Reveal Non‐Uniform Passivation on Lithium Metal Anode With Operando Spectroscopic Ellipsometry

ABSTRACT The passivity of lithium (Li) metal anode is crucial for the safety and long‐term cycling performance of Li metal batteries (LMBs), yet the mechanism and evolution of native passivation remain poorly understood. In this work, we use operando spectroscopic ellipsometry (SE) with a real‐time, spatially resolved thickness‐mapping function to study freshly prepared Li metal anodes with/without texture control. The native passivation reaction is monitored inside an Argon glovebox with precise control of O 2 , CO 2, and H 2 O gas environment. We discovered that the nucleation and anisotropic reactivity of Li metal are the root causes for the non‐uniform distribution of the native passivation layer. The equilibrium thickness has been studied for texture effects (i.e., Li with (200) and (110) out‐of‐plane orientations) and temperature effects. The growth of the native layer is fitted by Fehlner–Mott model. We correlate the thickness of the native passivation layer with subsequent SEI evolution by analyzing electrochemical impedance spectroscopy distribution of relaxation times (EIS‐DRT) and critical current densities (CCD) in three different electrolytes. By elucidating the fundamental growth mechanisms and roles of the native passivation layer, this work reveals the origin of non‐uniform reactivities on Li metal anodes, providing a new framework for engineering stable, dendrite‐free lithium metal interfaces.