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Poly(ethylene oxide)-alkali metal-silver halide salt systems with high ionic conductivity at room temperature
Solid State Ionics 21 (1986) 203-206 North-Holland, Amsterdam
POLY(ETHYLENE OXIDE)-ALKALI METAL-SILVER HALIDE SALT SYSTEMS WITH HIGH IONIC CONDUCTIVITY AT ROOM TEMPERATURE J.R. STEVENS Physics Department, University o f Guelph, Guelph, Ontario NI G 2 W1, Canada and
B.-E. MELLANDER Physics Department, Chalmers University of Technology, S.412 96 Goteborg, Sweden
Received 3 November 1985 ; revised manuscript received 10 April 1986 The electrical conductivity of poly(ethylene oxide)-MAg41s (M = Li, K, Rb) compounds has been measured for M : O ratios between 0.1 and 1.0. For P(EO)IKAg4Is the conductivity is 2 X 10 -3 i2 -1 cm-1 at room temperature and the activation energy is 0.16 eV.
1. Introduction It is well known that in a freshly prepared state the two solid electrolytes RbAg4I5 and KAg4I5 have unusually high ionic conductivity near room temperature, about 0.2 ~ - 1 cm-1 for both compounds [ 1 - 7 ] . These conductivities increase with temperature with activation energies of the order of 0.1 eV [1,4]. Unfortunately, these compounds are unstable to disproportionation, RbAg4I5 at temperatures below 27°C [8] and KAg4I5 at temperatures below 38°C [1 ]. In spite of this thermodynamic instability it is reported that RbAg4I 5 may be retained at room temperature for indet'mite periods of time in a dry atmosphere [ 9 - 1 1 ] . KAg4I5 is less stable but only qualitative estimates of this stability have been reported [10]. Both of these salts are hygroscopic and water catalyses the disproportionation. The objective of the work reported here was to incorporate AgI into complexes which would prove to be stable while maintaining high ionic conductivity. Because of all the attention being paid to poly(ethylene oxide) (PEO) [12,13] it seemed appropriate to incorporate the polymer in some way.
2. Preparation AgI is insoluble in organic solvents. Early prepara0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Phvsics Publishin~ Division)
tion of RbAg4I5 and KAg4I5 was by solid state reaction; for example stoichiometric amounts of AgI and RbI were combined and melted. AgI, RbI and KI are soluble in HI at about 50°C and crystals may be grown from solution [10]. In this work MAg4I5 (M = Li, K, Rb) were chemically complexed with high molecular weight PEO in a nitrogen atmosphere with concentrations ofM : O of 0.1,0.2, 0.4, 0.6 and 1.0. Stoichiometric amounts (exact to the milligram) of PEO, Agl and MI of 99.9% purity were mechanically mixed and then combined with water-free acetonitrile in the ratio of 1 : 25 by volume. The PEO was obtained from Polysciences Inc. and had a weight-average molecular weight of 600000. The mixture was heated to 50°C and stirred. Within a short time the compound P(EO)o : M MAg4 I5 precipitated out and the solvent was then evaporated ott. Depending on the errors in the stoichiometry traces of AgI and MI could be found. Precision stoichiometry is important because of the nature of the phase diagram [1,2] for the salt MxAgyI z (M = Rb, K). The solid electrolytes are formed precisely on the 20 mole% MI, 80 mole% AgI line which exists between 27°C and 228°C for RbAg415 and between 38°C and 253°C for KAg4I5. Because of the insolubility of AgI in acetonitrile we were not able to use this method to form AgI-PEO complexes. We could not find a reliable phase diagram for the LiI-AgI system in the literature.
