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FTIR spectroscopic studies of lithium tetrafluoroborate in propylene carbonate + diethyl carbonate mixtures
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 59–64
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FTIR spectroscopic studies of lithium tetrafluoroborate in propylene carbonate + diethyl carbonate mixtures Binbin Zhang a,b, Yuan Zhou a,⇑, Xiang Li a,b, Xiufeng Ren a,b, Hongen Nian a,b, Yue Shen a, Qiang Yun a,b a Key Laboratory of Salt Lake Resources and Chemistry of Chinese Academy of Sciences, Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, Qinghai 810008, China b University of Chinese Academy of Sciences, Beijing 100049, China
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analyzed for LiBF4 in PC, DEC and PC + DEC mixtures. + Strong interactions between Li and solvent molecules exist. + No preferential solvation of Li in LiBF4/PC + DEC solutions was detected.
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FTIR spectra have been collected and
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Article history: Received 17 September 2013 Received in revised form 9 November 2013 Accepted 11 November 2013 Available online 20 November 2013 Keywords: FTIR spectroscopy Lithium tetrafluoroborate Propylene carbonate (PC) Diethyl carbonate (DEC)
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a b s t r a c t FTIR (Fourier transformed infrared) spectra have been collected and analyzed for solutions of lithium tetrafluoroborate in propylene carbonate (PC), diethyl carbonate (DEC), and PC + DEC mixtures. It has been shown that the carbonyl stretch bands of PC and DEC, the ring of PC and the ether oxygen stretch bands of DEC are all very sensitive to the interaction between Li+ and the solvent molecules. New shoulders appear and the original bands split with the addition of LiBF4, indicating that a strong interaction between Li+ and molecules of PC and DEC exists through the oxygen group of C@O and ring of PC and both C@O oxygen and ether oxygen atoms of DEC. In addition, no preferential solvation of Li+ in LiBF4/PC + DEC solutions was detected. Ó 2013 Elsevier B.V. All rights reserved.
Introduction The study of organic electrolyte solutions in lithium ion battery continues to be an active field of research [1–10]. These electrolytes are of current interest because of their potential applications in lithium ion batteries [1,2]. Electrolyte of lithium ion battery consists of lithium salt (such as LiClO4, LiPF6, and LiBF4), mixture ⇑ Corresponding author. Address: Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, Qinghai 810008, China. Tel.: +86 971 6330483; fax: +86 971 6306002. E-mail address: [email protected] (Y. Zhou). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.11.054
of organic solvents (such as propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC)) and some functional additives. The structure and composition of electrolyte plays an important role in determining the battery performance, such as charge–discharge performance, operating temperature range and service life [1,2]. Therefore, starting from the perspective of microscopic structural elements angle, knowledge of ion–molecule and intermolecular interactions inside electrolytes is essential for the optimal choice of solvent and electrolyte. It is also very important and meaningful for the research and development of lithium ion battery with high performance.
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Vibrational spectroscopy has been proved to be a powerful technique for probing ion–molecule and intermolecular interactions through the changes of frequency, intensity and other bands properties [8]. Such studies [3,4,6,11–15,16–27] could help to identify the action mechanism of lithium salt and solvent inside electrolyte and confirm the factors that affect the general properties and performance of the electrolyte solutions. As lithium tetrafluoroborate (LiBF4) has a proper thermal and chemical stability, a series of investigations have been carried out to study the ion solvation and association of LiBF4-based electrolytes [28,22,23] which have an excellent low temperature battery performance. Xuan et al. [29] studied the ion solvation and association of lithium tetrafluoroborate in acetonitrile by vibrational spectroscopic and density functional methods. Alia and Edwards [30] studied the Raman spectra of LiBF4 in acrylonitrile and found the contact ion pairs and ion dimers. However, there have been no researchers studied the LiBF4/PC + DEC binary electrolyte system. We selected PC (64.40 at 298.15 K [31]) and DEC as the solvents because both of them are liquid at room temperature, and are commonly used in lithium ion batteries [1–6]. A number of commercial lithium ion batteries have adopted the mixed organic solvents containing PC [32] as their electrolytes. As LiBF4-based electrolytes have an excellent low temperature performance and are widely used in lithium ion battery, the study of LiBF4/PC + DEC is worthy and meaningful for the development of electrolyte with better low temperature performance. In this paper, in order to capture the nature of ion–molecule and intermolecular interactions inside LiBF4/PC + DEC binary electrolyte system, a systematic FTIR spectroscopic investigations of PC + DEC, LiBF4/PC, LiBF4/DEC and LiBF4/PC + DEC solutions are carried out. In order to separate out various interactions within these solutions, a series of systems with different components and concentrations are systematically studied, ranging from the simple pure components to