are ready to use, they are mixed at the required molar ratios (LiC104:DMSO). Contained in a sealed bottle, the mixture is heated at 100°C in a heating box for 5 h. The spectra are recorded at 20°C. Raman and IR spectrometer and the experimental geometry
The IR and Raman spectra shown in the paper are recorded with an IFS66 + FRA106 type Fourier transform IR-Raman spectrometer (Bruker Instruments Co., Germany). The laser power on the sample is 200 mW with a radiation spot of 0.1 mm in diameter. The excitation line for the Raman spectra is 1.064 pm. The scattered light is collected in the direction of 180” to the incident light (back scattering geometry). Each of the Raman spectra is the result of 200 scans, while that of the IR spectra is the result of 16 scans, at a speed of one scan per 2 s. The resolution of the spectrometer under this experimental setup is 4 cm-‘. A germanium detector of the scattered light is cooled in liquid nitrogen. Optical glass testing tubes are used only once in the Raman measurement to avoid the possible contamination to the following samples. For the IR measurement, a very small drop of the solution is evenly spread on the well-polished KBr crystal. Both the Raman and the IR measurements are carried out in the air. RESULTS
S==O stretching envelope and band fitting Pure DMSO at room temperature is highly self-associated. Associations may occur by means of O.....H bridging [15], S.....H bridging [IS], or S.....O bridging [l&18] of the DMSO molecules. Associations through O.....H and S.‘.‘.H bridgings appear improbable because of the low acidity of the C.....H bond in DMSO, whereas S.....O bridging seems to be more probable because of the strong polar character of the S.....O bond. Due to the self-association in pure DMSO, the S=O stretching band is a wide and structured envelope. It has been generally accepted that the band at ca. 1070 cm-’ belongs to the S=O stretching mode of free DMSO, whereas the two bands observed at ca. 1055 cm-’ and 1040 cm-’ belong to out-of-phase and in-phase S=O
AND DISCUSSION
As will be shown in the following, the intensities of many of the vibrational bands of different samples are changed significantly in the Raman and IR spectra. Therefore, it is necessary to choose a band as the internal intensity standard for comparison. For the accuracy of the comparison and calculation, the band used as the internal standard should be changed as little as possible, both in the vibrational frequency and in the intensity. Moreover, its intensity should not be too weak. It seems from Figs 1 and 2 that throughout the sample concentrations studied in the experiment, the CHj asymmetric deformation mode of DMSO at 1418 cm-’ remains a simple peak and no significant change is observed either in its Raman
I
1500
I
1240
I
1
980 720 Wavenumber(cm-‘)
I
460
I
200
Fig. 1. The Raman spectra of DMSO and DMSO with different mole ratios of LiClO4: (a) pure DMSO, (b) 0.05; (c) 0.1; (d) 0.2; (e) 0.3; (f) 0.4; (g) 0.6 (note: the bands with arrows are modes from Cl0.i).
Interaction
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between lithium perchlorate and dimethylsulfoxide
1150
1100
1050
1000
950
900
Wavenumber(cm-‘) Fig. 3. Band-fitting to the Raman (a) and IR (b) spectra of pure DMSO in the S==O stretching region (legend: solid line, the experimental envelope; circle, the band-fitting contour; short dashed line, the resolved component by band-fitting). I
1
1800
1520
1240
960
680
400
Wavenumber(cm-‘) Fig. 2. The IR spectra of DMSO and DMSO containing different mole ratios of LiClO4: (a) pure DMSO; (b) 0.05; (c) 0.1; (d) 0.2; (e) 0.3; (f) 0.4 (note: the bands with arrows are of the modes of CIO;).
stretching of the cyclic dimer. The low-frequency band at cu. 1025 cm-’ is assigned to the linear dimer and/or higher polymers. We indeed identified four components both in the Raman and IR spectra of pure DMSO by band-fitting technique (ORIGIN 3.5 of Microsoft Co.). However, as is shown in Fig. 3, only the latter three reported components were discriminated in the present study. No trace was observed about the 1070 cm-’ component, meaning that there are too few free molecules at room temperature to be detected. These band-fitting results in the Raman and IR spectra agree to each other and also coincide with the results of Rintoul and Shurvell [14] both in the frequency and the relative intensity. The component at ca. 1015 cm-’ under this envelope will be discussed in the third section. Figures 4 and 5 show the Raman and IR spectra of the S=O stretching mode of DMSO at different concentrations of LiClOd. It can be seen that the change of the contour of the spectra is more apparent in the Raman than in the IR. At low LiClO4 content in the solution, the intensity of the ca. 1025 cm-’ band is dominant. When LiClO4 is added into DMSO, the top of the contour of the S==G stretching mode moves towards the low frequency side. That is, the relative intensity of the 1025 cm-’ component
increases further, while the relative intensities of the modes related to the cyclic dimers decrease until the molar ratio of LiClO4 in the solution reaches 0.3 [Fig. 4(d)]. In the meantime, the 1070cm-’ band,
1180
1130 1080 Wavenumber(
1030 cm-*)
980
Fig. 4. The Raman spectra of the S=O mode of DMSO at different mole ratios of LiC104: (a) 0.05; (b) 0.1; (c) 0.2; (d) 0.3; (e) 0.4; (f) 0.6.