J.R. Stevens, B.-E. Mellander/P(EO)-alkali metal-silver halide salt systems
204
3. Experimental The samples were pressed in a cylindrical die o f 13 mm diameter using a pressure of 0.6 GPa. The samples were 1 to 2 mm long and perforated platinum or silver foils pressed on the sample surfaces served as electrodes. The samples were placed between two gold plates under spring pressure. The sample holder was placed inside a furnace and the sample was kept in a dry nitrogen atmosphere. The electrical conductivity was determined using complex impedance analysis. The complex impedance was measured using a computer controlled HP4274A LCR meter. The frequency range covered was 100 Hz to 100 kHz and the applied signal was 20 mV. A typical impedance plot is shown in fig. 1. The sample resistance was obtained from the intercept o f the straight line and the real axis. The DCS measurements were performed using a Rigaku DSC apparatus. The samples were contained in platinum cups; a protective atmosphere of dry nitrogen gas was used. The results of room temperature measurements of conductivity are presented in fig. 2 as lOgl0o versus the M : O ratio (M = K, Rb). It should be noted here that each K or Rb ion is accompanied by four Ag ions. Thus, if the ratios above are presented as metal ion to oxygen ratios they will be five times as high as those in fig. 2. For K : O ratios less than 0.2 the conductivity decreases rapidly and the trend is similar for the
T
/
E
A O
cA
o -5
k~ ID3
I
-10
1
M:O
Ratio
Fig. 2. Room temperature electrical conductivity o f P(EO)o:MMAg4I 5 for M = K (circles) and M = Rb (triangles). The dashed curve shows the calculated conductivity using percolation theory.
samples containing rubidium. The best conductivities for samples containing either potassium or rubidium salt are thus about 2 X 10 - 3 ~2-1 cm -1 . For P(EO)ILiAg4I 5 o= 1 × 10 -4 f2 -1 cm -1 at 20°C. In fig. 3 the temperature dependence of the conductivity
Temperature (~C) 75 50 25 0 ] D
100 Hz o/
A2
/
v"
/ /o
1",4
o/
0
i
J
[
X o
-
U
b-- - 0 . 5
\
\
/o
-
o
sO
,o ~)100kHz
0
o
I
I
1
2
.
ReZ ( k.Cl ) Fig. 1. Complex impedance plot for a P(EO)1KAg415 sample with silver electrodes.
-1.0 ~ 2.5
I 30 I O 0 0 / T (K -~)
Fig. 3. In(aT) versus 1000/Tplot for a
3.5
P(EO)IKAg4I 5
sample.
J.R. Stevens, B.-E. Mellander/P(EO) -alkali metal-silver halide salt systems
205
-2.5 DO A
7E
(a)
A
O
O
u
T
-3.C
I
-
O
b
150
200 250 Tempera-t-ure (°C)
150
200 250 Tempero-~ure (°C)
O)
-35
p
0
I
,
I
l
I
.
100 200 Time (doys)
Fig. 4. Electrical conductivity of P(EO)1MAg4Is as a function of time for M = K (circles) and M = Rb (triangles).
is shown as a ln(oT) versus 1/T plot for the P(EO)IKAg4I 5 sample. The activation energy is, in this case, 0.16 eV. The electronic contribution to the electrical conductivity has been measured using a dcpolarization method. For all samples tested the electronic conductivity is negligible compared to the ionic conductivity. The thermodynamic stability of the P(EO-)I MAg4I 5 samples has been monitored for more than 230 days, the samples being stored in a desiccator. Fig. 4 is a plot of lOgl0o versus time and shows a decrease in the conductivity of about 60% for each of the M = K, Rb samples over this period. These measurements were difficult to make since the electrodes had a tendency to peel off the pellets. The DCS measurements showed that a number of different phases could be detected in the samples. In fig. 5 some examples of DSC plots are shown. For pure PEO a large peak at about 60°C indicate the melting point of the polymer. This peak was not found in any of the samples where silver salts were present. Furthermore, in all samples, the peak corresponding to the phase transition at 147°C in pure AgI was detected. In addition to the peak at 253°C for KAg4I 5 and the peak at 228°C for RbAg4I 5 peaks at 238°C and 197°C indicated the nonstoichiometry of the samples, cf. the phase diagram [1,2]. One broad peak that could not be explained as a silver salt peak was observed at about 160°C. NMR measurements on pure PEO and P(EO)I KAg4I5 confirm the results of
(b)
Fig. 5. DSC plots for (a) P(EO)IKAg415 and (b) P(EO) 1 RbAg4Is -
the DSC measurements that the crystalline form of PEO does not remain in the samples where the PEO has been mixed with the salt.