the final systems of salt-solvent solutions. Experimental Lithium tetrafluoroborate (Aladdin, purity >99.99% metal basis) was dried under vacuum for 48 h at 120 °C. PC and DEC (Zhangjiagang Guotai-Huarong New Chemical Materials Co. Ltd., cell grade, purity >99.9%, moisture <20 ppm) were used as purchased. All electrolyte solution manufactures were carried out in nitrogen filled glove box (moisture <0.1 ppm, oxygen <20 ppm). As there is no obvious absorption bands were observed around 3500 cm 1 in the IR spectra, the effect of moisture in the liquid sample on ion–molecule and intermolecular interactions can be negligible. The concentration of the electrolyte solutions was expressed as amount-of-substance concentration (mol/L). Different volume ratios of VDEC/VPC solutions and different concentrations of LiBF4/PC, LiBF4/DEC and LiBF4/PC + DEC (v/v, 1:1) solutions were prepared to be systematically investigated. IR spectra were collected on a Thermo-Nicolet Nexus FTIR spectrometer (USA) equipped with a KBr crystal in the absorbance mode range from 400 to 4000 cm 1 with a resolution of 2 cm 1. All the measurements were made at room temperature. Results and discussion Characteristic peak assignments for PC and DEC Battisti et al. [4] studied the infrared and Raman spectroscopy of PC doped with various concentrations of lithium perchlorate (LiClO4) and Janz et al. [33] investigated the Raman spectrum of PC. Both of them deeply studied the structure and vibrational spectra of PC, and most of the vibrational peak assignments had been
made. Wang et al. [34] investigated the vibrational spectra of DEC, and gave detailed assignments for most of the IR bands. In addition, Xuan et al. [35] studied the IR vibrational spectra of LiBF4/ c-BL and gave their assignments for the bands observed, among which the m1 (BF4 ) of LiBF4 is about 765 cm 1. According to the IR spectra of pure PC and DEC, we make assignments of the characteristic group peaks for PC and DEC. It is presented that the band positions of C@O stretch, CAOAC stretch, symmetric ring deformation and ring deformation of PC molecule are located at 1791 cm 1, 1182 cm 1, 712 cm 1 and 777 cm 1, respectively. Simultaneously, the band positions of C@O stretch and CAOAC stretch of DEC molecule are respectively located at 1747 cm 1 and 1259 cm 1. The possible structures of PC (a) and DEC (b) molecule are presented in Fig. 1. PC+DEC mixture In order to deeply study the interaction between PC and DEC molecules, we prepared and investigated PC + DEC mixtures with different volume ratios of VDEC/VPC. Fig. 2 shows the dependence of position of C@O stretch for PC in PC + DEC mixtures on volume ratios of VDEC/VPC. It is shown in Fig. 2 that the position of the C@O stretch for PC shifts to higher wavenumber with the increasing of the volume ratios of VDEC/VPC. The maximum shift (22 cm 1) is observed for the C@O stretch of PC. Simultaneously, data fitting curve is shown in the figure. We discover that the rule of the data changes can match the formula as follows: Y = 0.0035X3 0.1964X2 + 3.6758X + 1788.8. We use R2 to express the correlation between this formula and the rule of experimental data changes. After calculation, we obtain that R2 = 0.9799 which is very close to 1 (meaning 100% fit). Therefore, the correlation of the experimental data and the fitting curve is excellent. Based on these observations, it can be suggested that an intermolecular interaction between PC and DEC molecules is really exists, and it corresponds with a certain formula. In addition, the excess molar enthalpy (HEm ) and excess molar volumes (V Em ) for PC + DEC binary mixtures have been determined by Franceconi et al. [36] at 298.15 and 313.15 K. They found that the V Em values are negative and the shape of HEm against the molar fraction of DEC is asymmetrical. Large negative deviation of excess dielectric constant from ideality for these mixtures has also been obtained by a pulse reflection technique [37]. These phenomena above, which are also in support of our FTIR results, can reasonably be interpreted by dipole–dipole interactions between PC and DEC molecules. LiBF4/PC and LiBF4/DEC solutions As known to us all, the interactions inside salt/solvent include cation–solvent and anion–solvent interactions. But in this paper, the interactions in LiBF4/PC and LiBF4/DEC solutions mainly refers to the interactions between Li+ cation and solvent molecules. Because both PC and DEC are poor anion solvators, changes of solvent molecular spectra are considered to be the result of interaction between Li+ and solvent molecules. The ion–molecule interaction in solutions of lithium tetrafluoroborate in propylene carbonate (LiBF4/PC) has been investigated in some detail [38]. Fig. 3 shows the IR spectra of C@O stretch for PC in solutions with different LiBF4 concentrations in the region of 1650–1950 cm 1. As is shown in Characteristic peak assignments for PC and DEC, the band of C@O stretch of pure PC is located at 1791 cm 1. With the increasing of LiBF4 concentration, it is obvious that the half-peak breadth of C@O stretch for PC increases quickly whilst a shoulder peak at 1816 cm 1 is clearly visible on the higher wavenumber side, especially on the concentration of
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Fig. 1. The possible structure of PC (a) and DEC (b) molecule.
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Fig. 4. The IR spectra of symmetric ring deformation and ring deformation for PC in solutions with different concentrations of LiBF4. (a) pure PC; (b) 0.2 mol/L LiBF4/PC; (c) 0.4 mol/L LiBF4/PC; (d) 1.2 mol/L LiBF4/PC.
