J . Phys. Chem. 1993, 97, 5826-5830
5826
Vibrational Studies of Lithium Perchlorate in Propylene Carbonate Solutions D.Battisti, G. A. " x i , B. IUassen, and R. Aroca' Materials and Surface Science Group, University of Windsor, Windsor, Ontario, Canada N9B 3P4 Received: November 17, 1992
Raman and infrared spectroscopic studies and conductivity and viscosity measurements of propylene carbonate (PC) doped with various concentrations of lithium perchlorate are reported. The assignment of the vibrational modes was supplemented by AM1 normal coordinate analysis. Both Raman and infrared spectra showed band splitting in the fundamental vibrational frequencies of PC and perchlorate anion. Spectral curve fitting within the totally symmetric perchlorate band shape showed contributions of free ion, solvent-shared ion pairs, and contact ion pairs. Strong Li+-PC interaction was observed for the PC ring deformation band at 712 cm-l. Ion pairing as deduced by spectroscopic techniques provided a rationale to account for conductivity and viscosity data.
Introduction Theuse of nonaqueous electrolytesin thedevelopment of lithium secondary batteries has been intensively inve~tigated.'-~Monomeric and polymeric organic materials doped with lithium salts show favorable conductivity with values reaching the 10-3 ohm-I cm-I mark. Such electrolytes are expected to be important in new battery technology.2" Numerous reports have demonstrated the utility of vibrational spectroscopy as a tool to probe saltsolvent interactions in electrolytic system^.^+^ The vibrational spectrum provides frequencies, intensities, and band properties which can be used to identify species and chemical processes.IO Such studies could help to identify factors that affect the general properties and the performance of the electrolyte. For example, Kakihana et a1.8 showed that the internal flexibility of poly(ethylene oxide) (PEO) and poly(propy1ene oxide) (PPO) doped with NaCF3SO3 had a greater effect on conductivity than ionion interactions. On the other hand, Chen et a1.6 outlined that adding an ion chelating agent to the PEO/NaCH3S03 system reduced ion pairing (as detected using Raman spectroscopy) and increased the conductivityof the polyelectrolyte. Recently McLin et al.11 considered ion associations in poly(propy1ene glycol)/ NaCF3S03fromviscosityand conductivity measurements. They showed that spectroscopic data complemented viscosity and conductivity measurements. The conductivity ( u ) of a liquid electrolytecan be approximated by12
= ( u / 6 ) ( N / V ) e 2 / K B T= ( N / V ) e 2 D / K B T (1) where N is the number of mobile ions of charge e contained in volume V,u is the jump frequency, and D is thediffusioncoefficient of the mobile ion. The Stokes-Einstein relation can be used to relate D to viscosity q: 0
D = KBT/6?rqrj (2) where rj is the Stokes radius of moleculej. From relations 1 and 2 it can be shown that the conductivity of the solution is inversely proportional to the viscosity of the solution. The equivalent conductivity (A) of a solution is described by
x = 1000u/c (3) where u is the conductivity (ohm-' cm-I) and c is the equivalent concentration (equiv/dm3). The high solubility of lithium perchlorate in PC, and the good physical properties of PC that include a high dielectric constant * To whom correspondence should be addressed. . .~. . .., 0022-3654,I93~, 12097-5826SO4.00 I O. ~
~
(64.9 at 25 OC),I3low freezing temperature (mp -49.2 "C), and high boiling point (241.7 "C), leads to an attractive system for electrolyte research and development. Yeager et al.I4originally reported IR spectroscopicstudies on ionic solvation in PC. They observed splitting in the 1182-and the 1798-cm-I infrared bands of PC in nitromethane doped with small cations (Le., Li+, Ag+, and Na+). The large cation tetra-n-butylammonium however did not produce spectral changes. In the present work, the ionic conductivity, the viscosity, and the FT-IR and Raman spectra are reported for propylene carbonate doped with different concentrations of LiC104. Inelastic light scattering is used for the first time to observe ionic associations in PC/LiC104 solutions. A normal coordinate analysis (NCA) of the PC molecule using the AM1 Hamiltonian was carried out in order to support the assignment of characteristic fundamental vibrational frequencies.15J6
