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Thermochemical investigations of the systems useful for lithium battery construction
49
J. Electroanal. Chem., 316 (1991) 49-56 Elsevier Sequoia S.A., Lausanne
JEC 01719
Thermochemical investigations of the systems useful for lithium battery construction Solution enthalpies of LiBF, and LiPF, in 1,2_dimethoxyethane + y-butyrolactone mixtures at 25 OC Alina Piekarska Department of Physical Chemistry, University of Ebdi, Pomorska 18, 91-416 E6di (Poland)
(Received 4 April 1991)
Abstract Solution enthalpies of LiBF, and LiPF, in the mixtures of 1,2-dimethoxyethane with y-butyrolactone have been measured over the whole range of the mixed solvent composition. The molar enthalpy of solution for both salts at the concentration 0.003 mol kg-’ goes through a minimum with the mixture composition corresponding to 90 mol% of 1,2_dimethoxyethane. Possible reasons for the presence of this minimum are discussed.
INTRODUCTION
Research of new, high efficiency power sources is an important task in contemporary electrochemistry. Construction of primary and secondary high energy lithium batteries has been a significant achievement in this area. These batteries, in most cases, contain a lithium salt with a large complex anion dissolved in a mixed solvent that consists of a high dielectric solvent (HDS) and a low viscosity solvent (LVS). For the HDS sulfolane W, dimethoxysulfoxide (DMSO), propylene carbonate (PC), ethylene carbonate (EC) and y-lactones are used. Tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), l,l- and 1,Zdialkoxyethanes and 1,3-dioxalane belong to the LVS group. The salts: LiBF,, LiPF,, L&F,, LiClO, are the most frequently employed electrolytes. Numerous papers have been devoted to the results of conductometric and potentiometric investigations of the systems useful for lithium battery construction. In addition, some viscosity, density and electric permittivity data concerning mixtures of HDS with LVS can be found in the literature. 0022-0728/91/$03.50
0 1991 - Elsevier Sequoia S.A. All rights reserved
50
However, only a little is known about the thermochemical properties of the solutions under discussion. In order to fill this gap at least partly we have undertaken some calorimetric investigations of the systems that are employed in the lithium batteries. In this paper, we present the measured dissolution enthalpies of lithium tetrafluoroborate (LiBF,) and lithium hexafluorophosphate (LiPF,) in the mixtures of y-butyrolactone (y-BL) with 1,Zdimethoxyethane (1,ZDME). The above-mentioned systems which are protected under many patents are the most promising ones for high energy battery construction in the opinion of some authors’ [l-41. EXPERIMENTAL
Lithium tetrafluoroborate, Aldrich 98% was dried for several days under a reduced pressure at 90-100 ’ C. Lithium hexafluorophosphate, Aldrich 97%, was kept for several days in a vacuum desiccator at 30-35°C. Both salts were then stored under dry argon. y-Butyrolactone and 1,2_dimethoxyethane, both Fluka AG, were dried with 0.4 nm molecular sieves and distilled in a water-free atmosphere c-y-BL under reduced pressure). The mixed solvents were prepared by weight in a dry box. Enthalpies of solution were measured with a “isoperibol” type calorimeter as described elsewhere [5]. The salts were contained in thin-walled ampoules made of hard glass. The ampoules were filled with the salts in a dry argon atmosphere directly before placing them in the calorimeter. It was particularly important in the case of LiPF, which affected the glass ampoules when it was left in them for a long time. Four to six independent measurements were made for each salt in each y-BL + 1,ZDME mixture investigated. The concentration range of the electrolyte was 0.003-0.01 mol kg-’ of the mixed solvent. RESULTS AND DISCUSSION