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1.2 mol/L LiBF4/PC solution (line d). Simultaneously, the intensity of the new shoulder peak gradually increases with the increasing of LiBF4 concentrations. Fig. 4 illustrates another two obvious trends concern the symmetric ring deformation (711–712 cm 1) and ring deformation (776–777 cm 1) of PC molecule under different LiBF4 concentrations in the region of 680–820 cm 1. Obvious band shifting of ring deformation (776–777 cm 1) from 776 cm 1 to 778 cm 1 can be seen in this figure. Simultaneously, the half-peak breadth of the band of symmetric ring deformation (711–712 cm 1) increases
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quickly whilst a shoulder peak at about 722 cm 1 is clearly visible on the higher wavenumber side, especially on the concentration of 1.2 mol/L LiBF4/PC (line d). After the addition of LiBF4, the shoulder is clearly visible when the concentration is 0.2 mol/L. And the intensity of the new shoulder increases with the increasing of the concentration. The intensity of the shoulder in line d (1.2 mol/L) is obviously higher than the intensity of line b (0.2 mol/L). According to the above vibrational spectroscopic studies of LiBF4/PC under different concentrations, it can be suggested that
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there is a strong interaction between Li+ and PC. This interaction occurs mainly on the carbonyl oxygen atom and the ring of PC molecule. The possible structure (a) of the interaction of Li+ with PC in LiBF4/PC solutions is presented in Fig. 7. Fig. 5 shows the IR spectra of C@O stretch for DEC in solutions with different LiBF4 concentrations. It is also obvious that the halfpeak width of the band of C@O stretch increases and band splitting occurs for the C@O stretch at 1747 cm 1. A shoulder at 1717 cm 1 is clearly visible on the lower wavenumber side at the concentration of 0.2 mol/L LiBF4/DEC. The intensity of the new band (1717 cm 1) increases with the increasing of LiBF4 concentrations. Fig. 6 shows another trend in the asymmetric stretch of CAOAC for DEC at 1259 cm 1. With the increasing of the LiBF4 concentrations, the half-peak width of the band of CAOAC asymmetric stretch increases and a new shoulder appears at 1276 cm 1, especially on high LiBF4 concentrations. At the same time, we can find
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that the intensity of the band at 1303 cm 1 increases with the increasing of the LiBF4 concentrations, but it do not overlaps the original one at 1259 cm 1. However, in LiClO4/DEC solutions, the new band overlaps the original one [34], which illustrates different lithium salt has different spectral changes and different band intensity. According to the above vibrational spectroscopic studies of LiBF4/DEC under different concentrations, it can be suggested that there is a strong interaction between Li+ and DEC. This interaction occurs mainly through the carbonyl oxygen atom and ether oxygen atom of DEC molecules. The possible structures (b, c) of the interaction of Li+ with DEC in LiBF4/DEC solutions are presented in Fig. 7.
LiBF4/PC+DEC solutions Fig. 8 illustrates the IR spectra of PC + DEC mixture solutions with different LiBF4 concentrations in the region of 650– 2000 cm 1. For the purpose of convenient comparison, the volume ratio of PC to DEC is fixed at 1:1 in the LiBF4/PC + DEC solutions. For convenient observation of the changes of the bands, we respectively enlarge the bands where changes happened in the following figures (Figs. 9–11). Fig. 9 shows the changes of IR spectra of C@O stretch of PC in PC + DEC solutions containing different LiBF4 concentrations. The carbonyl stretch of PC and DEC in the mixtures without LiBF4 are located at 1795 cm 1 and 1744 cm 1 which are different from those of pure PC and DEC (1791 cm 1 and 1747 cm 1, respectively) due to the interactions between PC and DEC molecules discussed above in PC+DEC mixture. The bands of C@O stretch change after the adding of LiBF4. Shoulder peaks appear and gradually more obviously visible with the increasing of LiBF4 concentrations. There are two obvious shoulders at 1806 cm 1 and 1720 cm 1, respectively, and the intensity of the new shoulders increases accompanied with the increasing of LiBF4 concentrations. The positions of these two shoulders are very close to those of PC in LiBF4/PC and DEC in LiBF4/DEC solutions, but not completely the same. This fact
Fig. 7. The possible structures of the interaction of Li+ with PC (a) or DEC (b) and (c) in LiBF4/PC or LiBF4/DEC solutions.
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shows that the interaction between PC and DEC molecules has been broken when LiBF4 is added. The IR spectra changes of CAOAC stretch of DEC in LiBF4/ PC + DEC solutions are shown in Fig. 10. The CAOAC stretch of DEC is also very sensitive to the interaction of Li+ with DEC in LiBF4/PC + DEC electrolyte solutions. This figure shows a change
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Wavenumber (cm-1) Fig. 11. The IR spectra of symmetric ring deformation and ring deformation of PC in PC + DEC solutions containing different LiBF4 concentrations. (a) PC-DEC (v/v, 1:1); (b) 0.5 mol/L LiBF4/PC + DEC; (c) 1.0 mol/L LiBF4/PC + DEC; (d) 1.2 mol/L LiBF4/ PC + DEC.
procedure which is similar to what is observed in the LiBF4/DEC solutions: an additional shoulder (at 1276 cm 1) appears, grows, simultaneously, the intensity of the band in 1303 cm 1 gradually increases with the increasing of LiBF4 concentrations. Fig. 11 expresses the IR spectra of symmetric ring deformation and ring deformation of PC in PC + DEC solutions containing different LiBF4 concentrations. The changes showed in this figure are almost the same as what is observed in the LiBF4/PC solutions: Band shifting of ring deformation can obviously be seen. At the same time, the half-peak breadth of symmetric ring deformation (711– 712 cm 1) increases quickly whilst a shoulder at about 722 cm 1 is clearly visible on the higher wavenumber side, especially on the concentration of 1.0 mol/L and 1.2 mol/L LiBF4/PC + DEC solutions. The intensity of the new band at 722 cm 1 increases at the expense of the band at 711 cm 1. At the concentration of 1.0 mol/L and 1.2 mol/L, the original band at 711 cm 1 becomes very weak compared to curve a (PC + DEC) and b (0.5 mol/L LiBF4/ PC + DEC) while the intensity of the new one at 722 cm 1 gradually increasing. According to the above analysis, it can be concluded that rather strong Li+-PC and Li+-DEC interactions appear in LiBF4/PC + DEC electrolyte solutions. This interaction affects the local structure of the solvent molecule, and causes their changes in spectra. The strong interaction occurs mainly through the oxygen atoms of carbonyl groups and ring deformation in PC molecules, and through both oxygen atoms of the carbonyl groups and ether oxygen groups in DEC molecules. The possible structures of the interaction of Li+ with PC and DEC in LiBF4/PC + DEC (v/v, 1:1) solutions are presented in Fig. 12. There are PC and DEC molecules in LiBF4/PC + DEC solutions. An interesting question is which kind of molecules interact preferentially with Li+? To answer this question, we compared the spectra of solutions with different solvent compositions. Fig. 12 shows the splitting of the 711 cm 1 symmetric ring deformation band of PC. It suggests that two kinds of PC molecules coexist in this solution. One kind, from the bulk solvent, described as ‘free’, generates the band at 711 cm 1. Another kind of solvent binds with Li+ and gives the band at 722 cm 1 [34]. Accordingly, the intensity of these bands should be proportional to the number of the corresponding solvent molecules. It is interesting to note that the value (1.16) of I712/I722 in LiBF4/PC solutions at the concentration of 1.2 mol/L is close to that (1.09) in LiBF4/PC + DEC solutions at the concentration of 1.2 mol/L and lower than that (1.35) in 0.5 mol/L LiBF4/PC + DECsolutions. It suggests that no obvious preferential solvation is
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Fig. 12. The possible structures of the interaction of Li+ with PC and DEC in LiBF4/PC + DEC (v/v, 1:1) solutions.
