Zinc Plating on Copper Proceeds via Breakdown of a Capacitive Electric Double Layer

The electric double layer (EDL) at solid-liquid interfaces governs electrochemical processes from plating to catalysis, yet its atomistic dynamics remain poorly defined. Using operando atomic-resolution transmission electron microscopy, we directly visualize EDL formation, growth, and collapse during zinc electroplating on copper in an ionic liquid electrolyte. Under galvanostatic conditions, the EDL appears as a dense amorphous layer that grows via charge accumulation, and dynamic surface erosion of the substrate releases surface atoms that nucleate transient metallic nanoparticles within the EDL. Enlargement of these particles locally short-circuits the capacitive layer, leading to abrupt dielectric breakdown, heat generation, and alloy deposition. Recurrent growth-breakdown cycles (240-520 s) produce ∼2 nm Cu/Zn alloy layers, with an activation free energy of ∼86 kJ mol^-1. Strikingly, brass nanoparticles form spontaneously at room temperature despite requiring ∼1000 °C in bulk, reflecting the large interfacial energy of nanoscale species. This breakdown-driven mechanism reframes electroplating as a discontinuous, chemically reactive, and electrostatically unstable process, providing a unifying explanation for the rough morphologies often observed in plated films. More broadly, our findings suggest that the dielectric breakdown of chemically active EDLs is a general phenomenon relevant to plating, energy storage, catalysis, and other interfacial transformations.