- Email: [email protected]
Effect of propylene carbonate as a plasticizer in high molecular weight PEOLiCF3SO3 electrolytes
SOLID STATE ELSEVIER
IONUS
Solid State Ionics 85 (1996) 61-66
Effect of propylene carbonate as a plasticizer in high molecular weight PEO-LiCF,SO, electrolytes Roger Frech”, Sangamithra
Chintapalli
Department of Chemistry and Biochemistry. The University of’ Oklahoma, Norman, OK 7.3’019. USA
Abstract The effect of propylene carbonate on the PEO-LiCF,SO, system was studied using infrared spectroscopy and ionic conductivity measurements. It has been observed that large amounts of propylene carbonate tend to decrease the degree of ionic association in polymer electrolytes, while the ionic conductivity increases by several orders of magnitude. Propylene carbonate interacts preferentially with the crystalline PEO phase relative to the (PEO),LiCF,SO, compound present in the PEO-LiCF3S0, salt complexes. Keywords: PEO; Propylene
carbonate;
Plasticizer;
Infrared
spectroscopy;
1. Introduction Current research trends have been concerned with lithium salt-based polymer electrolytes [l-8] due to their potential applications [8-121, with a particular interest in the development of novel polymer electrolytes with high ionic conductivities at ambient temperature. In order to achieve an understanding of the ionic conduction mechanism in polymer electrolytes, work has focused on the factors governing ionic conduction, such as segmental motion and ionic association. Much of this work has been with the poly(ethylene oxide)-lithium triflate (PEO-LiCF,SO,) system. In the phase diagram of PEOLiCF,SO,, a compound with the composition (PEO),LiCF,SO, has been identified [13]. Other compositions previously suggested for the compound are (PEO),.,LiCF,SO, [14] and (PEO),LiCF,SO, *Corresponding
author. Email: [email protected]
0167.2738/96/$15.00 Copyright P/I SO167-2738(96)00041-O
01996
Elsevier
LiCF,SO,;
Triflate
[15]. The crystal structure of (PEO),LiCF,SO, has been determined [ 161 and belongs to the space group P2 JC:,). In this compound, the Li’ cation is coordinated to three adjacent ether oxygens from the PEO chain and one from each of two adjacent triflate ions. The published phase diagrams show that at least three phases (a) crystalline PEO below the melting point at 60°C (b) the crystalline compound (PEO),LiCF,SO, and (c) amorphous PEO with dissolved PEO exist in the PEO-LiCF,SO, system. It is believed that the latter phase is responsible for ionic conduction while the former two inhibit ionic conductivity [l]. The (PEO),LiCF,SO, composition that is most widely used for battery applications has an ionic conductivity of 1.5 X 1O-4 S/cm at 120°C [10,17,18]. Electrolytes with higher room temperature conductivities can be realized by the introduction of additives (or plasticizers) such as propylene carbonate (PC), ethylene carbonate (EC) or low molecular
Science B.V. All rights reserved
62
R. Frech, S. Chintapalli
I Solid State Ionics 85 (1996) 61-66
weight polyethylene glycols and their derivatives [8,11,19-231. Studies involving differential scanning calorimetry (DSC), conductivity and electrochemical applications of plasticized PEO-LiCF,SO, electrolytes have been reported [24-261. Conductivity studies [24] reveal that additives decrease the crystalline PEO content in the electrolytes at room temperature while the glass transition temperature of PEO is lowered to a great extent. It has also been shown that the endothermic peak due to (PEO),LiCF,SO, does not change with the addition of plasticizer [24]. The role of PC as a plasticizer on the PEOLiCF,SO, system based on an infrared spectroscopic study is reported in this paper. Results of ionic conductivity measurements as a function of PC content are also presented.
