Lawrence Berkeley National Laboratory

Silicon is a promising alloying anode for lithium-ion batteries because of its high capacity and low cost. However, its use has been hampered by mechanical failure arising from the large volume change upon cycling and by an insufficiently stable solid electrolyte interphase (SEI). SEI formation depends on the Si surface, which is often an oxide (SiOx). In this study we compare three different Si surfaces using Si wafers: 1.3 nm native SiOx, 1.4 nm thermally grown SiO2, and a SiOx-free surface. The oxide-free surface showed the worst electrochemical performance, never exceeding 94% Coulombic efficiency (CE). It also exhibited the thickest SEI and the highest overpotential for lithiation, which correlated with uninhibited electrolyte reduction and the incorporation of P-F species into the SEI. The oxide-coated surfaces performed significantly better, demonstrating a CE above 99% beyond the second cycle, low overpotential for lithiation, and a thinner and more stable SEI. The oxides lower the onset potential for electrolyte reduction and yield an SEI with fewer P-F species. However, it was found that the CE with the native oxide surface decays from the fifth cycle onward and correlates with a resurgence of electrolyte reduction. A 1-2 nm thermal SiO2 coating is optimum for achieving a stable SEI that minimizes side reactions and sustains efficient cycling.