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Preparation and properties of polymeric solid electrolyte: Polyethylene oxide--sodium iodide complexes
Solid State lonics 9 & 10 (1983) 1121-1124 North-Holland PublishingCompany
1121
PREPARATION AND PROPERTIES OF POLYMERIC SOLID ELECTROLYTE: POLYETHYLENE OXIDE--SODIUM IODIDE COMPLEXES*~ C. K. Chiang, G. T. Davis, C. A. Harding and J. Aarons Polymer Science and Standards Division National Bureau of Standards Washington, DC 20234 USA The thermal and e l e c t r i c a l properties of mixtures of polyethylene oxide and sodium iodide were measured for concentrations of s a l t up to 25 mole %. A maximum in dc conductivity is observed at low concentrations of Nal, a region in which no c r y s t a l l i n e complex is formed as determined from DSC measurements. 1.
INTRODUCTION
The high ionic conductivity of a l k a l i metal halides complexed with polyethylene oxide has stimulated a search for the mechanism of ionic transport in this polymer which might then guide the development of other conductors [1-4]. Results presented here are p a r t i a l l y a confirmat i o n of e a r l i e r reports but they also show the effects of sodium iodide on the e l e c t r i c a l and thermal properties of polyethylene oxide over a range of concentrations from 1 to 25 mole %. 2.
PREPARATIONOF POLYMER/SALT MIXTURES
Poly(ethylene oxide), PEO, was obtained from Polysciences, Inc. in powdered form and used d i r e c t l y . The results reported here are for PEO with a molecular weight of 5x106. Reagent grade anhydrous sodium iodide (Fisher) was heated at 120 °C before dissolving in spectrograde a c e t o n i t r i l e (Kodak) from a freshly opened b o t t l e . A c e t o n i t r i l e containing the appropriate amount of completely dissolved s a l t was then used to dissolve approximately 2 wt.% PEO by s t i r r i n g in a closed vessel for 24 hours. Films were prepared by completely evaporating the solvent at 35 °C, and the resulting opaque films were stored in an evacuated dessicator. Samples prepared in t h i s manner exhibited c r y s t a l l i n e behavior c h a r a c t e r i s t i c of very low water content [5]. Salt concentrations in the PEO-NaI complexes are expressed in terms of mole percent rather than the O/Na r a t i o preferred by some authors. 3.
absent in the i n i t i a l heating cycle for NaI concentrations of 15 percent and greater (curves a and b) but subsequent cycles always e x h i b i t a peak in t h i s region (curve c). However, the magnitude of the peak decreases with increasing s a l t concentration. For s a l t concentration of 10 percent and greater, an endotherm at temperatures in the v i c i n i t y of 195 °C appears which has been a t t r i b u t e d to the melting of a PEOs a l t complex [1,7]. At temperatures above the melting point of the complex, we have observed microscopically what we believe to be phaseseparated crystals of NaI. Upon cooling to temperatures in the v i c i n i t y of 150 °C, one can f o l l o w the gradual disappearance of these cubic crystals over a period of hours as they are apparently re-incorporated into the PEO-NaI complex. As in other polymer systems, the composition of the phases present can be highly dependent upon thermal history. Therefore the concentration of NaI necessarily r e f l e c t s only
THERMALANALYSIS
D i f f e r e n t i a l scanning calorimeter traces were obtained using a Perkin Elmer DSC I I f o r samples in sealed aluminum pans. Rather i r r e g u l a r traces were often obtained on the f i r s t heating of a p a r t i c u l a r sample, but a f t e r once heating to 260 °C, subsequent traces were regular and reproducible. However, the nature of the sample a f t e r c r y s t a l l i z i n g from the melt can be considerably d i f f e r e n t from that as prepared by evaporation of solvent as can be seen in Figure 1. An endothermic peak in the v i c i n i t y of 68 °C, i n d i c a t i v e of uncomplexed PEO [ 6 ] , is usually 0 167-2738/83/0000
0000/$ 03.00 © 1983 North-Holland
i
I
i
100 TEMPERATURE
i
200
I
300
(°C)
Figure I. DSC traces of PEO containing 20 mole % NaI. Curves a and b are i n i t i a l heatings at 20 °C/min on two d i f f e r e n t samples. Curve c is typical of reproducible traces obtained a f t e r heating to 260 °C and cooling to -30 °C at 20 °C/min.