The measured enthalpies of solution, AH, of LiBF, and LiPF, in the mixtures of y-butyrolactone with 1,2-dimethoxyethane are given in Tables 1 and 2 as a function of both the electrolyte concentration and the mixed solvent composition. The dependence of the AH,, corresponding to m = 0.003 mol kg-’ electrolyte concentration on the mixed solvent composition is presented in Fig. 1. The molar solution enthalpy for both salts at the same concentration is more negative in pure 1,2-DME than that in pure y-BL. The 1,Zdimethoxyethane as well as y-butyrolactone are aprotic solvents. Therefore, the more exothermic solvation enthalpies in pure 1,2-DME. seem to be connected with the stronger solvation of the lithium cation in the above-mentioned solvent due to its higher donicity compared with the y-BL (the donor numbers, DN: for 1,2-DME is 24 [6]; and for y-BL, 15.9 1611.Moreover, in the opinion of some authors, mono- and di-ethers coordinate small alkali metal cations (Li+ and Na+) strongly and form
51 TABLE 1 Molar enthalpies of solution, AH, 298.15 K
m/
mol kg-’
m/
-
40.73 40.45 40.38 40.00 39.62
0.002985 0.003610 0.004875 0.006704 0.010987
+ 40 mol% of y-BL
+ 50 mol% of y-BL
- 39.33 - 39.25 -39.16 - 38.95 - 38.80
0.003191 0.004770 0.006335 0.009276
0.002476 0.003316 0.005229 0.006491 0.008730
kJ mol-’
+ 60 mol% of y-BL 0.003243 0.004190 0.005695 0.008130
mol kg-’
AH,/
kJ mol-’
+ 10 mol% of y-BL
+ 30 mol% of y-BL 0.003166 0.003859 0.004163 0.006293 0.008199
y-butyrolactone
AH,/
1,2-DME 0.003176 0.004831 0.005589 0.007665 0.008835
of LiBF, in l,Zdimethoxyethane+
- 36.07 -35.92 -35.58 -35.21
-41.80 -41.65 -41.46 -41.28 - 40.90
- 38.57 -38.31 -38.10 - 37.99
+ 70 mol% of y-BL 0.003256 0.004710 0.006235 0.008760
- 34.98 - 34.66 -34.31 -34.14
m/
mol kg-’
mixtures at
AH,/
k.l mol-’
+ 20 mol% of y-BL 0.003072 0.003347 0.004520 0.007329 0.007703 0.008137
-41.13 - 41.04 - 40.83 - 40.75 - 40.54 - 40.25
- 37.32 -37.17 - 36.80 - 36.50 .- 36.44
+ 90 mol% of y-BL 0.003085 0.004323 0.007132 0.008045
- 32.40 - 31.92 - 31.69 -31.42
100 mol% of y-BL 0.002985 0.003768 0.005156 0.007739
- 30.52 -30.31 - 29.96 - 29.50
chelate-type complexes [1,7-101. This process can also give an additional exothermic impact to the enthalpy of solvation of the lithium salts in pure 1,2-dimethoxyethane. The curves representing the enthalpies of solution for both LiBF, and LiPF, at the concentration m = 0.003 mol kg-’ (Fig. 1) exhibit their minimum at high 1,2-DME content (ca. 90 mol% of the 1,ZDME). The presence of this minimum does not seem to be related to any essential variation of the mixed solvent structure. Owing to the similar, aprotic character of both mixed solvent components, a gradual change from the structure typical for pure 1,ZDME to that typical for pure y-BL would be expected. The monotonic course of viscosity [1,2], density 111 and electric permittivity [l-3] functions (Fig. 2) for y-butyrolactone + 1,2dimethoxyethane mixtures within the whole composition range confirms the above conclusion. It can be supposed, therefore, that the appearance of the AH, minimum is due to the presence of ions in the solution, i.e. it is connected with ion-solvent and ion-ion interactions. The former results mainly from the “chelat-
52
TABLE 2 Mdlar enthalpies of solution, AH,
of LiPF, in l,Zdimethoxyethane+
y-butyrolactone
mixtures at
298.15 K m/ mol kg-
’
AH,/ kJ mol-’
12-DME 0.002888 0.003547 0.003551 0.006988 0.009524
- 72.76 - 72.21 - 72.00 -71.63 - 71.42
-
-
0.002988 0.003365 0.005418 0.006098 0.011198
-
0.002399 0.003231 0.005382 0.006595 0.010444
-
76.18 76.15 75.73 75.50 75.00
0.003017 0.004771 0.005168 0.010630
73.98 73.52 73.02 72.88 72.35
0.002998 0.003791 0.003848 0.005783 0.09300
0.003480 0.004815 0.006022 0.006048 0.007749
AH,/ kJ mol-’
-
- 62.80 -62.13 - 61.63 - 61.50 -61.42
-
69.70 69.22 69.13 68.66 68.21
+ 80 mol% of y-BL 0.002983 0.003057 0.005041 0.006270 0.006311 0.008302
- 57.25 - 57.22 - 56.48 - 56.00 - 55.90 -55.17
+ 90 mol% of y-BL
100 mol% of y-BL
0.003266 0.004538 0.005894 0.009797
0.001940 0.002213 0.002918 0.005239 0.007548
- 54.03 - 53.82 -53.14 - 52.21
75.75 74.60 74.35 73.22
+ 40 mol% of y-BL
