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Comparison of NMR and conductivity in (PEP)8LiClO4+γ-LiAlO2
SOLID STATE
Solid State Ionics 53-56 (1992} 1102-1105 Norlh-Holland
IOHICS
Comparison of NMR and conductivity in (PEO) 8LiC104+'f-LiA102 Wang Gang 1, j. Roos, D. Brinkmann 2 Physik-lnstitut, (,:niver~itdt Ziirich. 8001 Zurich. S~*itzerland
F. Capuano, F. Croce and B. Scrosati Dipartimenlo di Chimica Universita di Roma *'La Sapienza", O0185 Rome. Italy
Polymer electrolyteshave become materials of considerable interest due to the wide range of their possible applications. However. their significantlylow ionic conductivity at room temperature and their poor mechanical properties at temperatures above the crystalline-amorphousphase transition (around 65 °C) considerably limit their usage. Introducing an inert "filler" into the polymer electrolyte as a means for increasingtheir mechanicaland electrical properties, has been widely recognized. In this work we report the characteristics of (PEO) sLiC104+ x weight % y-LiA102with x = 0, 10, 20 and 30, by describing results obtained by nuclear magnetic resonance and conductivity studies.
I. Introduction Polymer electrolytes have become materials of considerable interest due to the wide range of their possible applications. However, their significantly low ionic conductivity at room temperature and their poor mechanical properties at temperatures above the crystalline-amorphous phase transition ( a r o u n d 65~C) considerably limit their usage [1 ]. Introducing an inert "filler" into the polymer electrolyte as a means for increasing their mechanical and electrical properties, has been widely recognized. For instance, (PEO)sLiCIO4 with inert fine-grade 7LiAIO2 has been found to be a very promising system [2]. Measurements of electrical conductivity, mechanical properties and differential scanning calorimetry ( D S C ) have been carried out [3]. It was found that the mechanical properties were improved [3,4] and that the ionic conductivity was enhanced [5]. The question arises whether the e n h a n c e m e n t of Permanent address: Institute of Physics, Academia Sinica, Beijing 100 080, China. : Authorto whom correspondence should be addressed.
conductivity is due to processes taking place at an atomic level or to semi-macroscopic effects like presewing the polymer amorphous phase. Nuclear magnetic resonance ( N M R ) can answer these questions. We therefore have studied 7Li N M R as a function of temperature in (PEO) 8LiCIO4 + x wt% y-LiAIO2 with x = O , 10, 20 and 30. We have measured the linewidth and the spin-lattice and s p i n - s p i n relaxation limes.
2. Experimental 2. I. Preparation q f f i / m s
All polymer electrolyte films were prepared in a glove box using an inert argon atmosphere. The appropriate weights of lithium perchlorate (from Fluka AG, dried in vacuum at 120°C for 48 h before use) and PEO (BDH, molecular weight over 5X 106 ) to yield a PEO: Li ratio o f 8 : l in the complex, were dissolved in carefully dried acetonitrile ( R P E ) . The solutions obtained were stirred for 20 h at room temperature in a stoppered flask. A known a m o u n t of 7-LiA102 powder ( ~ 1 ~tm particle size)
0167-2738/92/$ 05,00 © 1992 ElsevierScience Publishers B.V. All rights reserved.
I4( Gang et al. / NMR and conductivity in (PEO)sLiCTO~+ ~[-LiAIO:
was then added, the mixture was stirred continuously until complete homogenization had occurred. This mixture was cast on a flat polytetrafluoroethylene sheet and covered to allow slow evaporation of acetonitrile. Using this procedure, homogeneous films of the composite polymer electrolyte were obtained with a thickness of 50 to 100 lam and no optical evidence for powder agglomeration.
1103
EQUIVCRT code [6 ], thus yielding the equivalent circuit parameters of the samples.
