Hierarchical assemblies of conjugated ultrathin COF nanosheets for high-sulfur-loading and long-lifespan lithium-sulfur batteries: Fully-exposed porphyrin matters

Covalent organic frameworks (COFs) are promising electrode materials for next-generation energy storage devices such as Li-S batteries. However, most existing COF materials are produced as microcrystalline powders with densely packed structure and microporous texture, which leads to sluggish ion diffusion, insufficient active site utilization or limited active material loading, constraining practical implementation. Herein, we report the first bottom-up synthesis of 3D hierarchical COF superstructures comprising porphyrin-rich conjugated ultrathin nanosheets, as a multiscale engineering solution to the above obstacles for fully demonstrating the potential of COFs in Li-S batteries. As-made superstructure features minimum nanosheet restacking, large accessible surface area with unique macro-meso-micro-porosity, which not only turns diffusion-dominated redox kinetics in conventional COFs into charge-transfer controlled process, but also renders sufficient utilization of inherent abundant active sites for maximizing the chemisorptions of polysulfides and boosting sulfur utilization. Importantly, COF superstructure can be used as an idea model to probe the specific role of active functional groups and fully exposed porphyrin is revealed to act as unique anchoring sites with strong chemical binding to polysulfides. Thus, COF superstructures, as a new polymeric host, endow Li-S battery with large capacity, excellent rate capability and superb cycling stability particularly at high sulfur content or loading.