4. Discussion
The samples thus contain crystalline salt phases and possibly also a PEO-salt complex. We conclude that the high conductivity is probably due to the presence of MAg4I 5 since o decreases with decreasing MAg4I 5 content and since the activation energy is almost the same as for pure MAg4I 5 salt, while usually PEO-salt complexes have higher activation energies. It has been shown that the electrical conductivity of some twophase systems can be described using effective medium percolation theory [ 14,15 ]. Even if the present case is more complicated it can be of interest to compare the position of the percolation threshold with the rapid drop in conductivity observed for M : O ratios of about 0.1, see fig. 2. The theoretical conductivity of a two-phase system, salt and polymer can be calculated using percolation theory. Normalizing on the
206
J.R. Stevens, B.-E. Mellander/P(EO)-alkali metal-silver halide salt systems
highest conductivity for the complex (3 X 10 - 3 ( ~ cm) - 1 ) and using a value o f 1 X 10 - 9 (f~ cm) - 1 for the polymer the theoretical conductivity as a function o f M : O ratios follows the dashed line in fig. 2. It is interesting to note that the position o f the percolation threshold, which is not sensitive to the conductivit y values chosen, agrees well with the rapid drop in conductivity found for M : O ratios of about 0.1. The complexes being discussed here are more complicated than a simple two-phase mixture since traces o f b o t h AgI and MI has been detected and the behaviour o f the polymer is not known. The position o f the percolation threshold indicates, however, that percolation phenomena has to be taken into account in this case. Although o for P(EO)I KAg 415 has decreased somewhat over more than seven months it is certainly much more stable as a crystalline powder than reported by Gallagher and Klein [10] for the pure KAg4I 5 salt. These authors said that thin layers o f the salt began to discolor within 5 min. LiAg4I 5 quickly discolors as it picks up water from the air and disproportionates. However, P(EO)I LiAg4I 5 left in the open air discolored only slightly over one month. We have not been successful preparing samples with high o at room temperature using methanol as a solvent. The reason for this is not clear to us. We have examined P(EO)o:MKAg4I 5 compounds prepared using methanol and acetonitrile for C, H and N by elemental analysis. Within the accuracy o f the measurements there was no difference between compounds prepared using different solvents. The results reported here are preliminary and work will continue, especially in regard to long term thermodynamic stability.
Acknowledgement We would like to thank Professor Arnold Lund6n
for continuous support and many valuable discussions. We would also like to thank Dr. T. Hjertberg o f the Polymer Research Group for performing the NMR measurements and Drs. K. Holmberg and E. Andreasson o f Berol Kemi AB for supplying us with polymer samples and for stimulating discussions. This work has been supported financially b y the Swedish Natural Sciences Research Council and the Swedish Board of Technical Development which is gratefully acknowledged.
References [1 ] J.N. Bradley and P.D. Greene, Trans. Faraday Soc. 62 (1966) 2069. [2] J.N. Bradley and P.D. Greene, Trans. Faraday Soc. 63 (1967) 424. [3] B.B. Owens and G.R. Argue, Science 157 (1967) 308. [4] B.B. Owens and G.R. Argue, J. Electrochem. Soc. 117 (1970) 898. [5] D.O. Raleigh, J. Appl. Phys. 41 (1970) 1876. [6] B.B. Owens, in: Advances in electrochemistry and electrochemical engineering, ed. C.W. Tobias, Vol. 8 (Wiley, New York, 1971). [7] B. Scrosati, J. Appl. Chem. Biotechnol. 21 (1971) 223. [8] L.E. Topol and B.B. Owens, J. Phys. Chem. 72 (1968) 2106. [9] W.V. Johnston, H. Wiedersich and G.W. Lindberg, J. Chem. Phys. 51 (1969) 3739. [10] D.A. Gallagher and M.V. Klein, Phys. Rev. B 19 (1979) 4282. [ 11 ] B.B. Owens, B.K. Patel, P.M. Skarstad and D.L. Warburton, Solid State Ionics 9/10 (1983) 1241. [12] M. Armand, Solid State Ionics 9/10 (1983) 745. [13] G.T. Davis and C.K. Chiang, in: Conducting polymeric materials, ed. H. Sasabe (Chemical Marketing Co., Tokyo, 1984) p. 244. [14] D.G. Ast, Phys. Rev. Letters 33 (1974) 1042. [ 15 ] M.F. Bell, M. Sayer, D.S. Smith and P.S. Nicholson, Solid State Ionics 9/10 (1983) 731.
