Fundamental Mechanisms of Solvent Decomposition Involved in Solid-Electrolyte Interphase Formation in Sodium Ion Batteries

Prolonged decomposition of electrolytes forming a thick and unstable solid-electrolyte interphase (SEI) continues to be a major bottleneck in designing sodium-ion batteries (SIBs). We have carried out quantum chemistry simulations to investigate the fundamental mechanisms of reduction-induced decomposition of electrolyte solvents in the vicinity of a sodium ion. Kinetics and thermodynamics of several reaction pathways for one- and two-electron reduction of ethylene carbonate (EC) have been examined. Our calculations indicate that the high reduction potential and low barrier for the ring opening of EC is the main cause for the continuous growth of SEI observed in SIBs. The impact of two well-known electrolyte additives, vinyl carbonate (VC) and fluoroethylene carbonate (FEC), on SEI composition was evaluated by studying decomposition pathways of (1) VC and FEC molecules in the bulk EC solvent and (2) an EC molecule in a supermolecular cluster comprising an EC and the additive molecule. The additive molecules have significantly low barriers for decomposition and therefore decompose first. Additionally, the presence of an additive molecule was also shown to increase the barrier for decomposition of EC. Another observation suggests that the preferred reduction state of an EC molecule changes when it forms a dimer with additive molecules, and these reduction states have different decomposition pathways which leads to formation of different SEI compounds. On the basis of these observations, we predict that not only do the additive molecules protect solvent molecules from reductive decomposition but also they can promote alternate pathways for the decomposition, leading to qualitatively different and potentially stable SEI products.