Elucidating the Step‐Wise Solid Electrolyte Interphase Formation in Lithium‐Ion Batteries with Operando Raman Spectroscopy

Abstract The solid electrolyte interphase (SEI) is arguably one of the most critical components of the Li‐ion cell. Despite decades of studies of the SEI, its intrinsic complexity and the lack of suitable characterization tools still prevent a real consensus on the governing mechanisms to be reached. Herein, operando Raman spectroscopy supported by complimentary online electrochemical mass spectrometry is employed to study the SEI formation on Au in a model electrolyte based on LiClO 4 in ethylene carbonate (EC). Both the electrolyte itself and cell contaminants, such as O 2 , CO 2 , and H 2 O, contribute in stepwise electro‐/chemical processes to the build‐up of the SEI. Effects associated with electrode/electrolyte double‐layer charging, electrode adsorbate polarization (stark effect), and SEI dissolution are discerned. Lithium carbonate and lithium oxide are identified as major products formed already ≈2 V versus Li + /Li. Although Raman spectroscopy provides deeper insights into the underlying mechanisms, complementary techniques are necessary to support spectral interpretations. Classical challenges in the field of surface science, such as contaminations, have to be systematically addressed if the puzzle of the SEI ever will be completed.