Mechanical Properties and Chemical Reactivity of Li _xSiO _y Thin Films

Silicon (Si) is a commonly studied candidate material for next-generation anodes in Li-ion batteries. A native oxide SiO₂ on Si is often inevitable. However, it is not clear if this layer has a positive or negative effect on the battery performance. This understanding is complicated by the lack of knowledge about the physical properties of the SiO₂ lithiation products and by the convolution of chemical and electrochemical effects during the anode lithiation process. In this study, Li _xSiO _y thin films as model materials for lithiated SiO₂ were deposited by magnetron sputtering at ambient temperature, with the goal of (1) decoupling chemical reactivity from electrochemical reactivity and (2) evaluating the physical and electrochemical properties of Li _xSiO _y. X-ray photoemission spectroscopy analysis of the deposited thin films demonstrate that a composition close to previous experimental reports of lithiated native SiO₂ can be achieved through sputtering. Our density functional theory calculations also confirm that the possible phases formed by lithiating SiO₂ are very close to the measured film compositions. Scanning probe microscopy measurements show that the mechanical properties of the film are strongly dependent on lithium concentration, with a ductile behavior at a higher Li content and a brittle behavior at a lower Li content. The chemical reactivity of the thin films was investigated by measuring the AC impedance evolution, suggesting that Li _xSiO _y continuously reacts with the electrolyte, in part because of the high electronic conductivity of the film determined from solid-state impedance measurements. The electrochemical cycling data of the sputter-deposited Li _xSiO _y/Si films also suggest that Li _xSiO _y is not beneficial in stabilizing the Si anode surface during battery operation, despite its favorable mechanical properties.