Abstract
This work details the in situ characterization of the interface between a silicon electrode and an electrolyte using a linear
fluorinated solvent molecule, 0.1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in deuterated dimethyl
perfluoroglutarate (d6-PF5M2) (1.87 x 10-2 mS/cm). The solid electrolyte interphase (SEI) composition and thickness
determined via in situ neutron reflectometry (NR) and ex situ X-ray photoelectron spectroscopy (XPS) were compared. The
data show that SEI expansion and contraction (breathing) during electrochemical cycling was observed via both
techniques; however, ex situ XPS suggests that the SEI thickness increases during Si lithiation and decreases during
delithiation, while in situ NR suggests the opposite. The most likely cause of this discrepancy is the selective removal of SEI
components (top 20 nm of the SEI) during the electrode rinse process, required to remove electrolyte residue prior to ex
situ analysis, demonstrating the necessity of performing SEI characterizations in situ.
