Ladder‐Type Covalent Organic Frameworks with Highly Delocalized π‐Electrons and Dense Redox‐Active Sites toward Robust Aqueous Iron Organic Batteries

Abstract Aqueous iron‐ion batteries (AIIBs) have demonstrated fascinating advantages in large‐scale energy storage, whereas the development of high‐performance Fe 2+ hosting cathode materials is still at its infancy. Herein, two hexaazatrinaphthyalene (HATN)‐based poly(benzimidazobenzophenanthroline) (BBL)‐ladder‐type covalent organic frameworks (COFs) (namely HAQ‐COF and HAB‐COF) are synthesized and for the first time served them as cathodes for AIIBs. The rigid backbones and delocalized π ‐electron networks endow them with stable structure and fast charge transport, while the dense arrangement of redox‐active groups in HAQ‐COF generates more chelating sites, which can facilitate the storage of multivalent metal ions. As a result, HAQ‐COF cathodes for AIIBs delivers a high specific capacity of 226 mAh g −1 at 0.2 A g −1 , excellent rate capability, and long‐term cycling stability with 87% capacity retention over 26 000 cycles at 2 A g −1 . Combined experimental (in situ/ex situ spectroscopy) analyses and DFT calculations uncover a dual‐site 24‐electron redox mechanism with Fe 2+ sequentially coordination by carbonyl and imine moieties. This work not only establishes BBL‐type COFs as high‐performance cathodes for AIIBs but also provides mechanistic insight into Fe 2+ storage, thereby informing rational design of sustainable and high‐performance cathode materials for multivalent‐ion energy storage systems.