Valence‐Dependent Overscreening and Charging Pathways in Aqueous Electric Double Layers

Multivalent‐ion aqueous electrochemical systems often operate in strongly coupled regimes where classical mean‐field electric double‐layer descriptions become inadequate, yet a unified valence‐resolved microscopic picture of screening and charging remains incomplete. Here, we perform constant‐potential molecular dynamics simulations of 1 M aqueous NaCl, MgCl 2 , and AlCl 3 confined between Au electrodes under strictly matched conditions. We find that increasing cation valence drives a transition from a counterion‐dominated Stern layer (NaCl) to mixed‐ion Stern layers with substantial coion penetration (MgCl 2 /AlCl 3 ), which induces pronounced multilayer overscreening and enhanced interfacial cation–anion association. Valence also reshapes the charging pathway. NaCl follows counterion adsorption/ion exchange, whereas MgCl 2 and AlCl 3 increasingly charge through cooperative rearrangement of correlated ion pairs/clusters across multiple layers, accompanied by slower electrode charging dynamics. Water orientation analysis reveals more heterogeneous near‐surface solvent environments in multivalent electrolytes, consistent with correlation‐enabled mixed‐ion interfacial organization. These results establish mechanistic signatures linking coion penetration, overscreening, and pair/cluster‐mediated charging, and provide molecular‐level insight for future studies of additive‐regulated interfacial restructuring, desolvation, and deposition processes in aqueous Zn‐ion batteries.