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Electrochemical doping of polyparaphenylene with alkali metals in solid state cells
Solid State lonics 40/41 (1990) 985-987 North-Holland
ELECTROCHEMICAL DOPING OF POLYPARAPHENYLENE WITH ALKALI METALS IN SOLID STATE CELLS C. HI~ROLD, D. B I L L A U D Laboratoire de Chimie Min~rale Appliquke, UA-CNRS 158, Universitb de Nancy, L B.P. 239, 54506 Vandoeuvre-les-Nancy, France
and R. Y A Z A M I Laboratoire d'lonique et d'Electrochimie du Solide de Grenoble URA-CNRS D-1213, Ecole Nationale SupPrieure d'Electrochimie et d'Electrom~tallurgie de Grenoble, B.P. 75, 38402 St. Martin d'Hbres, France
Very thin films of polyparaphenylene are doped by Li +, Na + and K + by electrochemical means in PEO based solid state cells. The doping and undoping proceed in successive steps which should result from reversible phases formation. High doping levels were achieved and the doped form of polyparaphenylene were found stable.
I. Introduction
P o l y p a r a p h e n y l e n e ( P P P ) can be d o p e d by electron d o n o r s (n-type d o p i n g ) such as alkali metals [ 1,2 ] or t e t r a b u t y l a m m o n i u m [ 3,4] using an electrochemical technique in liquid electrolyte m e d i u m . Unlike heterocyclic polymers such as polypyrrole and polyaniline who are d e g r a d e d by the n-doping, P P P shows a good stability a n d can i n c o r p o r a t e a high a m o u n t o f dopant. On the o t h e r h a n d the slow diffusion o f the guest species within the p o l y m e r network being the limiting step in the h o m o g e n i z a t i o n process, the choice o f the electrolyte a n d the host p o l y m e r film thickness play a m a j o r role in the electrode reaction. In liquid electrolyte, inserted cation can be solvated by neutral molecules which increase their a p p a r e n t size a n d weight a n d thus reduce the diffusion rate a n d the d o p i n g deepness. In addition, an e n h a n c e m e n t o f the h o m o g e n e i t y in the d o p e d polymer is expected by using very thin films and thus makes it possible to d e t e r m i n e some t h e r m o d y n amical d a t a relative to the d o p i n g reaction. In this work, P P P films in the o r d e r o f 1000 A in thickness are d o p e d by Li ÷, N a + a n d K ÷ in electro0167-2738/90/$ 03.50 © Elsevier Science Publishers B.V. ( North-Holland )
chemical cells with polyethylene oxide ( P E O ) as solvating polymer electrolyte. Since the alkali cation M ÷ moves in PEO free from any solvent molecule, only binary c o m p o u n d s can be f o r m e d according to the scheme ( C6 H 4 ) .,: + x y M + + x y e - ~ (My+ (C6H4) - v )x .
(1)
We particularly focused on the reversibility o f the above reaction and t e m p t e d to extract some therm o d y n a m i c a l d a t a relative to the phase f o r m a t i o n in the case o f sodium.
2. Experimental
P P P films with thickness in the o r d e r o f 1000 A were grown by electrochemical p o l y m e r i z a t i o n on a stainless steel electrode having 20 m m on d i a m e t e r according to the F a u v a r q u e m e t h o d [5]. The elect r o d e was then m o u n t e d in a b u t t o n type cell using a P E O - L i X film o f 200 Jam in thickness. A freshly cut alkali metal was then pressed on a stainless steel disc o f 17 m m on d i a m e t e r and constituted the negative pole o f the battery. Three types o f cells were
986
C. tt('rold et al. / Electrochemical doping of polvparaphenylene
studied corresponding to the following chains: l{J
( - )st. s t e e l / M / P ( E O ) , , M X / P P P / s t ,
steel( + ) ,
where M = L i +, n = 8 and X = C 1 0 4 ; M = N a + or K +, n = 12 and X = CF3SO3. Cells were assembled u n d e r high purity argon atmosphere, then transferred into a furnace where a p e r m a n e n t flow o f dry argon was maintained. The t e m p e r a t u r e was adjusted to 80°C_+ I : C for Li and Na based systems and to 5 9 ~ C _+ 1 °C for the K based one. At that temperature, the conductivity o f the P E O - L i X films is 7 × 1 0 5, 3 . 3 × 1 0 - 4 a n d l . 3 × 1 0 -3 S c m - ~ for the Li, N a and K based systems respectively [ 6 ]. D o p i n g and u n d o p i n g reactions were studied by cyclic v o l t a m m e t r y with sweeping rate ranging from 0.02 to 1 m V / s .
i
.