Experimental Section Solutions containing propylene carbonate and lithium perchlorate were prepared in a drybox filled with Ar. The oxygen and moisture contentswere less than 1 ppmin the drybox. Lithium perchlorate was obtained from Alfa Products and vacuum dried at 150 OC. Propylene carbonate (Fluka Chemicals) was stored over 4-A molecular sieves for 48 h. Solutions of PC/LiC104 (mo1:mol) were degassed for 3 h under vacuum prior to any electrical or optical analysis. The 514.5-nm excitation line from a Spectraphysics Model 164 Ar+ laser was used for Raman excitation of PC and PC/ LiC104 solutions in glass capillaries. A polarization rotator (Spectraphysics Model 310-21) was mounted on the laser to produce S-polarized (TE) incident light. The scattered light was collected without an analyzer, giving the Raman spectra presented in Figures 2 and 5-7. Raman shifts were measured with a SPEX1403doublespectrophotometer. The spectral band-pass and laser power were 2 cm-I and 50 mW, respectively. The Raman spectrum was acquired in steps of 0.5 cm-l and 2 s/step delay. A Bruker FT-IR Model 98 spectrometer and an attenuated total reflectance (ATR) solution cell (with a ZnSe internal reflection element) were used to collect IR spectra. Conductivity measurements were recorded at 1000 Hz and 27 OC with a Digibridge (GenRad Model 1658) cell containing two nickel electrodes. The viscosity of the electrolyte solutions was measured at 27 OC with a Cannon viscosimeter, model T298. Raman files were imported into the Spectra Calc (Galactic Industries Corp.) software for data manipulation and spectral plotting. The program Data Fit by Square Tools (Spectrum Square Associates, Inc.) environment was loaded into Spectra 0 1993 American Chemical Society
Lithium Perchlorate in Propylene Carbonate Solutions 1
The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 5827 TABLE I: Characteristic Frequencb of Propylene Carbonate.
0
Raman frequency (cm-1) 455 m 712 s
Equilibrium
712 cm.'
849 vs
1782 cm"
Figure 1. EquilibriumgeometryofPC. The712- and 1782-cm-I normal modes of PC are illustrated graphically.
1119m 1147m 1336 m 1355 m 1391 w 1484 m 1782 m 2882 m 2938 vs 2990 s
infrared frequency (cm-1)
assignment
712 s 777 vs 849 m 1060 vs 1120vs 1148 vs 1177vs 1337 sh 1355 vs 1391 vs 1484 s 1800 vs 2882 w 2937 m 2990 s
ring deformation symmetric ring deformation ring deformation C7-H + 06-c5-c4 bending C5-H twist C4-H bending (2.1-H wag C4-H bending C5-C4 + c2-6 C1-03 stretch 06-c5 stretch (2.1-H wag C7-H bending C7-H bending C5-H wag C7-H bending C r H umbrella C 2 4 1stretching -CH2 stretching -CH3 stretching -CH, stretching
+
+
+
+
+
s = strong, m = medium, w = weak, v = very, sh = shoulder.
500
1000
+dOOI
1500
Wovenumber (cm- 1 )
-i"
5000'
Figure 2. ATR-IRS (top spectrum) and Raman spectra (515.5-nq excitation line, bottom spectrum) of propylene carbonate.
Calc and was used for band fitting. The NCA of propylene carbonate was carried out with the semiempiricalAM1 Hamiltonian17 using the IBM-PS/2 Version 4.0 of the MOPAC program.'"
Results and Discussion Vibrational Assignment of Propylene Carbonate. To assist in the interpretation of vibrational modes, vibrational frequencies of PC were calculatedusing the semiempirical AM 1 Hamiltonian using the MOPAC IBM-PS/2 Version 4.0 program. The program FORCE was used to obtain the harmonic frequencies and full Cartesian eigenvectors. The theoretical equilibrium geometry as defined in Figure 1 was obtained by energy minimization using the AM1 Hamiltonian. The AM1 method provided a listing of the atom-based Cartesian displacementswhich were transferred to a dynamic display program written in our laboratory to graphicallyobserve each normal mode on the monitor of an IBM PC. For example, the graphic displays for the experimental PC frequencies at 7 12 and 1782 cm-l are shown in Figure 1. Similar computer graphics programs to observe vibrational motions have been developed by others for the Macintosh computer.19 The calculated frequencieswereoverestimated in the AM 1calculation. However, using scaling factors such as those reported by Seeger et al.,19,20 the theoretical values were brought into closer agreement with the experimentaldata. The observed IR and Raman spectra of PC are shown in Figure 2. The PC molecule belongs to the Cl point group, and most Raman bands were polarized as previously found by Janz et al.15 Characteristic vibrational frequencies were the C-H stretches in the Raman, the C-H bendings in the IR, the C - 0 stretch in the IR, and the ring deformations in the IR and Raman spectra. Notably, the calculated value for the symmetric ring bending mode was 708 cm-1. Experimentally, the strong Raman and IR band at 712 cm-I was assigned to the locally symmetric ring deformation.I5 Characteristic group frequencies of PC and their assignment are given in Table I.
