Electrochemical properties of thermally-structurized polymers

Synthetic Metals, 51 (1992) 109 114

109

Electrochemical properties of thermally-structurized polymers L. Sawtschenko, K. Jobst, J. Froehner, L. Wuckel and G. Paasch Institut fiir Festk6rper- und Werkstofforschung, Helmholtzstrasse 20, 0-8027 Dresden (Germany)

Abstract Comparison of two conjugated polymers--thermally-structurized poly(acrylonitrile) (TSPAN) and poly(perinaphthalene) ( P P N ) - - w a s carried out by means of CV and c h a r g e - d i s c h a r g e experiments. Their redox potentials differ from those calculated using a Hueckel method. Distinct differences in the electrochemical behaviour can be attributed to the different molecular structures of these polymers.

Introduction

Conducting polymers with interesting electrical and electrochemical properties can be prepared by thermal treatment of various polymers or monomers [1, 2]. In recent years some investigations into the electrochemical properties of thermally-structurized poly(acrylonitrile) (TSPAN) have been published [3, 4]. Using a special treatment [5], materials of different porosity and specific surface area can be obtained. Experiments with Li/TSPAN cells (with a 1 M solution of LiC104 in dried, purified propylene carbonate as electrolyte) have clearly shown that these structural properties of TSPAN influence, essentially, the kinetics of its electrochemical reactions and specific capacity. Materials of higher specific surface and pore volume have an increased capability for charge storage due to the greater diffusion of electrolyte and dopant ions into the material of the electrode. There is, however, limited knowledge of the influence of the molecular structure of the thermally-treated polymers on their electrochemical behaviour. In this paper we present the first results of a comparison of the electrochemical properties of two polymers of different structure: TSPAN and poly(perinaphthalene) (PPN), prepared by thermal treatment of poly(acrylonitrile) and perylenetetracarboxylic dianhydride.

Materials and results

It has been determined by elemental analysis, IR spectroscopy, X-ray diffraction, and E S C A - X P S that TSPAN (700 °C) shows a complicated,

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110

TSPAN H H kC,,/C.ct.c/
I

I

I

I

I

/

J

/C.. C~ C. C. C C~ _ ~N / "%C/ %C/ %C/ %C/ ~,C/

'

I

I

I

I

\ c,"% c/C c/C. c

I

I

I

I

/

I

C C C ,C %N/ %C/ %C/ %C/ \ H H I

C

PPN

Fig. 1. Molecular structures of TSPAN and PPN.

crosslinked, layer-like s t r u c t u r e [2], whereas P P N (800 °C) has a relatively homogeneous, only slightly crosslinked, ladder-like s t r u c t u r e (Fig. 1). A highly porous TSPAN, with a specific surface of about 10001100 m 2 g-l, a pore volume of about 0.8-0.9 cm 3 g-l, and a specific conductivity of about 0 . 1 - 0 . 2 S c m -~, has been employed in our investigations. Most of the pores ( > 9 5 % ) in this material exhibit a diameter less t h a n 3 nm. The m e a n diameter of the particles was about 1-2/~m. The c h a r a c t e r i s t i c length of the material ( c h a r a c t e r i z i n g the average of the longest distance in the bulk from the pore surface) is of the order of a few nanometers, and it was expected t h a t almost the whole volume of the material would be involved in the electrochemical reactions. In Fig. 2, the results of CV m e a s u r e m e n t s with a scanning r a t e of 1 mV s -~ are depicted. It is seen t h a t TSPAN can be reversibly oxidized and r e d u c e d in the potential r a n g e between 4.5 and 0.5 V versus Li. The CV plot of T S P A N shows wide oxidation and r e d u c t i o n waves only, w i t h o u t distinct peaks. Moreover, the discharging of TSPAN, as shown in Fig. 3, does not occur at a c o n s t a n t potential. Both peculiarities lead to the conclusion t h a t the r e d u c t i o n and oxidation processes of T S P A N are not d e t e r m i n e d by a single defined r e d u c t i o n or by a single defined oxidation potential. This can be understood considering the fact t h a t the s t r u c t u r e of T S P A N is not uniform (Fig. 1). Therefore, the oxidation potential of 4.28 V versus Li, and the r e d u c t i o n potential of 2.66 V versus Li, c a l c u l a t e d supposing an ideal n a p h t h i r i d i n e s t r u c t u r e of TSPAN, and using an extended Hueckel m e t h o d [6], do not agree with experimental results.

111

,~ 3 -

--

TSPAN

. . . .

PPN

-1 -2 -3

o

i

~

i

i i portia//VvcLi

Fig. 2. CV plots of TSPAN and PPN in the potential range 4.5 0.5 V vs. Li. Scanning rate: 1 mV s-'.

~.0 • TSPAN x PPN

.~

i

I

~ 3.o

2.0

1.5-

~

,

1

¢.o

0

50

l O0

~ 150

200

250

spec. capacity / m A h . g - I Fig. 3. Discharge curves of TSPAN and PPN after charging at a constant voltage of 4.0 V. Current density: 0.01 m A c m 2.

