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 Fe²⁺ 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 Fe²⁺ 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 Fe²⁺ storage, thereby informing rational design of sustainable and high-performance cathode materials for multivalent-ion energy storage systems.