Conductivity, viscosity, and Ion Association in pC/UCI04 Solutions. The results of conductivity and fluidity (viscosity.') measurementsare shown in Figure 3 for PC/LiC104 electrolytes. The maximum ionic conductivitywas observed at 1.3 M LiCIOl which corresponds to 7-8 PC molecules per lithium ion. At a higher lithium salt concentration the conductivity drops and a more viscous solution is formed. The drop in ionic conductivity is related to higher viscosity which reduces the mobility of Li+ and the number of Li+ charge carriers. An increase in viscosity is typically attributed to the formation of ion pairs and ion multiples in the solution. The equivalent conductivity Xi of a liquid electrolyte at infinite dilution can be expressed by the Stokes-Einstein-Lorenz formalism1
Xi = ZiF5/6?rvri (4) where F is the Faraday constant and v the viscosity of the electrolyte. Although this model gives a qualitative agreement between conductivity and viscosity, it fails to include the solvent dielectric effects on the ion mobility. The effect of the solvent dielectricon the electrolyte conductivity was introduced by BOm,21 Fouss,22and Zwanzig,23where they added thedielectric relaxation drag to the viscosity drag. This modification renders a new equation which includes the static and high-frequency dielectric constants of the electrolyte
Xi =
Zip
6 4 r + C/r3]
(5)
with C = (2/3)(1/6r)(r/q)(Z2e2/e,)[(t,-e,)/e,], where is the dielectric relaxation time constant (D), and e, and e, are the static and high-frequency dielectric constants, respectively. Due to a strong dependence of X on ri, Xi has a maximum at X = A(33/4/40C1/4),corresponding to the radius R, = (3C)1/4 A. Although the theory qualitatively describesthe conductivitydata for dilute electrolytes, it fails to includeother complications such as ion pairs and ion aggregates. Plots of u vs v-1 or alternatively X vs v-1 can be used to examine the behavior of the electrolyte. The expected linear or 'ideal behavior" of u versus q-l was not observed for PC/LiC104 solutions presented in Figure 4, We present the plot IJvs q-l because it is more familar to practitioners in the field. The change in slope at the break point [LiClOs] = 1.3 M indicates significant dielectric effects as well as ion pairs and ion multiple formation. The conductivity and fluidity data show a similar trend. In dilute solutions where the ion pair and ion aggregate concentrations are low, the viscosity drag has a dominant role on ion mobility. However, at high concentrations the fluidity data as presented in Figure 4 show a maximum at ca. 1.3 M LiC104, indicating significant dielectric effects as well
Battisti et al.
5828 The Journal of Physical Chemistry, Vol. 97, No. 22, 199.3 P C U C I O ~(Mole Rotio)
36:1
5:l I
8:1
I
I
41
A
3 1 I
I
PC:LiClO, = 1O:l
0
1
2
5
4
[UCIOd (Mol/L)
Figure 3. Conductivity( 0 )and fluidity (0)measurementsin PC/LiC104 solutions. PCtiC1o4 (Mole Rotlo) 8:1
41 r
56:l
-1
I
950 Wavenumber (cm.')
900
Figure 5. Lorentzian curve fitting within the A , Raman band of perchlorate. The arrows indicate the PC bands that were also used for curve fitting.
OJ
0
10
20 30 l/Viscosity (Poise")
40
1
50
Figure 4. Conductivityversus fluidity in solutionsof propylene carbonate doped with lithium perchlorate.
