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 × 10(-2) mS cm(-1)). 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 were 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, which is required to remove the electrolyte residue prior to ex situ analysis, demonstrating the necessity of performing SEI characterization in situ.
Highlights
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 Â 10À2 mS cmÀ1)
This study further investigates the solid electrolyte interphase (SEI) formed by dimethyl perfluoroglutarate using in situ neutron reflectometry (NR) for comparison with ex situ X-ray photoelectron spectroscopy (XPS)
This study provides direct evidence for the contention that rinsing electrodes prior to ex situ characterization modifies the SEI, leading to potentially fallacious conclusions about the function, composition, and optimization of SEIs
Summary
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 Â 10À2 mS cmÀ1). Neutrons are deeply penetrating and sensitive to light elements (H and Li),[12,16,18,25,26,27,28] unlike X-rays, making NR suited for the characterization of SEIs and electrodes in Li-ion batteries.[12,16,18,25,26,27,28] A specialized cell has been developed at the Oak Ridge National Laboratory for use on a Liquids Reflectometer (Spallation Neutron Source, Beam Line 4B) for performing NR on thin films during electrochemical cycling Analysis of these data provides information on the thickness, roughness, and composition of various layers within the battery.[25] Prior work has revealed the formation of a 17 to 25 nm SEI layer in ethylene carbonate/dimethyl carbonate (EC/DMC) electrolytes whose composition and thickness change with cycling.[26] Other in situ NR studies have shown the formation of a 5.5 nm reaction layer on TiO2,29 a 20 nm thick layer that forms at the electrode/electrolyte interface on LiFePO4,30 and a 7 nm electrochemical SEI layer that is formed only upon delithiation, though the extent of lithiation was not clear.[21] More recently, we demonstrated the formation of a 3.3 nm Li-rich layer at open circuit voltage on the high voltage cathode LiMn1.5Ni0.5O4 which changed with charging to become a F-rich layer with a similar thickness[24] while others have focused on electrolyte decomposition over Cu metal as a function of potential.[22]
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