AN IN SITU EPR STUDY OF ELECTROCHEMICALLY DOPED TRANS POLYACETYLENE

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AN IN SITU EPR STUDY OF

ELECTROCHEMICALLY DOPED TRANS POLYACETYLENE

L. Kispert, J. Joseph, T. Jayaraman, L. Shacklette, R. Baughman

To cite this version:

L. Kispert, J. Joseph, T. Jayaraman, L. Shacklette, R. Baughman. AN IN SITU EPR STUDY OF

ELECTROCHEMICALLY DOPED TRANS POLYACETYLENE. Journal de Physique Colloques,

1983, 44 (C3), pp.C3-317-C3-320. �10.1051/jphyscol:1983362�. �jpa-00222714�

JOURNAL DE PHYSIQUE

Colloque C3, supplément au n°6, Tome 44, juin 1983 page C3-317

AN IN SITU EPR STUDY OF ELECTROCHEMICALLY DOPED TRANS POLYACETYLENE L.D. Kispert , J. Joseph , T.V. Jayaraman , L.W. Shacklette and

R.H. Baughman**

Chemistry Department, The University of Alabama, Tuscaloosa, AL 35486, U.S.A.

**Polymer Laboratory, Allied Corporation, Morristown, N.J. 07960, U.S.A.

Résumé - Nous avons étudié' par EPR in situ, pendant des cycles charge/décharge ré'pé'té's, le comportement d'une batterie comprenant une cathode de trans polyaciftylene

(PA) et une anode d'un alliage Li/Al. La diminution du rapport A/B observée est consistante avec une diminution de la conductivité* dans l'é"tat dope", tandis que l'invariance de la largeur de raie EPR, avant et après dopage, suggère qu'une disparition massive de la conjugaison des chaines ne se produit pas pendant les cycles charge/décharge-

Abstract - An in situ EPR study has been carried out during repeated charge/discharge cycles of a battery consisting of trans polyacetylene (PA) as the cathode and Li/Al alloy as the anode. The detected spin concentration reversibly increased by less than a factor of two and the maximum A/B ratio decreased with each repeated cycle for CIO4- in propylene carbonate and B F 4

in sulfolane as the dopants.

While this decreasing A/B ratio is consistent with decreasing conductivity in the doped state, the observed invariance of ESR linewidth before and after doping suggests that massive disruption of chain conjugation does not occur during battery cycling.

Introduction

An in situ electron spin resonance study has been carried out during the charge/discharge cycle of a battery consisting of trans-polyacety- lene (PA) as the cathode and Li/Al alloy as the anode. EPR measure- ments have been made to determine the unpaired electron spin

concentration, linewidth, and line asymmetry as a function of percent doping during repeated charge/discharge cycles over a preiod of 1 month for two different electrolyte solutions: LiClC>4 in propylene carbonte and L1BF4 in sulfolane.

Changes from symmetrical to Dysonian EPR line shapes are observed during electrochemical doping which reflect the conductivity changes of the polyacetylene [1]. If the thickness of the conductive

material is large with respect to the skin depth at microwave frequen- cies then an asymmetric EPR lineshape is expected [2,3]. The asym- metric EPR pattern can be measured in terms of the A/B ratio, the ratio of the positive peak height at low field (first derivative of the absorption spectrum) to the negative peak height at high field.

Given the value of A/B, the ratio can be determined (Tp = time it takes an electron to diffuse through the skin depth and T2 = spin-spin relaxation time). If the thickness of the sample is much smaller than

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983362

C3-318 JOURNAL DE PHYSIQUE

t h e s k i n d e p t h , t h e n a s y m m e t r i c a l L o r e n t z i a n l i n e w i t h a h a l f - w i d t h 1/T2 i n d e p e n d e n t o f s p i n d i f f u s i o n w i l l be o b s e r v e d . F o r s a m p l e s whose t h i c k n e s s is c o m p a r a b l e t o t h e s k i n d e p t h , t h e l i n e s h a p e is a complex f u n c t i o n o f , I ( T ~ / T ~ ) ~ / ~ , where h is t h e r a t i o o f t h e m a t e r i a l t h i c k n e s s t o s k i n d p e t h [ 4 ] . F o r maximum l e v e l s o f d o i n g i n t h e s e e x p e r i m e n t s t h e s k i n d e p t h is c o m p a r a b l e t o t h e PA f i l m t h i c k n e s s o f 80 m i c r o n s . Thus o u r m e a s u r e m e n t s f a l l i n t h e l a t t e r c a t e g o r y . E x p e r i m e n t a l

