Mechanistic insights into the thermodynamics and kinetics underlying the reductive decomposition of fluoroethylene and difluoroethylene carbonates for SEI formation in LIBs†

Abstract

Fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC) are electrolyte additives that significantly influence the formation of the solid electrolyte interphase (SEI) during the initial cycling of lithium-ion batteries (LIBs). While FEC has been partially explored, the reductive decomposition mechanism of DFEC, particularly its kinetic and thermodynamic behaviour, remains poorly understood. In this work, we employ density functional theory (DFT) simulations to systematically investigate the thermodynamic (free energy, ΔG) and kinetic (free energy barrier, ΔG^‡) parameters governing the reductive decomposition pathways of FEC and DFEC. The results indicate that both additives predominantly undergo direct two-electron reduction processes to form LiF and CO as the primary products. DFEC exhibits thermodynamic and kinetic behavior comparable to that of FEC. Notably, DFEC features a unique double-defluorination pathway that generates additional LiF, potentially enhancing SEI stability. Mayer bond order (MBO) and atomic dipole moment corrected Hirshfeld (ADCH) charge analyses further reveal that the Li⁺ coordination facilitates the defluorination process. These findings offer new insights into the decomposition of DFEC and confirm its ability to form LiF-rich SEI layers, highlighting DFEC as a promising electrolyte additive for stable and high-performance LIBs.