3. Results and discussion Fig. 1 shows the infrared spectra of (PEO),LiCF,SO, +y% PC with increasing wt.% PC in the region 775-745 cm- ’ . The bottom curve is the spectrum for (PEO),LiCF,SO,. The peak at 760 cm -’ corresponds to the symmetric deformation mode of CF, [denoted as &(CF,)] for the crystalline compound (PEO),LiCF,SO,. With increased addition of PC, peaks at 753 and 757 cm-’ become visible. These bands correspond to &(CF,) of “free” ions and ion pairs respectively. These assignments are consistent with earlier studies [27,28], an ab initio calculation [29] and a normal coordinate analysis [30]. At 150 wt.% PC, only a small intensity contribution from the 760 cm-’ band is present, suggesting that the ionic association is greatly altered
W-I?6 OF PC
2. Experimental PEO (MW 4X 106, Aldrich) was used as received while PC (Aldrich) was distilled under vacuum at -80°C. Lithium triflate (Aldrich) was dried at 100°C in a vacuum. A solution of the desired amounts of PEO, PC and lithium triflate dissolved in a sufficient amount of acetonitrile was stirred overnight at room temperature. After continuous stirring, the solution was allowed to stand at room temperature for 24 h to facilitate degassing. For infrared studies, thin films were made on CsI windows with the dimensions 38 X 19 X4 mm and blow-dried at room temperature to avoid loss of PC. The IR studies were carried out with a Digilab FTS-40 Bio-Rad infrared spectrometer in the absorbance mode in the region 4000-400 cm-’ and at a resolution of 2 cm-‘. Conductivity measurements were made using a Hewlett-Packard 4192A LF impedance analyzer in the frequency range 5 Hz-13 MHz. Films for this technique were directly cast in the sample holder. The PEO-Li ratio was maintained at 9:l and the amount of PC added is expressed as a weight percent (wt.%) of the PEO present; i.e. y%=[Wt(PC)lWt (PEO)]lOO. The composition is represented as (PEO),LiCF,SO, +y% PC.
760
755
750
Wavenumbers
(cti’)
Fig. 1. IR spectra of (PEO),LiCF, SO,+y% PC in the 6<(CF,) region. The wt.% of PC is indicated for each curve.
R. Frech, S. Chintapalli I Solid State Ionics 8.7 (1996) 61-66
only in the presence of large amounts of PC. It can be speculated that the addition of a high dielectric solvent such as PC alters the dielectric constant of the PEO matrix and thus affects the ionic association. Fig. 2 shows the infrared spectra in the CH,wagging region for the system (PEO),LiCF,SO, + v% PC. The peaks at 1360 and 1343 cm- ’ , indicated by dashed lines, correspond to the crystalline phase of pure PEO. The peak at 1353 and 1340 cm-’ are due to the crystalline compound (PE0)3LiCF,S0, [27]. With the addition of PC it can be seen that the peaks at 1360 and 1343 cm-’ decrease in intensity and disappear completely beyond 70 wt.% PC. However, the peaks due to the compound persist until 150 wt.% PC is added. These results indicate that there is no significant change occurring to the compound. The effect of PC on pure PEO was studied in order to note any differences in the behavior of pure PEO and (PEO),LiCF,SO, upon the addition of PC.
63
I
I
I
I
I
1380
1370
1360
1350
1340
Wavenumbers
1330
(cm-‘)
Fig. 3. IR spectra of (PEO),LiCF, SO, with PC as a plasticizer in the 1380-1330 cm ’ spectral region. The wt.% of PC is indicated for each curve.
13.80
13’70
1360 13.50 Wavenumbers
1340 (cd)
1330
Fig. 2. IR spectra of PEO with PC as a plasticizer in the 1380-1330 cm-’ spectral region. The wt.% of PC is indicated for each curve.
