Covalent organic framework-based core-shell sulfur cathode design for enhanced polysulfide immobilization and redox kinetics in lithium-sulfur batteries

The industrialization of lithium-sulfur (Li-S) batteries is largely hindered by the low cycle performance stemming from the polysulfide shuttle effect and slow reaction kinetics. In order to address these problems, an advanced design strategy for multifunctional sulfur cathode with core-shell structure is proposed by embedding transition metal-based porous carbon composites derived from metal-organic framework (MOF) within covalent organic framework (COF). The inner carbon/metal core within the composite serves both as sulfur trapping and catalyst for promoting sulfur reaction kinetics. The COF shell, rich in micropores and polar functional groups, further mitigates lithium polysulfide (LiPS) shuttling through its sieving effect. This strategy is demonstrated by fabricating Co-decorated MIL-125-NH2-derived Co/Ti-NC composite coated with a layer of TpPa-1 COF. Based on these advantages, the S/Co/Ti-NC@TpPa cathode delivers an outstanding cycling performance. The battery shows a high initial specific capacity of 1135 mAh g−1 at 0.2 C, and the capacity decay rate is 0.05% per cycle at 1 C after 500 cycles. The area capacity of the cell reaches 3.6 mAh cm−2 at a high sulfur loading of 4.46 mg cm−2. The MOF in COF core-shell design provides a promising strategy for developing practical sulfur cathode.