Film Properties of Electropolymerized Polypyrrole for a Sulfur/Ketjenblack Cathode in Lithium Secondary Batteries

A novel polypyrrole (PPy) film was investigated to determine the optimal conditions for operation in a Li/S battery. The PPy film was prepared by oxidative electropolymerization to improve the Li/S battery performance, as reported in our previous paper. In such a system, the PPy film was coated directly on the S/Ketjenblack cathode to solve the problem of polysulfide dissolution. The optimum PPy film preparation conditions to prevent polysulfide dissolution and to promote Li+ permeability were determined by varying the PPy polymerization bath composition and polymerization potential. As a result, the inclusion of 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in the polymerization bath (0.1 M pyrrole in 1-methyl-1-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide) was found to be the most important factor for producing a PPy film with a high Li+ transport number (tLi+ ≈ 1). A polymerization potential of 4.5 V versus Li/Li+ was shown to be optimum for the promotion of Li+ permeability. The mechanism by which the PPy film prevents polysulfide dissolution and increases Li+ permeability is discussed by analyzing the SEM, CV, XPS, and 13C solid-state NMR data.