Tuning the structure characteristic of the flexible covalent organic framework (COF) meets a high performance for lithium-sulfur batteries

Abstract

Rigid-based covalent organic frameworks (COFs) are always employed as sulfur-redox/confined host for adapting its varied states upon cycling of highly effective Li-S batteries. Restricted by the unalterable structure frameworks of rigid COFs, flexible COFs are considered to reserve greater capability in coping with active sulfur to meet higher energy demands, while no flexible COF host has been employed currently. In this work, flexible-based COFs with triazine-ring organic macromolecule blocks (COF-TPT(OH) and COF-TPT) are designed and developed as the host materials of Li-S batteries. The structural diversity of flexible-based COF could provoke the phenomenon of capacity increase, which endows the COF-TPT(OH)@S electrode with a high reversible capacity of 1100 mAh g-1 after 80 cycles at 0.1 C. Even after 1000 cycles at 0.5 C, it can still deliver 732 mAh g−1 with a low decay rate of 0.045 % per cycle. Moreover, the introduction of hydroxyl groups (-OH) equips COF-TPT(OH) with the characteristics of both rigid- and flexible- COFs, contributing to the enhanced abilities of the polysulfides conversion and limitation. Our findings will provide a new idea for the selection of host materials for Li-S batteries.