Suppression of polysulfide dissolution by polypyrrole modification of sulfur-based cathodes in lithium secondary batteries

In order to solve the problem of polysulfide dissolution into the electrolyte on sulfur-based cathodes, we propose a novel method of modifying the S cathode by coating it with a polypyrrole (PPy) film, which is prepared by oxidative electropolymerization using a solution consisting of, 1-methyl-1-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and pyrrole. The PPy film demonstrates a high Li⁺ transport number. The film also exhibits a superior ability to inhibit polysulfide dissolution into the electrolyte during the charge-discharge cycles. Furthermore, the charge-discharge properties of the coated cathode is evaluated using an electrolyte consisting of 1.0 M LiTFSI in a mixture of 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL), which is known to easily dissolve polysulfides. Because of the surface modification with the PPy film, the cathode exhibits excellent specific capacities of 823 and 354 mAh g^-1 at C-rates of 0.1 and 1.0 C, respectively, with high coulombic efficiency. Thus, the strategy of coating the S cathode with PPy is successful in inhibiting the polysulfides dissolution even in electrolytes known to easily dissolve polysulfides, besides allowing high C-rate operation. Further, the modification of the S cathode allows the selection of a suitable electrolyte based on the anode, rather than being limited by the cathode.