3. Results and discussion
3.1. Conductivity 2.2. N M R m e a s u r e m e n t s
The 7Li experiments were performed using a standard pulse spectrometer and a magnetic field of2.11 T corresponding to a Larmor frequency of 34.977 M Hz. The free-induction decay (FID) and spin-echo signals were digitized by transient recorders, accumulated and then Fourier transformed by on-line computers. The linewidth reported in this paper is lhe full width at half height (FWHH) of the square root of the power spectrum of the FID. The spin-lattice relaxation time T~ data were obtained from the FID signal using the saturation method (a n / 2 pulse follows a comb of saturating n~ 2 pulses). The spin-spin relaxation time T2 was deduced from the spin-echo decay. Temperatures were controlled to within _+0.5 K. The samples consisted of dried membranes cut into small pieces and sealed in pyrex tubes under vacuum. 2. 3. Conductivity m e a s u r e m e n t s
The electrical properties of the electrolyte films were measured by ac impedance spectroscopy at frequencies ranging from 10 3 to 105 Hz, using a Solartron 1255 FRA and a Solartron 1286 ECI, both coupled to a IBM PS/2 PC. In order to keep the system's response under linear condition, the peak-topeak potential difference of the stimulating signal was always maintained below 20 inV. Two parallel plane stainless-steel metallographicgrade polished electrodes with a surface area of 1.13 cm 2 were used. The cells which were housed in a Buchi rood. T-51 oven in order to provide constant temperature, were kept under constant mechanical pressure by using spring loaded terminals. The impedance data were fitted by a NLLSQ analysis program utilizing a modified Levemberg-Marquardt algorithm, adapted from Boukamp's
A detailed discussion of the electrical conductivity measurements has been reported elsewhere [ 3 ]. Fig. 1 shows the temperature dependence of the conductivity of the four samples. Both the 10 and the 20 wt% composite samples exhibit conductivities which are higher than those of the pure PEO salt complex. This fact is even more remarkable if we consider that the conductivity data have not been corrected for the "dilution effect", i.e. the decrease of the charge carrier concentration, in the constant volume samples, by the introduction of the ceramic filler particles. This enhancement of the conductivity has already been reported in literature [7] and is most probably due to an increase of the content of the amorphous phase of the composite polymer electrolyte. In other words, the conductivity of the composite samples is the result of two opposing effects: the ~'dilution" caused by the presence of the dispersoids depresses the ion transport while the stabilization of the amorphous phase enhances the ion transport. As a matter of fact, plotting conductivity versus fractional content of the ceramic filler yields the expected "bell"-shaped curve (fig. 2) with a maximum around 10 wt%. From a microscopic point of view, the recrystallization process may be regarded as a rearrangement of the polymer chains leading to a more ordered and stable state. The presence of dispersoid particles can hinder this process provided the particles are distributed homogeneously and their dimensions are comparable to the polymer chain length [8]. As the concentration of the ceramic powder increases, however, the ],-LiAIO2 particles start to agglomerate thus leading to regions free of the filler where recrystallization can readily occur.
1104
W. Gang et al. / NMR and conductivity in (PEO)sLiCIO4 + "~-LiAlOe
u
4
',',';5
Q~- :0~.
e G
I(
CS4
>
o
C, ..~ 0
lr
(: ; b
I u
:L' 0 /
1
AD 2 ~, ,0
"i,
'r :~
1000.",
;
I
K
L)
a ,
I
Fig. 1. Temperaturedependence of ionic conductivity of (PEO)8LiCIO4+ #-LiAIO2for various concentrations of y-LiAIO2: (©) 0 wt%, ( 0 ) 10wt%, ( A ) 20 wt%, ( A ) 30 wl%.
J
> 4-~ o
• '3
•
:'? '
-i-
,,O o
J
0
Z
(}
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ComposiLion,
5!},
4 ~)
wt %
Fig. 2. Composition dependence of the normalized ionic conductivity (a x% sample/a 0% sample) of (PEO)sLiCIO4+7-LiA102 at 60°C.
