Extended Endocyclic Conjugation and <i>N,N</i> ‐Bidentate Chelation Alleviate the Capacity–Stability Trade‐Off in Organic Magnesium Battery Cathodes

ABSTRACT Rechargeable Mg batteries represent an appealing post‐lithium energy‐storage technology, yet their advancement is hampered by the scarcity of cathode materials combining high capacity, rapid kinetics, and long‐term cycling stability. In this study, we propose a molecular design strategy integrating extended endocyclic conjugation with polydentate Mg 2+ coordination. Using hexaazatriphenylene (HATN), a rigid planar macrocycle featuring extensive π‐conjugation and N , N ‐bidentate chelating sites, as the Mg‐storage active center, we constructed polymer cathodes through monothioether and dithioether linkages. Theoretical and experimental analyses reveal that the HATN unit enables high‐capacity, multi‐electron reversible Mg 2+ storage while maintaining structural stability via efficient charge buffering through strong electron delocalization, offering a notable advantage in a “capacity‒delocalization” evaluation framework. The thioether linkage suppresses dissolution and yields high surface area with hierarchical porosity, boosting interfacial kinetics and Mg 2+ transport. The resulting polymer cathode delivers a high capacity of 370 mAh g ‒1 at 0.1 A g ‒1 , superior rate capability (94 mAh g ‒1 at 5.0 A g ‒1 ), and exceptional cycling stability (95% capacity retention over 500 cycles at 1.0 A g ‒1 ). This work presents an innovative molecular‐level design strategy for high‐performance organic Mg‐battery cathodes, advances the mechanistic understanding of multivalent‐ion storage, and provides a new paradigm for rational electrode engineering for multivalent battery systems.