+ 60 mol% of y-BL 66.89 66.52 66.04 65.71 65.45 65.07
m/ mol kg-’
+ 15 mol% of y-BL
+ 30 mol% of y-BL 75.21 74.68 73.88 73.80 72.85
+ 50 mol% of y-BL 0.002781 0.003272 0.004753 0.006257 0.006723 0.007459
AH,/ kJ mol-’
+ 10 mol% of y-BL
+ 20 mol% of y-BL 0.002377 0.003108 0.005294 0.005435 0.008468
m/ mol kg-’
- 52.15 - 52.04 - 51.88 -51.08 -51.68
ing” properties of 1,Zdimethoxyethane towards the lithium cation [1,7-lo]. The latter is an ionic association due to a very low electric permittivity of the mixtures with a high diether content. The analysis of conductometric, viscosimetric and spectroscopic data led Salomon and his coworkers to the conclusion that the lithium cation, despite having four active centers, was coordinated by only one molecule of 1,Zdimethoxyethane. For this reason, the “chelated” cation was still able to form a “contact” ionic pair [l] (Fig. 3). Faber, Irish and Petrucci [93 suggested that both “contact” and “solvent-separated” ionic pairs existed in 1,2-DME solution: Li+-DME-X-s Li+-X--DME (1) The ionic association equilibrium constants, KA for lithium salts in 1,ZDME at 25 ‘C are the following: 1.2 X 10’ dm3 mol-’ [ll] or 2.4 X 10’ dm3 mol-’ [12] for LiBF,, 7.1 x lo4 dm3 mol-’ [9,11] for LiAsF, and 2.5 X lo6 dm3 mol-’ 141 for
53
[ pBL]/
mol %
Fig. 1. Molar enthalpy of solution of LiBF, (a) and LiPF, (b) at m = 0.003 mol kg-’ in the mixtures of 1,Zdimethoxyethane with y-butyrolactone.
LiClO,. No data were found in the literature for KA of LiPF, in the solvent discussed; some information, however, points to similar behaviour of LiPF, and LiAsF, in many non-aqueous solutions. Among other properties, both these salts exhibit similar conductivities and very close ion pair formation constants in several y-BL + 1,2-DME mixtures at 25 o C [4]. Therefore, it can be assumed that KA for LiPF, in pure 1,2_dimethoxyethane is close to that for LiAsF,, i.e. it is of the order of lo4 dm3 mol-‘. The electric permittivity of y-BL + 1,2-DME mixtures increases along with the increase of the lactone content (Fig. 2) [l-31. As a result, the ionic association of the salts decreases in these mixtures. The literature data on this subject are scarce, but it was found that KA values for LiBF, and LiAsF, [l], as well as for LiCIO, [2] fulfil the Fuoss relation, i.e. log KA is a linear function of l/e of the mixture. Taking into account these observations we have calculated the KA values for LIBF, and LiAsF, (regarded as similar to that for LiPFJ in all the mixtures investigated here. The results are presented in Table 3. A rapid decrease of ionic association of the lithium salts (Table 3), when a small amount of y-BL is added to 1,2-DME releases a significant number of “free” Li+ ions, which in turn, can be “chelated” by the 1,Zdimethoxyethane molecules. This phenomenon probably brings about an increase in the exothermic ion solvation effect within the small
54
60 t 110
1.00
- 0.6 0
I
I
20
I
I
40
I I
60
I
I
80
1
090
0.4 100
[r_BL]/ mol %
Fig. 2. Relative permittivity k), density (doIand viscosity CT,) of 1,Zdimethoxyethane mixtures.
+ y-butyrolactone
y-butyrolactone content in the mixed solvent up to the minimum of AH, (Fig. 1). A further increase of the r-BL content (x, > 0.1) makes the ionic association still smaller, but now the Li+-DME complex formation is more difficult due to the increase of the electric permittivity of the mixture, and the interactions between -y-BL and 1,2-DME (dipole-dipole type [14]) and between Li+ and y-BL. As a result of such competition the dissolution enthalpy for both the salts increases (exothermic effect of the solution decreases) beyond 10 mol% of y-butyrolactone content. In order to determine the standard solution enthalpies, AH: of the electrolytes in the mixtures examined one must extrapolate the measured molar dissolution enthalpies as a function of the electrolyte concentration to an infinitely dilute
0
_ -
Fig. 3. Structure of the ionic pair with specifically solvated cation in 1,ZDME solution.
