J Solution Chem (2008) 37: 947–955 DOI 10.1007/s10953-008-9288-9
Ion Association and Solvation Behavior of Some Lithium Salts in Tetrahydrofuran. A Conductivity and Raman Spectroscopic Study Debashis Das
Received: 10 November 2007 / Accepted: 20 January 2008 / Published online: 29 April 2008 © Springer Science+Business Media, LLC 2008
Abstract Accurate measurements of electrical conductivities and laser Raman spectra of solutions of lithium chloride (LiCl), lithium bromide (LiBr), lithium tetrafluoroborate (LiBF4 ) and lithium perchlorate (LiClO4 ) in tetrahydrofuran are reported. The conductivity data have been analyzed by the Fuoss-Krauss theory, yielding values for the ion-pair and triple-ion formation constants. The Raman spectra suggest the presence of a new signal of perchlorate ClO− 4 ions in solution, whereas there is no such evidence for the other investigated anions. The observed processes have been interpreted by an Eigen multistep mechanism. For each salt, the predominant portion is found to remain in the form of ion pairs, leaving only a small fraction of triple ions. Keywords Electrical conductance · FT-Raman spectra · Lithium electrolytes · Tetrahydrofuran
1 Introduction Lithium has been studied for many years as an anode material for non-aqueous solution batteries [1]. In such systems, the choice of electrolyte solution and optimization of its salt concentration are two important factors. An electrolyte possessing high specific conductivity, and hence with minimal ion-ion interactions, is required to maintain low internal resistance in the cell. Knowledge of the state of association of the electrolytes and their interactions with the solvent molecules is essential for making an optimal choice of solvent and electrolyte in such systems. To this end a classical method, namely electrical conductivity, has been used to study the status of ion association and solvation in electrolyte solutions. Such studies are complemented with structural studies of the metal ion solvates using Raman spectroscopy. Hopefully, this will provide a molecular rationale for the choice of an electrolyte for battery construction. D. Das () Department of Chemistry, Dinhata College, Dinhata, 736135 Cooch-Behar, West Bengal, India e-mail:
[email protected]
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Recently, a comprehensive program [2–6] was initiated to study lithium ion solvation in different non-aqueous solvents using conductometric and Raman spectroscopic measurements. The study has now been extended to tetrahydrofuran (THF), a particularly relevant solvent for battery construction using lithium electrodes. Here we have investigated the solution behavior of four lithium salts, e.g., lithium chloride (LiCl), lithium bromide (LiBr), lithium tetrafluoroborate (LiBF4 ) and lithium perchlorate (LiClO4 ) in tetrahydrofuran (THF) using Raman spectroscopic and conductometric techniques, in an effort to provide a picture of the effect of these anions on ion complexation in this medium.
2 Experimental Section 2.1 Chemicals Tetrahydrofuran (Merck, India) was kept over KOH for several days, refluxed for 24 h and then distilled over LiAlH4 . The measured density and viscosity of the purified solvent at 298.15 K, 0.882 g·cm−3 and 0.463 mPa·S [6, 7], respectively, agree well with the literature values. The salts were of Fluka’s purum or puriss grade. Lithium chloride, lithium bromide and lithium tetrafluoroborate were dried under vacuum at 50–60 °C for 48 h and were then used without further purification. Lithium perchlorate was recrystallized three times from conductivity water and then dried under vacuum for 2–3 days [8]. 2.2 Apparatus and Procedure Conductivity measurements were carried out with a Systronics conductivity meter using a dip-type cell of cell constant 1.14 cm−1 with an precision of 0.1%. Measurements were made in a thermostatic bath maintained within ±0.005 K of the desired temperature. The details of the experimental procedure were described earlier [9]. The viscosity and density of the solvent were measured by means of a suspended-level Ubbelohde viscometer [10] and an Ostwald-Sprengel type pycnometer of about 25 mL capacity, respectively. Several independent solutions were prepared and runs were performed to ensure the reproducibility of the results [11]. Due correction was made for the specific conductivity of the solvent. The laser Raman spectroscopic measurements were made with a DILOR Z24 spectrometer using the 480 nm excitation frequency. The spectral slit width was kept at 4 cm−1 . The laser power used was 300 mW. The spectra were recorded by the Regional Sophisticated Instrumentation Centre, Indian Institute of Technology at Madras. All spectra were scanned at least twice to ensure repeatability. To avoid moisture pickup, all solutions were prepared in a dehumidified room with utmost care. The relative permittivity (ε = 7.58 at 298.15 K) of tetrahydrofuran was obtained from the literature [12].