3.2, N M R Fig. 3 shows the temperature dependence o f the 7Li linewidth. All four samples we have investigated exhibited a broad signal at low and a narrow signal at high temperature and a transition region around 270 K where the broad line narrows. In this transi-
tion region, the Li signal consists of a broad line with a narrower line superimposed. For determining the F W H H we used the baseline o f the broad signal. However, using width values at other heights o f the signal, resulted in the same temperature dependence. In order to make sure that the Li signals we have recorded do not arise from Li nuclei of the filler, we
W. Gang et al. / N M R and conductivity in (PEO)sLiCI04+ 7-LiAl02 I
r
I
I
16
"~ 12 -1-2~
0
•
JZ
"- -"8-
__1
" ' \ " xx
~,', •
%, i ','a 6',',
4
',~,~
0 '160
I
200
I
/
240 280 Temperature (K)
.. [
320
360
Fig. 3. Temperature dependence of the 7Li NMR linewidth of the polymer Li in (PEO)sLiCIO4+7-LiAIO2: ( O ) 0 wt%, (@) l0 wt%, (A) 20 wt%, (A) 30 wt%. have d e t e r m i n e d the following parameters: ( i ) the spin-lattice and s p i n - s p i n relaxation time Tl and T2, respectively, in ( P E O ) 8LiC104 + x wt% y-LiA102; (ii) the linewidth and T, in the filler 7-LiA102. T~ o f 7Li in the PEO c o m p o u n d depends only very weakly on temperature. For ( P E O ) 8LiCIO4 + 20 wt% 7-LiAIO2 a b r o a d m i n i m u m o f about 0.1 s occurs a r o u n d 340 K; at 410 K we measured T~ = 0 . 2 s. The other samples exhibit about the same values at temperatures above 340 K; no m e a s u r e m e n t s were done at lower temperatures. F o r T2 no clear pattern o f t e m p e r a t u r e dependence could be detected. F o r the pure sample, 1"2 increases from 6.6 ms at 334 K to 83 ms at 406 K. F o r the other samples, the t e m p e r a t u r e d e p e n d e n c e o f T2 seems to be weaker with T2 values a r o u n d 25 ms. F o r 7Li in the filler y-LiAIO2, we measured T~ = 9 s at r o o m t e m p e r a t u r e which is in accord with data o f Follsteadt and Biefeld [9]. F o r the linewidth o f the filler Li signal we o b t a i n e d about 10 kHz; line narrowing sets in a r o u n d 300°C [9]. Since in the
1105
p o l y m e r measurements the Li F I D signals were recorded with rapid accumulation and the T, data were obtained after saturating the Li spectrum, the long T~ o f the filler Li guarantees that we only measured properties o f the p o l y m e r Li. The interesting result o f the linewidth measurements is that line narrowing is observed in a temperature range that is about the same for all samples. The underlying mechanism for this narrowing is most likely local m o v e m e n t o f Li ions, for instance between different voids in the p o l y m e r framework. We then conclude that the local dynamics o f the Li ions, in particular the Li mobility, is not changed by adding the 7-LiAIO2 filler. This in turn supports the idea that the enhancement o f conductivity by adding a filler is caused by stabilizing and increasing the fraction o f the a m o r p h o u s phase in ( P E O ) s L i C 1 0 4 + x wt% y-LiA102.
Acknowledgement One o f us ( W . G . ) is grateful to the Schweizerischer N a t i o n a l f o n d s for financial support.
References [ 1] C.A. Vincent, Prog. Solid State Chem. 17 (1987) 145. [2] F. Croce, F. Bonino, S. Panero and B. Scrosati, Philos. Mag. B 59 (1989) 161. [3] F. Capuano, F. Croce and B. Scrosati, J. Electrochem. Soc. 138 (1991) 1918. [4] J.E. Weston and B.C.H. Steele, Solid State Ionics 7 (1982) 75. [ 5 ] W. Wieczorek, K. Such, H. Wyciglikand J. Ptocharski, Solid State Ionics 36 (1989) 255. [6] B.A. Boukamp, Solid State Ionics 20 (1986) 31. [7] W. Wieczorek, K. Such, J. Ptocharski and J. Przytuski, in: Proc. Second Intern. Symp. Polymer Electrolytes (ISPE-2), ed. B. Scrosati (Elsevier, London, 1990) p. 339. [8] N. Billon and J.M. Haudin, Ann. Chim. Ft. 15 (1990) 1. [9] D.M. Follstaedt and R.M. Biefeld, Phys. Rev. B 18 (1978) 5928.
