A nitrogen-rich two dimensional covalent organic framework with multiple carbonyls as a highly efficient anchoring material for lithium–sulfur batteries

Abstract

Lithium–sulfur (Li–S) batteries, as next generation energy storage systems, have gained considerable attention due to their high energy density, low cost, and environmental friendliness. However, the practical applications of Li–S batteries are presently hindered by several issues, such as the low conductivity of sulfur species, the shuttle effect of polysulfides, and poor conversion efficiency in discharging/charging processes. In this study, using density functional theory (DFT)-based computations, we propose a two dimensional (2D) covalent organic framework (COF) with triquinoxalinylene (TQ) and benzoquinone (BQ) units in its skeleton, namely, TQBQCOF, as a promising sulfur host material for high-performance Li–S batteries. We have found that the TQBQCOF is a semiconductor with a band gap of 1.16 eV. After the adsorption of LiPSs the TQBQCOF becomes metallic in nature, which ensures its excellent electronic conductivity. The moderate adsorption energies of the TQBQCOF to the soluble LiPSs can effectively suppress the shuttle effect of polysulfides. Notably, the TQBQCOF also shows high catalytic activity for the sulfur reduction reactions (SRR) in the discharge process and Li₂S decomposition in the charging process of Li–S batteries. The Gibbs free energy barrier for the SRR is 0.22 eV, while the decomposition barrier of a Li₂S molecule on the TQBQCOF is only 0.04 eV, ensuring the rapid charging and discharging processes of Li–S batteries.