Polydopamine-Assisted In Situ Formation of a Covalent Organic Framework on Single-Walled Carbon Nanotubes to Multifunctionalize Separators for Advanced Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries are regarded to be one of the most promising energy storage systems owing to their high energy density. Nevertheless, polysulfide shuttle and self-discharge severely hinder their commercial production. To address these issues, a novel multifunctional separator (COF-PDA/SWCNT/PP) is developed by modifying the commercial polypropylene (PP) separator via in situ growth of a covalent organic framework (COF) functional layer on the gutter layer made from single-walled carbon nanotubes (SWCNTs). SWCNTs in this configuration works as the gutter layer to reduce the pore size of the PP separator and offers lithium ion/electron transmission channels, while the COF-PDA barrier layer acts as a robust and efficient shield against polysulfide shuttling via both physical confinement and chemical interactions. Notably, the simultaneous generation of polydopamine (PDA) can get rid of defects in COFs by welding the COF nanoparticles, enhancing the mechanical property of the ultrathin COF-PDA layer. On the basis of the multilayer architecture, the modified separator not only effectively suppresses the shuttle effect but also realizes rapid lithium-ion transportation. As expected, the battery with a COF-PDA/SWCNT/PP separator presents a reversible specific capacity of 1031 mAh g–1 over 100 cycles at 0.2 C. Moreover, a high reversible capacity of 642 mAh g–1 over 500 cycles even at 1 C and outstanding anti-self-discharge behavior by a low capacity-attenuation of 4.6% over 7 days are also acquired for the cells implementing the as-developed separator. Hence, this proposed strategy of multilayer modified separators will open a new route to prepare high-performance Li–S batteries.