Solid State Ionics 53-56 (1992} 1102-1105 Norlh-Holland
IOHICS
Comparison of NMR and conductivity in (PEO) 8LiC104+'f-LiA102 Wang Gang 1, j. Roos, D. Brinkmann 2 Physik-lnstitut, (,:niver~itdt Ziirich. 8001 Zurich. S~*itzerland
F. Capuano, F. Croce and B. Scrosati Dipartimenlo di Chimica Universita di Roma *'La Sapienza", O0185 Rome. Italy
Polymer electrolyteshave become materials of considerable interest due to the wide range of their possible applications. However. their significantlylow ionic conductivity at room temperature and their poor mechanical properties at temperatures above the crystalline-amorphousphase transition (around 65 °C) considerably limit their usage. Introducing an inert "filler" into the polymer electrolyte as a means for increasingtheir mechanicaland electrical properties, has been widely recognized. In this work we report the characteristics of (PEO) sLiC104+ x weight % y-LiA102with x = 0, 10, 20 and 30, by describing results obtained by nuclear magnetic resonance and conductivity studies.
I. Introduction Polymer electrolytes have become materials of considerable interest due to the wide range of their possible applications. However, their significantly low ionic conductivity at room temperature and their poor mechanical properties at temperatures above the crystalline-amorphous phase transition ( a r o u n d 65~C) considerably limit their usage [1 ]. Introducing an inert "filler" into the polymer electrolyte as a means for increasing their mechanical and electrical properties, has been widely recognized. For instance, (PEO)sLiCIO4 with inert fine-grade 7LiAIO2 has been found to be a very promising system [2]. Measurements of electrical conductivity, mechanical properties and differential scanning calorimetry ( D S C ) have been carried out [3]. It was found that the mechanical properties were improved [3,4] and that the ionic conductivity was enhanced [5]. The question arises whether the e n h a n c e m e n t of Permanent address: Institute of Physics, Academia Sinica, Beijing 100 080, China. : Authorto whom correspondence should be addressed.
conductivity is due to processes taking place at an atomic level or to semi-macroscopic effects like presewing the polymer amorphous phase. Nuclear magnetic resonance ( N M R ) can answer these questions. We therefore have studied 7Li N M R as a function of temperature in (PEO) 8LiCIO4 + x wt% y-LiAIO2 with x = O , 10, 20 and 30. We have measured the linewidth and the spin-lattice and s p i n - s p i n relaxation limes.
2. Experimental 2. I. Preparation q f f i / m s
All polymer electrolyte films were prepared in a glove box using an inert argon atmosphere. The appropriate weights of lithium perchlorate (from Fluka AG, dried in vacuum at 120°C for 48 h before use) and PEO (BDH, molecular weight over 5X 106 ) to yield a PEO: Li ratio o f 8 : l in the complex, were dissolved in carefully dried acetonitrile ( R P E ) . The solutions obtained were stirred for 20 h at room temperature in a stoppered flask. A known a m o u n t of 7-LiA102 powder ( ~ 1 ~tm particle size)
0167-2738/92/$ 05,00 © 1992 ElsevierScience Publishers B.V. All rights reserved.
I4( Gang et al. / NMR and conductivity in (PEO)sLiCTO~+ ~[-LiAIO:
was then added, the mixture was stirred continuously until complete homogenization had occurred. This mixture was cast on a flat polytetrafluoroethylene sheet and covered to allow slow evaporation of acetonitrile. Using this procedure, homogeneous films of the composite polymer electrolyte were obtained with a thickness of 50 to 100 lam and no optical evidence for powder agglomeration.
1103
EQUIVCRT code [6 ], thus yielding the equivalent circuit parameters of the samples.
