Quantifying the Dynamic and Additives-Dependent Interface Evolution by Operando Neutron Reflectometry

The solid electrolyte interphase (SEI) governs key electrochemical properties in batteries. While additive-driven SEI engineering constitutes the most promising strategy for tailoring interfacial composition, the impact of specific additives on SEI's dynamic evolution remains unresolved. Herein, we performed <i>operando</i> neutron reflectometry (NR) to quantitatively resolve the SEI's structural dynamics under cycling conditions. Employing model additives with well-defined decomposition mechanisms, fluoroethylene carbonate (FEC) and vinylene carbonate (VC), we establish a robust <i>operando</i> NR framework that enables transferable mechanistic insights for emerging additive systems. Our data reveal contrasting SEI architectures: FEC produces a thin, inorganic-rich SEI (LiF-dominant) that enhances mechanical integrity and cycling stability, while VC yields a flexible organic-dominated SEI which mitigates stress-induced microcracking. These findings provide atomically resolved design principles for advanced electrolyte additives via <i>operando</i> interfacial analysis, advancing high-energy-density Li-ion batteries and beyond.