A p p r o x i m a t e l y 1 . 5 t o 2 mg o f c i s - r i c h p o l y a c e t y l e n e (PA) f i l m was m e c h a n i c a l l y a t t a c h e d t o a c o r r u g a t e d p l a t i n u m e l e c t r p d e i n a n i n e r t a t m o s p h e r e d r y box (N2) a t room t e m p e r a t u r e and h e a t e d i n v a c u o a t 160°C f o r 2 h o u r s i n o r d e r t o i s o m e r i z e PA. A f l a t 3 mrn w i d e aluminum e l e c t r o d e was c o a t e d w i t h a p o l y p r o p y l e n e s o l u t i o n . A f t e r d r y i n g , a l t e r n a t i n g s e c t i o n s o f t h e i n s u l a t i n g p o l y p r o p y l e n e c o a t i n g w e r e removed s o t h a t t h e r e m a i n i n g r i d g e s o f p o l y p r o p y l e n e c o u l d f u n c t i o n a s a s p a c e r b e t w e e n e l e c t r o d e s . The b a t t e r y was t h e n a s s e m b l e d i n a n i n e r t a t m o s p h e r e d r y box u s i n g a s t a n d a r d EPR f l a t c e l l a s t h e s a m p l e c o n t a i n e r . F i r s t , t h e PA f i l m was i n s e r t e d from o n e end i n t o t h e f l a t c e l l , f o l l o w e d by i n s e r t i o n o f t h e A 1 e l e c t r o d e f r o m t h e o t h e r end o f t h e c e l l s o t h a t t h e two e l e c t r o d e s were p a r a l l e l and o v e r l a p p e d o n e a n o t h e r . N e x t , t h e e l e c t r o l y t e s o l u t i o n ( e i t h e r 0 . 5 M LiC104 i n p r o - p y l e n e c a r b o n a t e , b a t t e r y A , o r 1 M LiBF4 i n s u l f o l a n e , b a t t e r y B ) was added t o j u s t c o v e r t h e PA and A 1 e l e c t r o d e . An NBS Ruby i n t e n s i t y s t a n d a r d was a t t a c h e d t o t h e o u t s i d e f a c e and c e n t e r o f t h e f l a t c e l l s o t h a t a n a b s o l u t e u n p a i r e d s p i n c o n c e n t r a t i o n c o u l d be d e t e r m i n e d i n s i t u f o r e a c h EPR s p e c t r u m r e c o r d e d . The b a t t e r y was c h a r g e d w i t h a c o n s t a n t v o l t a g e s o u r c e and d i s c h a r g e d w i t h a c o n s t a n t c u r r e n t s o u r c e . The p e r c e n t d o p i n g p e r CH u n i t was c a l c u l a t e d from t h e number o f c o u l o m b s o f c h a r g e e x t r a c t e d upon d i s c h a r g e . The PA f i l m was doped d u r i n g e a c h c h a r g i n g c y c l e u n t i l a D y s o n i a n EPR l i n e s h a p e a p p e a r e d w i t h a n A/B r a t i o o f -1.5 o r g r e a t e r . T h i s c r i t e r i a c o r r e s p o n d s t o d o p i n g o f a t l e a s t 1%. The b a t t e r y was t h e n d i s c h a r g e d and EPR s p e c t r a r e c o r d e d a t r e g u l a r t i m e i n t e r v a l s . D u r i n g c h a r g i n g , a u n i - f o r m c h a n g e o c c u r r e d i n t h e EPR l i n e s h a p e from t h a t o f a L o r e n t z i a n l i n e w i t h A/B

1 . 0 t o t h a t o f a Dysonian l i n e w i t h a maximum

A/B = 4.2 f o r 6% d o p i n g . Only a s i n g l e EPR l i n e was o b s e r v e d d u r i n g t h e c h a r g e / d i s c h a r g e c y c l e . No l i n e w i t h a s l i g h l t y d i f f e r e n t g v a l u e

( s u c h a s e x p e c t e d f r o m L i m e t a l ) was o b s e r v e d . The w e l l known c a t h o d e a n d a n o d e r e a c t i o n s t h a t t a k e p l a c e upon d i s c h a r g e ( u n d o p i n g ) a r e g i v e n below f o r b a t t e r i e s A and B [ 5 ] .

B a t t e r y A ( Z -

C 1 0 4 - ) , B a t t e r y B ( Z - = BFq-) C a t h o d e R e a c t i o n ^:

[ ( c H ) + Y ( Z - ) y ] x + yxe- + ( C H ) , + y x ( Z - ) Anode R e a c t i o n

E l e c t r o l y t e : 0 . 5 M LiC104 i n p r o p y l e n e c a r b o n a t e ( B a t t e r y A ) , 1 M LiBF4 i n s u l f o l a n e ( B a t t e r y B ) .

R e s u l t s

Some r e p r e s e n t a t i v e d a t a g i v e n i n T a b l e I were o b t a i n e d f o r b a t t e r y A ( L i C l 0 4 i n p r o p y l e n e c a r b o n a t e ) when t h e PA f i l m was doped t o

y

5 . 9 % .

