A Carboxyl-Engineered Organic Cathode for High-Performance Aqueous Iron-Ion Batteries via Dual Fe²⁺/H⁺ Coordination...

Aqueous iron batteries (AIBs) are promising candidates for large-scale stationary energy storage. However, their development is hindered by the lack of efficient cathodes capable of overcoming the inherent challenges of Fe²⁺ chemistry associated with large hydrate size and strong electrostatic interactions. Here, we report the molecular design of 5,6,11,12,17,18-hexaazatrinaphthylene-2,8,14-tricarboxylic acid (HATTA) as a high-performance organic cathode that simultaneously addresses capacity, voltage, and stability limitations. This is achieved by introducing carboxyl groups into the hexaazatrinaphthylene framework. Specifically, it creates additional active sites and reduces the lowest unoccupied molecular orbital energy that elevates working voltage to 0.55 V. Additionally, the incorporation of COOH groups extends π-conjugation, enhancing rate capability. As a result, the HATTA cathode demonstrates a high capacity of 126 mAh g^-1 at 0.1 A g^-1 and outstanding cycling stability (90 mAh g^-1 after 1500 cycles) that outperform recently reported AIBs cathodes. More importantly, through combined in situ spectroscopy and simulations, we reveal a dual-ion storage mechanism involving concurrent Fe(OTf)⁺ coordination and H⁺ insertion, along with dynamic formation of electroactive FeOOH species that contribute additional capacity. This work not only establishes HATTA as a promising cathode material for AIBs but also provides fundamental insights into molecular design principles for multivalent ion batteries.