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

Abstract 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 Li2CO3 while evolving predominantly H2, as observed by ECMS. Increasing LiFSI concentration favored making CO2 while suppressing Li2CO3. This concentration-dependent mechanism was further supported by depth-resolved (∼40 nm) XPS, where Li2CO3 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 Li2CO3 suppression as key CE descriptors to guide future electrolyte design.