Ethylene Carbonate-Based Electrolyte Decomposition and Solid-Electrolyte Interphase Formation on Ca Metal Anodes

The formation of a solid-electrolyte interphase (SEI) in multivalent ion batteries, resulting from the decomposition of organic solvents at the anode interface, is a major bottleneck to their development as it prevents ionic diffusion and reversible stripping and plating. To gain insight into SEI formation in these systems, we investigate the decomposition of pure ethylene carbonate (EC) and an EC/Ca(ClO₄)₂ electrolyte on a Ca metal surface using density functional theory and ab initio molecular dynamics calculations. We first find that CaCO₃, CaO, and Ca(OH)₂ are all primary inorganic SEI components. We then investigate the reaction mechanisms of this decomposition, finding that although a fast two-electron reduction producing CO₃^2- and C₂H₄ is thermodynamically and kinetically favorable, a reaction producing C₂H₄O₂^2- and CO dominates when multiple EC molecules are considered. Finally, we find similar results upon the inclusion of Ca(ClO₄)₂ salt.