.
.
.
7
i
Iv
+ "~,,, [ {
': i )
- I(}
i' 0 l!,
' A]
0
B'
A'
+
2
3. Results and discussion Fig. 1 displays the steady state cyclic v o l t a m m o grams o b t a i n e d after m a n y cycles with the Li ( a ) , N a ( b ) and K (c) based systems. The respective voltage sweeping rates were 0.15, 1 a n d 0.02 mV s and were chosen to m a k e possible a good observation o f current peaks. W h e n the cell potential is decreased from its initial OCV value i.e. 2 V versus L i ÷ / L i in ( a ) , 1.2 V versus N a + / N a in ( 2 ) ) and 1.2 5 V versus K ÷ / K in (c), successive doping peaks (A, B and C (for K o n l y ) ) are observed. Their associated u n d o p i n g peaks A ' , B' and C' (for K ) appear when the potential is reincreased. A c o m p a r a tive study o f the relative a m o u n t o f electricity involved in each step o f the d o p i n g / u n d o p i n g process ( y in eq. ( 1 ) ) is quite difficult since there rem a i n s an incertitude on the film thickness. H o w e v e r the shape o f peaks and their relative intensities vary from one system to the other. Sharper peaks are observed with the N a - b a s e d system (fig. l b ) indicating a higher diffusion rate o f this metal in P P P compared to Li and K. This should result from a better c o m b i n a t i o n o f the weight and the polarizing power o f N a ÷. On the other hand, the total a m o u n t o f intercalated N a + in the first and the second steps (A and B peaks respectively) are in the same o r d e r o f magnitude. This strongly suggests that the doping proceeds through "stage" f o r m a t i o n in a similar way
-50
i
(tJA)
A'
~
,
2
#
e
(V
v n .
K
/K I
-1 -2 -3
Fig. 1. Cyclovoltammograms obtained with (a) Li/ P(EO)sLiCIO4/PPP at 80°C under 0.15 mV s-~; (b) Na/ P(OE)j2NaCF3SO3/PPP at 80°C under 1 mV s -~ and (c) K/ P(EO) ~2KCF3SO3/PPP at 59°C under 0.02 mV s-~. Successive doping peaks (A, B and C (in K system) ) and undoping peaks (A', B' and C' (in K system ) ) are observed associated with possible phases formation.
987
C. H&ald et al. /Electrochemical doping of polyparaphenylene
Table 1 Successive peaks voltage during the doping (A, B and C) and corresponding undoping (A', B' and C' ) of PPP by Li, Na and K. The average values (e) are also reported. Alkali metal
Potentials (V versus M÷/ M ) e(A) e(A') (e)
e(B)
e(B')
(e)
e(C)
e(C')
(e)
Li Na K
0.800 0.450 0.800
0.175 0.100 0.500
0.375 0.300 0.850
0.275 0.200 0.675
0.300
-
-
0.400
0.350
1.200 0.900 1.300
1.00 0.675 1.05
than in the graphite c o m p o u n d s using comparable electrochemical cells [ 7 ]. Assuming the richest phase (or stage 1 ) that of the composition (Nao.sC6H4)x [8], the associated stage 2 would correspond to (Nao.25C6H4)x. Li (fig. l a ) and K (fig. l b ) based systems display much more broad peaks which makes their attribution to stages of formation more difficult. However in all these systems, noteworthy is the reproducibility of cyclic v o l t a m m o g r a m s over more than ten cycles and the stability of the doped forms of P P P even at a high degree of doping a n d at the low potentials (i.e. 0V versus M ÷ / M ) . In table 1 are reported the potentials of each peak and their calculated average value ( e ) which, in a first a p p r o x i m a t i o n can be considered as the therm o d y n a m i c a l OCV corresponding to the associated d o p i n g / u n d o p i n g steps. This a s s u m p t i o n is reasonable considering the good reversibility of the electrode reactions particularly in the Na-based system as discussed above. In this case, the stage formation relative to A / A ' and B / B ' peaks should correspond to the equilibria (2) and (3) respectively:
4. C o n c l u s i o n s
We have showed that PPP is reversibly doped by Li +, Na + and K + in solid state cells operated at 60 or 80 ° C. The slow scan cyclic voltammetry allowed the observation of successive steps in the doping and u n d o p i n g process. These steps were associated to the formation of intercalation "stages" by analogy to graphite compounds. Further investigations are needed to confirm this aspect particularly by in situ structure determination. The Na-based system led to the most accurate electrochemical measurements allowing some t h e r m o d y n a m i c a l data to be achieved. The good stability, the high doping level in the PPP and the low potential versus M ÷ / M reached during the richest phase formation increase its potential application as negative electrode in solid state alkali metals secondary batteries.