Zhaoziang Wang et
2614
I
1300
I
I
1210 1120 1030 Wavenumber(cm-‘)
I
940
1
850
Fig. 5. The IR spectra of the S=O stretching mode of DMSO at different mole ratios of LiClO4: (a) 0.05; (b) 0.1; (c) 0.2; (d) 0.3; (e) 0.4.
which represents the existence of free DMSO molecules, appears and a wide band with even higher frequency (the fifth band) shows up at ca. 1090 cm-’ [Fig. 4(b)]. This new band is assigned to the Li+-associated DMSO molecules (see the following). This means that the addition of LiC104 into DMSO has perturbed the DMSO molecules self-associated in the form of cyclic dimers. However, the dissociation of the cyclic dimers does not promote the increase of free DMSO molecules in the solution. Instead, the new wide band at 1090 cm-’ grows in intensity with the increasing content of LiC104 and moves towards the high frequency side of the spectra. At the molar ratio of 0.3, the new band has moved from 1072 cm-’ of free DMSO molecules [Fig. 4(a)] to 1089 cm-’ [Fig. 4(b)] and then to 1101 cm-’ [Fig. 4(d)]. At the molar ratio of 0.3 [Fig. 4(d)], the spectral intensities of the cyclic dimers and the linear dimers and/or higher polymers become comparable. However, when LiClO4 is further added into DMSO, the 1040 and 1055 cm-’ components decrease so much that they are almost negligible in intensity while that of the 1027 cm-’ component increases significantly [compare Fig. 4(d) with Figs 4(e) and (f)]. This implies that the molar ratio of ca. 0.3 is the critical concentration for the avalanche-like dissociation of the loosely self-associated cyclic dimers. The LiClOdperturbed cyclic dimers become free DMSO molecules and then associates with LiC104. Similar to the phenomena observed in Figs 4(a) and (b), as the concentration of the solution increases, the wide band
al.
due to the association of LiC104 and DMSO continues to move towards the high frequency side of the Raman spectra. At a molar ratio of ca. 0.6, this band moves to 1123 cm-‘. In the meantime, its relative intensity increases significantly. By the band-fitting technique, Ramana and Singh [12] identified a band at ca. lOOOcm-’ in the LiBrand LiI-DMSO solutions and they assigned this new component to the m stretching mode of Li+-DMSO complex species. However, as has been described above, no such 1000 cm-’ component was identified either in the Raman or IR spectra in the present investigation. It seems that the S=O stretching mode of Li+-DMSO is not located at ca. 1000 cm-’ in the LiCIOrDMSO solutions. In previous studies on the association of Li+ ions with EC [7, 81, PC [9], and DMF [ll], it was found that the Li+ ions associated with the oxygen atoms of EC, PC and DMF and with the nitrogen atom of DMF. The associations make the related Raman and IR bands move towards the high frequency side of the spectrum. This is reasonable. When Li+ ions associate with the 0 or N atoms of the solvents, the effective masses of the associates such as [Li:O=C]+ and/or [Li:O-Cl+ in EC and PC or [Li:N-C=O]+ and [Li:O=C-N]+ in DMF become much less than the original O-C or N-C bond though the association makes the C-O or C-N bonding less tight. Therefore, the total effect of the association makes the vibrational frequency of the bond increase because the vibrational frequency is proportional to fi, where p is the effective mass of the associates and K is proportional to the strength of the chemical bond. In the light of this viewpoint, we assign the wide band at ca. 1100 cm-’ in the LiClOpDMSO system to the S=O stretching mode of Li+-DMSO complex species in the solution. Although the Li+-DMSO association occurs mainly between Li+ and free DMSO molecules, the possibility cannot be ruled out that there may be interaction between Li+ ions and the self-associated DMSO molecules, especially the linear dimers and/or higher polymers. As a result, the environment around the Li+ ions is very complicated. Therefore, the width of the 1100 cm-’ band becomes larger when the solution becomes more concentrated and the species in the solution becomes more complicated. Raman spectra in the S=O bending region Figure 6 shows the Raman spectra in the S=O bending region of solutions of LiC104 in DMSO. In pure DMSO, three bands are observed at 383, 335 and 306 cm-‘, which have been assigned to the in-plane and the out-of-plane bending of the M bond and the in-plane CSC bending of DMSO, respectively. With the increase of LiC104 content, the positions of these three bands remain unchanged. These results agree quite well with those of Ramana and Singh [12] in the LiBr- and LiI-DMSO systems.
