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polymer rechargeable lithium solid-state cell
Svltthetic Metals. 28 (1989) C663 C668
ELECTROCHEMICAL LITHIUM
CHARACTERIZATION
SOLID-STATE
C663
OF A P O L Y M E R / P O L Y M E R
RECHARGEABLE
CELL.
C. A R B I Z Z A N I , M. M A S T R A G O S T I N O D i p a r t i m e n t o di Chimica "G. Ciamician", U n i v e r s i t ~ di Bologna, Italy S. P A N E R O , P. P R O S P E R I , B. SCROSATI D i p a r t i m e n t o di Chimica, U n i v e r s i t ~ di Roma "La Sapienza", Italy
ABSTRACT The characteristics of s o l i d -s t a t e lithium cells, using (PEO)20 LiCIO 4 as e l e c t r o l y t e and the e l e c t r o s y n t h e s i z e d p o l y m e r s polypyrrole, p o l y b i t h i o p h e n e and p o l y d i t h i e n o t h i o p h e n e as p o s i t i v e electrodes, were investigated by cyclic voltammetry, charged i s c h a r g e g a l v a n o s t a t i c cycles and a.c. impedance s p e c t r o s c o p y . T h e e x p e r i m e n t a l data suggest that s o l i d - s t a t e p o l y m e r i c systems can be e m p l o y e d in u l t r a t h i n b a t t e r i e s at 70°C o p e r a t i n g temperature.
INTRODUCTION Much attention is currently focused on the use of polyheterocyclic conducting polymers, like polypyrrole, p o l y t h i o p h e n e and their derivatives, as p o s i t i v e electrodes in lithium, liquid organic e l e c t r o l y t e r e c h e a r g e a b l e batteries EI,2~. D e s p i t e the good c y c l a b i l i t y of these polymer electrodes, the d i f f u s i o n of the c o u n t e r i o n s in the bulk of the polymer for maintaining e l e c t r o n e u t r a l i t y is an intrinsic l i m i t a t i o n in the d e s i g n of high-rate, h i g h - p o w e r batteries. Accordingly, it seems more p r o m i s i n g to direct the a p p l i c a t i o n of these polymers to the d e v e l o p m e n t of l o w - r a t e , s m a l l - s i z e batteries. In this connection, it would be desirable to replace the liquid with solid electrolyte, as an all solid-state cell would assure higher r e l i a b i l i t y and versatility. A solid p o l y m e r electrolyte, such as the complexes of polyeth y l e n e oxide (PEO) with lithium salts (e.g. LiCiO 4 ) w h i c h feature ionic c o n d u c t i v i t y with both anion and cation transport [3,42 , is p o t e n t i a l l y useful for advanced, t h i n - l a y e r designs of l i t h i u m b a t t e r i e s with c o n d u c t i n g polymers as p o s i t i v e electrodes. Some results of s o l i d - s t a t e lithium b a t t e r i e s with PEO-lithium salts e l e c t r o l y t e s and p o l y p y r r o l e have been r e p o r t e d [5,6~. At the present time the l i m i t a t i o n of these new s o l i d - s t a t e lithium b a t t e r i e s resides in the fact that they operate only above room temperature. The c o n d u c t i v i t y of " c o n v e n t i o n a l " P E O - l i t h i u m salt e l e c t r o l y t e s becomes a p p r e c i a b l e beyond 60°C, i.e. after t r a n s i t i o n in an amorphous phase D , 4 ~ . 0379-6779/89/$3~50
© Elsevier Sequoia/Printed in The Netherlands
C664
