Carboxyl-Substituted Organic Molecule Assembled with MXene Nanosheets for Boosting Aqueous Na⁺ Storage

Aqueous alkali-ion batteries have enormous promise as a kind of safe, reliable, and sustainable energy technologies for power supplies. Although organic molecules with tunable and diverse configurations are potential electroactive materials, their inadequate redox activity and electron affinity hinder the practical application for aqueous alkali-ion storage. Herein, a novel electron-withdrawing carboxyl-substituted dipyridophenazine (CDPPZ) organic molecule is designed and synthesized for aqueous Na⁺ storage. Significantly, the introduction of carboxyl functional groups not only serves as additional redox-active sites for reversible Na⁺ coordination, but also causes the rearrangement of intramolecular electron cloud density to reduce the energy level, thereby ensuring the high redox activity and superior electron affinity of the CDPPZ molecule. For portable electronics, a self-supporting, adhesive-free, and flexible CDPPZ@MXene electrode is further constructed by incorporating highly redox-active CDPPZ molecule with MXene nanosheets, which delivers a fast, stable, and unrivaled aqueous Na⁺ storage capability with a high reversible capacity of 172.6 mAh cm^-3 and excellent redox stability over 4000 cycles. In situ dynamic analysis combined with theoretical calculations illustrates the Na⁺ storage mechanism and corresponding coordinated pathway. Finally, a high-performance flexible aqueous Na-ion battery is fabricated with exceptional energy/power density and remarkable cycling lifespan, further confirming its promising application prospect.