Interaction
g 4L
between lithium perchlorate and dimethylsulfoxide
Nevertheless, the relative intensities of the two S=O bending modes decrease significantly when LiClO4 is added into DMSO, especially the 383 cm-’ band. Since only the bands related to the S=O bond changes significantly in the intensity, while the CSC bond-related band almost remains unchanged, it is possible that the intensity changes of the two bands are due to the interaction between Li+ ions and the S=O group of DMSO. However, Ramana and Singh [ 191 have pointed out that even in the system of tetrabutylammonia bromide (TBAB)-DMSO-d6 solution, such a phenomenon is observed. Therefore, it seems that the cation-molecule interaction alone are not responsible for this effect. Considering that the DMSO molecules are self-associated by the S. .. ..O bridging and the addition of LiClOd breaks down the self-association to some extent, it seems possible that the changes in the intensity of these bands might be related to the breaking down of the complex species by LiClOd. That is, both the cation and anion in the solution are responsible to the intensity changes of these two bands. This question will be further discussed in the following section. Raman and IR spectra related to the CHJ groups
Besides all the three components in the s---O stretching envelope discussed in the first section,
2615
f
e
4
I
3100
d ,1
I
3050 3000 2950 Wavenumber(cm-‘)
1
2900
2850
Fig. 7. The Raman spectra of the C-H symmetric stretching and asymmetric modes of DMSO at various contents of LiCIO4: (a) pure DMSO; (b) 0.05; (c) 0.1; (d) 0.2; (e) 0.3; (f) 0.4.
I
I
520
475
430 385 340 Wavenumber(cm-I)
295
1
250
Fig. 6. The Raman spectra of the S=O bending region of DMSO with different contents of LiC104: (a) pure DMSO; (b) 0.05; (c) 0.1; (d) 0.2; (e) 0.3; (f) 0.4; (g) 0.6.
another component identified is located at cu. 1015 cm-‘. Different authors assign this component differently. Ramana and Singh [12] assigned it to the polymeric species of DMSO while Rintoul and Shurvell [14] assigned it to the asymmetric CH3 rocking frequency because of its greater intensity in the IR spectrum and its higher frequency. However, it seems more reasonable to rule out the attribution of this component to the CHJ rocking mode of DMSO, according to the IR analysis of the deuterated DMSO [19]. From Figs 4 and 5, it can be seen that the relative intensity of this component increases with the LiClO4 content. Figure 7 shows the Raman spectra of the symmetric (at cu. 2910 cm-‘) and asymmetric (at cu. 3000 cm-‘) C-H stretching bands in DMSO with different LiClO4 concentrations, while Fig. 8 shows the LiClOd-concentration dependence of the Raman shifts of these two modes. It can be seen from these two figures that the Raman shifts of the symmetric and asymmetric C-H stretching bands at 2996 and 2912 cm-’ increase with increasing LiC104 molar ratio and the relationships of the Raman shift vs the LiClO4 concentration are almost linear, but the positions of their counterparts in the IR spectra (not shown here) seem to be constant, although the concentration of the solution changes greatly For the sake of clarity, the
2616
Zhaoziang Wang et al.
