On the kinetics of electrodeposition in a magnesium metal anode

Magnesium (Mg) metal batteries are promising for next-generation energy storage due to Mg’s abundance and potential for improved energy densities through two-electron redox in cathodes and thin film anodes supporting high current densities. However, reversible and uniform electrodeposition of Mg necessary for high-energy-density Mg batteries continues to be plagued by challenges traceable to energy barriers to desolvation in liquid electrolytes, the stabilization of passivation layers that impede Mg diffusion at electrified interfaces, and deleterious high-asperity electrodeposition morphologies. Thus, the specific impact and coupling of factors such as (i) composition of the salt solution, (ii) electrode overpotential, and (iii) reversible stripping/plating on the interfacial kinetics of electrodes remains an area of intense interest. This study investigates the effects of overpotential time-transients and electrolyte concentration using experimentally-guided phase-field methods and compares them with an electrochemical spherical cap model. Complex dynamics at the electrode/electrolyte interface involve transient changes in overpotential, influencing nucleation, diffusion, and crystallization, ultimately altering surface evolution. We find that the Damkohler number converges to approximately one for a symmetric Mg-Mg cell using 0.1 M Mg[TFSI] 2 in 1:1 v/v diglyme/dimethoxyethane suggesting a subtle balance between reactive and diffusive forces. The results indicate the formation of dendritic structures is primarily driven by limitations in electrolyte transport. In addition, we determine the limiting current density of this cell under various electrolyte concentrations, which is an essential step in successful battery design and operation. Our findings underscore the critical role of overpotentials in underpinning reaction kinetics and non-equilibrium growth dynamics during metal electrodepositon.