Electrolyte Additives for Controlling Roughness of Copper Electrodeposits Formed in Deep Eutectic Solvents

Due to their wide electrochemical stability window, non-toxicity, and highly solubility for metal salts, deep eutectic solvents (DESs) are attracting attention as new class of electrolytes for electrodeposition and energy storage applications. However, due to their viscous nature, DESs impose transport limitations implying rough or dendritic morphology of electrodeposits formed in them. In the present work, we investigated roughness evolution during copper electrodeposition in a DES containing choline chloride and ethylene glycol in a 1:2 molar ratio (ethaline). Cuprous chloride was added to the DES as the metal precursor. Further, polyethyleneimine (PEI) – a common electrolyte additive used in electrodeposition from aqueous media – was added to ethaline and its effects on roughness evolution were investigated. Steady-state polarization measurements confirmed that PEI suppresses copper electrodeposition kinetics. This manifests into micro-scale leveling of deposits and thus suppression of roughness amplification during electrodeposition, which was confirmed electrochemically as well as using optical profilometry. Effects of PEI concentration on electrochemical behavior and roughness suppression were investigated. Specifically, hysteretic response during cyclic voltammetry was observed under certain conditions. Implications of the PEI-induced hysteresis on suppression of roughness in high-efficiency rechargeable batteries will be discussed.