A Combined Computational and Experimental Approach to Determine Mg(BH4)2 Electrolyte Parameters

Metallic magnesium is a promising material for rechargeable battery anodes, as it offers higher theoretical volumetric capacities than lithium-ion chemistries and is less prone to dendrite formation upon repeated cycling [1]. Magnesium borohydride (Mg(BH₄)₂) in dimethoxyethane (DME) is a candidate electrolyte for metal-anode batteries because of its stability in highly reducing electrochemical environments. Additionally, magnesium borohydride is not corrosive towards typical electrode materials to the same extent as some halogenated electrolytes [2]. To date, however, little is known about the electrochemical behavior of this system.

We have developed a computational model capable of determining the redox and transport properties of an electrochemical system. In this model, used to simulate the cyclic voltammetry of an electrochemical cell, we solve the Nernst-Planck equation for electrolyte transport coupled with the Poisson equation for electrostatics. The Butler-Volmer equation is used to describe the electron transfer reaction at the electrode/electrolyte interface. This methodology is based on models previously presented in the literature, but enhancements have been made to account for the nucleation overpotential and coulombic efficiency.  These enhancements are required to predict the experimentally observed electrodeposition and stripping behavior [3,4].

To obtain the input physical parameters for this model, cyclic voltammetry experiments were performed with 75mM Mg(BH₄)₂ in DME. A gold microelectrode was used as the working electrode. The counter and reference electrodes were magnesium. Several scan rates were explored within the voltage range –1V to 1V vs. Mg/Mg²⁺. By matching the simulated voltammogram to those obtained experimentally, we determine the values for the heterogeneous rate constant, diffusion constants, and the extent of dissociation of Mg(BH₄)₂in the electrolyte. The results will be presented and discussed. These parameters enable future investigations of the cycling behavior of the electrolyte system, such as cyclability and rate capabilities.

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.

References:

[1] Hyun Deog Yoo, Ivgeni Shterenberg, Yosef Gofer, Gregory Gershinksy, Nir Pour and Doron Aurbach, Energy Environ. Sci., 6, 2265-2279 (2013).

[2] Rana Mohtadi, Masaki Matsui, Timothy S. Arthur and Son-Jong Hwang, Angew. Chem. Int. Ed., 51, 9780-9783 (2012).

[3] Timothy R. Brumleve and Richard P. Buck, J. Electroanal. Chem., 90, 1-31 (1978).

[4] Ian Streeter and Richard G. Compton, J. Phys. Chem. C, 112, 13716-13728 (2008).