Fig. 3 shows the spectra depicting the effect of PC on pure PEO in the CH,-wagging region. The dashed lines in this figure again point to the crystalline PEO peaks at 1360 and 1343 cm-‘. With an increase in PC wt.%, these peaks diminish in intensity and eventually disappear beyond 70 wt.% PC. A peak starts growing at 1352 cm- ’ which then dominates the spectral region as a broad band centered at 1354 cm- ’ The new peak corresponds to the C-H bending mode in PC, which appears at 1355 cm-’ m . pure PC. These results indicate that the effect of PC is identical in the cases of PEOLiCF,SO, and pure PEO. These studies are consistent with the results of the PEO-LiCF,SO, system with EC as a plasticizer (311. It has been found that EC preferentially interacts with the crystalline PEO rather than the crystalline (PEO),LiCF,SO,. The interaction between PC and LiCF,SO, can be observed in the symmetric ring deformation mode of PC [spectrum (1) of Fig. 41, which appears at 712
R. Frech, S. Chintapalli
I Solid State tonics 85 (1996) 61-66
A
(6)
I
i
&
8
(4)
A
(3)
/y_
v1
8
-
v-6
b
. (1)
i\ I
740
-8 I
I
720
700
Wavenumbers
68C
(cm-‘)
Fig. 4. Comparison of the symmetric ring deformation region of PC in (1) pure PC; (2) PC-LiCF,SO,; and (PEO),LiCF,SO,+ y% PC with (3) y=50; (4) y=70; (5) y=90 and (6) y=150.
-9’I
I
0
20
”
”
40
wT% Fig. 5. Plot of log 6 (r (PEO),LiCF,SO, +y% PC.
cm-’ [32]. An additional band appears at a higher complex frequency, 723 cm- ’ , in the PC-LiCF,SO, [spectrum (2)]. Similar results have been reported by Battisti et al. [32] in the PC-LiClO, system. The higher frequency component has been attributed to a Li+-PC interaction due to the concentration dependence of this peak [32]. Similar results have been observed in the EC-LiCF,SO, system [30]. The intensity of the peak at 723 cm-’ is small in the plasticized system [spectra (3) to (6) in Fig. 41, indicating that PC interacts weakly with Li+ in the presence of PEO. This provides additional evidence for the preferential interaction of PC with the crystalline PEO phase. Complex impedance measurements showed that addition of PC to (PEO),LiCF,SO, significantly improved the ionic conductivity. Fig. 5 shows a plot of log u vs. wt.% PC. With increasing PC concentration, the ionic conductivity increases by several orders of magnitude. A conductivity value of 5.0X lo-’ S/cm was obtained for the complex with
” 60 vs.
” 80
” 100
” 120
( 140
ofPC PC
wt.%
for
the
system
90 wt.% PC, which is the order of conductivity of (PE0)9LiCF,S0, at - 100°C. Further studies investigating the effect of plasticizers such as EC, tetraethylene glycol and tetraethylene glycol dimethyl ether are in progress.
4. Conclusions The infrared spectroscopic results show that the addition of PC to (PEO),LiCF,SO, tends to favor the formation of the less associated species, although at a 9:l 0:M ratio, the effect is prominent only at large wt.% of PC. With the addition of PC, the polymer-salt complexes become more amorphous at room temperature, i.e. the content of crystallinity due to pure PEO decreases. The conductivity increases by 3-4 orders of magnitude up to 90 wt.% PC, although the curve-fitting results of the S,(CF,)
R. Frech,
S. Chintapalli
I Solid State lonics
X.5 (1996)
65
61-66
region indicate that the corresponding increase in relative intensity of free ions is only about 2 1%. This argues that the increase in “free” ion charge carriers can only be a minor contribution to the dramatically increased conductivity. The infrared spectroscopic results show that PC preferentially interacts with the crystalline PEO phase. The (PEO),LiCF,SO, complex is a mixture of regions of crystalline PEO and regions of the 3:l compound, (PEO),LiCF,SO,. With the addition of plasticizer, the crystalline PEO regions become amorphous with some dissolved LiCF,SO, salt. Since neither crystalline PEO nor (PEO),LiCF,S03 exhibit significant ionic conduction, the connected regions of amorphous, plasticized PEO with dissolved salt become the ionically conducting pathways.
