SEI Film Formation Mechanism Based on Aggregation Configurations of Electrolyte Decomposed Products: DFT-MD Study
Authors: Sodeyama Keitaro , Ushirogata Keisuke , Okuno Yukihiro , Tateyama Yoshitaka 
Published: February 2020

Solid electrolyte interphase (SEI) is an important key of the stability and durability of the lithium ion batteries (LIB). It is generally accepted that molecules in the electrolyte solution reductively decompose to form various SEI ﬁlm components (SFCs), such as organic oligomers and inorganic moieties, and that the SFCs precipitate on the electrode surface to form a stable SEI ﬁlm with a thickness of several tens of nanometers. The formation processes of SEI have been typically assumed to involve “surface growth mechanism”. Decomposed products of the electrolyte molecules immediately precipitate to the electrode surface. At a certain thickness of the SFC aggregate, electron tunneling from the electrode to the electrolyte is suppressed. Growth of the SFC aggregate then stops and the formation of the SEI film is completed. However, there is a question as to how the reductive decompositions of the electrolyte are repeated until the thickness reaches several tens of nanometers. In this study, we investigated the solubility properties of SFCs into the EC solution, a measure of aggregation tendency, and the adhesion to the edges of possible reduced graphite by DFT-MD simulations. From these results, we discuss how SEI ﬁlms grow and when the growth stops in the present modeling of the SFCs.
In this work, we chose EC solvent for the electrolyte components. Aggregation tendency was evaluated on the basis of the dissolution energy of the SFC in the EC solution. The dissolution energy, Ediss, was calculated using the following formula,
Ediss = E(mEC + SFC) − m μEC − μSFC

where E(mEC + SFC) is the equilibrium total energy of the target system with mEC solvent molecules and one SFC, and μEC and μSFCcorrespond to the average chemical potentials of an EC molecule in the equilibrium EC solution and a SFC monomer in the SFC condensed phase.
We examined the adhesive energies of the SFC aggregates at the interface between the graphite electrode and EC solvent. The adhesive energy was deﬁned as the difference of the averaged total energy between the attached state on the surface and the dissolved states in the EC solution. As model surfaces, we used graphite electrodes terminated by H, mixed H/OH and mixed H/O. The SFC aggregate is composed of 12 SFC molecules. 60 EC molecules were stuffed into the remaining space of the supercell. The unit cells size is 14.913 × 53.811 × 14.824 angstrom.
We used Car-Parrinello type of DFT-MD simulations, with CPMD code. The system temperature was controlled using a Nose thermostat with a target temperature of 353 K.
Typical conﬁgurations in the SFC aggregates of EC decomposed product are shown in Figure 1. Two ROCO2edges mainly contribute to the so called four-fold coordination to the Li ion, suggesting that the networking with Li ion bridging is a typical feature. The estimated dissolution energies were +12.2 kcal/mol. This indicates that the SFC dissolution is energetically unfavorable. The SFC aggregation property is significantly affected by the Li glue effect.
The average adhesive energies of the SFC aggregate per component on H-terminated graphite edge is +3.2 kcal/mol. Snapshots of the adhesion are shown in figure 2. We found that the adhesion is energetically unfavorable. We also examined the different types of terminations and in the whole patterns, adhesion is unfavorable. The results seem inconsistent with the surface growth mechanism, which intrinsically assumes successive SFC adhesions.
We then propose a new mechanism named “near-shore aggregation mechanism”. In this mechanism, the electrolyte reductive decompositions always take place on the negative electrode surface, and the formed SFCs desorb into the electrolyte solution. Aggregation of the SFCs then proceeds in the “near-shore region” from the electrode, which is a crucial difference from the “surface growth” scenario. These processes are well explained by the calculated tendencies of aggregation and adhesion. When the SFC clusters grow to a certain size in the near-shore regions well as in the electrolyte solution and start to coalesce, the free electrolyte region between the electrode and the SFC aggregates shrinks due to further supply of SFCs on the electrode surface. This scenario can explain why the SEI thickness can reach on the order of 10 nm order despite the fact that the SFC aggregates have high electronic insulation. In this talk, we will also discuss the aggregation of the inorganic SFC moieties.
[1] K. Ushirogata, K. Sodeyama, Z. Futera, Y. Tateyama, Y. Okuno, J. Electorochem.Soc., 162(14), A1-A9 (2015).
[2] Y. Okuno, K. Ushirogata, K. Sodeyama, Y. Tateyama, Phys Chem Chem Phys., 18, 8643-8653 (2016).

Figure 1