The data from a c o m p a r a t i v e study of c y c l a b i l i t y and s t a b i l i t y of s o l i d - s t a t e cells using l i t h i u m as anode, (PEO)20 LiCIO 4 as e l e c t r o l y t e , and p o l y p y r r o l e (pPy), p o l y 2 , 2 ' b i t h i o p h e n e (pBT) and polydithieno(3,2-b;2',3'-d)thiophene (pDTT) as cathode, are r e p o r t e d and d i s c u s s e d herein. EXPERIMENTAL Dithienothiophene was synthesized as specified in ref.7. All the other c h e m i c a l c o m p o u n d s were r e a g e n t - g r a d e p r o d u c t s and p u r i f i e d b e f o r e use. The p o l y m e r films were galvanostatically grown on stainless steel e l e c t r o d e s (0.385 cm 2) in d e g a s s e d cells with separated compartments. The e l e c t r o s y n t h e s i s c o n d i t i o n were: pPy in p r o p y l e n e c a r b o n a t e (PC) - LiCIO4 0.2 M - p y r r o l e 0.2 M at I=7 mA cm-2 (room t e m p e r a t u r e ) . pBT in a c e t o n i t r i l e (ACN)- LiCiO 4 0.5 M b i t h i o p h e n e I0 mM at I=1.6 mA cm-2 (room t e m p e r a t u r e ) . p D T T in m e t h y l e n e c h l o r i d e - t e t r a b u t h y l a m m o n i u m p e r c h l o r a t e (TBAP) 0.2 M - d i t h i e n o t h i o p h e n e 2 mM at I=0.4 mA c m - 2 ( T = 1 5 ° C ) . All the p o l y m e r films were g r o w n w i t h ca. 200 mC c m - 2 i n order to have c o m p a r a b l e coverage. The p r e p a r a t i o n of (PEO)20 LiCIO 4 polymer electrolyte is d e s c r i b e d in ref. 4. The s o l i d - s t a t e p o l y m e r cells were a s s e m b l e d in a dry b o x by sandwiching a Li disk, a thin layer (ca. 200 ffm) of e l e c t r o l y t e and the p o l y m e r films. The cells were h o u s e d in a h e r m e t i c a l l y s e a l e d teflon c o n t a i n e r and taken out of the dry box for the electrochemical measurements. All the e l e c t r o c h e m i c a l tests were c a r r i e d out at 7 0 ± 2 ° C (after 1 h to ensure t e m p e r a t u r e e q u i l i b r i u m in the container) w i t h A M E L instrumentation interfaced with a microcomputer. The impedance study was c a r r i e d out w i t h a S o l a r t r o n f r e q u e n c y r e s p o n s e a n a l y s e r (FRA), m o d . 1 2 8 6 . R E S U L T S AND D I S C U S S I O N Figure 1 shows the cyclic v o l t a m m e t r y curves (20th cycle) of pPy, pBT and p D T T p o l y m e r i c e l e c t r o d e s in s o l i d - s t a t e cells. The v a l u e s of the i n t e g r a t e d charge under oxidation and r e d u c t i o n waves i n d i c a t e d o p i n g level (y) (calculated by r e d u c t i o n waves) for pPy, pBT and pDTT of 15, 24, 26 ~ (per pyrrole, 2,2'bithiophene, dithienothiophene unity), respectively and c o u l o m b i c e f f i c i e n c i e s (~) of 0.91, 0.90, 0.99 r e s p e c t i v e l y . The c y c l a b i l i t y of these s o l i d - s t a t e cells is v e r y g o o d and compares well with that of polymer electrodes in liquid electrolytic medium as shown in figure 2, w h i c h i l l u s t r a t e s the v o l t a m m e t r i c curves (20th cycle) for the same p o l y m e r e l e c t r o d e m a t e r i a l s in L i C I O 4 - P C l i q u i d s o l u t i o n cells. The rates of the d o p i n g - u n d o p i n g p r o c e s s are s l i g h t l y lower in the s o l i d - s t a t e cells, w h i c h is p r o b a b l y due to a slower ion diffusion both in the polymer electrode and in the polymer electrolyte. Despite this, the cyclability of solid devices r e m a i n s very p r o m i s i n g . F i g u r e 3 shows the capacity expressed in mC c m ~ ( o b t a i n e d in discharge) and the efficiency of subsequent charge-discharge g a l v a n o s t a t i c cycles at I=0.13 mA cm -2 w i t h s o l i d - s t a t e cells h a v i n g pPy, pBT and p D T T as p o s i t i v e electrodes. The r e s u l t s show that the c y c l e - l i f e of these s o l i d - s t a t e cells is r e m a r k a b l y good, as are the v a l u e s of c a p a c i t y and e f f i c i e n c y .