PC concentrations, as the PEO-salt complex becomes increasingly amorphous, do the PC interactions with the salt become significant. The increase in ionic conductivity of the plasticized PEO-salt complexes at higher PC concentrations is at the expense of mechanical stability.
5. Comments
References
5.1.
J. Barker and R. Koksbang
Technology
(Valence
Inc.) commented:
It was interesting to note that the addition of PC to the polymer electrolyte favored formation of a more amorphous complex at room temperature. However, could you comment on the consequence to the polymers physical properties of such large amounts of plasticizers. 5.2. 1.1. Olsen and R. Koksbang Technology
(Valence
Inc.) later added:
PC is not only a plasticizer for the polymer but also a solvent for the salt - as a result, the salt concentration in the polymer phase is reduced unless additional salt is added to compensate for this effect. This explains the increase in the ionic conductivity and the decrease in the ion association observed by the authors. 5.3. R.E. Frech
(University
of Oklahoma,
Norman)
replied:
Over much of the range of PEO-salt complex is not a Until about 50 wt.%, the PC with the pure crystalline PEO
PC addition, the PC+ homogeneous system. preferentially interacts phase. Only at higher
Acknowledgments This work was partially supported by funds from the National Science Foundation EPSCoR Advanced Development Program (Grant No. EHR-9 10877 1) and the American Chemical Society (Grant No. ACS-PRF 2548 1-AC7P).
[II M.B. Armand, J.M. Chabagno and M. Duclot, in: Fast ion Transport in Solids, eds. P. Vashista, J.N. Mundy and G.K. Shenoy (Elsevier Applied Science, London, and New York, 1979) p. 131. I21 J.-P Le Nest and A. Gandini, in: Proc. 2nd Int. Symposium on Polymer Electrolytes, ed. B. Scrosati (Elsevier. Amsterdam, 1990) p. 129. [31 S. Schantz, L.M. Tore11 and J.R. Stevens, J. Chem. Phys. 94 (1991) 6862. [41 J. Manning. 2245.
R. Frech and E. Hwang,
Polymer
31
( 1990)
[51 S. Schantz, J. Sadahl, L. Borjesson, L.M. Torell and J.R. Stevens, Solid State Ionics 128-30 (1988) 1047. [61 M. Gauthier, A. Belanger, B. Kapfer, G. Vassort and M. Armand, in: Polymer Electrolyte Reviews. eds. J.R. MacCallum and C.A. Vincent, Vol. 2 (Elsevier Applied Science, London, 1989) p. 285. 28 ( 1995) 1246. [71 R. Frech and W. Huang, Macromolecules Fundamentals and F31 F.M. Gray, Solid Polymer Electrolytes: Technological Applications, (VCH, New York. 1991). 191 B. Scrosati, in: Applications of Electroactive polymers, ed. B. Scrosati (Chapman and Hall, London, 1993) p, 25 I. [101 A. Hooper, M. Gauthier and A. Belanger, in: Electrochemical Science and Technology of Polymers, ed. R.G. Linford, Vol. 2 (Elsevier Applied Science, London, 1990) p. 375. M. Armand, J.Y. Sanchez, M. Gauthier and Y. Choquette, in: The Electrochemistry of Novel Materials. eds. J. Liplowski and P.N. Ross (VCH, New York, 1994) p. 65. B. Scrosati and R.J. Neat, in: Applications of Electroactive Polymers, ed. B. Scrosati (Chapman and Hall, London. 1993) p. 182. CD. Robitaille and D. Fauteux. J. Electrochem. Sot. 133 (1986) 315.
