Sulfur and Wavy‐Stacking Boosted Superior Lithium Storage in 2D Covalent Organic Frameworks

Abstract

Abstract2D conjugated covalent organic frameworks (c‐COFs) provide an attractive foundation as organic electrodes in energy storage devices, but their storage capability is long hindered by limited ion accessibility within densely π–π stacked interlayers. Herein, two kinds of 2D c‐COFs based on dioxin and dithiine linkages are reported, which exhibit distinct in‐plane configurations—fully planar and undulated layers. X‐ray diffraction analysis reveals wavy square‐planar networks in dithiine‐bridged COF (COF‐S), attributed to curved C─S─C bonds in the dithiine linkage, whereas dioxin‐bridged COF (COF‐O) features densely packed fully planar layers. Theoretical and experimental results elucidate that the undulated stacking within COF‐S possesses an expanded layer distance of 3.8 Å and facilitates effective and rapid Li+ storage, yielding a superior specific capacity of 1305 mAh g−1 at 0.5 A g−1, surpassing that of COF‐O (1180 mAh g−1 at 0.5 A g−1). COF‐S also demonstrates an admirable cycle life with 80.4% capacity retention after 5000 cycles. As determined, self‐expanded wavy‐stacking geometry, S‐enriched dithiine in COF‐S enhances the accessibility and redox activity of Li storage, allowing each phthalocyanine core to store 12 Li+ compared to 8 Li+ in COF‐O. These findings underscore the elements and stacking modes of 2D c‐COFs, enabling tunable layer distance and modulation of accessible ions.