present in LiBF4/PC + DEC electrolyte solutions. This fact can be explained by the donor–acceptor approach. Values of donor number (DN) are 16.1 for DEC and 15.1 for PC [31]. Their DN values are very similar. Therefore, no obvious preferential selective solvation of Li+ with PC and DEC molecules could be expected in LiBF4/PC + DEC solutions. Conclusions The electrolyte solutions of lithium tetrafluoroborate (LiBF4) in PC, DEC and PC + DEC solutions have been systematically investigated by FTIR spectroscopy study. The tentative conclusions can be summarized as follows: (i) The dipole–dipole interaction between PC and DEC molecules in PC + DEC mixtures causes the C@O stretch of PC shift to the higher wavenumber side with the increasing of volume ratio in PC + DEC solutions. (ii) Strong interactions of Li+-PC and Li+-DEC are observed in LiBF4/PC, LiBF4/DEC and LiBF4/PC + DEC electrolyte solutions. It is possible that the former interaction (Li+-PC) occurs through the oxygen atoms of carbonyl groups and ring deformation in PC molecules, and the latter interaction (Li+-DEC) through both oxygen atoms of the carbonyl groups and ether oxygen groups in DEC molecules. (iii) No obvious preferential selective solvation of Li+ was found in the LiBF4/PC + DEC electrolyte solutions due to their small difference of PC and DEC in DN values. Acknowledgements Financial support from the National 863 Program (2013AA110100), ‘‘Western light’’ talent cultivation plan joint scholars Program (Chinese Academy of Sciences) and Open fund of key laboratory of salt lakes resources and chemistry (Chinese Academy of Sciences) are gratefully acknowledged. References [1] J. Barthel, H.J. Gores, Pure Appl. Chem. 57 (1985) 1071–1082. [2] K.M. Abraham, S.B. Brumner, in: J.P. Gabano (Ed.), Lithium Batteries, Academic Press, New York, 1983. [3] H.L. Yeager, J.D. Fedyk, R.J. Parker, J. Phys. Chem. 77 (1973) 2407–2410. [4] D. Battisti, G.A. Nazri, B. Klassen, R. Aroca, J. Phys. Chem. 97 (1993) 5826–5830.
[5] K. Kondo, M. Sano, A. Hiwara, T. Omi, M. Fujita, A. Kuwae, M. Iide, K. Mogi, H. Yokoyama, J. Phys. Chem. B 104 (2000) 5040–5044. [6] B. Klassen, R. Aroca, M. Nazri, G.A. Nazri, J. Phys. Chem. B 102 (1998) 4795– 4801. [7] G.Y. Gu, S. Bouvier, C. Wu, R. Laura, M. Rzeznik, K.M. Abraham, Electrochim. Acta 45 (2000) 3127–3139. [8] X.P. Xuan, J.J. Wang, J.M. Tang, G.R. Qu, J.S. Lu, Spectrochim. Acta A 56 (2000) 2131–2139. [9] J.J. Wang, X.P. Xuan, J.S. Lu, N. Pei, Y.J. Mo, Z. Phys. Chem. 215 (2001) 437–446. [10] X.P. Xuan, J.J. Wang, J.M. Tang, G.R. Ou, J.S. Lu, Phys. Chem. Liq. 39 (2001) 327– 342. [11] Y. Matsuda, M. Morita, T. Yamashita, J. Electrochem. Soc. 131 (1984) 2821– 2827. [12] K. Matsubara, R. Kaneuchi, N. Maekita, J. Chem. Soc.-Faraday Trans. 94 (1998) 3601–3605. [13] W.W. Huang, R. Frech, P. Johansson, J. Lindgren, Electrochim. Acta 40 (1995) 2147–2151. [14] S.A. Hyodo, K. Okabayashi, Electrochim. Acta 34 (1989) 1551–1556. [15] S.A. Hyodo, K. Okabayashi, Electrochim. Acta 34 (1989) 1557–1561. [16] M.P. Foley, D.M. Seo, Christopher J. Worosz, P.D. Boyle, W.A. Henderson, H.C. De Long, P.C. Trulove, Physical and Analytical Electrochemistry (General Session) – 220th ECS Meeting, 41 (2012) 99–105. [17] O.O. Postupna, Y.V. Kolesnik, O.N. Kalugin, O.V. Prezhdo, J. Phys. Chem. B 115 (2011) 14563–14571. [18] S. Das, V.S. Bhadram, C. Narayana, A.J. Bhattacharyya, J. Phys. Chem. B 115 (2011) 12356–12361. [19] R.S. Sah, B. Sinha, M.N. Roy, Fluid Phase Equilib. 307 (2011) 216–221. [20] X. Fang, X.J. Wu, Phys. Test. Chem. Anal. 47 (47–49) (2011) 53. [21] X.J. Wu, Y. Pan, L.L. Song, Chem. J. Chin. Univ.-Chin. 31 (2010) 1741–1746. [22] H.W. Qiao, H.L. Luan, Z.M. Zhou, L.X. Bi, W. Yao, J.M. Li, C. Chen, J. Mol. Struct. 