References
(C6 H4) x + 0.25Na + + 0 . 2 5 e - ~ (Nao.25C6 H4)x •
(2) and (Na0.25C6H 4 )x + 0.2 5Na + + 0 . 2 5 e - ~ (Nao.sC6H4)x. The ( e ( ~ / 2 ) ) and ( e ( 2 / 1 ) ) being respectively 0.675 V and 0.200 V, the deduced free energy variation can be d e t e r m i n e d using the classical relation AG=-nF(e) and leads to A G ( ~ / 2 ) = - 6 5 . 1 kJ (mol N a ) -~ for the stage 2 formation a n d A G ( 2 / 1 ) = - 19.3 kJ (mol N a ) - ' for the stage 1 at 80°C. These values are in agreement with those o b t a i n e d with the same system from the galvanostatic titration curves and are presented elsewhere [8].
[ 1] L.W. Shacklette,J.E. Toth, N.S. Murphy and R.H. Baughman, J. Electrochem. Soc. 132 (1985) 1529. [2] T.R. Jow, L.W. Shacklette, M. Maxfield and D. Vernick, J. Electrochem. Soc. 134 (1987) 1730. [3] G. Foyer, Y. Pelous, F. Maurice, M.A. Petit, A. Digua and J.F. Fauvarque, Synth. Met. 21 (1987) 241. [4] M. Satoh, M. Tabata, F. Uesegi, K. Kaneto and K. Yoshino, Synth. Met. 17 (1987) 583. [5]J.F. Fauvarque, M.A. Petit, A. Digua and G. Foyer, Makromol. Chem. 188 ( 1987 ) 1833. [6 ] E.A. Rietmann, M.L. Kaplanand R.J. Cava, SolidState lonics 17 (1985) 67; 25 (1987) 41. [7] R. Yazami and Ph. Touzain, J. Power Sources 9 (1983) 365. [ 8 ] H. Fark, J. Fink, B. Scheerer, M. Stamm and B. Thieke, Synth. Met. 17 (1987) 583. [9] C. Herold, R. Yazami and D. Billaud, C.R. Acad. Sci. Paris, to be published.
ELECTROCHEMICAL DOPING OF POLYPARAPHENYLENE WITH ALKALI METALS IN SOLID STATE CELLS C. HI~ROLD, D. B I L L A U D Laboratoire de Chimie Min~rale Appliquke, UA-CNRS 158, Universitb de Nancy, L B.P. 239, 54506 Vandoeuvre-les-Nancy, France
and R. Y A Z A M I Laboratoire d'lonique et d'Electrochimie du Solide de Grenoble URA-CNRS D-1213, Ecole Nationale SupPrieure d'Electrochimie et d'Electrom~tallurgie de Grenoble, B.P. 75, 38402 St. Martin d'Hbres, France
Very thin films of polyparaphenylene are doped by Li +, Na + and K + by electrochemical means in PEO based solid state cells. The doping and undoping proceed in successive steps which should result from reversible phases formation. High doping levels were achieved and the doped form of polyparaphenylene were found stable.
I. Introduction
P o l y p a r a p h e n y l e n e ( P P P ) can be d o p e d by electron d o n o r s (n-type d o p i n g ) such as alkali metals [ 1,2 ] or t e t r a b u t y l a m m o n i u m [ 3,4] using an electrochemical technique in liquid electrolyte m e d i u m . Unlike heterocyclic polymers such as polypyrrole and polyaniline who are d e g r a d e d by the n-doping, P P P shows a good stability a n d can i n c o r p o r a t e a high a m o u n t o f dopant. On the o t h e r h a n d the slow diffusion o f the guest species within the p o l y m e r network being the limiting step in the h o m o g e n i z a t i o n process, the choice o f the electrolyte a n d the host p o l y m e r film thickness play a m a j o r role in the electrode reaction. In liquid electrolyte, inserted cation can be solvated by neutral molecules which increase their a p p a r e n t size a n d weight a n d thus reduce the diffusion rate a n d the d o p i n g deepness. In addition, an e n h a n c e m e n t o f the h o m o g e n e i t y in the d o p e d polymer is expected by using very thin films and thus makes it possible to d e t e r m i n e some t h e r m o d y n amical d a t a relative to the d o p i n g reaction. In this work, P P P films in the o r d e r o f 1000 A in thickness are d o p e d by Li ÷, N a + a n d K ÷ in electro0167-2738/90/$ 03.50 © Elsevier Science Publishers B.V. ( North-Holland )
chemical cells with polyethylene oxide ( P E O ) as solvating polymer electrolyte. Since the alkali cation M ÷ moves in PEO free from any solvent molecule, only binary c o m p o u n d s can be f o r m e d according to the scheme ( C6 H 4 ) .,: + x y M + + x y e - ~ (My+ (C6H4) - v )x .