55 TABLE 3 Relative permittivity, c and equilibrium canstants of the ionic pair formation, K, for LiBF, and LiAsF, in 1,2_dimethoxyethane + y-butyrolactone mixtures at 298.15 K [y-BL]/%mol
E
0 10.3 13.10 20.5 25.26 30.2 36.57 50.2 70.6 89.7 100
7.20 [ll 10.2 121 10.93 [ll 12.8 121 14.67 [ll 16.1 121 18.47 [l] 22.8 [21 29.8 [2] 36.8 [2] 41.77 [ll
KA
LiAsF,
LiBF, 1.2x 107 [ill 1.2x 10s
7.1 x 104 [91 6.6 x 103 3891 111 1320 672 111 363 194 ill 83C47.7[41) 36 22 12
1.2x 104 2050 260 (205 141) 80 40 28.33 [ 131
solution (m = 0). Owing to the low electric permittivity of the mixtures with a high 1,ZDME content an extrapolation method taking,into account the ionic association process should be employed for this purpose. One of them is based on an expression proposed by Barthel and coworkers 1151: AH,=AH,O=aL,(FI) +(1-CX)AH, (2) where: L,(FI) is the relative apparent molal heat content of a solution with “free” ions, A HA is the enthalpy of association and (Y is the degree of dissociation. The L,(FI) can be calculated from Debye-Hiickel theory [15] when the temperature coefficient of electric permittivity and thermal expansion coefficient of the solvent are known. Unfortunately, the appropriate data for the mixtures investigated are not available. The degree of dissociation can be calculated from the ion association constant, KA. Using the KA data from Table 3, we have calculated the values for LiBF, and L&F, (assumed to be close to that for LiPF,) in the solutions within the high 1,2-DME content for two different concentrations of the electrolyte examined in each mixture (Table 4). As can be seen from these data, the cy values in pure 1,2-DME are very small. Therefore, the aL,(FI) term is negligible in these TABLE 4 Degree of dissociation of LiBF, and LiAsF, in pure 1,2-DME and in 90 mol% of l,ZDME+ of y-BL Salt concentration/ mol kg-’
a LiBF,
L&F,
100
0.003 0.005
0.005 0.004
0.068 0.053
90
0.003 0.005
0.052 0.040
0.2247 0.174
[DME]/mol%
10 mol%
56
systems and eqn. (2) becomes the well known Wu and Friedman expression [16] that can be solved by plotting the measured AH, as a function of (1 -a). Unfortunately, the range of the electrolyte concentration investigated is too narrow to make possible the determination of the AH,” values in this way. However, taking into account the differences in (1 -a) values at the same electrolyte concentration in pure 1,2-DME and in 90 mol% of DME mixture and the ionic association enthalpies for different electrolyte-non-aqueous solvent systems reported in the literature (12-16 kJ mol-‘) it can be supposed that the extrapolation of AH, to m = 0 and (1 - a) = 0 would give a monotonic curve of AH,0 = f(x). ACKNOWLEDGEMENT
Financial support acknowledged.