POLY(ETHYLENE OXIDE)-ALKALI METAL-SILVER HALIDE SALT SYSTEMS WITH HIGH IONIC CONDUCTIVITY AT ROOM TEMPERATURE J.R. STEVENS Physics Department, University o f Guelph, Guelph, Ontario NI G 2 W1, Canada and
B.-E. MELLANDER Physics Department, Chalmers University of Technology, S.412 96 Goteborg, Sweden
Received 3 November 1985 ; revised manuscript received 10 April 1986 The electrical conductivity of poly(ethylene oxide)-MAg41s (M = Li, K, Rb) compounds has been measured for M : O ratios between 0.1 and 1.0. For P(EO)IKAg4Is the conductivity is 2 X 10 -3 i2 -1 cm-1 at room temperature and the activation energy is 0.16 eV.
1. Introduction It is well known that in a freshly prepared state the two solid electrolytes RbAg4I5 and KAg4I5 have unusually high ionic conductivity near room temperature, about 0.2 ~ - 1 cm-1 for both compounds [ 1 - 7 ] . These conductivities increase with temperature with activation energies of the order of 0.1 eV [1,4]. Unfortunately, these compounds are unstable to disproportionation, RbAg4I5 at temperatures below 27°C [8] and KAg4I5 at temperatures below 38°C [1 ]. In spite of this thermodynamic instability it is reported that RbAg4I 5 may be retained at room temperature for indet'mite periods of time in a dry atmosphere [ 9 - 1 1 ] . KAg4I5 is less stable but only qualitative estimates of this stability have been reported [10]. Both of these salts are hygroscopic and water catalyses the disproportionation. The objective of the work reported here was to incorporate AgI into complexes which would prove to be stable while maintaining high ionic conductivity. Because of all the attention being paid to poly(ethylene oxide) (PEO) [12,13] it seemed appropriate to incorporate the polymer in some way.
2. Preparation AgI is insoluble in organic solvents. Early prepara0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Phvsics Publishin~ Division)
tion of RbAg4I5 and KAg4I5 was by solid state reaction; for example stoichiometric amounts of AgI and RbI were combined and melted. AgI, RbI and KI are soluble in HI at about 50°C and crystals may be grown from solution [10]. In this work MAg4I5 (M = Li, K, Rb) were chemically complexed with high molecular weight PEO in a nitrogen atmosphere with concentrations ofM : O of 0.1,0.2, 0.4, 0.6 and 1.0. Stoichiometric amounts (exact to the milligram) of PEO, Agl and MI of 99.9% purity were mechanically mixed and then combined with water-free acetonitrile in the ratio of 1 : 25 by volume. The PEO was obtained from Polysciences Inc. and had a weight-average molecular weight of 600000. The mixture was heated to 50°C and stirred. Within a short time the compound P(EO)o : M MAg4 I5 precipitated out and the solvent was then evaporated ott. Depending on the errors in the stoichiometry traces of AgI and MI could be found. Precision stoichiometry is important because of the nature of the phase diagram [1,2] for the salt MxAgyI z (M = Rb, K). The solid electrolytes are formed precisely on the 20 mole% MI, 80 mole% AgI line which exists between 27°C and 228°C for RbAg415 and between 38°C and 253°C for KAg4I5. Because of the insolubility of AgI in acetonitrile we were not able to use this method to form AgI-PEO complexes. We could not find a reliable phase diagram for the LiI-AgI system in the literature.
