Near-Shore Aggregation Mechanism of Electrolyte Decomposition Products to Explain Solid Electrolyte Interphase Formation

Keisuke Ushirogata,Yukihiro Okuno,Yoshitaka Tateyama,Zdenek Futera,Keitaro Sodeyama

doi:10.1149/2.0301514jes

Abstract

To get insight of the formation mechanism of solid electrolyte interphase (SEI) film in Lithium-ion battery (LIB), we examine a probable scenario, referred to as “surface growth mechanism,” for electrolyte involving ethylene carbonate (EC) solvent and vinylene carbonate (VC) additive by using density functional theory (DFT). We first extracted stable SEI film components (SFCs) for the EC/VC electrolyte and constructed probable SFC aggregates via DFT molecular dynamics. We then examined their solubility in the EC solution, their adhesion to a model graphite electrode, and the electronic properties. The results showed that the SFC aggregates are characterized by “unstable adhesion” to the graphite surface and “high electronic insulation” against the EC solution. These characteristics preclude explaining SEI growth up to a typical thickness of several tens of nanometers based on the surface growth mechanism. With the present results, we propose “near-shore aggregation” mechanism, where the SFCs formed at the electrode surface desorb into the near-shore region and form aggregates. The SFC aggregates coalesce and come into contact with the electrode to complete the SEI formation. The present model provides a novel perspective for the long-standing problem of SEI formation.

Highlights

We found a large exothermic reaction between the intact vinylene carbonate (VC) and ethylene carbonate (EC) anion radical, which is accompanied by the CO2 gas evolution that was experimentally observed
Summary In this study, we used density functional theory (DFT) calculations to examine the ability of the “surface growth mechanism” to explain the formation of the solid electrolyte interphase (SEI) at the interface between the carbonate electrolyte solution and the negative electrode
We first extracted stable SEI film components (SFCs) for the EC/VC electrolyte and constructed probable SFC aggregates via DFT-molecular dynamics (MD). We examined their solubility in the EC solution, the adhesion to the model graphite electrodes, and the electronic properties

Summary

Results

SFCs in the presence of VC.— To extract the characteristic structures of VC-derived SFCs, we first examined the intermolecular reactions involving VC in detail. We calculated the average adhesive energies of single SFCs and SFC aggregates to the hydrogen-terminate graphite edge, a typical model of a negative electrode under reductive condition, by DFT-MD sampling In this analysis, we focused on Li2EDC and Li2DOB to clearly extract the intrinsic characteristics of EC- and VC-derived SFCs. We first described the adhesion of the single SFCs. For the adhesive state, we prepare an initial configuration where the SFC monomer has a “stand-up” structure where the molecular axis was perpendicular to Figure 7. We found that adhesion of the monomer on the graphite surface in the EC solvent is energetically unfavorable for both Li2EDC and Li2DOB The former adhesion appeared less probable, we concluded that there was no particular difference between the EC- and VC-derived SFCs. The results seem inconsistent with the surface growth mechanism, which intrinsically assumes successive SFC adhesions

SFC aggregate

Discussion

SEI film enlargement of SEI source