for this work from the CPBP-01.15 programme
is kindly
REFERENCES 1 M. Salomon and E.J. Plichta, Electrochim. Acta, 30 (1985) 113. 2 L. Werblan, A. Suzdorf, E. Lin, G. Szymadski and J. Lesihski, Bull. Acad. Polon. Sci. Ser. Sci. Chim., 33 (1985) 285. 3 L. Werblan, A. Balkowska, J. Lesidski and G. Szymabski, J. Electroanal. Chem., 250 (1988) 165. 4 A. Balkowska, G. Szymairski and L. Werblan, J. Electroanal. Chem., 287 (1990) 229. 5 A. Piekarska, H. Piekarski and S. Taniewska-Osihska, J. Chem. Sot., Faraday Trans. I, 82 (1986) 513. 6 V. Gutman, The Donor-Acceptor Approach To Molecular Interactions, pp. 13-33, Plenum Press, New York, 1978. 7 Y. Matsuda, M. Morita and K. Kosaka, J. Electrochem. Sot., 130 (1983) 101. 8 B.R. Sundheim and K.F.A. Cafesso, J. Chem. Phys., 31 (1959) 809. 9 H. Faber, D.E. Irish and S. Petrucci, J. Phys. Chem., 87 (1983) 3515. 10 Y. Matsuda, Nippon Kagaku Kaishi, (1989) 1. 11 H.E. Maaser, M. Delsignore, M. Newstein and S. Petrucci, J. Phys. Chem., 88 (1984) 5100. 12 J. Bartel, R. Gerber and H.J. Gores, Ber. Bunsenges. Phys. Chem., 88 (1984) 616. 13 G.M. Serebrannikowa, M.A. Aleshina and T.F. Vasekina, Zh. Fiz. Khim., 58 (1984) 2843. 14 V.F. Stolypin and A.I. Mishustin, Zh. Fiz. Khim., 61 (1987) 3233. 15 J. Bartel, H.J. Gores, G. Schmeer and R. Wachter, Top. Curr. Chem., 111 (1983) 33. 16 Y.C. Wu and H.L. Friedman, J. Phys. Chem., 70 (1966) 501.
J. Electroanal. Chem., 316 (1991) 49-56 Elsevier Sequoia S.A., Lausanne
JEC 01719
Thermochemical investigations of the systems useful for lithium battery construction Solution enthalpies of LiBF, and LiPF, in 1,2_dimethoxyethane + y-butyrolactone mixtures at 25 OC Alina Piekarska Department of Physical Chemistry, University of Ebdi, Pomorska 18, 91-416 E6di (Poland)
(Received 4 April 1991)
Abstract Solution enthalpies of LiBF, and LiPF, in the mixtures of 1,2-dimethoxyethane with y-butyrolactone have been measured over the whole range of the mixed solvent composition. The molar enthalpy of solution for both salts at the concentration 0.003 mol kg-’ goes through a minimum with the mixture composition corresponding to 90 mol% of 1,2_dimethoxyethane. Possible reasons for the presence of this minimum are discussed.
INTRODUCTION
Research of new, high efficiency power sources is an important task in contemporary electrochemistry. Construction of primary and secondary high energy lithium batteries has been a significant achievement in this area. These batteries, in most cases, contain a lithium salt with a large complex anion dissolved in a mixed solvent that consists of a high dielectric solvent (HDS) and a low viscosity solvent (LVS). For the HDS sulfolane W, dimethoxysulfoxide (DMSO), propylene carbonate (PC), ethylene carbonate (EC) and y-lactones are used. Tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), l,l- and 1,Zdialkoxyethanes and 1,3-dioxalane belong to the LVS group. The salts: LiBF,, LiPF,, L&F,, LiClO, are the most frequently employed electrolytes. Numerous papers have been devoted to the results of conductometric and potentiometric investigations of the systems useful for lithium battery construction. In addition, some viscosity, density and electric permittivity data concerning mixtures of HDS with LVS can be found in the literature. 0022-0728/91/$03.50
0 1991 - Elsevier Sequoia S.A. All rights reserved
50
However, only a little is known about the thermochemical properties of the solutions under discussion. In order to fill this gap at least partly we have undertaken some calorimetric investigations of the systems that are employed in the lithium batteries. In this paper, we present the measured dissolution enthalpies of lithium tetrafluoroborate (LiBF,) and lithium hexafluorophosphate (LiPF,) in the mixtures of y-butyrolactone (y-BL) with 1,Zdimethoxyethane (1,ZDME). The above-mentioned systems which are protected under many patents are the most promising ones for high energy battery construction in the opinion of some authors’ [l-41. EXPERIMENTAL