66
R. Frech, S. Chintapalli
I Solid State tonics 8.5 (19%)
[14] C. Berthier, W. Gore&i, M. Minier, M.B. Armand, J.M. Chabagno and P. Rigaud, Solid State Ionics I1 (1983) 91. [15] P.R. Sorensen and T. Jacobsen, Polym. Bull. 9 (1983) 47. [I61 P. Lightfoot, M.A. Mehta and P.G. Bruce, Science 262 (1993) 883. [17] D. Fauteux, J. Prud’homme and P.E. Harvey, Solid State Ionics 28-30 (1988) 923. [18] C.A.C. Sequeira, J.M. North and A. Hooper, Solid State Ionics 13 (1984) 175. [19] I. Kelly, J.R. Owen and B.C.H. Steele, J. Electroanal. Chem. Interfacial Electrochem. 168 (1984) 467. [20] M.H. Sheldon, M.D. Glasse, R.J. Latham and R.G. Linford, Solid State Ionics 34 (1989) 135. [21] M. Watanabe, M. Kanba, H. Matsuda, K. Tsunemi, K. Mizoguchi, E. Tsuchida and I. Shinohara, Makromol. Chem. Rapid Commun. 2 ( 1981) 741. [22] R.D.A. Paulmer and A.R. Kulkami, Solid State Ionics 68 (1994) 243. [23] R. Huq, R. Koksbang, P.E. Tonder and G.C. Farrington, Electrochim. Acta 37 (1992) 1681.
61-66
[24] C. Wang, Q. Liu, Q. Cao, Q. Meng and L. Yang, Solid State Ionics 53-56 (1992) 1106. [25] L. Yang, J. Lin, Z. Wang, C. Wang, R. Zhou and Q. Liu, Solid State Ionics 40-41 (1990) 616. [26] I. Kelly, J.R. Owen and B.C.H. Steele, J. Power Sources 14 (1985) 13. (271 M.A.K.L. Dissanayake and R. Frech, Macromolecules 28 (1995) 5312. [28] W. Huang, Ph.D. Thesis (The University of Oklahoma, OK, 1994). [29] W. Huang, R. Frech and R.A. Wheeler, J. Phys. Chem. 98 (1994) 100. [30] W. Huang, R.A. Wheeler and R. Frech, Spectrochim. Acta 50A (1994) 985. [31] S. Chintapalli and R. Frech, Macromolecules 29 (1996) 3499. [32] D. Battisti, G.A. Nazri, B. Klassen and R. Aroca, J. Phys. Chem. 97 (1993) 5826.
IONUS
Solid State Ionics 85 (1996) 61-66
Effect of propylene carbonate as a plasticizer in high molecular weight PEO-LiCF,SO, electrolytes Roger Frech”, Sangamithra
Chintapalli
Department of Chemistry and Biochemistry. The University of’ Oklahoma, Norman, OK 7.3’019. USA
Abstract The effect of propylene carbonate on the PEO-LiCF,SO, system was studied using infrared spectroscopy and ionic conductivity measurements. It has been observed that large amounts of propylene carbonate tend to decrease the degree of ionic association in polymer electrolytes, while the ionic conductivity increases by several orders of magnitude. Propylene carbonate interacts preferentially with the crystalline PEO phase relative to the (PEO),LiCF,SO, compound present in the PEO-LiCF3S0, salt complexes. Keywords: PEO; Propylene
carbonate;
Plasticizer;
Infrared
spectroscopy;
1. Introduction Current research trends have been concerned with lithium salt-based polymer electrolytes [l-8] due to their potential applications [8-121, with a particular interest in the development of novel polymer electrolytes with high ionic conductivities at ambient temperature. In order to achieve an understanding of the ionic conduction mechanism in polymer electrolytes, work has focused on the factors governing ionic conduction, such as segmental motion and ionic association. Much of this work has been with the poly(ethylene oxide)-lithium triflate (PEO-LiCF,SO,) system. In the phase diagram of PEOLiCF,SO,, a compound with the composition (PEO),LiCF,SO, has been identified [13]. Other compositions previously suggested for the compound are (PEO),.,LiCF,SO, [14] and (PEO),LiCF,SO, *Corresponding
author. Email: [email protected]
0167.2738/96/$15.00 Copyright P/I SO167-2738(96)00041-O
01996
Elsevier
LiCF,SO,;
Triflate