C665
1=0.4
I mA cm - 2
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)
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13.5 mC cm -2
197
/
mC c m - 2 k J '
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Fig. i. C y c l i c voltammetry c u r v e s at v=50 mV/s in cells, p P y ( F = 9 . 5 . 1 0 - T m o l p y r r o l e cm -2) from 2.2 to ( F = 8 . 5 1 0 - 7 m o l b i t h i o p h e n e cm -2) from 2.2 to 4.3 (F=9.4 1 0 - 7 m o l d i t h i e n o t h i o p h e n e cm-2)from 2.2 to 4.3 vs Li. T = 7 0 ° C .
I1=0.4
1=0.4 m A cm -2
rnA cm -2
/
~
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j
otpBT3 /
solid-state 3.8 V; pBT V; pDTT V. V v a l u e s
mA c m - 2 /
pDTT/ "0 0
g
4,
V 0
r--
c-
16.6 mC cm-2
a
mCcm-2V
Fig. 2. C y c l i c v o l t a m m e t r y c u r v e s at v = 5 0 m V / s in P C - L i C I O 4 1 M. p P y (F=9.5 l0 -7 m o l cm -2) f r o m 2.2 to 3.8 V; y = 1 8 %; ~=0.99. pBT ( P = 8 . 5 " I 0 -7 m o l cm -2) from 2.2 to 4.3 V; y = 3 8 %; ~ = 0 . 9 6 . p D T T (F=9.4 i0 -7 m o l cm -2) f r o m 2.2 to 4.3 V; y = 4 8 %; ~ = 0 . 9 4 . V v a l u e s vs Li. R o o m t e m p e r a t u r e .
C666
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Fig. 3. C a p a c i t y (,) and e f f i c i e n c y (-) at d i f f e r e n t number of g a l v a n o s t a t i c cycles (I=0.13 mA cm-2). S o l i d - s t a t e cells with pPy (F=9.5 10-7mol cm -2) from 2.2 to 3.8 V, pBT (F=8.5.10 -7 mol cm -2 ) from 2.2 to 4.2 V, and pDTT (~=8.0 10-Tmol c m - 2 ) f r o m 2.2 to 4.3 V, respectively. V values vs Li. T=70°C.
C667
One m a j o r p r o b l e m of p o l y m e r e l e c t r o d e s in l i q u i d - e l e c t r o l y t e b a t t e r i e s is their poor charge-retention capability [8J. In our tests of s o l i d - s t a t e cells, the e l e c t r o d e s t a b i l i t y was e v a l u a t e d u t i l i s i n g the values of e f f i c i e n c y (~) o b t a i n e d at 70°C w i t h g a l v a n o s t a t i c cycles at 0.13 mA c m - 2 at d i f f e r e n t e l a p s e d time after i n i t i a l charge and the corresponding OCV values before discharge. The cells were stored at room temperature and e q u i l i b r a t e d at 70°C for 1 hour b e f o r e d i s c h a r g e . The results are s h o w n in table i.
TABLE i Shelf-life Polymer
pPy pBT pDTT
Fxl07
charge
initially OCV ~; mol c m - 2 m C cm -2 V 8.4 14.0 3.46 0.98 8.5 9.6 3.90 0.98 8.2 14.0 4.00 0.96
1 hour OCV ~; V 3.30 0.81 3.78 0.65 3.67 0.55
3 days OCV ~ V 3.16 0.64 3.78 0.70 3.74 0.59
1 week OCV ~; V 3.02 0.46 3.63 0.51 3.59 0.43
It is to be n o t e d that m u c h faster d i s c h a r g e was found w h e n the cells were c o n t i n u o u s l y s t o r e d at 70oC. The n a t u r e of the s e l f - d i s c h a r g e m e c h a n i s m is still unknown. It is o n l y p o s s i b l e to state that this m e c h a n i s m does not involve an irreversible degradation of the p o l y m e r electrodes, as the cells can always be r e c h a r g e d to their o r i g i n a l c a p a c i t y values and, if immediately discharged, they always deliver their original e f f i c i e n c y . Our r e s u l t s d e n o t e that the rate of the s e l f - d i s c h a r g e process increases with temperature. Therefore, storage at low t e m p e r a t u r e is always to be r e c o m m e n d e d . Further information on the kinetics of the electrochemical p r o c e s s e s of polymer electrodes may be obtained by impedance analysis. F i g u r e 4 illustrates the c o m p l e x i m p e d a n c e plots of a Li/(PEO)20 L i C I O 4 / p P y s o l i d - s t a t e cell o b t a i n e d in the c h a r g e d (A) and d i s c h a r g e d (B) states. On the basis of the c o m m o n l y accepted interpretation models ~ J , the Z°'-Z ' i m p e d a n c e plot in figure 4A reveals, in p a s s i n g from h i g h to low frequency: an intercept with a real axis, Re, which identifies the resistivity of the polymer electrolyte b e t w e e n the Li and pPy e l e c t r o d e s ;an a r c w h i c h may be a s s o c i a t e d w i t h the e l e c t r o d e interfaces; and a 45 ° line, w h i c h c o n f i r m s that the k i n e t i c s of the e l e c t r o c h e m i c a l p r o c e s s may be c o n t r o l l e d by ion d i f f u s i o n . As expected, in the d i s c h a r g e d state (figure 4B) the resistance of the polymer electrode increases and the impedance response tends to i n d i c a t e a c a p a c i t a n c e effect at low f r e q u e n c i e s as o f t e n found in p o l y m e r e l e c t r o d e s [10,8J.