The current in the wave maximum is directly proportional to the square root of the scanning rate, indicating a diffusion-controlled doping process [7] and not merely the formation of a double layer on the surface of the electrode material. A distinct dependence of the charge-storage capacity on the discharging current density was found mainly at current densities below 0.3 mA cm -2. Under these conditions the dopant ions can diffuse into the bulk, leading to higher specific capacities. The doping level increased by a factor of about 2 on decreasing the current density from 0.05 to 0.002 mA cm -2. The doping level does not change significantly on increasing the current density above 0.05 mA cm -2 because, in this case, the dopant ions mainly penetrate into easily attainable positions. Perhaps,

112

).5o

L5 ~0

~.0 25

2.0

1.5

LO

0.5

0

0.5

1.0

1.5 20

reduction ~ L oxidat/on degree of doping/rno! %

Fig. 4. Doping (©) and undoping (Q) of TSPAN. Current density: 0.5 mAcm 2.

due to crosslinking in the layered structure, TSPAN is very stable in c h a r g e - d i s c h a r g e experiments, and more than 100 cycles between 4.0 and 2.0 V can be obtained. The equilibrium o.c voltage in the initial state (without doping) for L i - T S P A N cells was found to be about 3.3 V. By discharging between 3.3 and 2.0 V, relatively high degrees of doping, up to x = 0.07... 0.08 in C1H0.17N0.0~Li÷x can be obtained (n-doping). The degree of p-doping between 4.0 and 3.3 V amounts to 0.017. This leads to an overall degree of doping between 4.0 and 2.0 V of nearly 0.1. By discharging to voltages below 2.0 V, essentially higher doping levels can be reached. In Figs. 3 and 4 it can be seen that the discharge curves for p- and n-doping change continuously from one to another and cross each other without any step at the o.c. voltage. This can be explained by the assumption that, in consequence of the inhomogeneity of the crosslinked layered structure, and due to the different bonding relations in the TSPAN, additional energy levels between the reduction and oxidation potentials have been formed, which could also be occupied during electrochemical doping. Due to this specific behaviour of TSPAN both p- and n-doping can be used for charge storage, increasing the capacity as well as the mean discharging voltage. The structure of PPN (Fig. 1) is less crosslinked compared with that of TSPAN. Thus it was expected that the diffusion rate of the dopant ions would be sufficiently high, without additional treatment, to obtain a high specific surface. The specific surface area of the investigated PPN was nearly 50 m 2 g-1 and the specific conductivity was about 1 S cm -1. The CV curve of PPN (Fig. 2) differs greatly from that of TSPAN. The electrochemical reduction (n-doping) occurs only at potentials below 1.5 V. At higher potentials there are no electrochemical reactions, indi-

113

cating that no p-doping occurs up to a potential of 4.5 V versus Li. Therefore, PPN can only be reduced in the potential range between 0.5 and 4.5 V. The maximum of the waves for n-doping and undoping are not essentially shifted against each other, pointing to a sufficient diffusion rate of Li ÷ ions in PPN in spite of its relatively small specific surface area. The o.c. voltage of the P P N - L i cell in the initial state is nearly 3 V. The results of the discharging experiments (Fig. 3) are in agreement with the results of CV measurements. The voltage drops quickly from the starting voltage to about 1.8 V, without any essential capacity for the oxidized state (practically no p-doping of PPN between 4.5 and 3.0 V). From 1.8 to 1.0 V a relatively high capacity for n-doping of about 190 mA h g-~ is obtained. The potential curve is relatively flat in comparison with TSPAN because of the more homogeneous structure of this polymer. The theoretically-calculated reduction and oxidation potentials of 2.54 and 2.9 V, respectively, versus Li [8], do not agree with these experiments. Corresponding to the CV and discharging behaviour, the reduction potential must be nearly 1.7 V versus Li, and the oxidation potential higher than 4.5 V. The differences between theoretical and experimental values may be interpreted by the fact that the structure of the PPN is not ideal and contains some defects. The fast voltage drop on discharging from 4.0 or 4.5 V to 1.8 V shows that, in contrast to TSPAN, no remarkable additional energy levels between the oxidation and reduction potentials have been formed in this polymer due to the structure of PPN. Thus, a significant voltage difference between oxidized and reduced states exists. The mean discharging potential lies at 1.4V, which is essentially lower than that of TSPAN. The degree of n-doping is comparable with that of TSPAN in the range between 4 and 2V and reaches x = 0.09-0.1 in C1H0.46Li÷x. A dependence of the doping degree on current density was also found for PPN. By increasing the current density from 0.005 to 0.1 mA cm 2 the doping degree decreases by a factor of about 2. The cycle life of the PPN is to be investigated. The results of these experiments show that the molecular structure of materials prepared by thermal treatment of polymers, influences their electrochemical behaviour significantly. Using appropriate preparation methods, and choosing suitable monomers or polymers, one can obtain materials having different electrochemical properties.

References 1 Sh. Yata, T. Osaki, Y. Hato, N. Takehara, (3. Kinoshita, K. Tanaka and T. Yamabe, Synth. Met., 38 (1990) 177, 185. 2 K. Jobst, L. Sawtschenko, M. Schwarzenberg and L. Wuckel, Synth. Met., 41 43 (1991) 959.

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3 L. Sawtschenko, K. Jobst, M. Schwarzenberg, L. Wuckel and G. Paasch, Synth. Met., 41-43 (1991) 1165. 4 G. Paasch, M. Schwarzenberg, K. Jobst and L. Sawtschenko, Synth. Met., 41-43 (1991) 2953. 5 Patent DD, H 01 M, 319129 (1988). 6 G. Lehmann and B. Pietrass, Synth. Met., 28 (1988) 521. 7 J. Heinze, M. Dietrich and J. Mortensen, Makromol. Chem., Macromol. Symp., 8 (1987) 73. 8 L. Wuckel and G. Lehmann, Makromol. Chem., Macromol. Symp., 37 (1990) 195.