Computational Examination of Orientation-Dependent Morphological Evolution during the Electrodeposition and Electrodissolution of Magnesium

Abstract

Magnesium batteries provide a promising alternative to Li-ion batteries due to their potential for lower cost, higher energy density, and safer, dendrite-free operation. Magnesium is the eighth most abundant element in the earth’s crust, and replacing the relatively rarer lithium could enable reduced battery costs. Furthermore, metallic Mg anodes have a significantly higher theoretical specific volumetric capacity (3833 mAh/cm3) than either Li-graphite anodes (760 mAh/cm3) or metallic Li anodes (2046 mAh/cm3). Finally, metallic Li anodes often form dendrites during cycling that can short the battery, leading to thermal runaway. In contrast, metallic Mg anodes form compact faceted films during cycling with minimal risk of dendrite formation. Understanding the evolution of the metallic Mg anode is a critical factor in the development of a practical, high-performance Mg-metal battery. In this presentation, we describe a new model for simulating the evolution of the faceted Mg deposits, both during deposition and during dissolution. We apply the standard dilute solution equations to describe the evolution of the concentration and the distribution of the potential in the electrolyte. The evolution of the magnesium deposit is described using a phase-field approach with conserved dynamics and a source term along the deposit surface to account for deposition and dissolution. The deposition/dissolution rate is calculated using Butler-Volmer kinetics. Reaction rate constants that depend on the local orientation of the surface of the deposit allow for the simulation of facet development and evolution. We present two possible models relating the orientation dependence of the forward rate constant to that of the backward rate constant. We apply this model to a range of experimentally relevant scenarios to improve the understanding of the mechanisms governing the film morphology during cycling. We investigate the effect on the deposit morphology of changing the applied current density during growth, finding that the deposits become narrower and taller as the applied current increases. We present predictions of the characteristic deposit morphology during dissolution for the two orientation-dependent reaction rate models. These predictions provide a diagnostic criterion for future experiments to determine which of the two models is correct. Finally, we present a series of two-deposit simulations to illustrate the types of interactions between deposits and how those interactions influence the film morphology. Acknowledgement: This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.