Calcium-Ion Batteries: Identifying Ideal Electrolytes for Next-Generation Energy Storage Using Computational Analysis

Abstract

Calcium-ion batteries show promise as high-density, next-generation replacements for current lithium-ion batteries. The precise chemical structure of the carbonate electrolyte solvent has a large impact on calcium battery efficacy. In this computational study, we have investigated the solvation behavior of calcium tetrafluoroborate in both neat carbonates and carbonate mixtures using combined molecular dynamics simulations and quantum mechanical calculations. Our results indicate that both neat ethyl methyl carbonate and a mixture of ethylene carbonate and diethyl carbonate show the highest free energy of solvation for the Ca2+ ion, making them likely candidates for further focus. The cation’s interaction with the carbonyls of the coordinating solvents, rather than those with the tetrafluoroborate counterions, plays the primary role in delocalizing the charge on Ca2+. Detailed calculations indicate that the HOMO–LUMO energy gap (E g), electronic chemical potential (μ), and chemical hardness (η) of the calcium–carbonate complexes are directly proportional to the free energy of solvation of the complex. Comparison of these observed trends with our previous results from Li+, Na+, and Mg2+ ions shows that this correlation is also observed in solvated magnesium ions but not in lithium or sodium salts. This observation should assist in the rational design of next-generation battery materials in the rational selection of additives, counterions, or electrolyte solvents.

Current Understanding of Nonaqueous Electrolytes for Calcium‐Based Batteries

“…4b). 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.…”

“…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.…”

“…Note that it has been briefly mentioned that the higher temperatures used in the original EC:PC study may also aide overall desolvation kinetics. [16,57] Table 1 within Ref. [57].…”

“…The composition of the Ca 2+ solvation environment at dilute concentrations (0.01 M) of Ca(BF4)2 was recently investigated computationally by Shakourian-Fard et al [57] Seven neat solvents (EC, PC, EMC -ethyl methyl carbonate, DMC -dimethyl carbonate, DEC -diethyl carbonate, VC -vinylene carbonate, BC -butylene carbonate) and four solvent blends (EC:PC, EC:DMC, EC:EMC, EC:DEC) were analyzed. MD simulations indicated that Ca 2+ interacts strongly with carbonyl (-C=O) oxygens and more-weakly with ether oxygens (Fig.…”