Covalent organic frameworks with silsesquioxane and quinoline units as multifunctional separator modifier for stable Lithium-Sulfur batteries

The realization of practical lithium-sulfur (Li-S) batteries necessitates simultaneous manipulation the interfacial chemistry of sulfur cathode, lithium anodes and conduction paths to tackle the challenges associated with the electrochemical reversibility and conversion kinetics. Designing functional interlayers with well-defined directional channels and accessible active sites holds significant potential in this context. Here, we demonstrate an implementation of constructing quinoline-linked covalent organic frameworks (QSQ-COF) that relies on the selection of silsesquioxane cage-like building blocks and the aza-Diels-Alder cycloaddition approach. This design creates uniquely hierarchical porous channels along with abundant lithiophilic units anchored periodically within, synergistically offering efficient ion/electron conduction and stronger polysulfide trapping in Li-S batteries. Specifically, integrating such hybrid networks as a functional interlayer with separator help attaining boosted redox kinetics of sulfur electrode and improved cycling stability. The mechanically robust interlayer can also guide a homogeneous deposition of Li ion at the metal anode that achieves a smooth surface without dendrite growth. Thus, the full cell assembled with QSQ-COF interlayer demonstrates outstanding electrochemical performance by maintaining a capacity retention rate of 92.35 % after undergoing 200 cycles at a rate of 0.2C, and in Li-Li symmetric cells exhibiting Li+ transfer numbers up to 0.89.