Low-activation-energy bipolar organic nanostructures for high-capacity and ultralong-life aqueous calcium-ion batteries

Rechargeable aqueous calcium-ion batteries (CIBs) provide a promising solution to problems of large-scale energy storage due to their divalent-electron transfer, resource abundance, and high capacity. However, their advancement is challenged by suboptimal anode materials with low exposure of redox-active motifs in densely stacked and disorganized structures due to high spatial energy barriers, resulting in limited capacity and durability. We designed low-activation-energy bipolar organic nanostructures (BONs) through integrating dual-electron benzoquinone and 4,4'-azodianiline units into extended π-conjugated polymeric skeletons through multi-intermolecular H-bonds (N-H⋯O) and π-π interactions. The well-organized rod geometries of BONs delivered consecutive electron delocalization pathways to fully expose built-in multi-redox carbonyl/azo/amine motifs and strengthen the anti-dissolution ability in aqueous electrolytes. Consequently, stable 4 e^- Ca²⁺/H⁺/OTF^- storage was initiated in the BONs anode with an ultralow activation energy (0.22 eV), thereby liberating a state-of-the-art capacity (302 mAh g^-1) and lifespan (100 000 cycles) among all reported organics in CIBs. Besides, the BONs anode could be leveraged to design an advanced BONs‖KCoFe(CN)₆ full battery with superior capacity (210 mAh g^-1), high energy density (221 Wh kg^-1 anode) and long-lasting cycling stability (20 000 cycles). This work constitutes a major advance in designing multi-redox organic nanostructures for better CIBs.