Reduction and Solid Electrolyte Interphase Formation Mechanism of Prop‐1‐ene‐1,3‐Sultone

Despite the lack of fundamental understanding of their operating mechanisms, layer‐forming electrolyte additives are key to stabilizing modern Li‐ion cells. Herein, the reduction mechanism of sulfur‐containing additive prop‐1‐ene‐1,3‐sultone (PES) is investigated using a model inert electrode/electrolyte system to isolate additive‐driven reactions. Attenuated total reflection fourier‐transform infrared spectroscopy, online electrochemical mass spectrometry, electrochemical quartz crystal microbalance with dissipation monitoring, and electrochemical impedance spectroscopy (EIS) are applied operando along with ex situ X‐ray photoelectron spectroscopy. PES satisfies the main criteria for layer‐forming additives, to reduce at higher potentials than typical carbonate ester‐based electrolytes and to suppress further electrolyte decomposition. Inclusion of PES in the electrolyte significantly reduced gas evolution, forming a thin, dense, and Li + ‐conductive SEI. Despite significant charge passed already at 1.4 V vs. Li + /Li, no major interfacial change is observed until 1.0 V, where a high‐resistance, low‐capacitance interphase forms containing sulfur–oxygen species, such as ROSO 2 Li, RSO 3 Li, and Li 2 SO 3 . Such species primarily form in chemical steps after PES reduction, which in turn continue the layer growth long after the electrochemical PES reduction has ended. The combined operando methodology applied herein provides unique insights into electrode layer formation in batteries and thereby guides understanding and development of future electrolyte additives.