The “ethylene carbonate (EC)–propylene carbonate (PC) mystery” has puzzled electrochemists for decades. Surprisingly, the minor structural difference between PC and EC, a methyl vis‐à‐vis a proton, prevents PC unlike EC to form a stable solid electrolyte interphase (SEI) on carbon (C), which along with the popularity of PC has impeded the development of Li‐ion batteries with many years. Despite several hypotheses, the fundamental reason remains debated largely due to the lack of sufficient experimental evidence. Herein, SEI formed as a result of EC and PC reductions are analyzed by two state‐of‐the‐art operando techniques, online electrochemical mass spectrometry and electrochemical quartz crystal microbalance with dissipation monitoring. Although both EC‐ and PC‐based electrolytes appear to have virtually identical reaction pathways, PC is reduced much more extensively than EC and forms a much thicker SEI. However, while the SEI derived from EC remains on the electrode, PC reduction products redissolve in the electrolyte leaving the bare C electrode behind. The presented study illustrates the complex scheme of competing electro‐/chemical reactions behind SEI formation and provides further scientific details needed to eventually form a consensus of the processes governing electrode/electrolyte interphases in Li‐ion batteries.

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 LiClO4 in ethylene carbonate (EC). Both the electrolyte itself and cell contaminants, such as O2, CO2, and H2O, 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.