Two-dimensional covalent organic frameworks with p- and bipolar-type redox-active centers for organic high-performance Li-ion battery cathodes

Redox-active covalent organic frameworks (COFs) are considered to be promising cathode materials for various secondary batteries. In particular, the combination of building blocks containing p-type/bipolar-type redox-active centers with p-type/bipolar-type linkages into the COF skeleton is supposed to have advantages in outputting a high redox potential with increased capacities and good cycling stability, which, however, has not been reported, to the best of our knowledge. Herein, a bipolar-type 2D COF, namely TPPDA-CuPor-COF, has been fabricated through the condensation of tetraphenyl-p-phenylenediamine (TPPDA) with copper-5,10,15,20-tetrakis(4-formylphenyl)-porphyrin (Cu-TFPP). In TPPDA-CuPor-COF, the TPPDA moieties enable a p-type doping reaction while the Cu-TFPP moieties are able to act as bipolar-type active centers, favorable for a high redox potential and increased content of redox-active groups. Thus, in combination with the unhindered ion/electron diffusion paths caused by its stable and crystalline porous structure, the TPPDA-CoPor-COF cathode gives a high capacity of 142 mA h g(-1) with a high average output voltage of 2.7 V vs. Li/Li+ at 60 mA g(-1), excellent rate performance, and good cycling stability in LIBs, revealing the promising application of TPPDA-CuPor-COF as a cathode material for secondary batteries. In addition, the bipolar redox mechanism for the TPPDA-CuPor-COF cathode has been demonstrated by density functional theory (DFT) calculations and ex situ X-ray photoelectron spectroscopy (XPS) during its electrochemical process.