A lithium sulfonylimide COF-modified separator for high-performance Li-S batteries

Lithium-sulfur (Li-S) batteries are highly regarded as the next-generation high-energy-density secondary batteries due to their high capacity and large theoretical energy density. However, the practical application of these batteries is hindered mainly by the polysulfide shuttle issue. Herein, we designed and synthesized a new lithium sulfonylimide covalent organic framework (COF) material (COF-LiSTFSI, LiSTFSI = lithium (4-styrenesulfonyl) (trifluoromethanesulfonyl)imide), and further used it to modify the common polypropylene (PP) separator of Li-S batteries. The COF-LiSTFSI with sulfonylimide anion groups features stronger electronegativity, thus can effectively facilitate the lithium ion conduction while significantly suppress the diffusion of polysulfides via the electrostatic interaction. Compared with the unmodified PP separator, the COF-LiSTFSI modified separator results in a high ionic conductivity (1.50 mS·cm⁻¹) and Li⁺ transference number (0.68). Consequently, the Li-S battery using the COF-LiSTFSI modified separator achieves a high capacity of 1229.7 mAh·g⁻¹ at 0.2 C and a low decay rate of only 0.042% per cycle after 1000 cycles at 1 C, compared with those of 941.5 mAh·g⁻¹ and 0.061% using the unmodified PP separator, respectively. These results indicate that by choosing suitable functional groups, an effective strategy for COF-modified separators could be developed for high-performance Li-S batteries.