as ion pair and ion multiple formation. The drop in conductivity in this work is attributed to the formation of ion pairs and ion aggregates in solution as supported by the followingspectroscopic data. Changes in the band shape of the totally symmetric ( v , ) band of the tetrahedral free perchlorate anion centered at 933 cm-l have been attributed to ion associations in solution. In the Raman spectrum of perchlorate anions, the V I is the most intense while the other bands at 462 cm-I (VZ, E), 1102 cm-1 ( u j , F2), and 628 cm-I (u4, F2) are only observed in concentrated solutions. In a recent ATR-IRS report of LiC104 in nitromethane, it was shown that the v3 band of perchlorate anion was also sensitive to ion association^.^^ James et al.z5926 used band fitting to identify ionsolvent interactions in the Raman spectra of perchlorate solutions. Their analysis led to the identification of four species: (i) free perchlorate anion (Le., C104- at 933 cm-I), (ii) solvent-shared ions (i.e., Li+-soIvent-C1O4-) at 939 cm-I, (iii) contact ion pairs (i.e., Li+C104-) at 948 cm-I, and (iv) multiple ion aggregates (Le., (Li+C104-),,) at 955 cm-1. Curve fitting within the A, perchlorate anion band has also been used to identify free C104and solvent-shared ions9 in the poly(propy1ene glycol)/LiC104 electrolyte. Ion pairing in sodium perchlorate solutions was also identified by optical spectro~copy.~~ The program Datafit supplied with the Square Tools software was used to curve fit the A1 band of the perchlorate anion to Lorentzian peak shapes in PC/LiC104 solutions. In the most concentrated solution (PC:LiC104 = 1:l), three Lorentzian bands centered at 933,938, and 944 cm-I were required to adequately fit the band shape. The spectral locations of the curve-fitted bands were in agreement with free C104- (933 cm-I), solvent-
shared ion pairs (938 cm-I), and contact ion pairs (944 ~ m - ~ ) . ~ 5 - ~ ' At concentrations of PC:LiC104 = Xl (2 I X I 10) only two Lorentzian bands centered at 933 and 938 cm-l were required to completely deconvolute the AI envelope of C104- as shown in Figure 5. In solutions where PC:LiC104 = Xl (X> 10) a single bandat 933 cm-I wassufficient tofullydeconvolutethe perchlorate envelope. The assignment of the 938-cm-I band to Li+-PC-C104- was also evaluated in Raman spectroscopicstudies where the lithium cation was replaced by a tetrabutylammonium (Y) ion. In the solution of PC:YC104 = 50:1, the V I free perchlorate band was observed at 930 cm-I. The band shape, width, and frequency were also preserved and found unchanged in solutions of PC: YC104 = 1O:l and PC:YC104 = 5:l. The latter solution was close to the saturation limit of tetrabutylammonium perchlorate in PC. Since the 938-cm-' band was essential in the band-fitting analysis of the PC:LiC104 = 5:l solution and not required in the deconvolution of the PC:YC104 = 5:l solution, it can be concluded that the 938-cm-1 band must originate from a strong Li+-PC-C104- interaction. In light of the spectroscopicevidence, Figures 3 and 4 can also be further discussed. According to the curve-fitting discussion, free ions persisted in solutions of PC:LiC104 = Xl (X> 10). Solvent-shared ions and free ions were restricted to solutions of PC:LiC104 = 221 (2 5 X 5 10). In Figure 4, it therefore seems possible that the behavior for PC:LiC104 = Xl (8 I X 5 36) can be attributed mainly to free ion effects. For solutions where PC:LiC104 = Xl (4 IX I8), the cr vs 7-1 dependence can be attributed to effects caused by both free ions and solvent-shared ions. The increase in conductivity from dilute to PC:LiC104 = 8:l in Figure 3 may therefore be rationalized in terms of the gradual increase in free ion population as salt was added. Thereafter the solvent seems unable to solvate the free ions, and solvent-ion interactions increase with increasing salt concentration. The drop in conductivity can therefore be related to the increase in the number of solvent-ion interactions. Splitting of the 712-cm-1 or PC ring deformation band was also accompanied by the appearance of solvent-shared ion pairs
Lithium Perchlorate in Propylene Carbonate Solutions
650
700
8b0
750
960
920
The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 5829
940
Wavenumber (cm-I) Figure 6. Spectral perturbations observed in the Raman bands of PC (712 cm-I) and perchlorate (933 cm-I) in PC/LiC104 solutions.
A
- -
A
740
700
PC:LiCIO,= 10:1
A
PC:LiCIO,=2: 1
700 750 Wavenumber (cm.!)
Figure 7. Lorentzian band fitting within the 712-cm-I Raman envelope of PC.
as shown in Figure 6. The different band shape for the 712-cm-l band also suggesteddifferent degrees of ion-molecule interactions. Lorentzian deconvolution of the 7 12-cm-1envelope revealed two bands centered at 712 and 722 cm-1 as shown in Figure 7. The higher frequency component results from a Li+-PC interaction, since it showed concentration dependence. For example the 722-cm-1 band appeared as a shoulder in the curve fit of PC: LiC104 = 1O:l and increased in intensity in concentrated (i.e., PC:LiC104 = 1:l and PC:LiC104 = 2:l) solutions. The assignment of the 722-cm-I band was also supported by Raman data of PC/YClO4 solutions. There was no considerable band splitting or broadening in the 7 12-cm-1 Raman band in solutions of PC:YC104 = 50:1, PC:YC104 = lO:l, and PC:YC104 = 5 1 . Splitting of the ring deformation mode has also been observed in solutionsof ethylenecarbonatedoped with lithiumperchlorate.28
1420
1380
U I900
1700
Wavenumber (cm-l) Fipre8. ATR-IRS recorded for various PC:LiC104 =X:1. The topmost spectrum is the undoped or reference PC spectrum and the subsequent spectra are (A) X = 50, 10,8, 7,6, $ 4 , and 3, respectively, (B) X = 50, 20, 10,8,7,6, 5 4 , and 3, respectively, and (C) X = 50, 10,8,7,5,and 3,
respectively.
