Electrolyte Reactivity in the Double Layer in Mg Batteries: An Interface Potential-Dependent DFT Study

The
electrochemical degradation of two solvent-based electrolytes
for Mg-metal batteries is investigated through a grand canonical density
functional theory (DFT) approach. Both electrolytes are highly reactive
in the double layer region where the solvated species have no direct
contact with the Mg-surface, hence emphasizing that surface reactions
are not the only phenomena responsible for electrolyte degradation.
Applied to dimethoxyethane (DME) and ethylene carbonate (EC), the
present methodology shows that both solvents should thermodynamically
decompose in the double layer prior to the Mg2+/Mg0 reduction, leading to electrochemically inactive reaction
products. Based on thermodynamic considerations, Mg0 deposition
should not be possible, which contrasts with experiments, at least
for DME-based electrolytes. This apparent contradiction is here addressed
through the rationalization of the electrochemical mechanism underlying
solvent electroactivation. An extended operation potential
window (OPW) is extracted, in which the Mg2+/Mg0 reduction can compete with electrolyte decomposition, thus
enabling battery operation beyond the solvated species thermodynamic
stability. The chemical study of the degradation products is in excellent
agreement with experiments and offers rationale for the Mg-battery
failure in EC electrolyte and capacity fade in DME electrolyte. The
potential-dependent approach proposed herein is thus able to successfully
tackle the challenging problem of interface electrochemistry. Being
fully transferable to any other electrochemical systems, this methodology
should provide rational guidelines for the development of viable electrolytes
for multivalent batteries and, more generally, energy conversion and
storage devices.