3 Results and Discussion 3.1 Conductance The equivalent conductivity () data as a function of the molar concentration (c) are listed in Table 1 for LiCl, LiBr and LiClO4 solutions in tetrahydrofuran. When values of log10 are
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Table 1 Equivalent (molar) conductivities and corresponding molarities of the lithium salts in tetrahydrofuran at 298.15 K LiCl c × 104 /
mol·dm−3
LiBr
LiClO4
c × 104 /
/ S·cm2 ·mol−1
c × 104 /
/
mol·dm−3
S·cm2 ·mol−1
mol·dm−3
/ S·cm2 ·mol−1
70.8
0.0050
115.5
0.1999
79.4
0.8450
80.4
0.0042
133.2
0.1900
89.1
0.8125
96.5
0.0040
154.0
0.1826
100.0
0.7857
125.9
0.0038
177.8
0.1805
112.2
0.7614
160.8
0.0037
205.3
0.1795
125.9
0.7434
214.4
0.0036
237.1
0.1796
141.2
0.7205
260.8
0.0036
273.8
0.1809
158.5
0.7037
348.4
0.0036
316.2
0.1833
177.8
0.6895
482.4
0.0038
365.2
0.1871
199.5
0.6805
616.4
0.0040
421.7
0.1924
245.2
0.6633
696.8
0.0041
486.9
0.1933
316.2
0.6600
803.9
0.0042
562.3
0.2080
354.8
0.6604
910.9
0.0044
649.4
0.2188
446.7
0.6805
1071.9
0.0046
749.9
0.2320
588.8
0.7283
1258.9
0.0049
1000.0
0.2679
741.3
0.7850
1548.8
0.0053
1333.5
0.3219
891.2
0.8626
1812.3
0.0057
1778.3
0.4083
1000.0
0.9036
2021.6
0.0062
2053.5
0.4602
1122.0
0.9894
2514.4
0.0066
2440.6
0.5612
1258.9
1.0753
3015.5
0.0075
3162.3
0.8100
1479.1
1.2712
plotted as a function of log10 c for these electrolytes, a minimum was observed in each case and then the conductivity again increased, indicating the formation of triple ions (Fig. 1) [13–15]. The conductivity data have been analyzed by the Fuoss-Krauss triple-ion theory [16] in the form 0 T0 KT 1/2 c, (1) {g(c)}c = 1/2 + 1/2 1 − 0 KP KP
g(c) = β =
1/2 ) exp(− 2.303β 1/2 (c) 0
(1 −
S 1/2 )(1 − )1/2 3/2 (c) 0 0
,
1.8247 × 106 , (εT )3/2
S = α0 + β =
0.8206 × 106 82.501 0 + . 3/2 (εT ) η0 (εT )1/2
(2)
(3) (4)
In the above equations β is the Debye-Hückel activity coefficient and S is the limiting Onsager slope coefficient of the conductivity equation, = 0 − S(c/0 )1/2 , and the other terms have their usual significance. 0 and T0 are the limiting equivalent conductivities of
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Fig. 1 Electrical conductivities (as log10 ) as a function of concentration (as log10 c) for: (a) LiCl, (b) LiBr, and (c) LiClO4 in tetrahydrofuran
+ LiX and of two possible triple ions, LiX− 2 and Li2 X , and KP and KT are their ion-pair and triple-ion formation constants, respectively. The symmetrical approximation that the two possible formation constants for triple ions are equal to each other has been accepted.
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Fig. 2 Values of c1/2 /(S·cm2 ·mol−1/2 ·dm−3/2 ) as a function of concentration (c/mol·dm−3 ) for: (a) LiCl, (b) LiBr, and (c) LiClO4 in tetrahydrofuran
Neglecting /0 compared to 1, and assuming for interionic attraction that g(c) = 1 [15] in Eq. 1, we get c1/2 =
0 1/2 KP
+ T0
KT 1/2
KP
c.
(5)
For the present data, it was found that Eq. 5 was inadequate, with the data showing a downward curvature when plotted as c1/2 versus c (Fig. 2). On the contrary, Eq. 1 gives reasonably straight lines and the curvature has almost disappeared.
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Table 2 Conductivity parameters of lithium salts in tetrahydrofuran at 298.15 K Electrolyte
0
S·cm2 ·mol−1
KP
mol−1 ·dm3
KT
r2
mol−1 ·dm3
103 cP
104 cT
mol·dm3
mol·dm3 0.2
LiCl
137.7
2.49 ×1011
59.91
0.9989
301.4
LiBr
138.8
1.09 ×108
50.03
0.9997
313.6
8.5
LiBF4
122.6
1.13 ×107
44.36
0.9944
37.9
3.1
LiClO4
140.8
6.58 ×106
24.87
0.9998
146.1
5.5
To apply Eq. 1, it is necessary to have estimates of 0 and to assume a value of T0 . Internal consistency, however, requires comparison of relative values of K T for the same electrolyte in different solvents [15]. This has been the limiting criterion used here in letting T0 = 20 /3 and using the average Walden product values of (0 η0 ) at 298.15 K for LiCl, LiBr, and LiClO4 in methanol, acetonitrile, pyridine and dimethylformamide from the literature [17] and for THF η0 = 0.463 mPa·s at 298.15 K. The 0 values are found to be 137.7, 138.8, and 140.8 and the calculated S values from Eq. 4 are 86.11 × 105 , 86.79 × 105 and 88.04 × 105 , both respectively, for LiCl, LiBr, and LiClO4 . Table 2 reports the 0 , K P and KT values for these salts, as well as those of the square of the correlation coefficients (r 2 ). Also included in this table are the corresponding 0 , KP , KT and r 2 values for LiBF4 taken from an earlier study [6]. The uncertainty in the values of KT is within ±1 to ±3 in each case. The values are also comparable to those found earlier for some lithium salts in 2-methyltetrahydrofuran [13, 14], 1,2-dimethoxyethane [15] and dimethoxymethane [18]. The concentrations of the ion pairs and the triple ions (cP and cT , respectively) at the highest concentration for each electrolyte have been calculated using the following relations: cP = c (1 − α − 3αT ) , −1/2
, α = (KP cP ) 1/2 αT = KT /KP c1/2 , 1/2 cT = KT /KP c3/2 .
