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 (SiO_x). This work compares three different Si surfaces using Si wafers: 1.3 nm native SiO_x, 1.4 nm thermally grown SiO₂, and a SiO_x-free surface. It also determines the impact of SiO₂ coatings of different thicknesses, and examines how the SEI evolves during rest periods.

In comparison to oxide-free Si, ~1.4 nm thermal or native oxide greatly improve Coulombic efficiency (CE), lower the onset potential for electrolyte reduction, and yield a thinner and more stable SEI. Thicker SiO₂ coatings reduce electrolyte reduction further, but SiO₂ films thicker than 3 nm also prevent uniform lithiation. A rather sharp transition is observed at ~3 nm, and lithiation occurs exclusively at isolated pinholes for 4 and 5 nm SiO₂. Interestingly, Li does spread laterally once inside the Si, preferentially at the SiO₂/Si interface. Experiments conducted with prolonged rest periods after SEI growth indicate that high CEs achieved during continuous cycling may obscure some limitations of SEI stability on Si.

[1] ACS Applied Energy Materials (2020) DOI 10.1021/acsaem.0c01337

[2] ACS Applied Materials & Interfaces 12 (24), 27017 (2020)

[3] ACS Applied Materials & Interfaces 12 (23), 26593 (2020).