Pyrrole Copolymers with Enhanced Ion Diffusion Rates for Lithium Batteries
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ABSTRACT Copolymers of pyrrole with a polyether-substituted pyrrole were tested as cathodes for lithium batteries. The charge and discharge characteristics showed that anion transport was much faster in the copolymer than in polypyrrole. As a result these electrodes store and release much more charge at higher current densities but are similar to polypyrrole at low currents. Pulse and relaxation measurements of the ion diffusion showed that this difference was due to a ten-fold increase in the anion diffusion coefficient. INTRODUCTION Conducting polymers originally offered the prospect of stable, flexible, light conductors and semiconductors for many applications in electrical and electronic systems. With time it has become clear that these polymers are not as versatile as had been hoped. In battery applications, there are concerns of stability against oxidation and reduction at high positive or negative potentials. The charge storage capacity of conducting polymers is also not better than comparable transition metal oxides or sulfides on a weight basis. In addition, ion diffusion through conducting polymers is slow and this limits the rate at which current can be withdrawn from a battery.
In an effort to improve this last factor, we have prepared a number of copolymers of pyrrole with pyrrole substituted with polyether side groups. The ether groups were expected to provide ionophilic channels within the polypyrrole structure and so allow rapid diffusion. We have shown that these copolymers do have better charge/discharge characteristics than polypyrrole and that these changes can be attributed to rapid diffusion [1]. A similar ethermodified polyphenylene has also been described [2]. The charging reaction for a conducting polymer cathode is: Polypyrrole+C10 4 -Polypyrrole
รท+ClO 4 +e
This requires that an anion transfer from solution into the cathode and then diffuse through the cathode, down the concentration gradient from the surface, and the associated electron must be conducted out from the electrode. Given the very slow diffusion of ions in conducting polymers, the interpretation of charge/discharge data can be difficult. Firstly, the history of the material must lead to pre-existing gradients that can influence any new measurement. It is thus hard to ensure that an electrode at some surface potential is in the same state throughout or that the starting state is reproducible from one cycle to the next. Secondly, it is known from gravimetric studies [3] that charge and discharge cycles may occur through diffusion of small cations which neutralize embedded anions in the neutral polymer state. The small cations would move much more rapidly than the large anions. 485 Mat. Res. Soc. Symp. Proc. Vol. 496 01998 Materials Research Society
MATERIALS AND METHODS Two ether-substituted pyrrole monomers were formed, 3-(3,6-Dioxaheptyl)pyrrole (I) and 3-(3,6,9-trioxadecanyl)pyrrole (II): CH2CH2OCH2CH2OCH3
H
I
CH2CH2OCH2CH2OCH2CH2OCH3
H II
This was carried out via tosylation of pyrrole to protect the nitrogen, react
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