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

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 pi-electron delocalization over the whole frameworks of both COFs, endowing them with excellent conductivities of 3.2-3.5 x 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 m(2) 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.

Automated summary

In this study, two three-dimensional covalent organic frameworks (COFs) were successfully developed by condensation between an 8-connected three-dimensional building block and a 4-connected two-dimensional linker. The resulting COFs showed excellent conductivity and high porosity, making them promising materials for high-capacity, high-rate, and stable sulfur cathodes in lithium-sulfur batteries.