C. k Chiang et al. / Preparation and properties o f polymeric solid electrolyte
1122
the overall content of s a l t , not the composition of a p a r t i c u l a r phase. The difference in the DSC scans between the films as formed from solution and those resulting from heating above the complex melting point and recooling also show the importance of specifying thermal history. Figure 2 summarizes the results of the DSC analyses for heating rates of 20 °C/min in which the reproducible traces (previously heated to 260 °C cooled to -30 °C at 20 °C/min and reheated immediately) are shown for a l l concentrations investigated. For samples which have been previously melted, an endotherm near the melting point of PEO is seen for a l l concentrations of NaI that were investigated. The position of the peak changes systematically with salt concentration, passing through a minimum near 10% NaI. For salt contents of 10% and greater, a second endotherm appears in the v i c i n i t y of 195 °C which is attributed to the melting of a specific complex formed between Nal and PEO. This peak shifts to s l i g h t l y higher temperatures with increasing Nal content as shown in Figure 3. In these data, there is no conclusive evidence indicative of a glass trans i t i o n within the temperature range studied. 4.
ELECTRICALMEASUREMENTS
Electrical conductivity measurements were performed in two ways: dc conductivity was measured using a four probe technique and complex PEO-Nal Complexes
250~
200
150
100
50
0
10 Nal
20
30
(mol.%)
Figure 3. Summaryof DSC endothermic peaks as function of overall salt concentration, mole %. impedance measurements were made in the frequency range from 100 Hz to 13 MHz. Both types of measurement were made in vacuum at temperatures between room temperature and j u s t below the melting point of the complex. Temperature was varied in steps with e q u i l i b r a t i o n times of one to two hours allowed before measurements were taken. Results were reproducible under heating and cooling conditions but the samples were never completely molten before cooling. I n t h e case o f dc measurements, t h e measurement time was k e p t w i t h i n a few seconds i n o r d e r t o minimize p o l a r i z a t i o n e f f e c t s . The r e s u l t s were i n c l o s e agreement w i t h t h o s e d e r i v e d from impedance measurements.
2% ~
L___------------~f
lO%
15% 20% 25% ,
i
i
I
-Ioo
o
1oo
200
TEMPERATURE
I
300
(°C)
F i g u r e 2. DSC t r a c e s o f PEO c o n t a i n i n g t h e i n d i c a t e d mole % o f NaI heated a t 20 °C/min. All samples have been p r e v i o u s l y heated t o t h e v i c i n i t y o f 260 °C and c o o l e d t o -30 °C a t 20 ° C / min.
Figure 4 shows the e l e c t r i c a l conductivity of the PEO-Nal mixtures as a function of reciprocal temperature for several d i f f e r e n t concentrations of Nal. The data from samples for a l l concentrations investigated show a change in slope at a temperature near 60 °C similar to e a r l i e r work [6]. Furthermore, the activation energy in each region is about the same for a l l concentrations; viz. 0.35 eV at high temperatures and 1.45 eV at temperatures below 50 °C. I t would seem that this break in the curve on an Arrhenius plot is an important clue to the mechanism of ion transport but we are not aware of any clear explanations as to what happens at this point. The break coincides with the temperature at which uncomplexed PEO normally melts. I f one thinks of the PEO crystals as crosslinks which i n h i b i t large scale motions of the polymer that are suddenly removed at the melting point, then one has to explain why the
C.K. Chiang et al, / Preparation and properties of polymeric solid electrolyte higher-melting PEO-Nal complex crystals which are s t i l l present do not also act l i k e crosslinks. I t is tempting to visualize the system as a complex e l e c t r i c a l network in which the resistance of the PEO phase changes at i t s melting point. However, conductivity data obtained at 24 GHz, which supposedly measures conductivity i n t r i n s i c to a c r y s t a l l i t e rather than including effects of grain boundaries, also shows a change in slope at the same temperature [8]. Perhaps changes occur in the PEO-salt complex coincident with melting of the uncomplexed polymer, a suggestion somewhat d i f f e r e n t from one proposed e a r l i e r [7].
'E
1123
- ~75%
16 "~
.o c
>-
16'
16'
1-~> p-
o 160
16'
r~ Z
o o
2s~c -
16"
.
16 ... E
0
C
.,..
0
10 -3
10
2'0
30
16'
Na( (mol,%l
-4
10 ld
Figure 5. The conductivities of PEO-Nal complexes as function of Nal-salt concentrations.
ef~
s
o
>-
~%°o
66
N
1
0 :3
ld z
z 0 0
168 I0
5.
m 0~o
% .f
*
um• aO
-9 1
2.(
30 IO~IT
.........
