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 D 2h 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 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.

Proton‐Induced Defect‐Rich Vanadium Oxides as Reversible Polysulfide Conversion Sites for High‐Performance Lithium Sulfur Batteries

“…Moreover, their good conductivity also contributes to promoting electron transfer during the electrochemical process [25–28] . To overcome the poor affinity of polysulfides, polar materials (metal oxides/sulfides, single metal, and heteroatom doping) are emerging to trap polysulfides by chemical interaction and retard serious dissolution [29–35] . Among them, vanadium‐based compounds possess rich chemistry properties because of the variable valences of vanadium, and they are widely used as promising candidates for electrochemical catalysts [36–39] .…”

“…[25][26][27][28] To overcome the poor affinity of polysulfides, polar materials (metal oxides/sulfides, single metal, and heteroatom doping) are emerging to trap polysulfides by chemical interaction and retard serious dissolution. [29][30][31][32][33][34][35] Among them, vanadium-based compounds possess rich chemistry properties because of the variable valences of vanadium, and they are widely used as promising candidates for electrochemical catalysts. [36][37][38][39] Chemically active vanadium sites could serve as polysulfides trapper to mitigate the shuttle effect in Li-S batteries.…”

Rationally Fabricating Three-Dimensional Covalent Organic Frameworks for Propyne/Propylene Separation

“…However, to the best of our knowledge, applying COFs for C 3 H 4 /C 3 H 6 separation has not been reported yet. Compared with traditional 2D COFs, which usually possess large and noninterconnected pores, 3D COFs tend to possess more complex microporous structures, interconnected channels, and higher structural robustness and have more accessible active sites, which endow them with higher superiority for gas separation. − In recent years, 3D COFs have attracted increasing interest in the construction of novel topologies and functionalities. − ,,− However, hindered by the limited building blocks and difficulty in structure determination, using 3D COFs for efficient gas separation is still in its infancy. ,, Thus, developing 3D COFs with tailored microporous structures and accessible active sites for gas separation, especially C 3 H 4 /C 3 H 6 separation, is of great significance. To achieve such a goal, there are two feasible design approaches: (1) introducing the specific adsorption sites ,, and (2) forming appropriate pore apertures to enhance the interaction of gas molecules and frameworks …”

Post-synthetic Fully π-Conjugated Three-Dimensional Covalent Organic Frameworks for High-Performance Lithium Storage

“…A fully π-conjugated 3D COF was then obtained (Scheme ). Triazine and pyrazine moieties in the 3D COFs contain rich nitrogen atoms which can bind the Li ions and provide multiple ion adsorption active sites and ion storage space. , In addition, compared to the traditional 2D COFs with strong π–π interaction between the 2D layers, the 3D structure in this work could efficiently avoid the π–π interaction between the layers and some negative influence on the low redox site utilization and improve reversible energy storage capacity. , Attributed to the high chemical stability and π-conjugation degree, as well as the good electronic and ionic conductivities, the Li-ion batteries (LIBs) assembled with the PYTRI-COF-2 electrodes display high Li-ion storage capacity and good cycling stability, maintaining a specific capacity of 227.3 mA h g –1 after 450 cycles.…”