Unveiling the Dynamic Heterogeneity and Evolution of the Solid-Electrolyte Interphase during Cycling at the Atomic Scale

The solid-electrolyte interphase (SEI) is critical to the performance of lithium-metal batteries. Nevertheless, a detailed atomic-level understanding of the dynamic evolution of the SEI during electrochemical cycling remains incompletely understood. We probe the dynamic structural evolution of the SEI during cycling using a constant-potential reactive force field method. Inspired by Landau-Ginzburg phase transition theory, we introduce an order parameter analysis to characterize the spatial distribution of lithium, which clearly distinguishes the inner and outer regions of the SEI through distinct fluctuation behaviors. The inner SEI region exhibits an approximately linear order-parameter profile and plateaus upon charging, indicating a well-defined phase character. Furthermore, analysis of the SEI density and porosity reveals that SEI thickens gradually with more uniformly distributed pores during charging. This work establishes a clear theoretical distinction between the inner and outer SEI layers and systematically elucidates the microscopic effects of the charge-discharge cycling on SEI structure.