Near-Zero Degradation in a Ladder-Type Polymer Cathode via Zn²⁺-Buffered Proton Storage

Organic electrodes in aqueous Zn batteries often suffer from capacity decay due to unstable charge-compensation chemistry and structural relaxation during cycling. Here, we report an ultralong-life ladder-type polymer cathode based on poly(benzimidazobenzophenanthroline) (BBL), in which proton-coupled charge storage is sustained by Zn²⁺-mediated local proton buffering in ZnSO₄. Although the bulk electrolyte is only mildly acidic, redox titration reveals a lower-potential Zn²⁺-associated feature that exhibits comparatively weak pH dependence within the Zn²⁺-containing series, consistent with a locally buffered proton-activity regime rather than a separate Zn²⁺-insertion process. Ex situ XRD, XPS, and SEM show reversible/partially reversible Zn₄SO₄(OH)₆·5H₂O formation, supporting Zn²⁺ hydrolysis-assisted proton regulation. The rigid π-conjugated ladder backbone suppresses structural relaxation, while in situ FTIR and N 1s XPS support preferential participation of imine-containing nitrogen sites, whereas carbonyl groups are not the dominant electrochemically addressable sites under the present conditions. Consequently, the BBL cathode delivers ∼135 mAh g^-1 with ∼99% capacity retention over 46 000 cycles at 1 A g^-1, together with high rate capability and robust high-mass-loading performance. These results suggest that Zn²⁺-mediated local proton buffering can stabilize proton-coupled charge storage in a rigid ladder-polymer framework and provide a viable route toward long-life aqueous organic batteries.