4 . 7 % a n d 4 . 7 % d u r i n g c h a r g e / d i s c h a r g e c y c l e s I , I1 a n d I11 r e s p e c t i v e - l y . A s t h e A/B r a t i o i n c r e a s e d t h e p e a k t o p e a k l i n e w i d t h (AHpp) i n - c r e a s e d u n t i l a v a l u e o f 2.8 G a u s s was o b s e r v e d f o r a n A/B r a t l o o f 4 . 2 . Upon d i s c h a r g e t h e l i n e w i d t h r e t u r n e d t o a v a l u e o b s e r v e d b e f o r e T a b l e I. B a t t e r y A

C y c l e S p i n ( b )

(max.

S p e c t r u m

d o p e d

# V o l t a g e A/B

AHpp

I I 4 3.00 1.11 0.80

( 4 . 7 * . 3 % ) 11 3.40 1 . 2 0 1 . 1 0

3 2 ( a ) 3.50 3 . 6 8 2.30 8 . 0

6 4 3.00 1 . 6 8 1 . 8 0 7 . 5

9 6 0.06 1 . 1 4 1 . 1 0 5.6 1 d i s c h a r g e

I11 1 3.00 1 . 1 5 1 . 0 0 5 . 6

( 4 . 7 + . 3 % ) 9 ( a ) 3.50 2.64 2.00 6 . 1 3 c h a r q e

1 3 3.30 2.34 2 . 0 0

4 3 0 . 2 5 1 . 1 4 1 . 1 0 g: 6l I d i s c h a r g e

( a ) L a s t s p e c t r u m b e f o r e d i s c h a r g e

( b ) T o t a l EPR s p i n s o b s e r v e d f o r a 2.7 cm l e n g t h o f PA, n o r m a l i z e d p e r g r a m of PA.

c h a r g i n g ( 1 . 1 G a u s s ) . The s p i n c o n c e n t r a t i o n was o b s e r v e d t o i r r e v e r - s i b l y i n c r e a s e b y n e a r l y a f a c t o r o f t w o upon i n i t i a l c h a r g i n g . I n s u b s e q u e n t c y c l e s a n i n c r e a s e o c c u r r e d upon d o p i n g w h i c h w a s r e v e r - s i b l e upon u n d o p i n g . N o t e w o r t h y is t h e f a c t t h a t w i t h e a c h s u b s e q u e n t c h a r g e / d i s c h a r g e c y c l e , t h e A/B r a t i o o b s e r v e d a t f u l l c h a r g e

d e c r e a s e d . T h i s t r e n d p e r s i s t e d a f t e r d a i l y c y c l i n g f o r o n e week.

A f t e r t h e b a t t e r y was l e f t f u l l y d i s c h a r g e d f o r t h r e e w e e k s , a f u r t h e r d e c r e a s e i n t h e A/B r a t i o o c c u r e d upon r e p e a t e d c h a r g e / d i s c h a r g e

c y c l e s u n t i l A/B

1 . 5 a n d (AHpp) = 1 . 4 G a u s s a t f u l l c h a r g e . T a b l e I1 p r e s e n t s a s i m i l a r d a t a f o r b a t t e r y B, u s i n g 1 M LiBF4 i n s u l f o l a n e . A g a i n a s m a l l r e v e r s i b l e i n c r e a s e i n s p i n c o n c e n t r a t i o n o c c u r r e d upon d o p i n g (max. o f 1 . 9 t i m e s f o r a 3.8% d o p i n g ) y e t a c o n t i n u a l d e c r e a s e i n t h e maximum A/B a t i o was o b s e r v e d w i t h e a c h c h a r g e / d i s c h a r g e c y c l e .

T a b l e 11. B a t t e r y B

C y c l e # S p i n

Time max. % S p e c t r u m C e l l AH^^) ^C&C.

Days d o p e d # V o l t a g e A/B ( G a u s s ) 1019/g

0 # 1 1 3.00 1 . 0 0 1 . 0 5

( 1 . 3 % ) 3 1 3.60 1 . 4 0 1 . 7 0

3 5 3 . 6 5 ( a ) 1 . 6 7 1 . 9 0 5 . 6

39 3.00 1 . 1 6 1 . 6 0 6 . 1 d i s c h a r g e

4 6 0 . 1 0 1 . 0 7 0.90 6.1)

JOURNAL DE PHYSIQUE

# 4 4 3.00 1 . 0 9 0.90

( 1 . 3 % ) 2 3 3.60 1 . 6 4 1 . 9 0

29

2 . 7 5 1 . 1 6 1 . 5 5

3 . 6 5 ( a ) 2.20 2.00 1 0 . 3

4 4 9.9

5 1 0.10 1 . 1 3 1 . 0 5 8 . 0 i d i s c h a r g e

# 5 1 3.00 1 . 0 3 1 . 1 0

( 3 . 8 % ) 6 3.60 1 . 5 2 2.00 9 . 4 c h a r g e

1 3

2.60 1 . 1 0 1 - 0 0

0.10 1 . 0 2 1 . 0 0

3 . 6 5 ( a ) 1 . 6 2 2.00 1 2 . 7 6.

