The Role of Native Oxide in Silicon Anode Lithiation Investigated By in-Situ X-Ray Photoelectron Spectroscopy

Abstract

Understanding fundamental aspects of silicon anode lithiation is important for designing improved battery chemistries with more stable interfaces. The present study focuses on the role that native SiO2 plays in early-stage lithiation and its conversion into relevant inorganic components of the solid-electrolyte interphase (SEI) on a silicon anode. The study uses the virtual-electrode (VE) approach to deliver a square-wave pulsed flux of Li+ ions to a native-SiO2/Si(001) wafer anode surface while monitoring compositional and chemical-state changes via in situ X-ray photoelectron spectroscopy (XPS). Lithiation proceeds initially by conversion of the native SiO2 layer into two distinct LiySiOx phases. Consistent with the Li-Si-O equilibrium phase diagram, the onset of LixSi formation is observed only after all SiO2 has been converted to lithium silicate phases. As lithiation proceeds and an increasing fraction of the Si0 observable within the XPS information depth (~10nm) is converted to LixSi, evidence for a third LiySiOx phase was observed. As these phase transformations occurred, the pulsed VE-XPS approach revealed chemically resolved changes in overpotential via transient Li+ion-induced shifts of core-level binding-energies (BEs) associated with the various phases. Through this analysis, we show how nanoscale native SiO2 significantly contributes to net overpotential during the initial stages of lithiation, and how overpotential dramatically decreases as SiO2 converts to LiySiOx.