3.5
(K -I)
Figure 4. DC e l e c t r i c a l conductivity of PEO-Nal complexes as a function of reciprocal temperature. ~ , 1~; e, 2%; A, 5%; ©, 10%; o, 25%. I f one examines the values of conductivity for d i f f e r e n t concentrations of salt at a given temperature, a maximumoccurs at low concentrations as shown in Figure 5. Notice that this maximum occurs at hal concentrations for which no crystal complex is formed as judged from DSC data. This implies that a c r y s t a l l i n e complex need not be formed to achieve fast ion transport. Maxima in conductivity at low s a l t concentrations have been reported for other noncryst a l l i n e polymer-salt systems [9,10]. The high melting c r y s t a l l i n e complex may only be a manifestation of the strong interaction between the polymer and the cation. Mechanical properties of the films were not measured but the high molecular weight of the PEO prevented spontaneous flow of the samples above the low temperature melting peak.
REFERENCES
Work supported in part by Office of Naval Research, U.S.A. ? Commercial materials are i d e n t i f i e d to specify the experimental procedure. Such i d e n t i f i c a t i o n does not imply recommendat i o n or endorsement by the National Bureau of Standards. [1] P. V. Wright, Br. Polym. Z, 319 (1975). [2] M. Armand, J. M. Chabagno and N. J. Duclot, Proc. 2nd I n t l . Meeting on Solid Electrolytes, St. Andrews, Scotland, Sept. (1978), p. 6-5-1. [3] M. Armand, J. M. Chabagnoand N. J. Duclot, Fast Ion Transport in Solids, ed. by M. Vashishta, J. N. Mundy, and G. K. Shenoy, North-Holland, New York (1979), p. 131. [4] D. F. Shriver, B. L. Papke, M. A. Rather, R. Dupon, T. Wong, and M. Brodwin, Solid State lonics 5, 83 (1981). [5] J. E. Weston and B. C. H. Steele, Solid State lonics 2, 347 (1981); Z, 81 (1982). [6] B. L. Papke, R. Dupon, M. A. Ratner, and D. F. Shriver, Solid State Ionics 5, 685 (1981). [7] D. R. Payne and P. V. Wright, Polymer 2_~3, 690 (1982). [8] T. Wong, M. Brodwin, B. L. Papke, and D. F. Shriver, Solid State lonics 5, 689 (1981). [9] M. Watanabe, O. Ikeda, I. Shinokara, Polymer J. 1_55, 175 (1983). [10] A. K i l l i s , J. F. LeNest, H. Cherdame, A. Gandini, Makromol. Chem. 18_~3, 2835 (1982).
1121
PREPARATION AND PROPERTIES OF POLYMERIC SOLID ELECTROLYTE: POLYETHYLENE OXIDE--SODIUM IODIDE COMPLEXES*~ C. K. Chiang, G. T. Davis, C. A. Harding and J. Aarons Polymer Science and Standards Division National Bureau of Standards Washington, DC 20234 USA The thermal and e l e c t r i c a l properties of mixtures of polyethylene oxide and sodium iodide were measured for concentrations of s a l t up to 25 mole %. A maximum in dc conductivity is observed at low concentrations of Nal, a region in which no c r y s t a l l i n e complex is formed as determined from DSC measurements. 1.
INTRODUCTION
The high ionic conductivity of a l k a l i metal halides complexed with polyethylene oxide has stimulated a search for the mechanism of ionic transport in this polymer which might then guide the development of other conductors [1-4]. Results presented here are p a r t i a l l y a confirmat i o n of e a r l i e r reports but they also show the effects of sodium iodide on the e l e c t r i c a l and thermal properties of polyethylene oxide over a range of concentrations from 1 to 25 mole %. 2.
PREPARATIONOF POLYMER/SALT MIXTURES
Poly(ethylene oxide), PEO, was obtained from Polysciences, Inc. in powdered form and used d i r e c t l y . The results reported here are for PEO with a molecular weight of 5x106. Reagent grade anhydrous sodium iodide (Fisher) was heated at 120 °C before dissolving in spectrograde a c e t o n i t r i l e (Kodak) from a freshly opened b o t t l e . A c e t o n i t r i l e containing the appropriate amount of completely dissolved s a l t was then used to dissolve approximately 2 wt.% PEO by s t i r r i n g in a closed vessel for 24 hours. Films were prepared by completely evaporating the solvent at 35 °C, and the resulting opaque films were stored in an evacuated dessicator. Samples prepared in t h i s manner exhibited c r y s t a l l i n e behavior c h a r a c t e r i s t i c of very low water content [5]. Salt concentrations in the PEO-NaI complexes are expressed in terms of mole percent rather than the O/Na r a t i o preferred by some authors. 3.
