Computational Model of Magnesium Deposition and Dissolution for Property Determination via Cyclic Voltammetry

Abstract

The development of a practical magnesium-anode battery requires electrolytes that allow for highly efficient magnesium exchange while also being compatible with cathode materials. Here, a one-dimensional continuum-scale model is developed to simulate cyclic plating/stripping voltammetry of a model magnesium-based electrolyte system employing magnesium borohydride/dimethoxyethane [Mg(BH 4 ) 2 /DME] solutions on a gold substrate. The model is developed from non-electroneutral dilute-solution theory, using Nernst-Planck equations for the mass flux and Poisson's equation for the electrostatic potential. The electrochemical reaction is modeled with multistep Butler-Volmer kinetics, with a modified current/overpotential relationship that separately accounts for the portions of the current responsible for nucleating new deposits and propagating or dissolving existing ones. The diffusivities of the electrolyte species, standard heterogeneous rate constant, charge-transfer coefficient, formal potential, and nucleation overpotential are determined computationally by reproducing experimental voltammograms. The model is computationally inexpensive and therefore allows for broad parametric studies of electrolyte behavior that would otherwise be impractical. A rechargeable magnesium battery was first demonstrated 25 years ago when Gregory et al. 1 showed that magnesium could be reversibly deposited onto and dissolved from a magnesium-metal surface, as well as intercalated into and deintercalated from various host cathodes. Further interest in secondary magnesium batteries arose following the work of Aurbach et al., 2 which demonstrated a highly efficient organohaloaluminate electrolyte using a magnesium anode and a Mo 6 S 8 Chevrel-phase cathode. As a battery anode, magnesium metal offers important advantages over both intercalation compounds and lithium metal, including a higher theoretical volumetric energy capacity (3833 mAh/cm 3 vs. 2046 mAh/cm 3 for lithium metal and 760 mAh/cm 3 for graphite-based lithium-ion anodes), as well as a higher abundance in the earth's crust.3 Additionally, magnesium is less prone to dendrite formation than lithium when electrodeposited and therefore offers potential for improved battery cycle life and safety. 4 In addition to high-capacity electrode materials, a practical magnesium battery will also require an efficient electrolyte that is compatible with (i.e., chemically stable in contact with) these electrodes. Compared to lithium electrolytes, magnesium electrolytes remain in a relatively early developmental stage.1-29 Electrolytes formulated from Grignard reagents have been widely studied both in the battery and general electrochemistry communities. 1,6,28,[30][31][32][33][34][35] The speciation of these electrolytes is complex because, in addition to ionic dissociation, the reagents also undergo the Schlenk equilibrium process (a type of ligand exchange) and form multimeric species in many solvents. Both organohaloaluminates and the so-called magnesium aluminum chloride complex (MACC) a...

Dendrites and Pits: Untangling the Complex Behavior of Lithium Metal Anodes through Operando Video Microscopy

“…28 The numerical model solves the time-based evolution of the Poisson–Nernst–Planck system of equations (PNP equations) to describe the electrochemical mass transport and the electrostatic potential across the cell. In 1D, the Nernst–Planck equation takes the formwhere J i is the mass flux of the i th species, D i is the diffusivity of the species, z i and c i are the charge and concentration of the i th species, x is the position, F is Faraday’s constant, R is the ideal gas constant, T is the absolute temperature, and ϕ is the electrostatic potential.…”

“…To provide a theoretical description of the cell behavior, a onedimensional (1D) numerical continuum-scale model was developed based on previous efforts to study the deposition and dissolution of magnesium metal anodes. 28 The numerical model solves the time-based evolution of the Poisson−Nernst− Planck system of equations (PNP equations) to describe the electrochemical mass transport and the electrostatic potential across the cell. In 1D, the Nernst−Planck equation takes the form…”

Modeling of Electron‐Transfer Kinetics in Magnesium Electrolytes: Influence of the Solvent on the Battery Performance

“…In the case of magnesium and DME based chloride‐free electrolytes the cathodic symmetry factor seems to be significantly smaller than 0.5. [39] Consequently, the transition state during plating from DME solvated magnesium cations is more reactant‐ than product‐like, which indicates that the desolvation plays an important role during the magnesium deposition. Therefore, the solvent (and anion) might significantly influence the symmetry factor of the Butler‐Volmer kinetics.…”

Multifunctional Additives Improve the Electrolyte Properties of Magnesium Borohydride Toward Magnesium–Sulfur Batteries

“…Rechargeable magnesium (Mg) batteries have been regarded as one of the promising candidates to power future electric vehicles and support large-scale energy storage systems owing to the high theoretical volumetric capacity (3832 mA h cm –3 for the Mg metal anode vs 2062 mA h cm –3 for the Li metal anode), low cost, and large natural abundance of Mg. − Mg batteries might also raise less safety concerns because the electrochemical deposition of Mg does not involve the formation of either thick solid electrolyte interface layers or metallic dendrites. − However, the commercialization of such promising batteries has not yet been realized because of the unsatisfied properties of Mg-ion electrolytes, including the poor compatibility with cathode materials, insufficient electrochemical window, and undesired Mg plating/stripping properties. − …”