The Confined Protonated Pyridinium within Covalent Organic Frameworks Symmetrically Intensifying the Sulfur-Species-Related Redox Reactions in Lithium–Sulfur Batteries

Speeding up ion transfer in lithium-sulfur batteries (LSBs) and mitigating kinetic sluggishness are key strategies for high specific capacities. From the perspective of balancing and promoting redox reactions in LSBs, protonated pyridinium covalent organic frameworks (COFs) (PDTA-COF:TFSI-) are synthesized. PDTA-COF:TFSI- topologically grows and self-assembles into a one-dimensional (1D) fiber-like morphology. These 1D COFs assemblies finally form a three-dimensional (3D) network with protonated pyridinium confined in COFs hexagonal cavities of about 2.3 nm in diameter. Owing to these confined protonated pyridiniums, PDTA-COF:TFSI- assemblies serve as microreactors for sulfur-species-related reactions. PDTA-COF:TFSI- has high electrolyte affinity and guarantees the targeted ion transfer toward the confined protonated pyridinium. Due to the reduced mass transfer barrier of ions, the Li+ transference number and the ionic conductivity reach 0.81 and 1.62 mS cm-1 at 25 °C, respectively. Density functional theory (DFT) calculations and Tafel kinetic performances confirm that, owing to the integration of the aforementioned multiple functions at the protonated pyridinium within microreactors, the sulfur-species-related reactions are symmetrically intensified, mediated by thiosulfate through a δ+-charged transition state. The (-)Li|PDTA-COF:TFSI-@PP|S(+) cell demonstrates a high reversible specific capacity of 1330.8 mAh g-1 with a capacity retention rate of 96.4%, even after 400 cycles at 0.1C.