(1)
We particularly focused on the reversibility o f the above reaction and t e m p t e d to extract some therm o d y n a m i c a l d a t a relative to the phase f o r m a t i o n in the case o f sodium.
2. Experimental
P P P films with thickness in the o r d e r o f 1000 A were grown by electrochemical p o l y m e r i z a t i o n on a stainless steel electrode having 20 m m on d i a m e t e r according to the F a u v a r q u e m e t h o d [5]. The elect r o d e was then m o u n t e d in a b u t t o n type cell using a P E O - L i X film o f 200 Jam in thickness. A freshly cut alkali metal was then pressed on a stainless steel disc o f 17 m m on d i a m e t e r and constituted the negative pole o f the battery. Three types o f cells were
986
C. tt('rold et al. / Electrochemical doping of polvparaphenylene
studied corresponding to the following chains: l{J
( - )st. s t e e l / M / P ( E O ) , , M X / P P P / s t ,
steel( + ) ,
where M = L i +, n = 8 and X = C 1 0 4 ; M = N a + or K +, n = 12 and X = CF3SO3. Cells were assembled u n d e r high purity argon atmosphere, then transferred into a furnace where a p e r m a n e n t flow o f dry argon was maintained. The t e m p e r a t u r e was adjusted to 80°C_+ I : C for Li and Na based systems and to 5 9 ~ C _+ 1 °C for the K based one. At that temperature, the conductivity o f the P E O - L i X films is 7 × 1 0 5, 3 . 3 × 1 0 - 4 a n d l . 3 × 1 0 -3 S c m - ~ for the Li, N a and K based systems respectively [ 6 ]. D o p i n g and u n d o p i n g reactions were studied by cyclic v o l t a m m e t r y with sweeping rate ranging from 0.02 to 1 m V / s .
i
.
.
.
.
7
i
Iv
+ "~,,, [ {
': i )
- I(}
i' 0 l!,
' A]
0
B'
A'
+
2
3. Results and discussion Fig. 1 displays the steady state cyclic v o l t a m m o grams o b t a i n e d after m a n y cycles with the Li ( a ) , N a ( b ) and K (c) based systems. The respective voltage sweeping rates were 0.15, 1 a n d 0.02 mV s and were chosen to m a k e possible a good observation o f current peaks. W h e n the cell potential is decreased from its initial OCV value i.e. 2 V versus L i ÷ / L i in ( a ) , 1.2 V versus N a + / N a in ( 2 ) ) and 1.2 5 V versus K ÷ / K in (c), successive doping peaks (A, B and C (for K o n l y ) ) are observed. Their associated u n d o p i n g peaks A ' , B' and C' (for K ) appear when the potential is reincreased. A c o m p a r a tive study o f the relative a m o u n t o f electricity involved in each step o f the d o p i n g / u n d o p i n g process ( y in eq. ( 1 ) ) is quite difficult since there rem a i n s an incertitude on the film thickness. H o w e v e r the shape o f peaks and their relative intensities vary from one system to the other. Sharper peaks are observed with the N a - b a s e d system (fig. l b ) indicating a higher diffusion rate o f this metal in P P P compared to Li and K. This should result from a better c o m b i n a t i o n o f the weight and the polarizing power o f N a ÷. On the other hand, the total a m o u n t o f intercalated N a + in the first and the second steps (A and B peaks respectively) are in the same o r d e r o f magnitude. This strongly suggests that the doping proceeds through "stage" f o r m a t i o n in a similar way
-50
i
(tJA)
A'
~
,
2
#
e
(V
v n .
K
/K I
-1 -2 -3
Fig. 1. Cyclovoltammograms obtained with (a) Li/ P(EO)sLiCIO4/PPP at 80°C under 0.15 mV s-~; (b) Na/ P(OE)j2NaCF3SO3/PPP at 80°C under 1 mV s -~ and (c) K/ P(EO) ~2KCF3SO3/PPP at 59°C under 0.02 mV s-~. Successive doping peaks (A, B and C (in K system) ) and undoping peaks (A', B' and C' (in K system ) ) are observed associated with possible phases formation.