885 (2008) 89–96. [23] H.W. Qiao, H.L. Luan, Z.M. Zhou, X.P. Fang, W. Yao, X. Wang, J.M. Li, Chin. J. Chem. 26 (2008) 689–692. [24] D. Das, J. Solution Chem. 37 (2008) 947–955. [25] H.W. Qiao, H.L. Luan, Z.M. Zhou, W. Yao, Chin. J. Chem. 25 (2007) 461–465. [26] X.P. Xuan, J.J. Wang, Y. Zhao, H.C. Zhang, Spectrochim. Acta A 62 (2005) 500– 505. [27] X.P. Xuan, J.M. Tang, J.J. Wang, H.Q. Wang, Spectrosc. Spect. Anal. 25 (2005) 548–552. [28] H.W. Qiao, X.P. Fang, H.L. Luan, Z.M. Zhou, Y.K. Wu, W. Yao, X. Wang, J.M. Li, C. Chen, J. Mol. Liq. 138 (2008) 69–75. [29] X.P. Xuan, H.C. Zhang, J.J. Wang, H.Q. Qang, J. Phys. Chem. A 108 (2004) 7513– 7521. [30] J.M. Alia, H.G.M. Edwards, J. Solution Chem. 29 (2000) 781–797. [31] Y. Marcus, Ion Solvation, Wiley, New York, 1985. [32] S.I. Tobishima, M. Arakawa, J.I. Yamaki, Electrochim. Acta 33 (1988) 239–244. [33] G.J. Janz, J. Ambrose, J.W. Coutts, J.R. Downey, Spectrochim. Acta A 35 (1993) 175–179. [34] J.J. Wang, Y.P. Wu, X.P. Xuan, H.Q. Wang, Spectrochim. Acta A 58 (2002) 2097– 2104. [35] X.P. Xuan, H.C. Zhang, J.J. Wang, H.Q. Wang, J. Henan Normal Univ. (Nat. Sci.) 30 (2002). 126-126. [36] R. Francesconi, R. Comelli, J. Chem. Eng. Data 40 (1995) 811–814. [37] R. Payne, I.E. Theodorou, J. Phys. Chem. 76 (1972) 2892–2900. [38] B.B. Zhang, Y. Zhou, X. Li, X.F. Ren, H.E. Nian, Y. Shen, Q. Yun, Int. J. Electrochem. Sci. 8 (2013) 12735–12740.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
FTIR spectroscopic studies of lithium tetrafluoroborate in propylene carbonate + diethyl carbonate mixtures Binbin Zhang a,b, Yuan Zhou a,⇑, Xiang Li a,b, Xiufeng Ren a,b, Hongen Nian a,b, Yue Shen a, Qiang Yun a,b a Key Laboratory of Salt Lake Resources and Chemistry of Chinese Academy of Sciences, Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, Qinghai 810008, China b University of Chinese Academy of Sciences, Beijing 100049, China
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analyzed for LiBF4 in PC, DEC and PC + DEC mixtures. + Strong interactions between Li and solvent molecules exist. + No preferential solvation of Li in LiBF4/PC + DEC solutions was detected.
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FTIR spectra have been collected and
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Article history: Received 17 September 2013 Received in revised form 9 November 2013 Accepted 11 November 2013 Available online 20 November 2013 Keywords: FTIR spectroscopy Lithium tetrafluoroborate Propylene carbonate (PC) Diethyl carbonate (DEC)
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a b s t r a c t FTIR (Fourier transformed infrared) spectra have been collected and analyzed for solutions of lithium tetrafluoroborate in propylene carbonate (PC), diethyl carbonate (DEC), and PC + DEC mixtures. It has been shown that the carbonyl stretch bands of PC and DEC, the ring of PC and the ether oxygen stretch bands of DEC are all very sensitive to the interaction between Li+ and the solvent molecules. New shoulders appear and the original bands split with the addition of LiBF4, indicating that a strong interaction between Li+ and molecules of PC and DEC exists through the oxygen group of C@O and ring of PC and both C@O oxygen and ether oxygen atoms of DEC. In addition, no preferential solvation of Li+ in LiBF4/PC + DEC solutions was detected. Ó 2013 Elsevier B.V. All rights reserved.
Introduction The study of organic electrolyte solutions in lithium ion battery continues to be an active field of research [1–10]. These electrolytes are of current interest because of their potential applications in lithium ion batteries [1,2]. Electrolyte of lithium ion battery consists of lithium salt (such as LiClO4, LiPF6, and LiBF4), mixture ⇑ Corresponding author. Address: Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, Qinghai 810008, China. Tel.: +86 971 6330483; fax: +86 971 6306002. E-mail address: [email protected] (Y. Zhou). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.11.054
of organic solvents (such as propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC)) and some functional additives. The structure and composition of electrolyte plays an important role in determining the battery performance, such as charge–discharge performance, operating temperature range and service life [1,2]. Therefore, starting from the perspective of microscopic structural elements angle, knowledge of ion–molecule and intermolecular interactions inside electrolytes is essential for the optimal choice of solvent and electrolyte. It is also very important and meaningful for the research and development of lithium ion battery with high performance.
