The buildup of the nanometer-thick SEI layer

Leung and Leenheer stated in "How voltage drops are manifested by lithium-ion configurations at interfaces and in thin films on battery electrodes" that in the initial stage of Solid Electrolyte Interface(SEI) formation, the solvent molecules absorbed on the surface of the charged electrode will greatly reduce the voltage and alter the structure of the layer under the applied electric field. This means that as the solvent covers the bare electrode, the mechanism of SEI buildup will change. This is crucial because it was generally assumed that the SEI formation would take the "surface growth" mechanism which does not encompass the whole procedure of SEI formation.

To address such an issue, Ushirogata et al. using AIMD simulation to observe the SEI formation proposed a "near-shore aggregation"(Fig.1 b) rather than a conventional surface growth mechanism(Fig.1 a). To clarify the term used here; the "surface growth" mechanism refers to a simple view of SEI formation based on precipitation of reductive decomposition products of the electrolyte molecules. It involves precipitation of the SEI film components on the negative electrode interfaces, and progress(grows) from the surface nucleation; until the SEI film is thick enough to prevent further reduction reaction. However, it was observed that the SEI film component aggregate on the negative electrode is less stable in the EC solution. Such results suggest that the "surface growth" mechanism is not a major player in the SEI formation. Thus new mechanism, the "near-shore aggregation" mechanism is proposed. The key difference between the newly proposed mechanism and the conventional one is that the reduction reaction always takes place on the negative electrode surface, and the aggregation of such products happen in the "near-shore region" from the electrode. When the SFC clusters grow to a certain size, they coalesce, and the free electrolyte region between the negative electrode and clusters shrinks due to the further supply of SFCs on the electrode surface. Such an explanation is supported by the fact that SEI thickness can reach 10nm despite SFCs having high electronic insulations. Once the clusters cover the negative electrode surface, the SFC supply from the electrode slows down and eventually stops.

 

Reference

Ushirogata, Keisuke & Sodeyama, Keitaro & Futera, Zdenek & Tateyama, Yoshitaka & Okuno, Yukihiro. (2015). Near-Shore Aggregation Mechanism of Electrolyte Decomposition Products to Explain Solid Electrolyte Interphase Formation. Journal of The Electrochemical Society. 162. 2670-2678. 10.1149/2.0301514jes].