Covalent Organic Framework with Multiple Redox Active Sites for High-Performance Aqueous Calcium Ion Batteries

Organicmaterials are promising for cation storage in calcium ionbatteries (CIBs). However, the high solubility of organic materialsin an electrolyte and low electronic conductivity remain the key challengesfor high-performance CIBs. Herein, a nitrogen-rich covalent organicframework with multiple carbonyls (TB-COF) is designed as an aqueousanode to address those obstacles. TB-COF demonstrates a high reversiblecapacity of 253 mAh g(-1) at 1.0 A g(-1) and long cycle life (0.01% capacity decay per cycle at 5 A g(-1) after 3000 cycles). The redox mechanism of Ca2+/H+ co-intercalated in COF and chelating withC O and C N active sites is validated. In addition,a novel C C active site was identified for Ca2+ ionstorage. Both computational and empirical results reveal that perTB-COF repetitive unit, up to nine Ca2+ ions are storedafter three staggered intercalation steps, involving three distinctCa(2+) ion storage sites. Finally, the evolution processof radical intermediates further elucidates the C C reactionmechanism.

Automated summary

In this study, a nitrogen-rich covalent organic framework (TB-COF) was designed as an aqueous anode for calcium ion batteries, addressing the challenges of organic materials' solubility and low electronic conductivity. TB-COF demonstrated a high reversible capacity and long cycle life, with a validated redox mechanism and identification of a novel active site for calcium ion storage.