C668
B
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400
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E
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r-
0
kJ I
I
200
./
200
f
f
"1
I
0
200
400
Z'/Ohm Fig. 4. C o m p l e x i m p e d a n c e plots of a Li/ at 70°C: A) Charged state; B) D i s c h a r g e d
0
200 Z ' / Ohm
(PEO)20LiC104 state.
/pPy cell
ACKNOWLEDGEMENTS The authors thank ENI for the R e s e a r c h F e l l o w s h i p and Prof. P. Ferloni for kindly p r o v i d i n g the (PEO)20 LiCIO 4 film. The r e s e a r c h was funded by a grant from CNR, Progetto Finalizzato E n e r g e t i c a 2 (n.86.00876.59/ 87.02265.59).
REFERENCES 1 R. Bittihn et al., Makrom. Chem,, Macrom. Symp., 8 (1987) 51 2 M. M a s t r a g o s t i n o et al., E l e c t r o c h i m i c a Acta, 32 (1987) 1589 3 M. Armand, Solid State Ionics, 9 & 10 (1983) 745 4 P. Ferloni et al., Solid State Ionics 18 & 19 (1986) 265 5 S. Panero et al., E l e c t r o c h i m i c a Acta, 32 (1987) 1461 6 P. N o v a k et al., J . - P o w e r Sources, 21 (1987) 17 7 F. de Jong et al., J. Org. Chem.,36 (1971) 1645 8 B. Scrosati, Progress in Solid State C h e m i s t r y 18 (1988) 1 9 J. Ross Macdonald, I m p e d a n c e Spectroscopy, John W i l e y and Sons, New York, 1987 i0 N. M e r m i l l o d et al., J. Electrochem. Soc.,133 (1986) 1073
ELECTROCHEMICAL LITHIUM
CHARACTERIZATION
SOLID-STATE
C663
OF A P O L Y M E R / P O L Y M E R
RECHARGEABLE
CELL.
C. A R B I Z Z A N I , M. M A S T R A G O S T I N O D i p a r t i m e n t o di Chimica "G. Ciamician", U n i v e r s i t ~ di Bologna, Italy S. P A N E R O , P. P R O S P E R I , B. SCROSATI D i p a r t i m e n t o di Chimica, U n i v e r s i t ~ di Roma "La Sapienza", Italy
ABSTRACT The characteristics of s o l i d -s t a t e lithium cells, using (PEO)20 LiCIO 4 as e l e c t r o l y t e and the e l e c t r o s y n t h e s i z e d p o l y m e r s polypyrrole, p o l y b i t h i o p h e n e and p o l y d i t h i e n o t h i o p h e n e as p o s i t i v e electrodes, were investigated by cyclic voltammetry, charged i s c h a r g e g a l v a n o s t a t i c cycles and a.c. impedance s p e c t r o s c o p y . T h e e x p e r i m e n t a l data suggest that s o l i d - s t a t e p o l y m e r i c systems can be e m p l o y e d in u l t r a t h i n b a t t e r i e s at 70°C o p e r a t i n g temperature.
