First Principles Investigation into The Dynamics at Ca Anodes and Electrolyte Interfaces

Motivated by recent experimental progress, we have utilized an approach combining density functional theory (DFT) and ab initio molecular dynamics (AIMD) to gain atomistic perspectives into the dynamic processes governing anode-electrolyte interactions. First principles based molecular dynamics allow one to capture the bond-breaking/forming processes, and interfacial interactions between the anode and the electrolyte, while preserving theoretical accuracy independent of empirical parameterizations. The DFT approach allows one to study the charge transfer during reactions energetics, capture transition states, and evaluate the potential energy landscapes in detail.<br/><br/>The transition to sustainable energy systems is predominantly influenced by the effectiveness of energy storage technologies. While Li-ion batteries have been the industry standard, there is a growing interest in multivalent ion battery systems. These alternatives offer advantages such as higher storage capacities, abundant material resources, and reduced geopolitical risks. Specifically, Ca-ion batteries garner attention as a promising option for high-energy density, low-cost batteries. Still at a nascent stage of development, understanding the interactions between the electrolyte and the Ca anode at the interface is crucial for optimizing the performance of these emerging battery technologies.<br/><br/>To gain insights into the initial formation of the solid−electrolyte interphase (SEI) at the interface, we first looked into the decomposition of pure ethylene carbonate (EC) and an EC/Ca(ClO₄)₂ electrolyte on a Ca metal surface. We found that CaCO₃, CaO, and Ca(OH)₂are the most common byproducts of the initial interfacial reactions. A fast two-electron reduction reaction producing CO₃²⁻ and C₂H₄ is found to be thermodynamically and kinetically favorable, while a reaction producing C₂H₄O₂²⁻ and CO takes precedence when multiple EC molecules are considered. Investigating the EC-based electrolyte decomposition at Li, Ca, and Al anode interfaces, we found that pure EC only decomposes to CO and C₂H₄O₂²⁻ species on each surface. However, upon the introduction of a salt molecule into the electrolyte, a second EC decomposition route resulting in the formation of CO₃²⁻ and C₂H₄ begins to occur. Li and Ca surfaces are more active in EC breakdown than Al due to the rate of charge transfer being much faster due to lower electronegativity and ionization energies.<br/><br/>Thereafter, we looked at the use of preformed solid electrolyte interphase (SEI) using Al₂O₃ to retard the interfacial reactions with the electrolyte (EC) in Ca anodes. The amorphous oxide layer was found to be thermodynamically stable while allowing for the diffusion of Ca through the layer. The oxide coating remained insulating up to the equilibrium stoichiometry. More significantly, the calcinated oxide layer slowed or prevented the decomposition of solvent molecules.<br/><br/>Lastly, we examined the ether and ester electrolyte stability on Ca anodes with different solvent and salt combinations. Tetrahydrofuran (THF) was observed to be the most stable relative to EC, even in a highly reductive environment. We attribute this to the charge distribution in THF on its backbone while the EC concentrates charge on its ester oxygens and carbonyl carbon, resulting in decomposition. We find Ca(BH₄)₂ and THF to be the most stable solution in our investigation, corroborating experimental evidence of its suitability as a CIB electrolyte.<br/><br/>More recently, inspired by the recent advances in machine learning-assisted techniques for computational materials research, we have been actively investigating the interactions of Ca metal anodes with S-based cathodes. We report on our current progress in modeling the Ca anode interactions with polysulfides.