J.R. Stevens, B.-E. Mellander/P(EO)-alkali metal-silver halide salt systems
204
3. Experimental The samples were pressed in a cylindrical die o f 13 mm diameter using a pressure of 0.6 GPa. The samples were 1 to 2 mm long and perforated platinum or silver foils pressed on the sample surfaces served as electrodes. The samples were placed between two gold plates under spring pressure. The sample holder was placed inside a furnace and the sample was kept in a dry nitrogen atmosphere. The electrical conductivity was determined using complex impedance analysis. The complex impedance was measured using a computer controlled HP4274A LCR meter. The frequency range covered was 100 Hz to 100 kHz and the applied signal was 20 mV. A typical impedance plot is shown in fig. 1. The sample resistance was obtained from the intercept o f the straight line and the real axis. The DCS measurements were performed using a Rigaku DSC apparatus. The samples were contained in platinum cups; a protective atmosphere of dry nitrogen gas was used. The results of room temperature measurements of conductivity are presented in fig. 2 as lOgl0o versus the M : O ratio (M = K, Rb). It should be noted here that each K or Rb ion is accompanied by four Ag ions. Thus, if the ratios above are presented as metal ion to oxygen ratios they will be five times as high as those in fig. 2. For K : O ratios less than 0.2 the conductivity decreases rapidly and the trend is similar for the
T
/
E
A O
cA
o -5
k~ ID3
I
-10
1
M:O
Ratio
Fig. 2. Room temperature electrical conductivity o f P(EO)o:MMAg4I 5 for M = K (circles) and M = Rb (triangles). The dashed curve shows the calculated conductivity using percolation theory.
samples containing rubidium. The best conductivities for samples containing either potassium or rubidium salt are thus about 2 X 10 - 3 ~2-1 cm -1 . For P(EO)ILiAg4I 5 o= 1 × 10 -4 f2 -1 cm -1 at 20°C. In fig. 3 the temperature dependence of the conductivity
Temperature (~C) 75 50 25 0 ] D
100 Hz o/
A2
/
v"
/ /o
1",4
o/
0
i
J
[
X o
-
U
b-- - 0 . 5
\
\
/o
-
o
sO
,o ~)100kHz
0
o
I
I
1
2
.
ReZ ( k.Cl ) Fig. 1. Complex impedance plot for a P(EO)1KAg415 sample with silver electrodes.
-1.0 ~ 2.5
I 30 I O 0 0 / T (K -~)
Fig. 3. In(aT) versus 1000/Tplot for a
3.5
P(EO)IKAg4I 5
sample.
J.R. Stevens, B.-E. Mellander/P(EO) -alkali metal-silver halide salt systems
205
-2.5 DO A
7E
(a)
A
O
O
u
T
-3.C
I
-
O
b
150
200 250 Tempera-t-ure (°C)
150
200 250 Tempero-~ure (°C)
O)
-35
p
0
I
,
I
l
I
.
100 200 Time (doys)
Fig. 4. Electrical conductivity of P(EO)1MAg4Is as a function of time for M = K (circles) and M = Rb (triangles).
is shown as a ln(oT) versus 1/T plot for the P(EO)IKAg4I 5 sample. The activation energy is, in this case, 0.16 eV. The electronic contribution to the electrical conductivity has been measured using a dcpolarization method. For all samples tested the electronic conductivity is negligible compared to the ionic conductivity. The thermodynamic stability of the P(EO-)I MAg4I 5 samples has been monitored for more than 230 days, the samples being stored in a desiccator. Fig. 4 is a plot of lOgl0o versus time and shows a decrease in the conductivity of about 60% for each of the M = K, Rb samples over this period. These measurements were difficult to make since the electrodes had a tendency to peel off the pellets. The DCS measurements showed that a number of different phases could be detected in the samples. In fig. 5 some examples of DSC plots are shown. For pure PEO a large peak at about 60°C indicate the melting point of the polymer. This peak was not found in any of the samples where silver salts were present. Furthermore, in all samples, the peak corresponding to the phase transition at 147°C in pure AgI was detected. In addition to the peak at 253°C for KAg4I 5 and the peak at 228°C for RbAg4I 5 peaks at 238°C and 197°C indicated the nonstoichiometry of the samples, cf. the phase diagram [1,2]. One broad peak that could not be explained as a silver salt peak was observed at about 160°C. NMR measurements on pure PEO and P(EO)I KAg4I5 confirm the results of
(b)
Fig. 5. DSC plots for (a) P(EO)IKAg415 and (b) P(EO) 1 RbAg4Is -
the DSC measurements that the crystalline form of PEO does not remain in the samples where the PEO has been mixed with the salt.
