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

Electrochemical lithiation/delithiation of a nonstoichiometric silicon oxide (SiOx) thin-film electrode on a Li6.6La3Zr1.6Ta0.4O12 were analyzed using operando X-ray photoelectron spectroscopy (XPS). At the pristine SiOx surface, bulk Si and SiOx peaks were observed and remained unchanged in the capacity density from 0 to ∼1300 mAh gSi-1. At the capacity density of ∼1400 mAh gSi-1, however, new peaks corresponding to Li∼2.0Si and Li silicates appeared simultaneously with a substantial decrease in the bulk Si and SiOx peaks. These results imply that, in the initial stage, lithiation of SiOx occurred at the SiOx/Li6.6La3Zr1.6Ta0.4O12 interface to form LiySi 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 LiySi peak reversibly responded to the state of charge because lithiation/delithiation occurred uniformly across the SiOx thin film.