Theoretical evaluation of ethylene carbonate anion transport and its impact on solid electrolyte interphase formation

Despite its critical role in determining the performance of lithium ion batteries (LIBs), evolution of the chemical composition and morphology of the solid electrolyte interphase (SEI) formed on graphite negative electrodes remains poorly described even for a conventional electrolyte consisting of 1 M LiPF₆ dissolved in a mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC), which is considered here. The EC⁻ radical anion produced from one-electron reduction of EC is known to be a key intermediate responsible for the formation of two commonly found products in the SEI, carbonate (CO₃²⁻) and ethylene dicarbonate (EDC²⁻). We evaluate the diffusion behavior of EC⁻ associated with solvent reorganization near the graphite electrode using molecular dynamics (MD) and metadynamics simulations. The predicted free energy profiles of EC⁻ transport at the onset of SEI formation show two distinct minima, depending on the EC:DMC ratio of the solvent and the voltage applied to the electrode. EC⁻ radical intermediates trapped in the free energy well located at z = 0.65 nm (or z = 1.5 nm) from the electrode surface may favorably undergo further reduction to CO₃²⁻ (or a combination reaction with another EC⁻ to form EDC²⁻). Moreover, suppressed EC⁻ migration could contribute to the formation of a thin, compact SEI layer. This study highlights that in addition to the kinetics and thermodynamics of solvent reduction, the near-electrode transport of reaction intermediates/products should be considered in modeling of SEI structure and growth.