3. Results and discussion
3.1. Conductivity 2.2. N M R m e a s u r e m e n t s
The 7Li experiments were performed using a standard pulse spectrometer and a magnetic field of2.11 T corresponding to a Larmor frequency of 34.977 M Hz. The free-induction decay (FID) and spin-echo signals were digitized by transient recorders, accumulated and then Fourier transformed by on-line computers. The linewidth reported in this paper is lhe full width at half height (FWHH) of the square root of the power spectrum of the FID. The spin-lattice relaxation time T~ data were obtained from the FID signal using the saturation method (a n / 2 pulse follows a comb of saturating n~ 2 pulses). The spin-spin relaxation time T2 was deduced from the spin-echo decay. Temperatures were controlled to within _+0.5 K. The samples consisted of dried membranes cut into small pieces and sealed in pyrex tubes under vacuum. 2. 3. Conductivity m e a s u r e m e n t s
The electrical properties of the electrolyte films were measured by ac impedance spectroscopy at frequencies ranging from 10 3 to 105 Hz, using a Solartron 1255 FRA and a Solartron 1286 ECI, both coupled to a IBM PS/2 PC. In order to keep the system's response under linear condition, the peak-topeak potential difference of the stimulating signal was always maintained below 20 inV. Two parallel plane stainless-steel metallographicgrade polished electrodes with a surface area of 1.13 cm 2 were used. The cells which were housed in a Buchi rood. T-51 oven in order to provide constant temperature, were kept under constant mechanical pressure by using spring loaded terminals. The impedance data were fitted by a NLLSQ analysis program utilizing a modified Levemberg-Marquardt algorithm, adapted from Boukamp's
A detailed discussion of the electrical conductivity measurements has been reported elsewhere [ 3 ]. Fig. 1 shows the temperature dependence of the conductivity of the four samples. Both the 10 and the 20 wt% composite samples exhibit conductivities which are higher than those of the pure PEO salt complex. This fact is even more remarkable if we consider that the conductivity data have not been corrected for the "dilution effect", i.e. the decrease of the charge carrier concentration, in the constant volume samples, by the introduction of the ceramic filler particles. This enhancement of the conductivity has already been reported in literature [7] and is most probably due to an increase of the content of the amorphous phase of the composite polymer electrolyte. In other words, the conductivity of the composite samples is the result of two opposing effects: the ~'dilution" caused by the presence of the dispersoids depresses the ion transport while the stabilization of the amorphous phase enhances the ion transport. As a matter of fact, plotting conductivity versus fractional content of the ceramic filler yields the expected "bell"-shaped curve (fig. 2) with a maximum around 10 wt%. From a microscopic point of view, the recrystallization process may be regarded as a rearrangement of the polymer chains leading to a more ordered and stable state. The presence of dispersoid particles can hinder this process provided the particles are distributed homogeneously and their dimensions are comparable to the polymer chain length [8]. As the concentration of the ceramic powder increases, however, the ],-LiAIO2 particles start to agglomerate thus leading to regions free of the filler where recrystallization can readily occur.
1104
W. Gang et al. / NMR and conductivity in (PEO)sLiCIO4 + "~-LiAlOe
u
4
',',';5
Q~- :0~.
e G
I(
CS4
>
o
C, ..~ 0
lr
(: ; b
I u
:L' 0 /
1
AD 2 ~, ,0
"i,
'r :~
1000.",
;
I
K
L)
a ,
I
Fig. 1. Temperaturedependence of ionic conductivity of (PEO)8LiCIO4+ #-LiAIO2for various concentrations of y-LiAIO2: (©) 0 wt%, ( 0 ) 10wt%, ( A ) 20 wt%, ( A ) 30 wl%.
J
> 4-~ o
• '3
•
:'? '
-i-
,,O o
J
0
Z
(}
10
2,'~
ComposiLion,
5!},
4 ~)
wt %
Fig. 2. Composition dependence of the normalized ionic conductivity (a x% sample/a 0% sample) of (PEO)sLiCIO4+7-LiA102 at 60°C.