INTRODUCTION Much attention is currently focused on the use of polyheterocyclic conducting polymers, like polypyrrole, p o l y t h i o p h e n e and their derivatives, as p o s i t i v e electrodes in lithium, liquid organic e l e c t r o l y t e r e c h e a r g e a b l e batteries EI,2~. D e s p i t e the good c y c l a b i l i t y of these polymer electrodes, the d i f f u s i o n of the c o u n t e r i o n s in the bulk of the polymer for maintaining e l e c t r o n e u t r a l i t y is an intrinsic l i m i t a t i o n in the d e s i g n of high-rate, h i g h - p o w e r batteries. Accordingly, it seems more p r o m i s i n g to direct the a p p l i c a t i o n of these polymers to the d e v e l o p m e n t of l o w - r a t e , s m a l l - s i z e batteries. In this connection, it would be desirable to replace the liquid with solid electrolyte, as an all solid-state cell would assure higher r e l i a b i l i t y and versatility. A solid p o l y m e r electrolyte, such as the complexes of polyeth y l e n e oxide (PEO) with lithium salts (e.g. LiCiO 4 ) w h i c h feature ionic c o n d u c t i v i t y with both anion and cation transport [3,42 , is p o t e n t i a l l y useful for advanced, t h i n - l a y e r designs of l i t h i u m b a t t e r i e s with c o n d u c t i n g polymers as p o s i t i v e electrodes. Some results of s o l i d - s t a t e lithium b a t t e r i e s with PEO-lithium salts e l e c t r o l y t e s and p o l y p y r r o l e have been r e p o r t e d [5,6~. At the present time the l i m i t a t i o n of these new s o l i d - s t a t e lithium b a t t e r i e s resides in the fact that they operate only above room temperature. The c o n d u c t i v i t y of " c o n v e n t i o n a l " P E O - l i t h i u m salt e l e c t r o l y t e s becomes a p p r e c i a b l e beyond 60°C, i.e. after t r a n s i t i o n in an amorphous phase D , 4 ~ . 0379-6779/89/$3~50
© Elsevier Sequoia/Printed in The Netherlands
C664
The data from a c o m p a r a t i v e study of c y c l a b i l i t y and s t a b i l i t y of s o l i d - s t a t e cells using l i t h i u m as anode, (PEO)20 LiCIO 4 as e l e c t r o l y t e , and p o l y p y r r o l e (pPy), p o l y 2 , 2 ' b i t h i o p h e n e (pBT) and polydithieno(3,2-b;2',3'-d)thiophene (pDTT) as cathode, are r e p o r t e d and d i s c u s s e d herein. EXPERIMENTAL Dithienothiophene was synthesized as specified in ref.7. All the other c h e m i c a l c o m p o u n d s were r e a g e n t - g r a d e p r o d u c t s and p u r i f i e d b e f o r e use. The p o l y m e r films were galvanostatically grown on stainless steel e l e c t r o d e s (0.385 cm 2) in d e g a s s e d cells with separated compartments. The e l e c t r o s y n t h e s i s c o n d i t i o n were: pPy in p r o p y l e n e c a r b o n a t e (PC) - LiCIO4 0.2 M - p y r r o l e 0.2 M at I=7 mA cm-2 (room t e m p e r a t u r e ) . pBT in a c e t o n i t r i l e (ACN)- LiCiO 4 0.5 M b i t h i o p h e n e I0 mM at I=1.6 mA cm-2 (room t e m p e r a t u r e ) . p D T T in m e t h y l e n e c h l o r i d e - t e t r a b u t h y l a m m o n i u m p e r c h l o r a t e (TBAP) 0.2 M - d i t h i e n o t h i o p h e n e 2 mM at I=0.4 mA c m - 2 ( T = 1 5 ° C ) . All the p o l y m e r films were g r o w n w i t h ca. 200 mC c m - 2 i n order to have c o m p a r a b l e coverage. The p r e p a r a t i o n of (PEO)20 LiCIO 4 polymer electrolyte is d e s c r i b e d in ref. 4. The s o l i d - s t a t e p o l y m e r cells were a s s e m b l e d in a dry b o x by sandwiching a Li disk, a thin layer (ca. 200 ffm) of e l e c t r o l y t e and the p o l y m e r films. The cells were h o u s e d in a h e r m e t i c a l l y s e a l e d teflon c o n t a i n e r and taken out of the dry box for the electrochemical measurements. All the e l e c t r o c h e m i c a l tests were c a r r i e d out at 7 0 ± 2 ° C (after 1 h to ensure t e m p e r a t u r e e q u i l i b r i u m in the container) w i t h A M E L instrumentation interfaced with a microcomputer. The impedance study was c a r r i e d out w i t h a S o l a r t r o n f r e q u e n c y r e s p o n s e a n a l y s e r (FRA), m o d . 