Effect of Polymerization Anion on Electrochemical Properties of Polypyrrole and on Li/LiClO₄/Polypyrrole Battery Performance

Electrochemical doping-undoping kinetics of electropolymerized polypyrrole (PPy) and the charge-discharge characteristics of Li/LiClO₄/PPy battery were found to show an interesting dependency on the kind of anions used for the preparation of PPy film. The relationship between the doping charge (anodization charge estimated from cyclic voltammogram) of PPy film and the polymerization potential revealed that the optimum potential for getting the largest doping capacity depends strongly on the kind of anions. The optimum potential for PF₆“-and CF₃S0₃“-formed PPy films was 0.32V vs. Ag/Ag⁺. On the other hand, ClO₄ ^--formed PPy film had the largest doping charge at 0.84V preparation over an examined potential range (0.30-0.84V). An appropriate polymerization scheme was discussed on the assumption that the presence of electron donating species (electrolyte anions) may stabilize a radical cation intermediate, by which the follow-up reaction to yield a polymeric film is favored. The postulated polymerization scheme is supported by the results of cross-sectional SEM micrographs of various anion formed PPy films. The charge-discharge characteristics of Li/LiClO₄/PPy (formed at the optimal potential) batteries were also investigated, in an attempt to clarify the relationship between battery performance and basic electrochemical behavior of the PPy electrode. From observing the charge-discharge behavior at various depths of charge and the current densities, the PF₆ ^--formed PPy was confirmed to be superior as a cathode material to the PPy films formed with other anions, viz., C1O₄ ^-BF₄ ^-and CF₃SO₃ ^-.