The ATR-IR spectra presented in Figure 8A also showed splitting of the 7 12-cm-l PC mode to give an additional component at 721 cm-I. The 722-cm-I Raman mode as determined by curve fitting therefore agrees with the observed IR band at 721 cm-I. The carbonyl PC stretching bands in the Raman and IR spectra were also perturbed at high salt concentrations. The full width at half-height of the C=O stretching at 1782cm-I was broadened by 27 cm-I in the Raman spectrum of the PC:LiC104 = 1:l solution. The frequency of the C=O solvent band was observed unchanged in PC/LiC104 solutions. The IR carbonyl band profile for PC/LiC104 solutionsis presented in Figure 8C. It is apparent that the carbonyl band in the IR showed a behavior similar to what was observed in the Raman: band broadening with increased LiC104 concentration. However, there was no band splitting as was reported in IR studies of PC/LiC104 in 11itr0methane.l~The other IR bands that were sensitive to salt were the 712- and 1391-cm-1 bands which are shown in Figure 8A,B. The latter band splits to give an additional component at 1405 cm-I. In summary, the Li-PC interactions strongly affect the vibrational fundamentals of the PC ring. Conclusions Raman spectroscopicstudies of PC/LiC104 solutions revealed free ions at 933 cm-l, solvent-shared ion pairs at 938 cm-I, and contact ion pairs at 944 cm-I (only observed in the PC:LiC104 = 1:1 solution). The spectroscopic studies complemented conductivity and viscosity data. Spectroscopic results support the claim that ion pairing in PC/LiC104 solutions reduces the conductivity of the electrolyte. Splitting of the 712-cm-' PC band was observed at high salt concentration, and it was assigned to a Li+-PC interaction. The normal coordinate analysis of propylene carbonate was carried out using the AM1 Hamiltonian in order to complement the vibrational assignments of characteristic group frequencies.
5830 The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 Acknowledgment. Financial assistance from NSERC of Canada and the General Motors Research Laboratories and EnvironmentalStaff, Warren, MI, is gratefully appreciated. This work was presented as a poster at the XI11 International Conference on Raman Spectroscopy, Wurzburg, Germany, August 1992. References and Notes (1) Aurbach,D.;Daroux,M.L,Faguy,P. W.;Yeager,E.J.Electrochem. SOC.1987, 134, 1611. (2) Nazri,G. A.; MacArthur, D. M.; OGara, J. F.;Aroca,R. In Materials Research Society Symposium Proceedings; Nazri, G. A., Shriver, D. F., Huggins, R. A., Balkanski, M., Eds.; Materials Research Society: Pittsburg, 1990; Vol. 210, p 163. (3) Abraham, K. M.; Alamgir, M.J. Electrochem.Soc. 1990,137,1657. (4) Croce, F.: Panero, S.;Scrosati, B. In Materials Research Society Symposium Proceedings; Nazri, G. A., Shriver, D. F., Huggins, R. A:, Balkanski, M., Eds.; Materials Research Society: Pittsburg, 1990; Vol. 210, p 179. (5) Mastragostino, M. In Materials Research Society Symposium Proceedings; Nazri, G. A,, Shriver, D. F., Huggins, R. A., Balkanski, M., Eds.; Materials Research Society: Pittsburgh, 1990; Vol. 210, p 191. (6) Chen, K.; Doan, K.; Ganapathiappan, S.; Ratner, M.; Shriver, D. F. In Materials Research Society Symposium Proceedings; Nazri, G. A., Shriver, D. F., Huggins, R. A., Balkanski, M., Eds.; Materials Research Society: Pittsburg, 1990; Vol. 210, p 191. (7) Torell, L. M.;Schantz, S.; Jacobson, P. In Materials Research Society Symposium Proceedings; Nazri, G. A., Shriver, D. F., Huggins, R. A., Balkanski, M., Eds.; Materials Research Society: Pittsburg, 1990; Vol. 210, p 221. (8) Kalikihana, M.; Sandahl, J.; Schantz, S.; Torell, L. M. In Second International Symposium on Polymer Electrolytes; Scrosati, B., Ed.; Elsevier: London, 1990; p 1.
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