In‐Operando FTIR Study to Investigate the Effect of Varying Lithium Salts on Solid Electrolyte Interface (SEI) Evolution in Lithium Metal Batteries

ABSTRACT Lithium Metal Batteries (LMBs) offer exceptional energy density but suffers from unstable Solid Electrolyte Interface (SEI). This study employs in‐operando Fourier transform infrared (FTIR) spectroscopy, coupled with post‐mortem XPS, to deconstruct the real‐time evolution of the SEI in Li||Li symmetric cells at a practical current density of 3 mA/cm 2 . We investigate three lithium salts in a DME: DOL solvent – 1 m LiTFSI, 1 m LiClO 4 , and 1 m LiDFOB. In‐ operando FTIR reveals distinct, salt‐dependent SEI formation dynamics. The results reveal that LiTFSI interacts instantaneously with the lithium surface, creating a robust, hybrid SEI composed of both organic (e.g., ROLi, ROCO 2 Li) and inorganic species (e.g., Li 2 O, Li 3 N, LiF). This stable interface correlates with superior electrochemical performance, exhibiting the lowest and most stable overpotential. In contrast, LiClO 4 shows a delayed interaction, with SEI formation commencing minutes into plating and featuring decomposition products such as LiCl. The LiDFOB electrolyte proves to be the least effective, yielding an unstable, organic‐rich SEI that results in high overpotential and poor performance. This study establishes a clear correlation between SEI composition and electrochemical performance, offering a molecular‐level understanding to inform rational electrolyte design and salt selection strategies for stable lithium metal batteries.