absent in the i n i t i a l heating cycle for NaI concentrations of 15 percent and greater (curves a and b) but subsequent cycles always e x h i b i t a peak in t h i s region (curve c). However, the magnitude of the peak decreases with increasing s a l t concentration. For s a l t concentration of 10 percent and greater, an endotherm at temperatures in the v i c i n i t y of 195 °C appears which has been a t t r i b u t e d to the melting of a PEOs a l t complex [1,7]. At temperatures above the melting point of the complex, we have observed microscopically what we believe to be phaseseparated crystals of NaI. Upon cooling to temperatures in the v i c i n i t y of 150 °C, one can f o l l o w the gradual disappearance of these cubic crystals over a period of hours as they are apparently re-incorporated into the PEO-NaI complex. As in other polymer systems, the composition of the phases present can be highly dependent upon thermal history. Therefore the concentration of NaI necessarily r e f l e c t s only
THERMALANALYSIS
D i f f e r e n t i a l scanning calorimeter traces were obtained using a Perkin Elmer DSC I I f o r samples in sealed aluminum pans. Rather i r r e g u l a r traces were often obtained on the f i r s t heating of a p a r t i c u l a r sample, but a f t e r once heating to 260 °C, subsequent traces were regular and reproducible. However, the nature of the sample a f t e r c r y s t a l l i z i n g from the melt can be considerably d i f f e r e n t from that as prepared by evaporation of solvent as can be seen in Figure 1. An endothermic peak in the v i c i n i t y of 68 °C, i n d i c a t i v e of uncomplexed PEO [ 6 ] , is usually 0 167-2738/83/0000
0000/$ 03.00 © 1983 North-Holland
i
I
i
100 TEMPERATURE
i
200
I
300
(°C)
Figure I. DSC traces of PEO containing 20 mole % NaI. Curves a and b are i n i t i a l heatings at 20 °C/min on two d i f f e r e n t samples. Curve c is typical of reproducible traces obtained a f t e r heating to 260 °C and cooling to -30 °C at 20 °C/min.
C. k Chiang et al. / Preparation and properties o f polymeric solid electrolyte
1122
the overall content of s a l t , not the composition of a p a r t i c u l a r phase. The difference in the DSC scans between the films as formed from solution and those resulting from heating above the complex melting point and recooling also show the importance of specifying thermal history. Figure 2 summarizes the results of the DSC analyses for heating rates of 20 °C/min in which the reproducible traces (previously heated to 260 °C cooled to -30 °C at 20 °C/min and reheated immediately) are shown for a l l concentrations investigated. For samples which have been previously melted, an endotherm near the melting point of PEO is seen for a l l concentrations of NaI that were investigated. The position of the peak changes systematically with salt concentration, passing through a minimum near 10% NaI. For salt contents of 10% and greater, a second endotherm appears in the v i c i n i t y of 195 °C which is attributed to the melting of a specific complex formed between Nal and PEO. This peak shifts to s l i g h t l y higher temperatures with increasing Nal content as shown in Figure 3. In these data, there is no conclusive evidence indicative of a glass trans i t i o n within the temperature range studied. 4.
ELECTRICALMEASUREMENTS
Electrical conductivity measurements were performed in two ways: dc conductivity was measured using a four probe technique and complex PEO-Nal Complexes
250~
200
150
100
50
0
10 Nal
20
30
(mol.%)
Figure 3. Summaryof DSC endothermic peaks as function of overall salt concentration, mole %. impedance measurements were made in the frequency range from 100 Hz to 13 MHz. Both types of measurement were made in vacuum at temperatures between room temperature and j u s t below the melting point of the complex. Temperature was varied in steps with e q u i l i b r a t i o n times of one to two hours allowed before measurements were taken. Results were reproducible under heating and cooling conditions but the samples were never completely molten before cooling. I n t h e case o f dc measurements, t h e measurement time was k e p t w i t h i n a few seconds i n o r d e r t o minimize p o l a r i z a t i o n e f f e c t s . The r e s u l t s were i n c l o s e agreement w i t h t h o s e d e r i v e d from impedance measurements.
2% ~
L___------------~f
lO%
15% 20% 25% ,
i
i
I
-Ioo
o
1oo
200
TEMPERATURE
I
300
(°C)
F i g u r e 2. DSC t r a c e s o f PEO c o n t a i n i n g t h e i n d i c a t e d mole % o f NaI heated a t 20 °C/min. All samples have been p r e v i o u s l y heated t o t h e v i c i n i t y o f 260 °C and c o o l e d t o -30 °C a t 20 ° C / min.