Lithium tetrafluoroborate, Aldrich 98% was dried for several days under a reduced pressure at 90-100 ’ C. Lithium hexafluorophosphate, Aldrich 97%, was kept for several days in a vacuum desiccator at 30-35°C. Both salts were then stored under dry argon. y-Butyrolactone and 1,2_dimethoxyethane, both Fluka AG, were dried with 0.4 nm molecular sieves and distilled in a water-free atmosphere c-y-BL under reduced pressure). The mixed solvents were prepared by weight in a dry box. Enthalpies of solution were measured with a “isoperibol” type calorimeter as described elsewhere [5]. The salts were contained in thin-walled ampoules made of hard glass. The ampoules were filled with the salts in a dry argon atmosphere directly before placing them in the calorimeter. It was particularly important in the case of LiPF, which affected the glass ampoules when it was left in them for a long time. Four to six independent measurements were made for each salt in each y-BL + 1,ZDME mixture investigated. The concentration range of the electrolyte was 0.003-0.01 mol kg-’ of the mixed solvent. RESULTS AND DISCUSSION
The measured enthalpies of solution, AH, of LiBF, and LiPF, in the mixtures of y-butyrolactone with 1,2-dimethoxyethane are given in Tables 1 and 2 as a function of both the electrolyte concentration and the mixed solvent composition. The dependence of the AH,, corresponding to m = 0.003 mol kg-’ electrolyte concentration on the mixed solvent composition is presented in Fig. 1. The molar solution enthalpy for both salts at the same concentration is more negative in pure 1,2-DME than that in pure y-BL. The 1,Zdimethoxyethane as well as y-butyrolactone are aprotic solvents. Therefore, the more exothermic solvation enthalpies in pure 1,2-DME. seem to be connected with the stronger solvation of the lithium cation in the above-mentioned solvent due to its higher donicity compared with the y-BL (the donor numbers, DN: for 1,2-DME is 24 [6]; and for y-BL, 15.9 1611.Moreover, in the opinion of some authors, mono- and di-ethers coordinate small alkali metal cations (Li+ and Na+) strongly and form
51 TABLE 1 Molar enthalpies of solution, AH, 298.15 K
m/
mol kg-’
m/
-
40.73 40.45 40.38 40.00 39.62
0.002985 0.003610 0.004875 0.006704 0.010987
+ 40 mol% of y-BL
+ 50 mol% of y-BL
- 39.33 - 39.25 -39.16 - 38.95 - 38.80
0.003191 0.004770 0.006335 0.009276
0.002476 0.003316 0.005229 0.006491 0.008730
kJ mol-’
+ 60 mol% of y-BL 0.003243 0.004190 0.005695 0.008130
mol kg-’
AH,/
kJ mol-’
+ 10 mol% of y-BL
+ 30 mol% of y-BL 0.003166 0.003859 0.004163 0.006293 0.008199
y-butyrolactone
AH,/
1,2-DME 0.003176 0.004831 0.005589 0.007665 0.008835
of LiBF, in l,Zdimethoxyethane+
- 36.07 -35.92 -35.58 -35.21
-41.80 -41.65 -41.46 -41.28 - 40.90
- 38.57 -38.31 -38.10 - 37.99
+ 70 mol% of y-BL 0.003256 0.004710 0.006235 0.008760
- 34.98 - 34.66 -34.31 -34.14
m/
mol kg-’
mixtures at
AH,/
k.l mol-’
+ 20 mol% of y-BL 0.003072 0.003347 0.004520 0.007329 0.007703 0.008137
-41.13 - 41.04 - 40.83 - 40.75 - 40.54 - 40.25
- 37.32 -37.17 - 36.80 - 36.50 .- 36.44
+ 90 mol% of y-BL 0.003085 0.004323 0.007132 0.008045
- 32.40 - 31.92 - 31.69 -31.42
100 mol% of y-BL 0.002985 0.003768 0.005156 0.007739
- 30.52 -30.31 - 29.96 - 29.50
chelate-type complexes [1,7-101. This process can also give an additional exothermic impact to the enthalpy of solvation of the lithium salts in pure 1,2-dimethoxyethane. The curves representing the enthalpies of solution for both LiBF, and LiPF, at the concentration m = 0.003 mol kg-’ (Fig. 1) exhibit their minimum at high 1,2-DME content (ca. 90 mol% of the 1,ZDME). The presence of this minimum does not seem to be related to any essential variation of the mixed solvent structure. Owing to the similar, aprotic character of both mixed solvent components, a gradual change from the structure typical for pure 1,ZDME to that typical for pure y-BL would be expected. The monotonic course of viscosity [1,2], density 111 and electric permittivity [l-3] functions (Fig. 2) for y-butyrolactone + 1,2dimethoxyethane mixtures within the whole composition range confirms the above conclusion. It can be supposed, therefore, that the appearance of the AH, minimum is due to the presence of ions in the solution, i.e. it is connected with ion-solvent and ion-ion interactions. The former results mainly from the “chelat-
