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.