Neutron Reflectometry Study of Solid Electrolyte Interphase Formation in Highly Concentrated Electrolytes

Highly concentrated electrolytes have been found to improve the cycle life and Coulombic efficiency of lithium metal anodes, as well as to suppress dendrite growth. However, the mechanism for these improvements is not well understood. Partly, this can be linked to the difficulty of accurately characterizing the solid electrolyte interphase (SEI), known to play an important role for anode stability and stripping/plating efficiency. Herein, in situ neutron reflectometry is used to obtain information about SEI formation in a highly concentrated ether‐based electrolyte. With neutron reflectometry, the thickness, scattering length density (SLD), and roughness of the SEI layer formed on a Cu working electrode are nondestructively probed. The reflectivity data point to the formation of a thin (5 nm) SEI in the highly concentrated electrolyte (salt:solvent ratio 1:2.2), while a considerably thicker (13 nm) SEI is formed in an electrolyte at lower salt concentration (salt:solvent ratio 1:13.7). Further, the SEI formed in the electrolyte with high salt concentration has a higher SLD, suggesting that the chemical composition of the SEI changes. The results from neutron reflectometry correlate well with the electrochemical data from SEI formation. By increasing the electrolyte salt concentration, several performance metrics of lithium metal batteries can be improved, but the mechanism for this improvement is still poorly understood. Herein, in situ neutron reflectometry is performed to characterize the solid electrolyte interphase (SEI) and demonstrate that the SEI layer becomes thinner when the lithium bis(trifluoromethanesulfonyl)imide salt concentration in 1,2‐dimethoxyethane is increased.