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 operando 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 operando 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 operando interfacial analysis, advancing high-energy-density Li-ion batteries and beyond.