Free-standing sulfur-polypyrrole cathode in conjunction with polypyrrole-coated separator for flexible Li-S batteries

Abstract

A free-standing sulfur-polypyrrole cathode and a polypyrrole coated separator were designed for flexible Li-S batteries. The free-standing sulfur-polypyrrole cathode was prepared by directly pasting a sulfur-coated polypyrrole (S@PPy) nanofiber composite on a flexible and conductive polypyrrole (PPy) film. Compared with carbonaceous matrixes, PPy has a strong interaction with polysulfides to mitigate the dissolution of polysulfides due to its unique chain structure and the lone pair electrons in the nitrogen atoms in PPy. In addition, the as-prepared PPy film not only shows excellent mechanical elasticity, but also possesses a rough surface, which can accommodate volume expansion, enhance the adhesion of active materials, and further trap the dissolved polysulfides. Due to the synergistic effect provided by the PPy film, the free-standing sulfur-polypyrrole cathode shows better electrochemical performance than the traditional cathode with S@PPy composite coated on Al foil. In order to further improve the cycling stability of Li-S batteries, a PPy coated separator was prepared, which acts as a fishing net to capture polysulfides and alleviate the shuttle effect, leading to a stable cycling performance. Moreover, the PPy layer coated on commercial separator is much lighter than many other free-standing interlayers reported previously. Considering the flexibility of the free-standing sulfur cathode and PPy coated separator, a soft-packaged flexible Li-S battery based on them has been designed and fabricated to power a device consisting of 24 light emitting diode (LED) lights. After repeated bending, the flexible Li-S battery can still maintain good performance, indicating the excellent mechanical flexibility of the designed Li-S battery.