The Influence of Water on the Cycling Behavior of the Polypyrrole Electrode in Lithium Cells

Abstract

Two different types of polypyrrole (PPy) were synthesized: the “dry” form from in propylene carbonate (PC): and the “wet” one from in PC. The polymerization of pyrrole in both electrolytes starts at +3.46V (vs. Li/Li⁺) and follows nearly the same E‐I curve. However, during the first discharge clear differences in electrochemical behavior can already be seen. During the first 1,000 cycles performed on 1‐μm thin PPy layers in the dry (PC) electrolyte, the coulombic capacity of the “dry” form of PPy dropped to about 80% of its initial value. The cycling behavior of the “wet” form is even better—the doping level is slightly higher and the capacity drops to 80% of its initial value after more than 20,000 cycles, when cycled in the optimum potential interval from 2.0 to 4.0V (vs. Li/Li⁺). Cyclic voltammograms and in situ FTIR measurements revealed that there is a gradual, slight loss of the most negative redox sites with increasing cycle number, accompanied by an increase in the amount of the irreversibly (over)oxidized PPy found in the neutral form of the polymer. The self‐discharge loss on ≈50‐μm‐thick PPy foils was found to be about 1% per day.