First Principles Study of Electrolyte Interactions in Calcium Ion Batteries

Multivalent ion batteries, which use species such as Ca as the working ion, are gaining increasing attention. Compared to other multivalent ions, Ca exhibits a reduction potential close to that of Li, high volumetric capacity, and faster diffusion. However, the breakdown of organic electrolytes causes a passivating layer (the solid electrolyte interphase, or SEI) to form on the electrode surface, blocking Ca diffusion and preventing reversible plating and stripping of the Ca anode. In this work we use density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations to study the interaction of these solvents, including ethylene carbonate (EC), propylene carbonate (PC), and tetrahydrofuran (THF), with Ca ions and investigate their breakdown on the anode surface. [1,2] We find that Ca forms a large first solvation shell with EC and PC and a slightly smaller one in THF. We then compute the diffusion coefficient of Ca in each solvent using AIMD, and find that it diffuses fastest in THF, and slower in EC and PC. Finally, we use AIMD to study the decomposition of EC on Li, Ca, and Al surfaces and identify the principle components of the SEI on each.
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[1] J. Young and M. Smeu, J. Phys. Chem. Lett. 9 3295 (2018)
[2] J. Young et al., ACS Appl. Energy. Mater. 2 1676 (2019)