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

Calcium ion batteries show promise as a high-density, next generation replacement 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, play the primary role in delocalizing the charge on Ca2+. Detailed calculations indicate that the HOMO-LUMO energy gap (Eg), 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 show 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 solvent