Surface SiO₂ Thickness Controls Uniform-to-Localized Transition in Lithiation of Silicon Anodes for Lithium-Ion Batteries

Silicon is a promising anode material for lithium-ion batteries because of its high capacity, but its widespread adoption has been hampered by a low cycle life arising from mechanical failure and the absence of a stable solid–electrolyte interphase (SEI). Understanding SEI formation and its impact on cycle life is made more complex by the oxidation of silicon materials in air or during synthesis, which leads to SiOx coatings of varying thicknesses that form the true surface of the electrode. In this paper, the lithiation of SiO2-coated Si is studied in a controlled manner using SiO2 coatings of different thicknesses grown on Si wafers via thermal oxidation. SiO2 thickness has a profound effect on lithiation: below 2 nm, SEI formation followed by uniform lithiation occurs at positive voltages versus Li/Li+. Si lithiation is reversible, and SiO2 lithiation is largely irreversible. Above 2 nm SiO2, voltammetric currents decrease exponentially with SiO2 thickness. For 2–3 nm SiO2, SEI formation above 0.1 V is suppressed, but a hold at low or negative voltages can initiate charge transfer whereupon SEI formation and uniform lithiation occur. Cycling of Si anodes with an SiO2 coating thinner than 3 nm occurs at high Coulombic efficiency (CE). If an SiO2 coating is thicker than 3–4 nm, the behavior is totally different: lithiation at positive voltages is strongly inhibited, and lithiation occurs at poor CE and is highly localized at pinholes which grow over time. As they grow, lithiation becomes more facile and the CE increases. Pinhole growth is proposed to occur via rapid transport of Li along the SiO2/Si interface radially outward from an existing pinhole, followed by the lithiation of SiO2 from the interface outward.