Conjugated Three-Dimensional High-Connected Covalent Organic Frameworks for Lithium–Sulfur Batteries

Abstract

Developing conjugated three-dimensional (3D) covalent organic frameworks (COFs) still remains an extremely difficult task due to the lack of enough conjugated 3D building blocks. Herein, condensation between an 8-connected pentiptycene-based D2h building block (DMOPTP) and 4-connected square-planar linkers affords two 3D COFs (named 3D-scu-COF-1 and 3D-scu-COF-2). A combination of the 3D homoaromatic conjugated structure of the former building block with the 2D conjugated structure of the latter linking units enables the π-electron delocalization over the whole frameworks of both COFs, endowing them with excellent conductivities of 3.2–3.5 × 10–5 S cm–1. In particular, the 3D rigid quadrangular prism shape of DMOPTP guides the formation of a twofold interpenetrated scu 3D topology and high-connected permanent porosity with a large Brunauer–Emmett–Teller (BET) surface area of 2340 and 1602 m2 g–1 for 3D-scu-COF-1 and 3D-scu-COF-2, respectively, ensuring effective small molecule storage and mass transport characteristics. This, in combination with their good charge transport properties, renders them promising sulfur host materials for lithium–sulfur batteries (LSBs) with high capacities (1035–1155 mA h g–1 at 0.2 C, 1 C = 1675 mA g–1), excellent rate capabilities (713–757 mA h g–1 at 5.0 C), and superior cycling stability (71–83% capacity retention at 2.0 C after 500 cycles), surpassing the most of organic LSB cathodes reported thus far.