Transition between growth of dense and porous films: Theory of dual-layer SEI

TL;DR

This paper develops a continuum model explaining the formation of dual-layer structures in passivating films like the SEI in lithium-ion batteries, highlighting a transition from dense to porous growth driven by electron transport dynamics.

Contribution

The work introduces a universal continuum model that explains the transition from dense to porous layers in passivating films, linking morphology evolution to transport and surface energies.

Findings

Model predicts initial dense SEI growth followed by porous layer formation.

Transition driven by slowing electron transport as film thickens.

Theory aligns with experimental observations of dual-layer structures.

Abstract

The formation of passivating films is a common aging phenomenon, for example in weathering of rocks, silicon, and metals. In many cases, a dual-layer structure with a dense inner and a porous outer layer emerges. However, the origin of this dual-layer growth is so far not fully understood. In this work, a continuum model is developed, which describes the morphology evolution of the solid-electrolyte interphase (SEI) in lithium-ion batteries. Transport through the SEI and a growth reaction governed by the SEI surface energies are modelled. In agreement with experiments, this theory predicts that SEI grows initially as a dense film and subsequently as a porous layer. This dynamic phase transition is driven by the slowing down of electron transport as the film thickens. Thereby, the model offers a universal explanation for the emergence of dual-layer structures in passivating films.