Zn Metal Electroplating via Transient SEI Pulsed Methods: From µm to mm Scales

Electrochemical metal deposition (electroplating) is central to numerous industrial processes, including integrated circuit metallization, electrowinning of critical minerals, and surface finishing. Beyond bath composition, the design of task-specific current protocols offers significant opportunities for tailoring the structural features of plated metals. In particular, the present work is motivated by the role of a Transient Solid–Electrolyte Interphase (T-SEI) formed during fast electro-dissolution, which arises from local supersaturation and salt precipitation. The formation of such a T-SEI can remove the heterogeneous native passivation layer, expose fresh metal grains for electrochemical growth, and fundamentally alter electrodeposition morphology by promoting compact, uniform growth upon redeposition. Using Zn electroplating—relevant to both battery anodes and corrosion-resistant coatings—as a model system, we investigate a new T-SEI pulsed protocol for fabricating metallic coatings up to 100 mAh/cm2, and compare it against conventional direct current (DC), pulsed current (PC), and pulse-reversed current (PRC) methods. In the moderate-thickness regime (~5 mAh/cm2, a few microns), conventional protocols promote independent nucleation, producing small grains and increasing surface roughness due to the native passivation layer—highlighting the importance of surface cleaning. In contrast, the T-SEI protocol exposes fresh grain surfaces, yielding larger grains and flatter morphologies, indicative of preferential growth of existing grains. Optimal deposition occurs at moderate current densities (~100 mA/cm2); excessively low currents (~1 mA/cm2) lead to mossy growth, while high currents (~1 A/cm2) cause dendritic growth. By contrast, in the ultra-high-capacity (UHC) regime (~100 mAh/cm2), even with optimized current densities, growth under all four methods becomes dominated by a small number of colossal grains with favorable orientations that exhibit high growth rates, resulting in large grain sizes and roughness metrics over an order of magnitude higher than in the thin-film regime. This roughness persists despite periodic reapplication of the T-SEI step, indicating that non-uniform surface passivation is no longer the dominant factor in the UHC regime.