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Vibrational spectroscopy has been proved to be a powerful technique for probing ion–molecule and intermolecular interactions through the changes of frequency, intensity and other bands properties [8]. Such studies [3,4,6,11–15,16–27] could help to identify the action mechanism of lithium salt and solvent inside electrolyte and confirm the factors that affect the general properties and performance of the electrolyte solutions. As lithium tetrafluoroborate (LiBF4) has a proper thermal and chemical stability, a series of investigations have been carried out to study the ion solvation and association of LiBF4-based electrolytes [28,22,23] which have an excellent low temperature battery performance. Xuan et al. [29] studied the ion solvation and association of lithium tetrafluoroborate in acetonitrile by vibrational spectroscopic and density functional methods. Alia and Edwards [30] studied the Raman spectra of LiBF4 in acrylonitrile and found the contact ion pairs and ion dimers. However, there have been no researchers studied the LiBF4/PC + DEC binary electrolyte system. We selected PC (64.40 at 298.15 K [31]) and DEC as the solvents because both of them are liquid at room temperature, and are commonly used in lithium ion batteries [1–6]. A number of commercial lithium ion batteries have adopted the mixed organic solvents containing PC [32] as their electrolytes. As LiBF4-based electrolytes have an excellent low temperature performance and are widely used in lithium ion battery, the study of LiBF4/PC + DEC is worthy and meaningful for the development of electrolyte with better low temperature performance. In this paper, in order to capture the nature of ion–molecule and intermolecular interactions inside LiBF4/PC + DEC binary electrolyte system, a systematic FTIR spectroscopic investigations of PC + DEC, LiBF4/PC, LiBF4/DEC and LiBF4/PC + DEC solutions are carried out. In order to separate out various interactions within these solutions, a series of systems with different components and concentrations are systematically studied, ranging from the simple pure components to the final systems of salt-solvent solutions. Experimental Lithium tetrafluoroborate (Aladdin, purity >99.99% metal basis) was dried under vacuum for 48 h at 120 °C. PC and DEC (Zhangjiagang Guotai-Huarong New Chemical Materials Co. Ltd., cell grade, purity >99.9%, moisture <20 ppm) were used as purchased. All electrolyte solution manufactures were carried out in nitrogen filled glove box (moisture <0.1 ppm, oxygen <20 ppm). As there is no obvious absorption bands were observed around 3500 cm 1 in the IR spectra, the effect of moisture in the liquid sample on ion–molecule and intermolecular interactions can be negligible. The concentration of the electrolyte solutions was expressed as amount-of-substance concentration (mol/L). Different volume ratios of VDEC/VPC solutions and different concentrations of LiBF4/PC, LiBF4/DEC and LiBF4/PC + DEC (v/v, 1:1) solutions were prepared to be systematically investigated. IR spectra were collected on a Thermo-Nicolet Nexus FTIR spectrometer (USA) equipped with a KBr crystal in the absorbance mode range from 400 to 4000 cm 1 with a resolution of 2 cm 1. All the measurements were made at room temperature. Results and discussion Characteristic peak assignments for PC and DEC Battisti et al. [4] studied the infrared and Raman spectroscopy of PC doped with various concentrations of lithium perchlorate (LiClO4) and Janz et al. [33] investigated the Raman spectrum of PC. Both of them deeply studied the structure and vibrational spectra of PC, and most of the vibrational peak assignments had been
made. Wang et al. [34] investigated the vibrational spectra of DEC, and gave detailed assignments for most of the IR bands. In addition, Xuan et al. [35] studied the IR vibrational spectra of LiBF4/ c-BL and gave their assignments for the bands observed, among which the m1 (BF4 ) of LiBF4 is about 765 cm 1. According to the IR spectra of pure PC and DEC, we make assignments of the characteristic group peaks for PC and DEC. It is presented that the band positions of C@O stretch, CAOAC stretch, symmetric ring deformation and ring deformation of PC molecule are located at 1791 cm 1, 1182 cm 1, 712 cm 1 and 777 cm 1, respectively. Simultaneously, the band positions of C@O stretch and CAOAC stretch of DEC molecule are respectively located at 1747 cm 1 and 1259 cm 1. The possible structures of PC (a) and DEC (b) molecule are presented in Fig. 1. PC+DEC mixture In order to deeply study the interaction between PC and DEC molecules, we prepared and investigated PC + DEC mixtures with different volume ratios of VDEC/VPC. Fig. 2 shows the dependence of position of C@O stretch for PC in PC + DEC mixtures on volume ratios of VDEC/VPC. It is shown in Fig. 2 that the position of the C@O stretch for PC shifts to higher wavenumber with the increasing of the volume ratios of VDEC/VPC. The maximum shift (22 cm 1) is observed for the C@O stretch of PC. Simultaneously, data fitting curve is shown in the figure. We discover that the rule of the data changes can match the formula as follows: Y = 0.0035X3 0.1964X2 + 3.6758X + 1788.8. We use R2 to express the correlation between this formula and the rule of experimental data changes. After calculation, we obtain that R2 = 0.9799 which is very close to 1 (meaning 100% fit). Therefore, the correlation of the experimental data and the fitting curve is excellent. Based on these observations, it can be suggested that an intermolecular interaction between PC and DEC molecules is really exists, and it corresponds with a certain formula. In addition, the excess molar enthalpy (HEm ) and excess molar volumes (V Em ) for PC + DEC binary mixtures have been determined by Franceconi et al. [36] at 298.15 and 313.15 K. They found that the V Em values are negative and the shape of HEm against the molar fraction of DEC is asymmetrical. Large negative deviation of excess dielectric constant from ideality for these mixtures has also been obtained by a pulse reflection technique [37]. These phenomena above, which are also in support of our FTIR results, can reasonably be interpreted by dipole–dipole interactions between PC and DEC molecules. LiBF4/PC and LiBF4/DEC solutions As known to us all, the interactions inside salt/solvent include cation–solvent and anion–solvent interactions. But in this paper, the interactions in LiBF4/PC and LiBF4/DEC solutions mainly refers to the interactions between Li+ cation and solvent molecules. Because both PC and DEC are poor anion solvators, changes of solvent molecular spectra are considered to be the result of interaction between Li+ and solvent molecules. The ion–molecule interaction in solutions of lithium tetrafluoroborate in propylene carbonate (LiBF4/PC) has been investigated in some detail [38]. Fig. 3 shows the IR spectra of C@O stretch for PC in solutions with different LiBF4 concentrations in the region of 1650–1950 cm 1. As is shown in Characteristic peak assignments for PC and DEC, the band of C@O stretch of pure PC is located at 1791 cm 1. With the increasing of LiBF4 concentration, it is obvious that the half-peak breadth of C@O stretch for PC increases quickly whilst a shoulder peak at 1816 cm 1 is clearly visible on the higher wavenumber side, especially on the concentration of
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Fig. 1. The possible structure of PC (a) and DEC (b) molecule.
