Nanoporous Phenazine-Based Covalent Organic Frameworks as Electrocatalytic Sulfur Hosts for Lithium–Sulfur Batteries

The design of sulfur hosts in lithium–sulfur batteries (LSBs) plays a central role in enhancing the battery performance, stability, and sustainability. Here, we report a redox-active, nanoporous phenazine-based covalent organic framework (COF) termed DAPh-TFP COF, synthesized via an organic solvent-free terracotta method using (2,7-diaminophenazine-benzophenoneimine (DAPh-Bnzph) and Tri formyl phloroglucinol (TFP). By embedding phenazine units into the COF backbone, this metal-free host enables dual functionality: structural confinement and catalytic polysulfide conversion. To isolate the contribution of redox-active sites, a control COF, DAA-TFP COF, was synthesized based on redox-inactive 2,6-diaminoanthracene (DAA). Comparative electrochemical analysis of S@DAPh-TFP COF and S@DAA-TFP COF cathodes reveals that the phenazine-functionalized framework significantly enhances redox kinetics, capacity retention, and long-term cycling stability. Impressively, the S@DAPh-TFP COF exhibits a high capacity of 1079 mAh g–1 at 0.05 C and retains 76% of its initial capacity after 1000 cycles at 1 C, with a Coulombic efficiency >95% and a low-capacity fading rate of 0.025% per cycle. This study demonstrates the potential of metal-free redox-active COFs as multifunctional sulfur hosts, combining structural confinement with electrochemical activity for advanced LSBs applications, and serves as a viable approach for the metal-free cathode fabrication in rechargeable battery technologies.