The benzothiadiazole-based COF/CNT composite as host for efficient Li-S batteries with excellent cycling stability

Abstract

The cycle stability and dynamic performance of lithium-sulfur (Li-S) batteries have been major challenges, which primarily arise from the dissolution and migration of the soluble reaction intermediates during cycling, as well as the low conductivity of sulfur. In this study, a covalent organic framework containing heteroatoms (B/N/S) is uniformly wrapped on the outer surface of carbon nanotubes to form a nanocomposite (COF@CNT). The composite combines the excellent electrical conductivity of CNTs and the COF material containing evenly distributed heteroatoms that can adsorb and catalyze polysulfides. B atom on COF can be chemically adsorbed by the negatively charged SX2−, S and N atoms can interact with Li+ to reduce the “shuttle effect”. At the same time, the heteroatoms promote the liquid-to-solid phase conversion of the soluble long-chain Li2Sx (4 ≤ x ≤ 8), and the subsequent conversion of solid Li2S2 to Li2S, thereby enhancing the dynamic kinetics. As a cathode material, COF@CNT/S exhibits an initial specific capacity of 1176 mAh/g at 0.2 C, maintaining a capacity of 688 mAh/g after 1000 cycles at 1 C with a Coulombic efficiency of 99.7 %. Notably, a quite low capacity decay rate of 0.029 % per cycle has been achieved, making the COF@CNT/S cathode one of the most stable electrode material for Li-S batteries. This work provides insights into improving cycle performance of Li-S batteries, and also enriches the current study on host materials.