Novel covalent organic framework/carbon nanotube composites with multiple redox-active sites for high-performance Na storage

Covalent organic frameworks (COFs) have attracted great attention as promising energy storage materials due to their exceptional crystallinity, designable periodic skeletons, adjustable porous distribution, and ordered accessible nano-channels. However, the reported COF-based cathodes are hindered by unsatisfying capacity and limited rate performance because of their limited utilization of redox-active groups and poorer electrical conductivity. Herein, a novel TP-OH-COF with rich redox-active groups integrated with carbon nanotube (TP-OHCOF@CNT50) is prepared in a one in-situ polycondensation. The few-layered TP-OH-COF with abundant active groups (C--O) wrapped on the surface of CNT can accommodate more Na-ions and shorten the ion/electron diffusion distance. As a sodium-ion batteries (SIBs) cathode, the TP-OH-COF@CNT50 delivers a high specific capacity of 256.4 mAh g-1 at 0.1 A g-1, ultra-long cycling stability (100 % retention after 3000 cycles at 2 A g-1), and excellent rate performance (103 mAh g-1 at 10 A g-1). The combination of in (ex) situ experiments manifests the high reversible surface-dominated Na-storage mechanism and structural stability with lower energy barrier for Na-ions diffusion in TP-OH-COF@CNT50 during Na-ions insertion/extraction. The theoretical calculations unveil the reaction sites and processes of Na-ions storage in TP-OH-COF@CNT50. The results provide an effective strategy for designing new COFs with high energy storage for SIBs.