[15]. The crystal structure of (PEO),LiCF,SO, has been determined [ 161 and belongs to the space group P2 JC:,). In this compound, the Li’ cation is coordinated to three adjacent ether oxygens from the PEO chain and one from each of two adjacent triflate ions. The published phase diagrams show that at least three phases (a) crystalline PEO below the melting point at 60°C (b) the crystalline compound (PEO),LiCF,SO, and (c) amorphous PEO with dissolved PEO exist in the PEO-LiCF,SO, system. It is believed that the latter phase is responsible for ionic conduction while the former two inhibit ionic conductivity [l]. The (PEO),LiCF,SO, composition that is most widely used for battery applications has an ionic conductivity of 1.5 X 1O-4 S/cm at 120°C [10,17,18]. Electrolytes with higher room temperature conductivities can be realized by the introduction of additives (or plasticizers) such as propylene carbonate (PC), ethylene carbonate (EC) or low molecular
Science B.V. All rights reserved
62
R. Frech, S. Chintapalli
I Solid State Ionics 85 (1996) 61-66
weight polyethylene glycols and their derivatives [8,11,19-231. Studies involving differential scanning calorimetry (DSC), conductivity and electrochemical applications of plasticized PEO-LiCF,SO, electrolytes have been reported [24-261. Conductivity studies [24] reveal that additives decrease the crystalline PEO content in the electrolytes at room temperature while the glass transition temperature of PEO is lowered to a great extent. It has also been shown that the endothermic peak due to (PEO),LiCF,SO, does not change with the addition of plasticizer [24]. The role of PC as a plasticizer on the PEOLiCF,SO, system based on an infrared spectroscopic study is reported in this paper. Results of ionic conductivity measurements as a function of PC content are also presented.
3. Results and discussion Fig. 1 shows the infrared spectra of (PEO),LiCF,SO, +y% PC with increasing wt.% PC in the region 775-745 cm- ’ . The bottom curve is the spectrum for (PEO),LiCF,SO,. The peak at 760 cm -’ corresponds to the symmetric deformation mode of CF, [denoted as &(CF,)] for the crystalline compound (PEO),LiCF,SO,. With increased addition of PC, peaks at 753 and 757 cm-’ become visible. These bands correspond to &(CF,) of “free” ions and ion pairs respectively. These assignments are consistent with earlier studies [27,28], an ab initio calculation [29] and a normal coordinate analysis [30]. At 150 wt.% PC, only a small intensity contribution from the 760 cm-’ band is present, suggesting that the ionic association is greatly altered
W-I?6 OF PC
2. Experimental PEO (MW 4X 106, Aldrich) was used as received while PC (Aldrich) was distilled under vacuum at -80°C. Lithium triflate (Aldrich) was dried at 100°C in a vacuum. A solution of the desired amounts of PEO, PC and lithium triflate dissolved in a sufficient amount of acetonitrile was stirred overnight at room temperature. After continuous stirring, the solution was allowed to stand at room temperature for 24 h to facilitate degassing. For infrared studies, thin films were made on CsI windows with the dimensions 38 X 19 X4 mm and blow-dried at room temperature to avoid loss of PC. The IR studies were carried out with a Digilab FTS-40 Bio-Rad infrared spectrometer in the absorbance mode in the region 4000-400 cm-’ and at a resolution of 2 cm-‘. Conductivity measurements were made using a Hewlett-Packard 4192A LF impedance analyzer in the frequency range 5 Hz-13 MHz. Films for this technique were directly cast in the sample holder. The PEO-Li ratio was maintained at 9:l and the amount of PC added is expressed as a weight percent (wt.%) of the PEO present; i.e. y%=[Wt(PC)lWt (PEO)]lOO. The composition is represented as (PEO),LiCF,SO, +y% PC.
