Concentration-Dependent Solid Electrolyte Interphase in Lithium Bis(fluorosulfonyl)imide and Fluoroethylene Carbonate Electrolytes

Understanding solid electrolyte interphase (SEI) formation on Li metal in fluorinated electrolytes is critical for improving Coulombic efficiency (CE). We combined in situ Fourier transform infrared spectroscopy (FTIR), in situ Raman, ex situ X-ray photoelectron spectroscopy (XPS), and electrochemical mass spectrometry (ECMS) to investigate concentration-dependent SEI with lithium bis(fluorosulfonyl)imide (LiFSI) and fluoroethylene carbonate (FEC) electrolytes. In situ Raman and FTIR revealed VC and poly(VC) in FEC-rich electrolyte (50 mM), indicating non-ring-opening polymerization, with poly(VC) decreasing at higher LiFSI concentration. In parallel, FEC underwent multiple ring-opening pathways: at 50 mM, FEC formed Li 2 CO 3 while evolving predominantly H 2, as observed by ECMS. Increasing LiFSI concentration favored making CO 2 while suppressing Li 2 CO 3 . This concentration-dependent mechanism was further supported by depth-resolved (∼40 nm) XPS, where Li 2 CO 3 resided predominately in the outer SEI and decreased with sputter depth and at higher LiFSI concentration. Critically, LiF was the dominant salt-derived species, which increased sharply with LiFSI concentration. These results elucidate how FEC and LiFSI shape the SEI, and identify both SEI fluorination and Li 2 CO 3 suppression as key CE descriptors to guide future electrolyte design.