1 2 8 6 . R E S U L T S AND D I S C U S S I O N Figure 1 shows the cyclic v o l t a m m e t r y curves (20th cycle) of pPy, pBT and p D T T p o l y m e r i c e l e c t r o d e s in s o l i d - s t a t e cells. The v a l u e s of the i n t e g r a t e d charge under oxidation and r e d u c t i o n waves i n d i c a t e d o p i n g level (y) (calculated by r e d u c t i o n waves) for pPy, pBT and pDTT of 15, 24, 26 ~ (per pyrrole, 2,2'bithiophene, dithienothiophene unity), respectively and c o u l o m b i c e f f i c i e n c i e s (~) of 0.91, 0.90, 0.99 r e s p e c t i v e l y . The c y c l a b i l i t y of these s o l i d - s t a t e cells is v e r y g o o d and compares well with that of polymer electrodes in liquid electrolytic medium as shown in figure 2, w h i c h i l l u s t r a t e s the v o l t a m m e t r i c curves (20th cycle) for the same p o l y m e r e l e c t r o d e m a t e r i a l s in L i C I O 4 - P C l i q u i d s o l u t i o n cells. The rates of the d o p i n g - u n d o p i n g p r o c e s s are s l i g h t l y lower in the s o l i d - s t a t e cells, w h i c h is p r o b a b l y due to a slower ion diffusion both in the polymer electrode and in the polymer electrolyte. Despite this, the cyclability of solid devices r e m a i n s very p r o m i s i n g . F i g u r e 3 shows the capacity expressed in mC c m ~ ( o b t a i n e d in discharge) and the efficiency of subsequent charge-discharge g a l v a n o s t a t i c cycles at I=0.13 mA cm -2 w i t h s o l i d - s t a t e cells h a v i n g pPy, pBT and p D T T as p o s i t i v e electrodes. The r e s u l t s show that the c y c l e - l i f e of these s o l i d - s t a t e cells is r e m a r k a b l y good, as are the v a l u e s of c a p a c i t y and e f f i c i e n c y .
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Fig. i. C y c l i c voltammetry c u r v e s at v=50 mV/s in cells, p P y ( F = 9 . 5 . 1 0 - T m o l p y r r o l e cm -2) from 2.2 to ( F = 8 . 5 1 0 - 7 m o l b i t h i o p h e n e cm -2) from 2.2 to 4.3 (F=9.4 1 0 - 7 m o l d i t h i e n o t h i o p h e n e cm-2)from 2.2 to 4.3 vs Li. T = 7 0 ° C .
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Fig. 3. C a p a c i t y (,) and e f f i c i e n c y (-) at d i f f e r e n t number of g a l v a n o s t a t i c cycles (I=0.13 mA cm-2). S o l i d - s t a t e cells with pPy (F=9.5 10-7mol cm -2) from 2.2 to 3.8 V, pBT (F=8.5.10 -7 mol cm -2 ) from 2.2 to 4.2 V, and pDTT (~=8.0 10-Tmol c m - 2 ) f r o m 2.2 to 4.3 V, respectively. V values vs Li. T=70°C.
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One m a j o r p r o b l e m of p o l y m e r e l e c t r o d e s in l i q u i d - e l e c t r o l y t e b a t t e r i e s is their poor charge-retention capability [8J. In our tests of s o l i d - s t a t e cells, the e l e c t r o d e s t a b i l i t y was e v a l u a t e d u t i l i s i n g the values of e f f i c i e n c y (~) o b t a i n e d at 70°C w i t h g a l v a n o s t a t i c cycles at 0.13 mA c m - 2 at d i f f e r e n t e l a p s e d time after i n i t i a l charge and the corresponding OCV values before discharge. The cells were stored at room temperature and e q u i l i b r a t e d at 70°C for 1 hour b e f o r e d i s c h a r g e . The results are s h o w n in table i.