987
C. H&ald et al. /Electrochemical doping of polyparaphenylene
Table 1 Successive peaks voltage during the doping (A, B and C) and corresponding undoping (A', B' and C' ) of PPP by Li, Na and K. The average values (e) are also reported. Alkali metal
Potentials (V versus M÷/ M ) e(A) e(A') (e)
e(B)
e(B')
(e)
e(C)
e(C')
(e)
Li Na K
0.800 0.450 0.800
0.175 0.100 0.500
0.375 0.300 0.850
0.275 0.200 0.675
0.300
-
-
0.400
0.350
1.200 0.900 1.300
1.00 0.675 1.05
than in the graphite c o m p o u n d s using comparable electrochemical cells [ 7 ]. Assuming the richest phase (or stage 1 ) that of the composition (Nao.sC6H4)x [8], the associated stage 2 would correspond to (Nao.25C6H4)x. Li (fig. l a ) and K (fig. l b ) based systems display much more broad peaks which makes their attribution to stages of formation more difficult. However in all these systems, noteworthy is the reproducibility of cyclic v o l t a m m o g r a m s over more than ten cycles and the stability of the doped forms of P P P even at a high degree of doping a n d at the low potentials (i.e. 0V versus M ÷ / M ) . In table 1 are reported the potentials of each peak and their calculated average value ( e ) which, in a first a p p r o x i m a t i o n can be considered as the therm o d y n a m i c a l OCV corresponding to the associated d o p i n g / u n d o p i n g steps. This a s s u m p t i o n is reasonable considering the good reversibility of the electrode reactions particularly in the Na-based system as discussed above. In this case, the stage formation relative to A / A ' and B / B ' peaks should correspond to the equilibria (2) and (3) respectively:
4. C o n c l u s i o n s
We have showed that PPP is reversibly doped by Li +, Na + and K + in solid state cells operated at 60 or 80 ° C. The slow scan cyclic voltammetry allowed the observation of successive steps in the doping and u n d o p i n g process. These steps were associated to the formation of intercalation "stages" by analogy to graphite compounds. Further investigations are needed to confirm this aspect particularly by in situ structure determination. The Na-based system led to the most accurate electrochemical measurements allowing some t h e r m o d y n a m i c a l data to be achieved. The good stability, the high doping level in the PPP and the low potential versus M ÷ / M reached during the richest phase formation increase its potential application as negative electrode in solid state alkali metals secondary batteries.
References
(C6 H4) x + 0.25Na + + 0 . 2 5 e - ~ (Nao.25C6 H4)x •
(2) and (Na0.25C6H 4 )x + 0.2 5Na + + 0 . 2 5 e - ~ (Nao.sC6H4)x. The ( e ( ~ / 2 ) ) and ( e ( 2 / 1 ) ) being respectively 0.675 V and 0.200 V, the deduced free energy variation can be d e t e r m i n e d using the classical relation AG=-nF(e) and leads to A G ( ~ / 2 ) = - 6 5 . 1 kJ (mol N a ) -~ for the stage 2 formation a n d A G ( 2 / 1 ) = - 19.3 kJ (mol N a ) - ' for the stage 1 at 80°C. These values are in agreement with those o b t a i n e d with the same system from the galvanostatic titration curves and are presented elsewhere [8].
[ 1] L.W. Shacklette,J.E. Toth, N.S. Murphy and R.H. Baughman, J. Electrochem. Soc. 132 (1985) 1529. [2] T.R. Jow, L.W. Shacklette, M. Maxfield and D. Vernick, J. Electrochem. Soc. 134 (1987) 1730. [3] G. Foyer, Y. Pelous, F. Maurice, M.A. Petit, A. Digua and J.F. Fauvarque, Synth. Met. 21 (1987) 241. [4] M. Satoh, M. Tabata, F. Uesegi, K. Kaneto and K. Yoshino, Synth. Met. 17 (1987) 583. [5]J.F. Fauvarque, M.A. Petit, A. Digua and G. Foyer, Makromol. Chem. 188 ( 1987 ) 1833. [6 ] E.A. Rietmann, M.L. Kaplanand R.J. Cava, SolidState lonics 17 (1985) 67; 25 (1987) 41. [7] R. Yazami and Ph. Touzain, J. Power Sources 9 (1983) 365. [ 8 ] H. Fark, J. Fink, B. Scheerer, M. Stamm and B. Thieke, Synth. Met. 17 (1987) 583. [9] C. Herold, R. Yazami and D. Billaud, C.R. Acad. Sci. Paris, to be published.