In summary, this study demonstrates clear differences in Li metal protection ability between a standard organic carbonate-based formulation (CCE) and a baseline localized high-concentration electrolyte (LHCE) via complementary operando and ex situ techniques. In particular, ellipsometry combined with a modified electrochemical cell setup provides real-time, quantitative tracking of the SEI growth, allowing for direct correlation of growth kinetics with electrochemical features. After refining the optical parameters of the components, a potentiodynamic protocol enables the determination of distinct SEI growth modes as a function of the electrolyte chemistry. With the LHCE, a rapid SEI formation is observed at the reductive decomposition peak of FSI- at 1.5 V vs. LiǀLi+, followed by a strongly suppressed growth rate even at 0.05 V vs. LiǀLi+. In contrast, the organic carbonate-based electrolyte provides ineffective protection of the sputtered Cu electrode, as an accelerated SEI growth rate is observed during the cathodic potential sweep, accompanied by a roughly six-fold higher current density than in the LHCE-based cell. A 12 h potential hold experiment at 0.05 V vs. LiǀLi+ further highlights the differences in the protecting ability of the electrolyte-derived interphases. While the thickness of the LHCE-derived SEI already stabilizes at 32 nm (drough: 6 nm) and only partially dissolves down to 21 nm (drough: 1 nm) upon open circuit potential (OCP), the organic carbonate-based interphase grows up to a thickness of 75 nm (drough: 11 nm) and contracts to roughly 41 nm (drough: 3 nm) under the open circuit conditions. Further discoveries include a decrease in surface roughness during non-growth regimes (OCP or at ≥ 1 V vs. LiǀLi+), highlighting SEI compaction likely due to dissolution and reorganization of organic-rich SEI components. Ex situ atomic force microscopy (AFM) experiments further confirm the more homogeneous, less rough nature of the SEI with the LHCE, which ultimately leads to a denser Li-deposition morphology and more efficient Li stripping compared to the organic carbonate-based system. The study further highlights the effectiveness of FSI–derived SEI formation in terms of reaction kinetics, protective abilities, and the promotion of efficient Li electrodeposition/dissolution, and identifies operando ellipsometry as a powerful analytical tool for elucidating SEI growth modes through real-time correlation with electrochemical responses.