Ab-Initio MD Simulations of Redox Reactions of Liquid Electrolytes and SEI Formation

Abstract

Atomistic understanding of solid electrolyte interphase (SEI) formed at the interface between electrode and liquid electrolyte is still an issue of greatest importance. However, difficulties in experimental in-situ observations and simulations considering the liquid dynamics (fluctuation) and reaction have inhibited elucidation of physics and chemistry of SEIs. We have addressed such unresolved issues by means of accurate large-scale ab-initio molecular dynamics (AIMD) samplings and free energy calculations on supercomputers. The SEI formation is initiated by electrochemical decomposition of the electrolyte molecules such as solvents and additives. Slight change of the electrolyte component often exhibits a large impact on the SEI quality. We first examined the reductive decomposition of the electrolyte molecules at charging (reductive) environment, using a typical electrolyte with ethylene carbonate (EC) solvents and vinylene carbonate (VC) additives. We carried out AIMD free energy calculations for possible decomposition pathways, and found that VC passivation to EC anion radical is a more appropriate mechanism than the conventional scenario with sacrificial reductive decomposition of VC (Fig. (a))1. This novel mechanism can explain the decrease of irreversible capacity and the CO2 evolution with a small amount of VC additives observed experimentally. Next, we extracted stable organic SEI film components (SFCs) from the products of the reductive decompositions of the electrolyte. We then investigated the characteristic of the probable SFC amorphous aggregates on the adhesion to the graphite electrode and the interfacial electronic states. The results showed that the SFC aggregates have characters of “unstable adhesion” to the graphite surface and “high electronic insulation”. With these results, we discussed that a near-shore aggregation mechanism of SFCs is promising for the SEI film formation.2 Very recently, we examined inorganic SFCs as well to elucidate their roles in the SEI formation.3 These new findings with supercomputer simulations give important perspectives for more efficient and reliable batteries. 1. K. Ushirogata, K. Sodeyama, Y. Okuno, Y. Tateyama, J. Am. Chem. Soc. 135, 11967-11974 (2013). 2. K. Ushirogata, K. Sodeyama, Z. Futera, Y. Tateyama, Y. Okuno, J. Electrochem. Soc. 162, A2670-2678 (2015). 3. Y. Okuno, K. Ushirogata, K. Sodeyama, Y. Tateyama, submitted. Figure (a) Proposed role of VC additive in reductive decomposition of the EC-based electrolyte. (b) Schematic picture of the near-shore aggregation mechanism for SEI formation on a graphitic electrode. Figure 1