4. Discussion
The samples thus contain crystalline salt phases and possibly also a PEO-salt complex. We conclude that the high conductivity is probably due to the presence of MAg4I 5 since o decreases with decreasing MAg4I 5 content and since the activation energy is almost the same as for pure MAg4I 5 salt, while usually PEO-salt complexes have higher activation energies. It has been shown that the electrical conductivity of some twophase systems can be described using effective medium percolation theory [ 14,15 ]. Even if the present case is more complicated it can be of interest to compare the position of the percolation threshold with the rapid drop in conductivity observed for M : O ratios of about 0.1, see fig. 2. The theoretical conductivity of a two-phase system, salt and polymer can be calculated using percolation theory. Normalizing on the
206
J.R. Stevens, B.-E. Mellander/P(EO)-alkali metal-silver halide salt systems
highest conductivity for the complex (3 X 10 - 3 ( ~ cm) - 1 ) and using a value o f 1 X 10 - 9 (f~ cm) - 1 for the polymer the theoretical conductivity as a function o f M : O ratios follows the dashed line in fig. 2. It is interesting to note that the position o f the percolation threshold, which is not sensitive to the conductivit y values chosen, agrees well with the rapid drop in conductivity found for M : O ratios of about 0.1. The complexes being discussed here are more complicated than a simple two-phase mixture since traces o f b o t h AgI and MI has been detected and the behaviour o f the polymer is not known. The position o f the percolation threshold indicates, however, that percolation phenomena has to be taken into account in this case. Although o for P(EO)I KAg 415 has decreased somewhat over more than seven months it is certainly much more stable as a crystalline powder than reported by Gallagher and Klein [10] for the pure KAg4I 5 salt. These authors said that thin layers o f the salt began to discolor within 5 min. LiAg4I 5 quickly discolors as it picks up water from the air and disproportionates. However, P(EO)I LiAg4I 5 left in the open air discolored only slightly over one month. We have not been successful preparing samples with high o at room temperature using methanol as a solvent. The reason for this is not clear to us. We have examined P(EO)o:MKAg4I 5 compounds prepared using methanol and acetonitrile for C, H and N by elemental analysis. Within the accuracy o f the measurements there was no difference between compounds prepared using different solvents. The results reported here are preliminary and work will continue, especially in regard to long term thermodynamic stability.
Acknowledgement We would like to thank Professor Arnold Lund6n
for continuous support and many valuable discussions. We would also like to thank Dr. T. Hjertberg o f the Polymer Research Group for performing the NMR measurements and Drs. K. Holmberg and E. Andreasson o f Berol Kemi AB for supplying us with polymer samples and for stimulating discussions. This work has been supported financially b y the Swedish Natural Sciences Research Council and the Swedish Board of Technical Development which is gratefully acknowledged.
References [1 ] J.N. Bradley and P.D. Greene, Trans. Faraday Soc. 62 (1966) 2069. [2] J.N. Bradley and P.D. Greene, Trans. Faraday Soc. 63 (1967) 424. [3] B.B. Owens and G.R. Argue, Science 157 (1967) 308. [4] B.B. Owens and G.R. Argue, J. Electrochem. Soc. 117 (1970) 898. [5] D.O. Raleigh, J. Appl. Phys. 41 (1970) 1876. [6] B.B. Owens, in: Advances in electrochemistry and electrochemical engineering, ed. C.W. Tobias, Vol. 8 (Wiley, New York, 1971). [7] B. Scrosati, J. Appl. Chem. Biotechnol. 21 (1971) 223. [8] L.E. Topol and B.B. Owens, J. Phys. Chem. 72 (1968) 2106. [9] W.V. Johnston, H. Wiedersich and G.W. Lindberg, J. Chem. Phys. 51 (1969) 3739. [10] D.A. Gallagher and M.V. Klein, Phys. Rev. B 19 (1979) 4282. [ 11 ] B.B. Owens, B.K. Patel, P.M. Skarstad and D.L. Warburton, Solid State Ionics 9/10 (1983) 1241. [12] M. Armand, Solid State Ionics 9/10 (1983) 745. [13] G.T. Davis and C.K. Chiang, in: Conducting polymeric materials, ed. H. Sasabe (Chemical Marketing Co., Tokyo, 1984) p. 244. [14] D.G. Ast, Phys. Rev. Letters 33 (1974) 1042. [ 15 ] M.F. Bell, M. Sayer, D.S. Smith and P.S. Nicholson, Solid State Ionics 9/10 (1983) 731.