Surface Film Formation and Dissolution in Si/C Anodes of Li-Ion Batteries: A Glow Discharge Optical Emission Spectroscopy Depth Profiling Study

Addition of a certain amount of Si to state of the art graphite anodes has become the most prominent option to increase the energy density of Li-ion cells. However, the distribution of Si in the depth of Si/C anodes is difficult to measure with established methods. In this paper, we present a semiquantitative depth profiling method based on glow discharge optical emission spectroscopy (GD-OES). The calibration of this method covers 0-100 wt % Si content in the anode and is validated by pilot-line-coated Si/C anodes with known Si contents. The quantified depth profiles with different pristine anodes show a homogeneous distribution of Si before contact with electrolyte. In contrast to that, pilot-line-coated electrodes after formation and long-term cycled cells with Si/C composite anodes from a commercially available 18650-type cell, as control measurement, reveal a peak near the anode surface, which corresponds to a new aging mechanism. This aging mechanism is verified by interrupted GD-OES sputtering. Raman spectroscopy and ICP-OES substantiate the dissolution of Si species in the electrolyte.