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

ABSTRACT 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.