Sulfhydryl-functionalized COF-based electrolyte strengthens chemical affinity toward polysulfides in quasi-solid-state Li-S batteries

Abstract

For lithium-sulfur batteries (Li-S batteries), a high-content electrolyte typically can exacerbate the shuttle effect, while a lean electrolyte may lead to decreased Li-ion conductivity and reduced catalytic conversion efficiency, so achieving an appropriate electrolyte-to-sulfur ratio (E/S ratio) is essential for improving the battery cycling efficiency. A quasi-solid electrolyte (COF-SH@PVDF-HFP) with strong adsorption and high catalytic conversion was constructed for in situ covalent organic framework (COF) growth on highly polarized polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) fibers. COF-SH@PVDF-HFP enables efficient Li-ion conductivity with low-content liquid electrolyte and effectively suppresses the shuttle effect. The results based on in situ Fourier-transform infrared, in situ Raman, UV–Vis, X-ray photoelectron, and density functional theory calculations confirmed the high catalytic conversion of COF-SH layer containing sulfhydryl and imine groups for the lithium polysulfides. Lithium plating/stripping tests based on Li/COF-SH@PVDF-HFP/Li show excellent lithium compatibility (5 mAh cm^–2 for 1400 h). The assembled Li-S battery exhibits excellent rate (2 C 688.7 mAh g^–1) and cycle performance (at 2 C of 568.8 mAh g^–1 with a capacity retention of 77.3% after 800 cycles). This is the first report to improve the cycling stability of quasi-solid-state Li-S batteries by reducing both the E/S ratio and the designing strategy of sulfhydryl-functionalized COF for quasi-solid electrolytes. This process opens up the possibility of the high performance of solid-state Li-S batteries.

Keywords

lithium-sulfur batteries / low electrolyte-to-sulfur ratio / polysulfide shuttle / PVDF-HFP/COF

Author summay

Linnan Bi and Jie Xiao contributed equally to this study.