Operando Ellipsometric Monitoring of Solid Electrolyte Interphase Evolution on Copper: Effects of Electrolyte Chemistry on Interphase Growth Kinetics

The formation and properties of the solid electrolyte interphase (SEI) are considered crucial for cell performance, particularly for negative electrode chemistries that undergo considerable volume and morphological changes during cell operation, such as lithium metal. In this work, the interphase formation kinetics and thickness evolution of the SEI on Cu are monitored via operando ellipsometry and correlated with key electrolyte‐dependent electrochemical and interphase properties of related lithium metal‐based cells. The quantification of SEI growth under potentiodynamic control revealed that the localized high‐concentration electrolyte (LHCE) promotes rapid SEI formation at the FSI ‐ reduction potential (1.5 V vs. LiǀLi + ), followed by a subsequent decrease in growth rate as the Li redox potential is approached. In contrast, the conventional organic carbonate‐based electrolyte (CCE) leads to continuously accelerated SEI growth during the cathodic potential sweeps, accompanied by roughly a sixfold higher electrical charge and a thicker interphase of 75 vs. 31 nm with the LHCE. Ex situ synchrotron infrared nano spectroscopy (SINS) confirmed a more homogeneous, less rough SEI formation with the LHCE, which, in combination with the identified fast formation kinetics and effectively protective FSI ‐ ‐derived interphase, promotes efficient electrodeposition/‐dissolution of Li with a compact morphology.