Comparative Study of Ethylene Carbonate-Based Electrolyte Decomposition at Li, Ca, and Al Anode Interfaces

One of the major bottlenecks to the development of alternatives to existing Li ion battery technology, such as Li metal or multivalent ion (Mg, Ca, Zn, or Al) batteries, has to do with the layer of inorganic and organic compounds that forms at the interface between the metallic anode and electrolyte via solvent and salt decomposition (the solid-electrolyte interphase or SEI). In Li metal batteries the growth of dendrites causes continual formation of new SEI, while in multivalent ion batteries the SEI does not allow for the diffusion of the ions. Finding appropriate electrolytes for such systems and gaining an understanding of SEI formation is therefore critical to the development of secondary Li metal and multivalent ion cells. In this work, we use ab initio molecular dynamics simulations to investigate the initial stages of decomposition of organic electrolytes based on ethylene carbonate (EC) and formation of the SEI on Li, Ca, and Al metal surfaces. We first find that pure EC only decomposes to CO and C₂H₄O₂2^- species on each type of surface. However, when a salt molecule is introduced to form an electrolyte, a second EC decomposition route resulting in the formation of CO₃2^- and C₂H₄ begins to occur; furthermore, a variety of different inorganic compounds, depending on the chemical composition of the salt, form on the surfaces. Finally, we find that EC breaks down more quickly on Li and Ca surfaces than on Al and show that this is because the rate of charge transfer is much faster owing to their lower electronegativity and ionization energies. The molecular level understanding of decomposition and SEI formation generated by this computational modeling can lead to the design of new electrolytes for beyond-Li ion batteries.