(6) (7) (8) (9)
Their values are also included in Table 2. 3.2 Raman Spectra The Raman spectra of pure tetrahydrofuran and of some representative salt solutions, in the wave number range of 4000 to 100 cm−1 , are presented in Figs. 3 and 4. The observed principal bands are listed in Table 3. Partial band assignments for the pure solvent as well as the electrolyte solutions have been made and are discussed accordingly. From Fig. 3, it is seen that THF exhibits its C-O stretching frequency at 922.8 cm−1 . The asymmetric C-H stretching mode of the solvent appears in the wave number range 3000 to 2800 cm−1 . These are the two major regions in the Raman spectra of tetrahydrofuran. No appreciable shifts of these Raman bands were observed in the salt solutions investigated here. This observation is similar to those found in the Raman spectra of 1,2dimethoxyethane [4]. The spectra indicate that the cation is associated with the anion in an intimate way in the form of contact ion pairs in THF solutions [19], but another possibility
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Table 3 Raman frequencies in cm−1a THF
LiCl
LiBr
LiClO4
LiBF4
–
466.1 (w)
464.1 (w)
465.7 (w)
543.5 (w)
922.8
923.6 (w)
922.8 (s)
920.3 (s)
924.5 (s)
–
–
–
939.1 (m)
–
2873.1 (s)
2874.2 (s)
2875.1 (s)
2877.3 (s)
2878.1 (s)
2912.7 (s)
2910.1 (s)
2914.2 (s)
2912.5 (s)
2915.1 (s)
2928.4 (s)
2927.8 (s)
2926.9 (s)
2928.0 (s)
2928.2 (s)
2963.6 (s)
2964.5 (s)
2963.4 (s)
2963.0 (s)
2966.1 (s)
a s = strong, m = medium, w = weak
Fig. 3 Raman spectrum of pure tetrahydrofuran
is that the cation might have some solvent molecules in its near-neighbor environment based on geometrical considerations [15]. Besides this band, the spectra of the salt solutions do not show any new signals with the exception of the LiClO4 solution that exhibits a peak of medium intensity at 939.1 cm−1 . This new non-degenerate band is attributed to the infrared forbidden, totally symmetric, stretching vibration of the perchlorate ion [20, 21]. This new ClO− 4 signal in tetrahydrofuran solutions has been assigned to the solvent-separated ion-pair Li+ · THF·ClO− 4 and/or to − + the solvent-separated dimer Li+ ·THF·ClO− · · ·Li ·THF·ClO , which are spectroscopically 4 4 indistinguishable from each other. Thus, for LiClO4 , both contact and solvent-separated ion pairs may exist in tetrahydrofuran solution. This observation may be interpreted in terms of
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Fig. 4 Raman spectrum of LiClO4 in tetrahydrofuran
the following Eigen multistep mechanism: − − + + Li+ + ClO− 4 Li ·THF·ClO4 Li ·ClO4 ,
(10)
− − + + 2Li+ ·THF·ClO− 4 Li ·THF·ClO4 · · · Li ·THF·ClO4 ,
(11)
+
2Li
·ClO− 4
(Li
+
·ClO− 4 )2 .
(12)
The other three salts, namely, LiCl, LiBr, and LiBF4 , are found to remain essentially in the form of contact ion pairs, no solvent-separated ion pair being detectable, and the relevant equilibrium are: Li+ + B− Li+ ·B− , +
−
+
−
2Li ·B (Li ·B )2 ,
(13) (14)
where B = Cl− , Br− , or BF− 4 . The concentration of contact ion pairs and hence of contact quadruplets should increase with increasing KP values for the electrolyte solutions.
4 Conclusion It may thus be concluded that all of the electrolytes investigated in the present study, namely + LiCl, LiBr, LiBF4 , and LiClO4 , form symmetrical triple ions, LiX− 2 and Li2 X , in tetrahydrofuran solutions, along with ion pairs. In addition, a predominant proportion of each of these electrolytes would exist in the form of ion pairs along with quadrupoles of the innersphere type. LiClO4 , on the other hand, is found to exist in tetrahydrofuran solutions as solvent-separated ion pairs and/or solvent-separated dimers.
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Acknowledgements The author is thankful to the University Grants Commission (Eastern Regional Office, Kolkata) for sanctioning a Minor Research Project via letter No. PSW/085(05-06), 21st March 2006.
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