Assembling self-adaptive covalent organic frameworks by rigid-flexible blocks to enhance potassium storage

Abstract

Owing to its ability to reversibly accommodate K+ ions through structural adjustment, self-adaptive covalent organic frameworks (COFs) are compatible with the dynamic process of K+ ions interaction/deinteraction, allowing them to harness more active sites within a finite space while minimizing structural evolution. To obtain self-adaptive COFs anodes with high capacity and conductivity, assembling both rigid and flexible blocks into frameworks is perceived as an ideal strategy. Herein, a novel self-adaptive two-dimensional COF (sa-2D-COF) is assembled by combining rigid blocks tris(1H-imidazol-1-yl)triazine (TIT) and flexible blocks 1,6-di(N-p-chlorophenyl-biguanidino)hexane (CHX). Combining the intrinsic electron delocalization of rigid TIT with the structure flexibility of flexible CHX, the sa-2D-COF anode exhibits good conductivity and a high reversible capacity of 237.7 mAh g−1 at 50 mA g−1 for potassium-ion batteries. Interestingly, a capacity increment was detected during the long-term cycles (230.7 mAh g−1 after 2400 cycles at 200 mA g−1 with 127.7 % capacity retention). Due to the prolonged cycling test, the structure of sa-2D-COF gradually adjusts to its optimal structure, while still maintaining an enlarged and stabilized capacity. The storage mechanisms were characterized by ex-situ characterizations and an electrostatic potential map, and the result shows that aromatic C=N, guanidinium C=N groups, and cation−π effect serve as active sites.