Hexaazatrinaphthylene self-fused multi-N-heterocycles for high-capacity and long-life aqueous magnesium-ion batteries

Polymers overcome the solubility issue of organic small molecules and offer superior structure-function designability compared to inorganic counterparts, representing a promising class of anode materials for aqueous magnesium-ion batteries (MIBs). However, their capacity remains relatively low (<200 mAh g^-1) due to the insufficient density of accessible redox-active motifs and restricted electron transfer, thereby resulting in an inherent trade-off between electrochemical stability and capacity. Herein, we demonstrate a capacity-stability trade-off-breaking design of hexaazatrinaphthylene self-fused multi-N-heterocycles (PDP) through single-component self-dehalogenation polymerization of the six-electron-transfer 2,8,14-tribromodiquinoxalino[2,3-a:2',3'-c]phenazine (DP) acceptor. The hexaazatrinaphthylene extended π-conjugated structure of PDP maximizes the density of accessible imine sites and enables extensive electron delocalization with a narrow bandgap of 2.48 (vs. 3.13 eV for DP). These features activate rapid multielectron Mg²⁺ redox reactions at N-heterocyclic motifs with a low activation barrier (0.28 eV), delivering an impressive capacity (375 mAh g^-1) for the PDP anode. Moreover, the intramolecular π-π interaction in PDP (-10.2 kcal mol^-1) is stronger than the water solvation force (4.9 kcal mol^-1), conferring excellent structural anti-dissolution in aqueous electrolytes for long-life MIBs (20 000 cycles). When paired with a high-voltage Prussian blue cathode, the high-capacity PDP anode delivers state-of-the-art energy density (252 Wh kg^-1) and cycling stability (88.9% capacity retention over 20 000 cycles). This work broadens the structural diversity of multi-active and stable polymers, marking a good start for advanced MIBs.