Operando X-ray Absorption Spectroscopy Reveals Solid Electrolyte Interphase Formation on Silicon Anodes for Li-ion Batteries

Solid electrolyte interphase (SEI) formation is critical to the long-term performance of Li-ion battery anodes, however probing the decomposition processes occurring at the buried electrode-electrolyte interface is a significant challenge. We demonstrate here for the first time, the use of operando soft X-ray Absorption spectroscopy in total electron yield (TEY) mode to resolve the chemical evolution of the SEI during electrochemical formation in a Li-ion cell. Interface-sensitive O, F, and Si K-edge spectra, acquired as a function of potential, reveal when key reactions occur on high capacity amorphous Si anodes cycled in conventional carbonate-based electrolytes (LP30) with and with- out fluoroethylene carbonate (FEC) as an additive. Density functional theory (DFT) calculations provide insight into the species observed and the origins of their spectral features. LiF is observed as the dominant F-containing SEI product, forming at ∼0.6 V for LP30 and at a higher potential of ∼1.0 V for LP30+FEC. The formation of organic SEI components is detected at lower potentials, with the emergence of a distinct peak related to -(C=O)O- containing species. This sequential formation of inorganic and organic components is implicated in layering of the SEI. Operando TEY-XAS offers new insights into the formation mechanisms of electrode-electrolyte interphases and their stability for a wide variety of electrode materials and electrolyte formulations.