52
TABLE 2 Mdlar enthalpies of solution, AH,
of LiPF, in l,Zdimethoxyethane+
y-butyrolactone
mixtures at
298.15 K m/ mol kg-
’
AH,/ kJ mol-’
12-DME 0.002888 0.003547 0.003551 0.006988 0.009524
- 72.76 - 72.21 - 72.00 -71.63 - 71.42
-
-
0.002988 0.003365 0.005418 0.006098 0.011198
-
0.002399 0.003231 0.005382 0.006595 0.010444
-
76.18 76.15 75.73 75.50 75.00
0.003017 0.004771 0.005168 0.010630
73.98 73.52 73.02 72.88 72.35
0.002998 0.003791 0.003848 0.005783 0.09300
0.003480 0.004815 0.006022 0.006048 0.007749
AH,/ kJ mol-’
-
- 62.80 -62.13 - 61.63 - 61.50 -61.42
-
69.70 69.22 69.13 68.66 68.21
+ 80 mol% of y-BL 0.002983 0.003057 0.005041 0.006270 0.006311 0.008302
- 57.25 - 57.22 - 56.48 - 56.00 - 55.90 -55.17
+ 90 mol% of y-BL
100 mol% of y-BL
0.003266 0.004538 0.005894 0.009797
0.001940 0.002213 0.002918 0.005239 0.007548
- 54.03 - 53.82 -53.14 - 52.21
75.75 74.60 74.35 73.22
+ 40 mol% of y-BL
+ 60 mol% of y-BL 66.89 66.52 66.04 65.71 65.45 65.07
m/ mol kg-’
+ 15 mol% of y-BL
+ 30 mol% of y-BL 75.21 74.68 73.88 73.80 72.85
+ 50 mol% of y-BL 0.002781 0.003272 0.004753 0.006257 0.006723 0.007459
AH,/ kJ mol-’
+ 10 mol% of y-BL
+ 20 mol% of y-BL 0.002377 0.003108 0.005294 0.005435 0.008468
m/ mol kg-’
- 52.15 - 52.04 - 51.88 -51.08 -51.68
ing” properties of 1,Zdimethoxyethane towards the lithium cation [1,7-lo]. The latter is an ionic association due to a very low electric permittivity of the mixtures with a high diether content. The analysis of conductometric, viscosimetric and spectroscopic data led Salomon and his coworkers to the conclusion that the lithium cation, despite having four active centers, was coordinated by only one molecule of 1,Zdimethoxyethane. For this reason, the “chelated” cation was still able to form a “contact” ionic pair [l] (Fig. 3). Faber, Irish and Petrucci [93 suggested that both “contact” and “solvent-separated” ionic pairs existed in 1,2-DME solution: Li+-DME-X-s Li+-X--DME (1) The ionic association equilibrium constants, KA for lithium salts in 1,ZDME at 25 ‘C are the following: 1.2 X 10’ dm3 mol-’ [ll] or 2.4 X 10’ dm3 mol-’ [12] for LiBF,, 7.1 x lo4 dm3 mol-’ [9,11] for LiAsF, and 2.5 X lo6 dm3 mol-’ 141 for
53
[ pBL]/
mol %
Fig. 1. Molar enthalpy of solution of LiBF, (a) and LiPF, (b) at m = 0.003 mol kg-’ in the mixtures of 1,Zdimethoxyethane with y-butyrolactone.
LiClO,. No data were found in the literature for KA of LiPF, in the solvent discussed; some information, however, points to similar behaviour of LiPF, and LiAsF, in many non-aqueous solutions. Among other properties, both these salts exhibit similar conductivities and very close ion pair formation constants in several y-BL + 1,2-DME mixtures at 25 o C [4]. Therefore, it can be assumed that KA for LiPF, in pure 1,2_dimethoxyethane is close to that for LiAsF,, i.e. it is of the order of lo4 dm3 mol-‘. The electric permittivity of y-BL + 1,2-DME mixtures increases along with the increase of the lactone content (Fig. 2) [l-31. As a result, the ionic association of the salts decreases in these mixtures. The literature data on this subject are scarce, but it was found that KA values for LiBF, and LiAsF, [l], as well as for LiCIO, [2] fulfil the Fuoss relation, i.e. log KA is a linear function of l/e of the mixture. Taking into account these observations we have calculated the KA values for LIBF, and LiAsF, (regarded as similar to that for LiPFJ in all the mixtures investigated here. The results are presented in Table 3. A rapid decrease of ionic association of the lithium salts (Table 3), when a small amount of y-BL is added to 1,2-DME releases a significant number of “free” Li+ ions, which in turn, can be “chelated” by the 1,Zdimethoxyethane molecules. This phenomenon probably brings about an increase in the exothermic ion solvation effect within the small
54
60 t 110
1.00
- 0.6 0
I
I
20
I
I
40
I I
60
I
I
80
1
090
0.4 100
[r_BL]/ mol %
Fig. 2. Relative permittivity k), density (doIand viscosity CT,) of 1,Zdimethoxyethane mixtures.
