Probing Surface Dynamics\nof SiO<sub><i>x</i></sub> Thin-Film Electrodes during Cycling\nthrough X‑Ray Photoemission\nSpectroscopy and Operando X‑Ray Reflectivity

SiO<sub><i>x</i></sub> electrodes are promising\nfor high-energy-density\nlithium-ion batteries (LIBs) due to their ability to mitigate volume\nexpansion-induced degradation. Here, we investigate the surface dynamics\nof SiO<sub><i>x</i></sub> thin-film electrodes cycled in\ndifferent carbonate-based electrolytes using a combination of ex situ\nX-ray photoelectron spectroscopy (XPS) and operando synchrotron X-ray\nreflectivity analyses. The thin-film geometry allows us to probe the\ndepth-dependent chemical composition and electron density from surface\nto current collector through the solid electrolyte interphase (SEI),\nthe active material, and the thickness evolution during cycling. Results\nreveal that SiO<sub><i>x</i></sub> lithiation initiates\nbelow 0.4 V vs Li<sup>+</sup>/Li and indicate a close relationship\nbetween SEI formation and SiO<sub><i>x</i></sub> electrode\nlithiation, likely due to the high resistivity of SiO<sub><i>x</i></sub>. We find similar chemical compositions for the SEI\nin FEC-containing and FEC-free electrolytes but observe a reduced\nthickness in the former case. In both cases, the SEI thickness decreases\nduring delithiation due to the removal or dissolution of some carbonate\nspecies. These findings give insights into the (de)lithiation of SiO<sub><i>x</i></sub>, in particular, during the formation stage,\nand the effect of the presence of FEC in the electrolyte on the evolution\nof the SEI during cycling.