Figure 4 shows the e l e c t r i c a l conductivity of the PEO-Nal mixtures as a function of reciprocal temperature for several d i f f e r e n t concentrations of Nal. The data from samples for a l l concentrations investigated show a change in slope at a temperature near 60 °C similar to e a r l i e r work [6]. Furthermore, the activation energy in each region is about the same for a l l concentrations; viz. 0.35 eV at high temperatures and 1.45 eV at temperatures below 50 °C. I t would seem that this break in the curve on an Arrhenius plot is an important clue to the mechanism of ion transport but we are not aware of any clear explanations as to what happens at this point. The break coincides with the temperature at which uncomplexed PEO normally melts. I f one thinks of the PEO crystals as crosslinks which i n h i b i t large scale motions of the polymer that are suddenly removed at the melting point, then one has to explain why the
C.K. Chiang et al, / Preparation and properties of polymeric solid electrolyte higher-melting PEO-Nal complex crystals which are s t i l l present do not also act l i k e crosslinks. I t is tempting to visualize the system as a complex e l e c t r i c a l network in which the resistance of the PEO phase changes at i t s melting point. However, conductivity data obtained at 24 GHz, which supposedly measures conductivity i n t r i n s i c to a c r y s t a l l i t e rather than including effects of grain boundaries, also shows a change in slope at the same temperature [8]. Perhaps changes occur in the PEO-salt complex coincident with melting of the uncomplexed polymer, a suggestion somewhat d i f f e r e n t from one proposed e a r l i e r [7].
'E
1123
- ~75%
16 "~
.o c
>-
16'
16'
1-~> p-
o 160
16'
r~ Z
o o
2s~c -
16"
.
16 ... E
0
C
.,..
0
10 -3
10
2'0
30
16'
Na( (mol,%l
-4
10 ld
Figure 5. The conductivities of PEO-Nal complexes as function of Nal-salt concentrations.
ef~
s
o
>-
~%°o
66
N
1
0 :3
ld z
z 0 0
168 I0
5.
m 0~o
% .f
*
um• aO
-9 1
2.(
30 IO~IT
.........
3.5
(K -I)
Figure 4. DC e l e c t r i c a l conductivity of PEO-Nal complexes as a function of reciprocal temperature. ~ , 1~; e, 2%; A, 5%; ©, 10%; o, 25%. I f one examines the values of conductivity for d i f f e r e n t concentrations of salt at a given temperature, a maximumoccurs at low concentrations as shown in Figure 5. Notice that this maximum occurs at hal concentrations for which no crystal complex is formed as judged from DSC data. This implies that a c r y s t a l l i n e complex need not be formed to achieve fast ion transport. Maxima in conductivity at low s a l t concentrations have been reported for other noncryst a l l i n e polymer-salt systems [9,10]. The high melting c r y s t a l l i n e complex may only be a manifestation of the strong interaction between the polymer and the cation. Mechanical properties of the films were not measured but the high molecular weight of the PEO prevented spontaneous flow of the samples above the low temperature melting peak.
REFERENCES
Work supported in part by Office of Naval Research, U.S.A. ? Commercial materials are i d e n t i f i e d to specify the experimental procedure. Such i d e n t i f i c a t i o n does not imply recommendat i o n or endorsement by the National Bureau of Standards. [1] P. V. Wright, Br. Polym. Z, 319 (1975). [2] M. Armand, J. M. Chabagno and N. J. Duclot, Proc. 2nd I n t l . Meeting on Solid Electrolytes, St. Andrews, Scotland, Sept. (1978), p. 6-5-1. [3] M. Armand, J. M. Chabagnoand N. J. Duclot, Fast Ion Transport in Solids, ed. by M. Vashishta, J. N. Mundy, and G. K. Shenoy, North-Holland, New York (1979), p. 131. [4] D. F. Shriver, B. L. Papke, M. A. Rather, R. Dupon, T. Wong, and M. Brodwin, Solid State lonics 5, 83 (1981). [5] J. E. Weston and B. C. H. Steele, Solid State lonics 2, 347 (1981); Z, 81 (1982). [6] B. L. Papke, R. Dupon, M. A. Ratner, and D. F. Shriver, Solid State Ionics 5, 685 (1981). [7] D. R. Payne and P. V. Wright, Polymer 2_~3, 690 (1982). [8] T. Wong, M. Brodwin, B. L. Papke, and D. F. Shriver, Solid State lonics 5, 689 (1981). [9] M. Watanabe, O. Ikeda, I. Shinokara, Polymer J. 1_55, 175 (1983). [10] A. K i l l i s , J. F. LeNest, H. Cherdame, A. Gandini, Makromol. Chem. 18_~3, 2835 (1982).