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Fig. 4. The IR spectra of symmetric ring deformation and ring deformation for PC in solutions with different concentrations of LiBF4. (a) pure PC; (b) 0.2 mol/L LiBF4/PC; (c) 0.4 mol/L LiBF4/PC; (d) 1.2 mol/L LiBF4/PC.
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1.2 mol/L LiBF4/PC solution (line d). Simultaneously, the intensity of the new shoulder peak gradually increases with the increasing of LiBF4 concentrations. Fig. 4 illustrates another two obvious trends concern the symmetric ring deformation (711–712 cm 1) and ring deformation (776–777 cm 1) of PC molecule under different LiBF4 concentrations in the region of 680–820 cm 1. Obvious band shifting of ring deformation (776–777 cm 1) from 776 cm 1 to 778 cm 1 can be seen in this figure. Simultaneously, the half-peak breadth of the band of symmetric ring deformation (711–712 cm 1) increases
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quickly whilst a shoulder peak at about 722 cm 1 is clearly visible on the higher wavenumber side, especially on the concentration of 1.2 mol/L LiBF4/PC (line d). After the addition of LiBF4, the shoulder is clearly visible when the concentration is 0.2 mol/L. And the intensity of the new shoulder increases with the increasing of the concentration. The intensity of the shoulder in line d (1.2 mol/L) is obviously higher than the intensity of line b (0.2 mol/L). According to the above vibrational spectroscopic studies of LiBF4/PC under different concentrations, it can be suggested that
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there is a strong interaction between Li+ and PC. This interaction occurs mainly on the carbonyl oxygen atom and the ring of PC molecule. The possible structure (a) of the interaction of Li+ with PC in LiBF4/PC solutions is presented in Fig. 7. Fig. 5 shows the IR spectra of C@O stretch for DEC in solutions with different LiBF4 concentrations. It is also obvious that the halfpeak width of the band of C@O stretch increases and band splitting occurs for the C@O stretch at 1747 cm 1. A shoulder at 1717 cm 1 is clearly visible on the lower wavenumber side at the concentration of 0.2 mol/L LiBF4/DEC. The intensity of the new band (1717 cm 1) increases with the increasing of LiBF4 concentrations. Fig. 6 shows another trend in the asymmetric stretch of CAOAC for DEC at 1259 cm 1. With the increasing of the LiBF4 concentrations, the half-peak width of the band of CAOAC asymmetric stretch increases and a new shoulder appears at 1276 cm 1, especially on high LiBF4 concentrations. At the same time, we can find
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that the intensity of the band at 1303 cm 1 increases with the increasing of the LiBF4 concentrations, but it do not overlaps the original one at 1259 cm 1. However, in LiClO4/DEC solutions, the new band overlaps the original one [34], which illustrates different lithium salt has different spectral changes and different band intensity. According to the above vibrational spectroscopic studies of LiBF4/DEC under different concentrations, it can be suggested that there is a strong interaction between Li+ and DEC. This interaction occurs mainly through the carbonyl oxygen atom and ether oxygen atom of DEC molecules. The possible structures (b, c) of the interaction of Li+ with DEC in LiBF4/DEC solutions are presented in Fig. 7.
LiBF4/PC+DEC solutions Fig. 8 illustrates the IR spectra of PC + DEC mixture solutions with different LiBF4 concentrations in the region of 650– 2000 cm 1. For the purpose of convenient comparison, the volume ratio of PC to DEC is fixed at 1:1 in the LiBF4/PC + DEC solutions. For convenient observation of the changes of the bands, we respectively enlarge the bands where changes happened in the following figures (Figs. 9–11). Fig. 9 shows the changes of IR spectra of C@O stretch of PC in PC + DEC solutions containing different LiBF4 concentrations. The carbonyl stretch of PC and DEC in the mixtures without LiBF4 are located at 1795 cm 1 and 1744 cm 1 which are different from those of pure PC and DEC (1791 cm 1 and 1747 cm 1, respectively) due to the interactions between PC and DEC molecules discussed above in PC+DEC mixture. The bands of C@O stretch change after the adding of LiBF4. Shoulder peaks appear and gradually more obviously visible with the increasing of LiBF4 concentrations. There are two obvious shoulders at 1806 cm 1 and 1720 cm 1, respectively, and the intensity of the new shoulders increases accompanied with the increasing of LiBF4 concentrations. The positions of these two shoulders are very close to those of PC in LiBF4/PC and DEC in LiBF4/DEC solutions, but not completely the same. This fact
Fig. 7. The possible structures of the interaction of Li+ with PC (a) or DEC (b) and (c) in LiBF4/PC or LiBF4/DEC solutions.
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Wavenumber (cm-1) Fig. 9. The IR spectra of C@O stretch of PC in PC + DEC solutions containing different LiBF4 concentrations. (a) PC-DEC (v/v, 1:1); (b) 0.5 mol/L LiBF4/PC + DEC; (c) 1.0 mol/L LiBF4/PC + DEC; (d) 1.2 mol/L LiBF4/PC + DEC.