760
755
750
Wavenumbers
(cti’)
Fig. 1. IR spectra of (PEO),LiCF, SO,+y% PC in the 6<(CF,) region. The wt.% of PC is indicated for each curve.
R. Frech, S. Chintapalli I Solid State Ionics 8.7 (1996) 61-66
only in the presence of large amounts of PC. It can be speculated that the addition of a high dielectric solvent such as PC alters the dielectric constant of the PEO matrix and thus affects the ionic association. Fig. 2 shows the infrared spectra in the CH,wagging region for the system (PEO),LiCF,SO, + v% PC. The peaks at 1360 and 1343 cm- ’ , indicated by dashed lines, correspond to the crystalline phase of pure PEO. The peak at 1353 and 1340 cm-’ are due to the crystalline compound (PE0)3LiCF,S0, [27]. With the addition of PC it can be seen that the peaks at 1360 and 1343 cm-’ decrease in intensity and disappear completely beyond 70 wt.% PC. However, the peaks due to the compound persist until 150 wt.% PC is added. These results indicate that there is no significant change occurring to the compound. The effect of PC on pure PEO was studied in order to note any differences in the behavior of pure PEO and (PEO),LiCF,SO, upon the addition of PC.
63
I
I
I
I
I
1380
1370
1360
1350
1340
Wavenumbers
1330
(cm-‘)
Fig. 3. IR spectra of (PEO),LiCF, SO, with PC as a plasticizer in the 1380-1330 cm ’ spectral region. The wt.% of PC is indicated for each curve.
13.80
13’70
1360 13.50 Wavenumbers
1340 (cd)
1330
Fig. 2. IR spectra of PEO with PC as a plasticizer in the 1380-1330 cm-’ spectral region. The wt.% of PC is indicated for each curve.
Fig. 3 shows the spectra depicting the effect of PC on pure PEO in the CH,-wagging region. The dashed lines in this figure again point to the crystalline PEO peaks at 1360 and 1343 cm-‘. With an increase in PC wt.%, these peaks diminish in intensity and eventually disappear beyond 70 wt.% PC. A peak starts growing at 1352 cm- ’ which then dominates the spectral region as a broad band centered at 1354 cm- ’ The new peak corresponds to the C-H bending mode in PC, which appears at 1355 cm-’ m . pure PC. These results indicate that the effect of PC is identical in the cases of PEOLiCF,SO, and pure PEO. These studies are consistent with the results of the PEO-LiCF,SO, system with EC as a plasticizer (311. It has been found that EC preferentially interacts with the crystalline PEO rather than the crystalline (PEO),LiCF,SO,. The interaction between PC and LiCF,SO, can be observed in the symmetric ring deformation mode of PC [spectrum (1) of Fig. 41, which appears at 712
R. Frech, S. Chintapalli
I Solid State tonics 85 (1996) 61-66
A
(6)
I
i
&
8
(4)
A
(3)
/y_
v1
8
-
v-6
b
. (1)
i\ I
740
-8 I
I
720
700
Wavenumbers
68C
(cm-‘)
Fig. 4. Comparison of the symmetric ring deformation region of PC in (1) pure PC; (2) PC-LiCF,SO,; and (PEO),LiCF,SO,+ y% PC with (3) y=50; (4) y=70; (5) y=90 and (6) y=150.