3.2, N M R Fig. 3 shows the temperature dependence o f the 7Li linewidth. All four samples we have investigated exhibited a broad signal at low and a narrow signal at high temperature and a transition region around 270 K where the broad line narrows. In this transi-
tion region, the Li signal consists of a broad line with a narrower line superimposed. For determining the F W H H we used the baseline o f the broad signal. However, using width values at other heights o f the signal, resulted in the same temperature dependence. In order to make sure that the Li signals we have recorded do not arise from Li nuclei of the filler, we
W. Gang et al. / N M R and conductivity in (PEO)sLiCI04+ 7-LiAl02 I
r
I
I
16
"~ 12 -1-2~
0
•
JZ
"- -"8-
__1
" ' \ " xx
~,', •
%, i ','a 6',',
4
',~,~
0 '160
I
200
I
/
240 280 Temperature (K)
.. [
320
360
Fig. 3. Temperature dependence of the 7Li NMR linewidth of the polymer Li in (PEO)sLiCIO4+7-LiAIO2: ( O ) 0 wt%, (@) l0 wt%, (A) 20 wt%, (A) 30 wt%. have d e t e r m i n e d the following parameters: ( i ) the spin-lattice and s p i n - s p i n relaxation time Tl and T2, respectively, in ( P E O ) 8LiC104 + x wt% y-LiA102; (ii) the linewidth and T, in the filler 7-LiA102. T~ o f 7Li in the PEO c o m p o u n d depends only very weakly on temperature. For ( P E O ) 8LiCIO4 + 20 wt% 7-LiAIO2 a b r o a d m i n i m u m o f about 0.1 s occurs a r o u n d 340 K; at 410 K we measured T~ = 0 . 2 s. The other samples exhibit about the same values at temperatures above 340 K; no m e a s u r e m e n t s were done at lower temperatures. F o r T2 no clear pattern o f t e m p e r a t u r e dependence could be detected. F o r the pure sample, 1"2 increases from 6.6 ms at 334 K to 83 ms at 406 K. F o r the other samples, the t e m p e r a t u r e d e p e n d e n c e o f T2 seems to be weaker with T2 values a r o u n d 25 ms. F o r 7Li in the filler y-LiAIO2, we measured T~ = 9 s at r o o m t e m p e r a t u r e which is in accord with data o f Follsteadt and Biefeld [9]. F o r the linewidth o f the filler Li signal we o b t a i n e d about 10 kHz; line narrowing sets in a r o u n d 300°C [9]. Since in the
1105
p o l y m e r measurements the Li F I D signals were recorded with rapid accumulation and the T, data were obtained after saturating the Li spectrum, the long T~ o f the filler Li guarantees that we only measured properties o f the p o l y m e r Li. The interesting result o f the linewidth measurements is that line narrowing is observed in a temperature range that is about the same for all samples. The underlying mechanism for this narrowing is most likely local m o v e m e n t o f Li ions, for instance between different voids in the p o l y m e r framework. We then conclude that the local dynamics o f the Li ions, in particular the Li mobility, is not changed by adding the 7-LiAIO2 filler. This in turn supports the idea that the enhancement o f conductivity by adding a filler is caused by stabilizing and increasing the fraction o f the a m o r p h o u s phase in ( P E O ) s L i C 1 0 4 + x wt% y-LiA102.
Acknowledgement One o f us ( W . G . ) is grateful to the Schweizerischer N a t i o n a l f o n d s for financial support.
References [ 1] C.A. Vincent, Prog. Solid State Chem. 17 (1987) 145. [2] F. Croce, F. Bonino, S. Panero and B. Scrosati, Philos. Mag. B 59 (1989) 161. [3] F. Capuano, F. Croce and B. Scrosati, J. Electrochem. Soc. 138 (1991) 1918. [4] J.E. Weston and B.C.H. Steele, Solid State Ionics 7 (1982) 75. [ 5 ] W. Wieczorek, K. Such, H. Wyciglikand J. Ptocharski, Solid State Ionics 36 (1989) 255. [6] B.A. Boukamp, Solid State Ionics 20 (1986) 31. [7] W. Wieczorek, K. Such, J. Ptocharski and J. Przytuski, in: Proc. Second Intern. Symp. Polymer Electrolytes (ISPE-2), ed. B. Scrosati (Elsevier, London, 1990) p. 339. [8] N. Billon and J.M. Haudin, Ann. Chim. Ft. 15 (1990) 1. [9] D.M. Follstaedt and R.M. Biefeld, Phys. Rev. B 18 (1978) 5928.