Electronic Structure Insights into the Solvation of Magnesium Ions with Cyclic and Acyclic Carbonates

Abstract

A computational framework to rank the solvation behavior of Mg(2+) in carbonates by using molecular dynamics simulations and density functional theory is reported. Based on the binding energies and enthalpies of solvation calculated at the M06-2X/6-311++G(d,p) level of theory and the free energies of solvation from ABF-MD simulations, we find that ethylene carbonate (EC) and the ethylene carbonate:propylene carbonate (EC:PC) binary mixture are the best carbonate solvents for interacting with Mg(2+) . Natural bond orbital and quantum theory of atoms in molecules analyses support the thermochemistry calculations with the highest values of charge transfer, perturbative stabilization energies, electron densities, and Wiberg bond indices being observed in the Mg(2+) (EC) and Mg(2+) (EC:PC) complexes. The plots of the noncovalent interactions indicate that those responsible for the formation of Mg(2+) carbonate complexes are strong-to-weak attractive interactions, depending on the regions that are interacting. Finally, density of state calculations indicate that the interactions between Mg(2+) and the carbonate solvents affects the HOMO and LUMO states of all carbonate solvents and moves them to more negative energy values.

Current Understanding of Nonaqueous Electrolytes for Calcium‐Based Batteries

“…Intriguingly, BF4was found to pair directly with Ca 2+ even at 0.01 M; depending on the carbonate, 1-2 BF4coordinated Ca 2+ (out of typical solvation numbers of 6-9). Remarkably, this was uniquely the case for Ca 2+ , but not Li + , Na + , and Mg 2+ , which did not interact with their respective anions (PF6 -, ClO4 -, and ClO4 -) [58][59][60] at 0.01 M and were instead completely solvated by the carbonates.…”

“…Solvation free energies, determined by density functional quantum chemical calculations for each MD-simulated Ca 2+ -solvent complex, ranged from -385 to -472 kcal/mol, [57] substantially more negative than Na + , Li + , and even Mg 2+ (-215 to -329 kcal/mol). [58][59][60] While higher solvation energies do promote solubility and thus have been argued to be a desirable feature of Ca electrolytes previously, [57] they also represent significant energy barriers that need to be overcome during Ca 2+ reduction. Of note, in a separate study, [61] increases in ion solvation strength of Li + when using high donor-number solvents (e.g., DMSO) were found experimentally and computationally to lead to an almost -600 mV shift in the Li + /Li redox potential vs. a Me10Fc + /Me10Fc reference, compared to less-strongly solvating solvents such as acetonitrile.…”

“…4a) as determined via molecular dynamics, MD, simulation. [57][58][59][60] Thus, the solvating environment surpasses others in terms of chemical complexity, yet, optimistically, also in degrees of freedom when considering possible future design of improved Ca 2+ /Ca redox. In addition to solvation number, the total solvation strength of Ca 2+ in a fixed set of neat or blended carbonates has been calculated [57] to be substantially higher than its Li + , Na + , and Mg 2+ counterparts (Fig.…”

“…(a) Ionic radius and typical number of solvating molecules in the first solvation shell (symbol denotes average; error bars denote reported ranges across varying carbonate solvents). Data obtained from Refs [11],[57]-[60]…”

Nanobelts of Beta-Sodium Vanadate as Electrode for Magnesium and Dual Magnesium-Sodium Batteries

“…ethylene carbonate) in the primary solvation sphere. 21 Unfortunately, the electrode of Mg seems to be incompatible with many solvents, such as water and carbonates which can passivate the surface of Mg. Thus, another choice can be the use of dual salt electrolyte and hybrid batteries.…”