Manipulating Kinetic Competition and Electrocrystallization at Electrochemical Interfaces in Aqueous Zn Batteries

Electrochemical stability of metal electrodes in liquid electrolytes is of fundamental interest in the development of safe, long-duration electrochemical energy storage technologies. It is known that the morphology and reversibility of the electrocrystallized metal can be influenced by the electrodeposition rate, interface chemistry, and substrate crystallography. Little is known about how these factors contribute to producing unstable, out-of-plane, mossy metal deposit growth below the classical diffusion limit. Herein, we report that sluggish solid-state ion transport across an emergent solid electrolyte interphase (SEI) produces a transport-limited electrokinetic regime at current densities well below the diffusion-limited value. Electrochemical analysis in a rotating disk electrochemical cell using Zn substrates with dominant (002) textures reveals that the kinetics of parasitic reactions between electrolyte components and substrate is key to understanding the new transport regime, as well as for controlling nonplanar, mossy growth of metal electrocrystals. The ultimate structure of the SEI is determined by competitive chemical kinetics of the metal electroreduction and parasitic reactions, is influenced by the cation solvation environment, and is essentially insensitive to the crystallography of the Zn anode. Finally, we report on the discovery of a critical electrodeposition rate (j_critical), above which the metal electrocrystallization reactions are generally favored, evidenced by a discontinuous increase in the Coulombic efficiency from values as low as 55% to above 95% in the same electrolyte. The practical relevance of our findings is evaluated in Zn||I₂ electrochemical cells in various configurations (coin cells ∼1.9 mAh cm^-2, pouch cells ∼1 mAh cm^-2 at 10 mA cm^-2).