concentration dependences of the Raman shifts of these two bands are shown in Fig. 8. These observations cannot be due to the experimental error since there are many bands in the Raman spectra whose positions remain constant throughout the concentrations investigated here and therefore, their positions can be used as the internal frequency standards, such as the bands at 1418, 951 (CHj rocking) and 931 cm-’ (the vI mode of CIO; anion) in the Raman spectra. The IR and Raman spectra of solutions of TBAB in DMSO-& [ 19, 201 and alkali metals and tetrabutylammonia halides in acetonitrile [ 12,21-241 have
3020
1
3017 3014 -3011 ; g 3006 V < 3005 .d A m 3002 9 g 2999 d
p: 2996
-
2993
-
2990
’
’
-0.10.0
’
’
Molar 2g30
’
’
’
’
0.1 0.2 0.3 0.4 0.5 0.6 0.7
ratio
I
2924 t
shown the possible interactions between the CHJ group of the solvents and the halides anions. These interactions result in the anion-molecular complex species in the solution and, therefore, the vibrational frequencies of the C-H stretching bands move towards the low-frequency side in the spectra. However, as Yearger et al. [25] have pointed out, the solvation effect of ClO; anions in the nonaqueous solution is very weak, therefore, the influence of ClO; on DMSO is too weak to be detected. Even if considering the perturbation of the solvent to the ClO; anions, such as the splittings of the v4 mode (at 624cm-‘) and the VI mode (at 931 cm-‘) in the Raman spectra and the activation of the vI mode in the IR spectra, which will be discussed elsewhere, it is still difficult to expect that the ClO;-CHJ group interaction will lead to the frequency increase of the C-H stretching modes. Instead, such an interaction will make the C-H stretching frequency decrease. This phenomenon also rules out the possibility that the intensity changes of the 383 and 335 cm-’ bands come from the anion-molecular interactions. The vibrational frequencies of the C-S symmetric stretching modes of DMSO at 699 and 669 cm-‘, respectively, also move towards the high frequency side in the Raman spectra (Fig. 1), where the v4mode of the ClO; anion at 624cm-’ can be used as the internal frequency standard). The relationship between the position of this band with the concentration of LiClO4 in the solution has been plotted in Fig. 9. These changes are continuous in the Raman shift with the increase of the LiC104 contents and during this process, no new components are discriminated from the original bands. This changing process indicates that there is no change in the symmetry of the CH3 structure or the CH+S-CH, structure during the addition of LiC104 into DMSO. What has been changed is the gradual and continuous increase of the strength of the chemical bond or the electronic nebula. Therefore, it is proposed that the increase of the Li+ concentration in the solution not only results in the formation of Li+-DMSO associates, but also makes the electronic nebula of the C-H bond increase by way of the induction effect transferred along the molecular chain. This mechanism of interaction can also explain the change features in the frequency of the C-S stretching modes at ca. 699 and 669 cm-’ as well as the C-H stretching modes. CONCLUSIONS
-0.10.0
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Molar
ratio
Fig. 8. The LiCIOd-content dependence of the Raman shifts of the symmetric (A) and asymmetric (B) CH3 stretching modes of DMSO.
The Raman and IR spectra of pure DMSO and DMSO containing different amounts of LiClOd have been investigated. With the addition of LiClO4 into the solvent, the self-associates of DMSO molecules are disassembled. Both the Raman and the IR spectra indicate that with the increase of LiClO4 content in the solvent, the loosely associated cyclic dimers in the solvent are dissociated. The interaction between the
Interaction
between lithium perchlorate and dimethylsulfoxide
725
2617
ACKNOWLEDGEMENTS
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%
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,
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This work was supported by Ford & NSFC, National Science Foundation of China (Contract No. 09412303). Z.-X.Wang is also thankful to Dr T. Wang of Bruker Instruments Co., Germany (Beijing Office) for his kind help in recording the spectra and Professor Y.-L. Liu of the Raman Laboratory, Institute of Physics, CAS, for his helpful discussion. REFERENCES 1. P. V. Wright, Br. Polym. .I. 7, 319 (1975). 2. P. V. Wright, J. Polym. Sci. Polym. Phys., Ed. 14, 955 (1976). 3. M. Armand, J. M. Chabagno and M. J. Duclot, 2nd International Conference on Solid Electrolytes, St Andrews, paper, 6, 5 (1978). 4. B.-Y. Huang, R.-J. Xue, L.-Q. Chen and F.-S. Wang, Ph.D Thesis (Institute of Physics, Chinese Academy of Sciences) 1995. 5. K. M. Abraham and M. Alamgir, J. Electrochem. Sot.
ratio
137, 1657 (1990).
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0.1 0.2 0.3 0.4 0.5 0.6 0.7
Molar
M. I. Sastry and S. Singh, J. (19X41. \-- - ./-
r3---0.10.0
D. Battisti, G. A. Nazri, B. Klassen and R. Aroca, J. Phys. Chem. 97, 5826 (1993).
ratio
Fig. 9. The LiCIOa-content dependence of the Raman shifts symmetric (A) and asymmetric (B) modes of
of the C-S DMSO.
Li+ ions and the s---O groups of the molecules result in the formation of Lit-DMSO associates. Different from the result of the simple halide anions, it seems that the effect of the ClO; anions on the molecules is too weak to be observed. Further spectroscopic study on the C-H stretching modes and the S-C stretching modes indicates that the interaction of the Li+ ions and the s--O groups of the molecules can also have an induction effect on the C-S bonding and the C-H bonding and makes the latter bonding stronger.
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(Edited by A. D.
Buckingham, E. Lippert and S. Bratos) p. 293. Willey, New York (1978). 22. M. I. Sastry and S. Singh, Curr. Sci. 55, 1157 (1986). 23. K. V. Ramana and S. Singh, Spectrochim. Acta A 44, 277 (1988).
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