Heteroatom-substituted phenazine-based pyridine isomers as high-performance organic cathodes for aqueous zinc-ion batteries

PPPQ exhibited a high theoretical specific capacity and strong electron-withdrawing ability. • PPPQ showed narrow band gap, low solubility and excellent stability in 3 M ZnSO 4 . • PPPQ electrode exhibited electrolyte wettability, which enabled the improvement of interface regulation. Aqueous zinc-ion batteries (AZIBs) have attracted considerable attention because of their high safety, low cost, and environmental compatibility. However, the practical application of organic cathode materials is still limited by high solubility, low electronic conductivity, and restricted theoretical capacity. Herein, a heteroatom-substitution strategy was proposed to regulate the molecular structure of phenazine-based cathodes by replacing benzene rings with pyridine groups and tuning the nitrogen-doping positions. Two phenazine-based pyridine isomers, pyrido[2,3- b ]pyrido[3′,2′:5,6]pyrazino[2,3- f ]pyrido[3′,2′:5,6]pyrazino[2,3- h ]quinoxaline (PPPQ) and pyrido[3,4- b ]pyrido[4′,3′:5,6]pyrazino[2,3- f ]pyrido[4′,3′:5,6]pyrazino[2,3- h ]quinoxaline (PQNI), were designed and synthesized. Theoretical calculations reveal that the introduction of pyridinic N atoms modulates the electron-cloud distribution, narrows the HOMO–LUMO gap, enhances electron delocalization, and creates a negatively charged molecular environment, thereby improving redox activity and interfacial stability. Compared with PQNI, the ortho-linked PPPQ exhibits stronger π-conjugation, more ordered π–π stacking, lower dissolution, and faster charge-transfer kinetics. As a result, the PPPQ cathode delivers a high specific capacity of 354 mAh g −1 at 0.1 A g −1 and maintains 95% capacity retention after 2000 cycles at 5 A g −1 . In situ/ex situ characterizations and density functional theory calculations further demonstrate that the C N sites serve as redox-active centers and that PPPQ undergoes a proton-dominated three-step 6e − charge-storage process. This work clarifies the structure–property relationship of heteroatom-substituted phenazine-based organic cathodes and provides a molecular design strategy for high-performance AZIBs. Download: Download high-res image (131KB) Download: Download full-size image