Operando Attenuated Total Reflectance Far-Ultraviolet Spectroscopic Analysis at the Cathode/Electrolyte Interface.

Understanding cathode/electrolyte interfacial reactions is critical for improving the stability of high-voltage Li-ion batteries; however, the molecular mechanisms governing cathode/electrolyte interphase (CEI) formation remain poorly understood due to the buried and dynamically evolving nature of the interface. Here, we develop an operando attenuated total reflectance far-ultraviolet (ATR-FUV) spectroscopic method to directly probe cathode/electrolyte interfacial chemistry during electrochemical cycling. With the aid of principal component analysis, we isolate a voltage-dependent interfacial spectral component that emerges at ∼3.9 V, well below the onset potential of electrochemical electrolyte oxidation. Supported by quantum chemical calculations, this component is assigned to dehydrogenated ethylene carbonate (de-H EC) and vinylene carbonate (VC), revealing a chemically driven proton-abstraction pathway induced by reactive surface oxygen species generated upon Li+ deintercalation. This process recurs upon charging, demonstrating a voltage-driven yet chemically initiated interfacial reaction distinct from direct electrochemical oxidation. These findings establish operando ATR-FUV spectroscopy as a powerful tool for resolving transient cathode interfacial chemistry and provide new molecular-level insights into CEI formation relevant to the design of next-generation high-voltage Li-ion batteries.