Mechanistic Study of the Effect of Epoxy Groups on Ethylene Carbonate Decomposition Reaction on Carbon Anodes of Sodium-Ion Batteries

The formation of solid-electrolyte interphase (SEI) layers which results from the decomposition of organic solvents in the electrolyte on the anode of sodium-ion batteries (SIBs) is crucial and must be addressed to make SIBs well positioned in commercialization because the SEI layer has profound effects on SIBs' initial capacity loss, life cycle, and safety. SEI properties such as chemical reactivity, thermal reactivity, mechanical stability, and durability have an impact on the overall performance of the batteries. Carbon-based anode materials are commonly used in SIBs and usually contain many types of defects and oxygenated functional groups. To gain insight into the influence of oxygenated functional groups of carbon-based materials on solvent decomposition mechanisms on the carbon surface, we perform density functional theory (DFT) calculations to investigate the effect of an epoxy group on decomposition mechanisms of ethylene carbonate (EC), which is a common solvent used in SIBs. We find that the presence of the epoxy group on the graphene surface diminishes EC decomposition as evidenced by a significant increase of reaction energies and reaction barriers. The EC decomposition mechanism yielding CO3 and C2H4 is most kinetically favorable. A similar effect of the epoxy group is also exhibited when the Na concentration increases. However, the increase of Na concentration affects the reaction barriers of each elementary step differently. More possible mechanisms were found when the explicit solvent molecules in the first solvation shell of Na are included. The additional pathway that an epoxy group reacts with a solvent molecule is found to be the most energetically favorable one. Thus, the epoxy group could promote EC decomposition through these pathways.