Energy landscape of the charge transfer reaction at the complex Li/SEI/electrolyte interface

Abstract

The charge transfer reaction is the fundamental reaction for rechargeable batteries. The energy landscape of this reaction depicts the equilibrium and kinetics of the electrochemical process. Typically, a Li-metal electrode is always covered by a thin layer of solid electrolyte interphase (SEI), forming a complex Li/SEI/electrolyte interface. In this paper, a new modeling framework was developed to predict the energy landscape of the lithiation/delithiation charge transfer reaction at a Li/Li₂CO₃/EC-electrolyte interface, with combined density functional theory (DFT) and tight-binding (DFTB) calculations. It was found that Li⁺ ions are much more energetically favorable to be dissolved in the electrolyte on a zero-charged Li-metal electrode, indicating that the SEI is a necessary kinetic barrier to prevent complete solvation of Li-metal into the electrolyte. During delithiation, Li⁺ ions would be stripped from the surface non-uniformly and form a large void on the Li-metal surface. During lithiation, it was demonstrated that the annihilation of Li⁺ ions and electrons occurs at the Li/SEI interface, for an idealized defect free SEI. Furthermore, at the experimentally defined zero voltage for Li⁺/Li⁰, the Li-metal surface is negatively charged (−0.62 ± 0.12 e nm⁻²) to maintain the electrochemical equilibrium. The electric field created by the negatively charged surface can reorient the electrolyte into an ordered structure, lower the Li⁺ ion desolvation energy barrier, and help the Li⁺ ion transport through the SEI. The charge transfer coefficient, α, in the Butler–Volmer equation was directly computed to be ∼0.22 from the simulated energy landscape, consistent with the experimental measurements. Thus, this model enables a bottom-up multiscale modeling approach for ion-transfer electrochemical reactions.