1 9

39 :: : ] d i s c h a r g e

( a ) L a s t s p e c t r u m b e f o r e d i s c h a r g e ( b ) Same as ( b ) , T a b l e I.

D i s c u s s i o n

The p e a k t o p e a k l i n e w i d t h (AHpp) o f t h e PA f i l m u s e d i n e i t h e r

s o l v e n t e q u a l l e d 1.1 G a u s s b e f o r e d o p i n g , a l i n e w i d t h r e p o r t e d b y p r e - v i o u s i n v e s t i g a t i o n s . However, s t u d i e s 161 h a v e shown t h a t t h e

l i n e w i d t h f o r PA p r e p a r e d u n d e r t h e m o s t r i g o r o u s c o n d i t i o n s e q u a l s 0.44G, s u g g e s t i n g i n c o r p o r a t i o n o f trace 0 2 c o n c e n t r a t i o n i n t h e PA u s e d h e r e . F o r t h e h i g h e s t d o p i n g l e v e l ( 5 . 9 % b a t t e r y A ) , A/B

4.2 a n d (AHpp)

2.8 G. Assuming t h e t h i c k metal a p p r o x i m a t i o n is v a l i d

[ 2 , 3 , 7 ] TD

20 n s a n d T2 = 3 5 n s . From T a b l e I a n d 11, it is n o t e d t h a t t h e l i n e w i d t h r e t u r n s t o t h e same v a l u e f o l l o w i n g e a c h

c h a r g e / d i s c h a r g e c y c l e . S i n c e t h e l i n e w i d t h d o e s n o t i n c r e a s e , m a s s i v e d i s r u p t i o n o f c h a i n c o n j u g a t i o n d o e s n o t o c c u r upon c y c l i n g . On t h e o t h e r h a n d , t h e d e c r e a s i n g A/B r a t i o s u g g e s t s d e c r e a s i n g c o n - d u c t i v i t y w i t h e a c h c y c l e . C o n d u c t i v i t y is e x p e c t e d t o b e p a r - t i c u l a r l y s e n s i t i v e t o c o n j u g a t i o n l e n g t h a n d a l s o t o t h e d e g r e e o f c r y s t a l l i n i t y . M. Druy h a s p r e v i o u s l y r e p o r t e d [ 8 ] t h a t t h e c o n d u c - t i v i t y o f PA e l e c t r o c h e m i c a l l y d o p e d w i t h Cl04- d e c r e a s e s s u b s t a n - t i a l l y a s a f u n c t i o n o f s t o r a g e t i m e i n vacuum ( b y 50% i n o n e m o n t h ) . C o n s e q u e n t l y , t h e e f f e c t s o f d i s c h a r g e c y c l e a n d s t o r a g e t i m e - c a n n o t b e d i f f e r e n t i a t e d . The s m a l l , r e v e r s i b l e i n c r e a s e i n s p i n c o n c e n - t a t i o n d u r i n g a n y c y c l e r e p r e s e n t s a maximum i n c r e a s e o f 0.02 s p i n s d e t e c t e d p e r d o p a n t m o l e c u l e i n t h e s a m p l e ( c y c l e # 5 , b a t t e r y B).

However, s i n c e t h e s k i n d e p t h a t h i g h d o p a n t c o n c e n t r a t i o n is l e s s t h a n t h e s a m p l e t h i c k n e s s , a l l s p i n s i n t h e s a m p l e w i l l n o t b e d e t e c t e d .

R e f e r e n c e s

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[ 2 ] FEHER G. a n d KIP A.F., P h y s . Rev. 98 ( 1 9 5 5 ) 337.

[ 3 ] DYSON F . J . , P h y s . Rev. - 98 ( 1 9 5 5 ) 349.

[ 4 ] KODERA H . , J. P h y s . S o c . J a p a n 28 ( 1 9 7 0 ) 89.

[ 5 ] MACINNES D. JR. , DRUY M. A. , N I G ~ Y P. J. , NAIRNS D. P. , MACDIARMID A.G., a n d HEEGER A.J. , J. Chem. S o c . . Chem. Comm. 1 9 8 1 , 317.

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[ 7 ] GLAUNSIRER W.S., a n d SIENKO M. J . , J. Magn. R e s o n a n c e 10 ( 1 9 7 3 ) 253.

[ 8 ] DRUY M.A., Ph.D. T h e s i s , 1 9 8 1 , Univ. o f P e n n s y l v a n i a ,

P h i l a d e l p h i a , PA.