Probing Surface Dynamics of SiOx Thin-Film Electrodes during Cycling through X-Ray Photoemission Spectroscopy and Operando X-Ray Reflectivity

SiO_x electrodes are promising for high-energy-density lithium-ion batteries (LIBs) due to their ability to mitigate volume expansion-induced degradation. Here, we investigate the surface dynamics of SiO_x thin-film electrodes cycled in different carbonate-based electrolytes using a combination of ex situ X-ray photoelectron spectroscopy (XPS) and operando synchrotron X-ray reflectivity analyses. The thin-film geometry allows us to probe the depth-dependent chemical composition and electron density from surface to current collector through the solid electrolyte interphase (SEI), the active material, and the thickness evolution during cycling. Results reveal that SiO_x lithiation initiates below 0.4 V vs Li⁺/Li and indicate a close relationship between SEI formation and SiO_x electrode lithiation, likely due to the high resistivity of SiO_x. We find similar chemical compositions for the SEI in FEC-containing and FEC-free electrolytes but observe a reduced thickness in the former case. In both cases, the SEI thickness decreases during delithiation due to the removal or dissolution of some carbonate species. These findings give insights into the (de)lithiation of SiO_x, in particular, during the formation stage, and the effect of the presence of FEC in the electrolyte on the evolution of the SEI during cycling.