Insights on SEI Growth and Properties in Na‐Ion Batteries via Physically Driven Kinetic Monte Carlo Model

Abstract Sodium‐ion batteries (SIBs) show promise for the next generation of energy storage technology but face significant challenges in regards to stability due in part to uncontrolled degradation of the solid electrolyte interphase (SEI). Kinetic Monte Carlo (kMC) modeling is uniquely suited to provide molecular‐scale insight on the phenomena that influence SEI growth and behavior in SIBs over full charge. In this work, spatially‐ and time‐dependent electrical potential is incorporated into kMC modeling for the first time, which enables the precise study of electrochemical reactivity and SEI growth during charging. A reaction network for a carbonate/NaPF 6 electrolyte developed using density functional theory is used to power the kMC simulations. The decomposition of NaPF 6 and formation of NaF is unfavorable at standard conditions, suggesting that water or other contaminants are required to facilitate the reaction. The SEI is shown to be primarily made of Na 2 CO 3 . SEIs with low electric conductivities exhibit the most ideal behavior and high C‐rates generate thinner SEIs with greater fractions of organic species. Dissolution of SEI species is shown to occur rapidly, even during formation. The results of the model correspond well to the SEI behavior known in the literature, and reveal the fundamental mechanisms that influence cell behavior.