+ y-butyrolactone
y-butyrolactone content in the mixed solvent up to the minimum of AH, (Fig. 1). A further increase of the r-BL content (x, > 0.1) makes the ionic association still smaller, but now the Li+-DME complex formation is more difficult due to the increase of the electric permittivity of the mixture, and the interactions between -y-BL and 1,2-DME (dipole-dipole type [14]) and between Li+ and y-BL. As a result of such competition the dissolution enthalpy for both the salts increases (exothermic effect of the solution decreases) beyond 10 mol% of y-butyrolactone content. In order to determine the standard solution enthalpies, AH: of the electrolytes in the mixtures examined one must extrapolate the measured molar dissolution enthalpies as a function of the electrolyte concentration to an infinitely dilute
0
_ -
Fig. 3. Structure of the ionic pair with specifically solvated cation in 1,ZDME solution.
55 TABLE 3 Relative permittivity, c and equilibrium canstants of the ionic pair formation, K, for LiBF, and LiAsF, in 1,2_dimethoxyethane + y-butyrolactone mixtures at 298.15 K [y-BL]/%mol
E
0 10.3 13.10 20.5 25.26 30.2 36.57 50.2 70.6 89.7 100
7.20 [ll 10.2 121 10.93 [ll 12.8 121 14.67 [ll 16.1 121 18.47 [l] 22.8 [21 29.8 [2] 36.8 [2] 41.77 [ll
KA
LiAsF,
LiBF, 1.2x 107 [ill 1.2x 10s
7.1 x 104 [91 6.6 x 103 3891 111 1320 672 111 363 194 ill 83C47.7[41) 36 22 12
1.2x 104 2050 260 (205 141) 80 40 28.33 [ 131
solution (m = 0). Owing to the low electric permittivity of the mixtures with a high 1,ZDME content an extrapolation method taking,into account the ionic association process should be employed for this purpose. One of them is based on an expression proposed by Barthel and coworkers 1151: AH,=AH,O=aL,(FI) +(1-CX)AH, (2) where: L,(FI) is the relative apparent molal heat content of a solution with “free” ions, A HA is the enthalpy of association and (Y is the degree of dissociation. The L,(FI) can be calculated from Debye-Hiickel theory [15] when the temperature coefficient of electric permittivity and thermal expansion coefficient of the solvent are known. Unfortunately, the appropriate data for the mixtures investigated are not available. The degree of dissociation can be calculated from the ion association constant, KA. Using the KA data from Table 3, we have calculated the values for LiBF, and L&F, (assumed to be close to that for LiPF,) in the solutions within the high 1,2-DME content for two different concentrations of the electrolyte examined in each mixture (Table 4). As can be seen from these data, the cy values in pure 1,2-DME are very small. Therefore, the aL,(FI) term is negligible in these TABLE 4 Degree of dissociation of LiBF, and LiAsF, in pure 1,2-DME and in 90 mol% of l,ZDME+ of y-BL Salt concentration/ mol kg-’
a LiBF,
L&F,
100
0.003 0.005
0.005 0.004
0.068 0.053
90
0.003 0.005
0.052 0.040
0.2247 0.174
[DME]/mol%
10 mol%
56
systems and eqn. (2) becomes the well known Wu and Friedman expression [16] that can be solved by plotting the measured AH, as a function of (1 -a). Unfortunately, the range of the electrolyte concentration investigated is too narrow to make possible the determination of the AH,” values in this way. However, taking into account the differences in (1 -a) values at the same electrolyte concentration in pure 1,2-DME and in 90 mol% of DME mixture and the ionic association enthalpies for different electrolyte-non-aqueous solvent systems reported in the literature (12-16 kJ mol-‘) it can be supposed that the extrapolation of AH, to m = 0 and (1 - a) = 0 would give a monotonic curve of AH,0 = f(x). ACKNOWLEDGEMENT
Financial support acknowledged.
for this work from the CPBP-01.15 programme
is kindly
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