Formation and Evolution of the Solid Electrolyte Interphase on Silicon Electrodes from Fluorine‐Free Electrolytes

Abstract With the increasing attention to energy storage solutions, a growing emphasis has been placed on environmentally compatible electrolytes tailored for lithium‐ion batteries. This study investigates the surface behavior of Si wafers as model systems cycled with a fluorine‐free electrolyte based on lithium bis(oxalato)borate (LiBOB), with and without the additive vinylene carbonate (VC). By utilizing operando X‐ray reflectivity (XRR) and ex situ X‐ray photoelectron spectroscopy (XPS), the intricate processes involved in solid electrolyte interphase (SEI) formation is elucidated, SiO ₂ /Si (de)lithiation, and the impact of the VC additive. Three distinct stages in SEI evolution during lithiation and delithiation are identified: SEI formation, subsequent densification and growth, and decrease in SEI thickness during delithiation, which collectively demonstrate the breathing behavior of the SEI during cycling. The addition of VC is found to mitigate LiBOB decomposition during cycling and promote a smoother SEI layer. Moreover, lithium trapping within the Si wafer post‐delithiation is observed for both electrolytes but to a lesser extent with the addition of VC. This study offers structural and chemical insights into the fundamental processes governing SEI formation and Si wafer (de)lithiation in LiBOB‐based electrolytes, with implications for designing environmentally friendly lithium‐ion batteries.