Covalent Triazine Frameworks for Lithium–Sulfur Batteries: Confinement, Catalysis, and Redox Mediation

Lithium–sulfur (Li–S) batteries offer high theoretical specific energy and low-cost sulfur chemistry, but their practical application is limited by polysulfide shuttle, the electronic insulation of sulfur and Li2S, and sluggish liquid–solid and solid–liquid conversion. Covalent triazine frameworks (CTFs) combine nitrogen-rich pore walls, tunable porosity, chemical robustness, and molecular designability, making them promising platforms for regulating sulfur species. This review focuses on strictly defined CTFs and closely related CTF-derived systems used in Li–S batteries. Rather than treating polysulfide adsorption or improved cycling as evidence of catalysis, the functional roles of CTFs are separated into confinement, catalysis, and redox mediation, while the strength of mechanistic evidence is evaluated independently. Representative sulfur hosts, conductive hybrids, functional separators, and redox-active CTFs are compared with emphasis on Li2S nucleation and growth, deposition morphology, sulfur-conversion kinetics, direct Li2S precipitation/decomposition tests, and practical cell parameters. The analysis identifies balanced polysulfide affinity, electronic connectivity, wet-state pore accessibility, active-site attribution, and framework stability as key structure–activity descriptors. Future progress requires rigorous mechanistic controls, scalable and sustainable synthesis, and testing under high-sulfur-loading and lean-electrolyte conditions. CTFs are therefore best regarded as molecularly tunable platforms for verifiable sulfur-redox regulation rather than universal catalysts.