Extended Endocyclic Conjugation and N,N-Bidentate Chelation Alleviate the Capacity-Stability Trade-Off in Organic...

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 Mg²⁺ 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 Mg²⁺ 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 Mg²⁺ 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.