-9’I
I
0
20
”
”
40
wT% Fig. 5. Plot of log 6 (r (PEO),LiCF,SO, +y% PC.
cm-’ [32]. An additional band appears at a higher complex frequency, 723 cm- ’ , in the PC-LiCF,SO, [spectrum (2)]. Similar results have been reported by Battisti et al. [32] in the PC-LiClO, system. The higher frequency component has been attributed to a Li+-PC interaction due to the concentration dependence of this peak [32]. Similar results have been observed in the EC-LiCF,SO, system [30]. The intensity of the peak at 723 cm-’ is small in the plasticized system [spectra (3) to (6) in Fig. 41, indicating that PC interacts weakly with Li+ in the presence of PEO. This provides additional evidence for the preferential interaction of PC with the crystalline PEO phase. Complex impedance measurements showed that addition of PC to (PEO),LiCF,SO, significantly improved the ionic conductivity. Fig. 5 shows a plot of log u vs. wt.% PC. With increasing PC concentration, the ionic conductivity increases by several orders of magnitude. A conductivity value of 5.0X lo-’ S/cm was obtained for the complex with
” 60 vs.
” 80
” 100
” 120
( 140
ofPC PC
wt.%
for
the
system
90 wt.% PC, which is the order of conductivity of (PE0)9LiCF,S0, at - 100°C. Further studies investigating the effect of plasticizers such as EC, tetraethylene glycol and tetraethylene glycol dimethyl ether are in progress.
4. Conclusions The infrared spectroscopic results show that the addition of PC to (PEO),LiCF,SO, tends to favor the formation of the less associated species, although at a 9:l 0:M ratio, the effect is prominent only at large wt.% of PC. With the addition of PC, the polymer-salt complexes become more amorphous at room temperature, i.e. the content of crystallinity due to pure PEO decreases. The conductivity increases by 3-4 orders of magnitude up to 90 wt.% PC, although the curve-fitting results of the S,(CF,)
R. Frech,
S. Chintapalli
I Solid State lonics
X.5 (1996)
65
61-66
region indicate that the corresponding increase in relative intensity of free ions is only about 2 1%. This argues that the increase in “free” ion charge carriers can only be a minor contribution to the dramatically increased conductivity. The infrared spectroscopic results show that PC preferentially interacts with the crystalline PEO phase. The (PEO),LiCF,SO, complex is a mixture of regions of crystalline PEO and regions of the 3:l compound, (PEO),LiCF,SO,. With the addition of plasticizer, the crystalline PEO regions become amorphous with some dissolved LiCF,SO, salt. Since neither crystalline PEO nor (PEO),LiCF,S03 exhibit significant ionic conduction, the connected regions of amorphous, plasticized PEO with dissolved salt become the ionically conducting pathways.
PC concentrations, as the PEO-salt complex becomes increasingly amorphous, do the PC interactions with the salt become significant. The increase in ionic conductivity of the plasticized PEO-salt complexes at higher PC concentrations is at the expense of mechanical stability.
5. Comments
References
5.1.
J. Barker and R. Koksbang
Technology
(Valence
Inc.) commented:
It was interesting to note that the addition of PC to the polymer electrolyte favored formation of a more amorphous complex at room temperature. However, could you comment on the consequence to the polymers physical properties of such large amounts of plasticizers. 5.2. 1.1. Olsen and R. Koksbang Technology
(Valence
Inc.) later added:
PC is not only a plasticizer for the polymer but also a solvent for the salt - as a result, the salt concentration in the polymer phase is reduced unless additional salt is added to compensate for this effect. This explains the increase in the ionic conductivity and the decrease in the ion association observed by the authors. 5.3. R.E. Frech
(University
of Oklahoma,
Norman)
replied:
Over much of the range of PEO-salt complex is not a Until about 50 wt.%, the PC with the pure crystalline PEO
PC addition, the PC+ homogeneous system. preferentially interacts phase. Only at higher
Acknowledgments This work was partially supported by funds from the National Science Foundation EPSCoR Advanced Development Program (Grant No. EHR-9 10877 1) and the American Chemical Society (Grant No. ACS-PRF 2548 1-AC7P).
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