Proton‐Mediated Dynamic Interfacial Regulation in a Covalent Polymer Anode for Stable Aqueous Calcium‐Ion Batteries

ABSTRACT In aqueous battery systems, proton co‐storage commonly accompanies the insertion of charge carriers, yet its influence on interfacial electrochemistry remains poorly understood. Here, a covalent polymer (PCD) is reported as an anode material for aqueous calcium‐ion batteries, operating through a Ca 2+ /H + storage mechanism associated with redox‐active C═N moieties. Proton adsorption dynamically modifies the interfacial microenvironment and induces the reversible formation of a Ca(OH) 2 surface phase. Rather than impairing performance, this proton‐mediated interfacial Ca(OH) 2 effectively suppresses hydrogen evolution, enabling stable operation at extended negative potentials. As a result, the PCD anode delivers high capacity, rapid charge–discharge response, and exceptional cycling stability. A 43 mAh pouch cell retains 80.4% of its capacity over 1900 cycles (>1000 h), representing a significant step forward in aqueous Ca 2+ storage. These findings reveal that proton‐induced interfacial phases can be harnessed to regulate parasitic reactions, offering a new paradigm for the design of stable aqueous battery electrodes.