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shows that the interaction between PC and DEC molecules has been broken when LiBF4 is added. The IR spectra changes of CAOAC stretch of DEC in LiBF4/ PC + DEC solutions are shown in Fig. 10. The CAOAC stretch of DEC is also very sensitive to the interaction of Li+ with DEC in LiBF4/PC + DEC electrolyte solutions. This figure shows a change
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Wavenumber (cm-1) Fig. 11. The IR spectra of symmetric ring deformation and ring deformation of PC in PC + DEC solutions containing different LiBF4 concentrations. (a) PC-DEC (v/v, 1:1); (b) 0.5 mol/L LiBF4/PC + DEC; (c) 1.0 mol/L LiBF4/PC + DEC; (d) 1.2 mol/L LiBF4/ PC + DEC.
procedure which is similar to what is observed in the LiBF4/DEC solutions: an additional shoulder (at 1276 cm 1) appears, grows, simultaneously, the intensity of the band in 1303 cm 1 gradually increases with the increasing of LiBF4 concentrations. Fig. 11 expresses the IR spectra of symmetric ring deformation and ring deformation of PC in PC + DEC solutions containing different LiBF4 concentrations. The changes showed in this figure are almost the same as what is observed in the LiBF4/PC solutions: Band shifting of ring deformation can obviously be seen. At the same time, the half-peak breadth of symmetric ring deformation (711– 712 cm 1) increases quickly whilst a shoulder at about 722 cm 1 is clearly visible on the higher wavenumber side, especially on the concentration of 1.0 mol/L and 1.2 mol/L LiBF4/PC + DEC solutions. The intensity of the new band at 722 cm 1 increases at the expense of the band at 711 cm 1. At the concentration of 1.0 mol/L and 1.2 mol/L, the original band at 711 cm 1 becomes very weak compared to curve a (PC + DEC) and b (0.5 mol/L LiBF4/ PC + DEC) while the intensity of the new one at 722 cm 1 gradually increasing. According to the above analysis, it can be concluded that rather strong Li+-PC and Li+-DEC interactions appear in LiBF4/PC + DEC electrolyte solutions. This interaction affects the local structure of the solvent molecule, and causes their changes in spectra. The strong interaction occurs mainly through the oxygen atoms of carbonyl groups and ring deformation in PC molecules, and through both oxygen atoms of the carbonyl groups and ether oxygen groups in DEC molecules. The possible structures of the interaction of Li+ with PC and DEC in LiBF4/PC + DEC (v/v, 1:1) solutions are presented in Fig. 12. There are PC and DEC molecules in LiBF4/PC + DEC solutions. An interesting question is which kind of molecules interact preferentially with Li+? To answer this question, we compared the spectra of solutions with different solvent compositions. Fig. 12 shows the splitting of the 711 cm 1 symmetric ring deformation band of PC. It suggests that two kinds of PC molecules coexist in this solution. One kind, from the bulk solvent, described as ‘free’, generates the band at 711 cm 1. Another kind of solvent binds with Li+ and gives the band at 722 cm 1 [34]. Accordingly, the intensity of these bands should be proportional to the number of the corresponding solvent molecules. It is interesting to note that the value (1.16) of I712/I722 in LiBF4/PC solutions at the concentration of 1.2 mol/L is close to that (1.09) in LiBF4/PC + DEC solutions at the concentration of 1.2 mol/L and lower than that (1.35) in 0.5 mol/L LiBF4/PC + DECsolutions. It suggests that no obvious preferential solvation is
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Fig. 12. The possible structures of the interaction of Li+ with PC and DEC in LiBF4/PC + DEC (v/v, 1:1) solutions.
present in LiBF4/PC + DEC electrolyte solutions. This fact can be explained by the donor–acceptor approach. Values of donor number (DN) are 16.1 for DEC and 15.1 for PC [31]. Their DN values are very similar. Therefore, no obvious preferential selective solvation of Li+ with PC and DEC molecules could be expected in LiBF4/PC + DEC solutions. Conclusions The electrolyte solutions of lithium tetrafluoroborate (LiBF4) in PC, DEC and PC + DEC solutions have been systematically investigated by FTIR spectroscopy study. The tentative conclusions can be summarized as follows: (i) The dipole–dipole interaction between PC and DEC molecules in PC + DEC mixtures causes the C@O stretch of PC shift to the higher wavenumber side with the increasing of volume ratio in PC + DEC solutions. (ii) Strong interactions of Li+-PC and Li+-DEC are observed in LiBF4/PC, LiBF4/DEC and LiBF4/PC + DEC electrolyte solutions. It is possible that the former interaction (Li+-PC) occurs through the oxygen atoms of carbonyl groups and ring deformation in PC molecules, and the latter interaction (Li+-DEC) through both oxygen atoms of the carbonyl groups and ether oxygen groups in DEC molecules. (iii) No obvious preferential selective solvation of Li+ was found in the LiBF4/PC + DEC electrolyte solutions due to their small difference of PC and DEC in DN values. Acknowledgements Financial support from the National 863 Program (2013AA110100), ‘‘Western light’’ talent cultivation plan joint scholars Program (Chinese Academy of Sciences) and Open fund of key laboratory of salt lakes resources and chemistry (Chinese Academy of Sciences) are gratefully acknowledged. References [1] J. Barthel, H.J. Gores, Pure Appl. Chem. 57 (1985) 1071–1082. [2] K.M. Abraham, S.B. Brumner, in: J.P. Gabano (Ed.), Lithium Batteries, Academic Press, New York, 1983. [3] H.L. Yeager, J.D. Fedyk, R.J. Parker, J. Phys. Chem. 77 (1973) 2407–2410. [4] D. Battisti, G.A. Nazri, B. Klassen, R. Aroca, J. Phys. Chem. 97 (1993) 5826–5830.
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