TABLE i Shelf-life Polymer
pPy pBT pDTT
Fxl07
charge
initially OCV ~; mol c m - 2 m C cm -2 V 8.4 14.0 3.46 0.98 8.5 9.6 3.90 0.98 8.2 14.0 4.00 0.96
1 hour OCV ~; V 3.30 0.81 3.78 0.65 3.67 0.55
3 days OCV ~ V 3.16 0.64 3.78 0.70 3.74 0.59
1 week OCV ~; V 3.02 0.46 3.63 0.51 3.59 0.43
It is to be n o t e d that m u c h faster d i s c h a r g e was found w h e n the cells were c o n t i n u o u s l y s t o r e d at 70oC. The n a t u r e of the s e l f - d i s c h a r g e m e c h a n i s m is still unknown. It is o n l y p o s s i b l e to state that this m e c h a n i s m does not involve an irreversible degradation of the p o l y m e r electrodes, as the cells can always be r e c h a r g e d to their o r i g i n a l c a p a c i t y values and, if immediately discharged, they always deliver their original e f f i c i e n c y . Our r e s u l t s d e n o t e that the rate of the s e l f - d i s c h a r g e process increases with temperature. Therefore, storage at low t e m p e r a t u r e is always to be r e c o m m e n d e d . Further information on the kinetics of the electrochemical p r o c e s s e s of polymer electrodes may be obtained by impedance analysis. F i g u r e 4 illustrates the c o m p l e x i m p e d a n c e plots of a Li/(PEO)20 L i C I O 4 / p P y s o l i d - s t a t e cell o b t a i n e d in the c h a r g e d (A) and d i s c h a r g e d (B) states. On the basis of the c o m m o n l y accepted interpretation models ~ J , the Z°'-Z ' i m p e d a n c e plot in figure 4A reveals, in p a s s i n g from h i g h to low frequency: an intercept with a real axis, Re, which identifies the resistivity of the polymer electrolyte b e t w e e n the Li and pPy e l e c t r o d e s ;an a r c w h i c h may be a s s o c i a t e d w i t h the e l e c t r o d e interfaces; and a 45 ° line, w h i c h c o n f i r m s that the k i n e t i c s of the e l e c t r o c h e m i c a l p r o c e s s may be c o n t r o l l e d by ion d i f f u s i o n . As expected, in the d i s c h a r g e d state (figure 4B) the resistance of the polymer electrode increases and the impedance response tends to i n d i c a t e a c a p a c i t a n c e effect at low f r e q u e n c i e s as o f t e n found in p o l y m e r e l e c t r o d e s [10,8J.
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ACKNOWLEDGEMENTS The authors thank ENI for the R e s e a r c h F e l l o w s h i p and Prof. P. Ferloni for kindly p r o v i d i n g the (PEO)20 LiCIO 4 film. The r e s e a r c h was funded by a grant from CNR, Progetto Finalizzato E n e r g e t i c a 2 (n.86.00876.59/ 87.02265.59).
REFERENCES 1 R. Bittihn et al., Makrom. Chem,, Macrom. Symp., 8 (1987) 51 2 M. M a s t r a g o s t i n o et al., E l e c t r o c h i m i c a Acta, 32 (1987) 1589 3 M. Armand, Solid State Ionics, 9 & 10 (1983) 745 4 P. Ferloni et al., Solid State Ionics 18 & 19 (1986) 265 5 S. Panero et al., E l e c t r o c h i m i c a Acta, 32 (1987) 1461 6 P. N o v a k et al., J . - P o w e r Sources, 21 (1987) 17 7 F. de Jong et al., J. Org. Chem.,36 (1971) 1645 8 B. Scrosati, Progress in Solid State C h e m i s t r y 18 (1988) 1 9 J. Ross Macdonald, I m p e d a n c e Spectroscopy, John W i l e y and Sons, New York, 1987 i0 N. M e r m i l l o d et al., J. Electrochem. Soc.,133 (1986) 1073