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

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 Ca²⁺/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 Ca²⁺ 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.