Electrochemical Lithiation and Delithiation of Amorphous Nonstoichiometric Silicon Oxide Thin-Film Electrode Studied by <i>Operando</i> X-ray Photoelectron Spectroscopy

Electrochemical lithiation/delithiation of a nonstoichiometric silicon oxide (SiO_<i>x</i>) thin-film electrode on a Li_6.6La₃Zr_1.6Ta_0.4O₁₂ were analyzed using <i>operando</i> X-ray photoelectron spectroscopy (XPS). At the pristine SiO_<i>x</i> surface, bulk Si and SiO_<i>x</i> peaks were observed and remained unchanged in the capacity density from 0 to ∼1300 mAh g_Si^-1. At the capacity density of ∼1400 mAh g_Si^-1, however, new peaks corresponding to Li_∼2.0Si and Li silicates appeared simultaneously with a substantial decrease in the bulk Si and SiO_<i>x</i> peaks. These results imply that, in the initial stage, lithiation of SiO_<i>x</i> occurred at the SiO_<i>x</i>/Li_6.6La₃Zr_1.6Ta_0.4O₁₂ interface to form Li_<i>y</i>Si and Li silicates, which was beyond the probing depth of XPS. Subsequently, lithiation gradually propagated into the bulk and approached the probing depth of XPS as the composition reached Li_∼2.0Si, thereby elongating the ion conductive pathway. Thereafter, the position of the Li_<i>y</i>Si peak reversibly responded to the state of charge because lithiation/delithiation occurred uniformly across the SiO_<i>x</i> thin film.