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

Abstract

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.

Low‐Temperature Charging and Aging Mechanisms of Si/C Composite Anodes in Li‐Ion Batteries: An Operando Neutron Scattering Study

“…A lower maximum and a broader distribution was observed for the Si distribution of the aged cell, which indicated that the formed film extended more into the depth of the anode compared with the anode from the fresh cell. These results for the cell cycled at 25 °C were consistent with depth profiling results for the same cell type cycled at 0 °C and 45 °C reported previously . The most likely formed Li silicates consume cyclable Li, which was consistent with the decelerated capacity fade curve in Figure .…”

“…These results for the cell cycled at 25 °C were consistent with depth profiling results for the same cell type cycled at 0 °C and 45 °C reported previously . The most likely formed Li silicates consume cyclable Li, which was consistent with the decelerated capacity fade curve in Figure . The loss of cyclable Li has also been observed for other cells without Si compound in the anode .…”

“…Discharged Si/C anodes of a) fresh and h) aged cells. Charging and relaxation of the fresh cell at b–d) −10 °C/0.75 C and e–g) −23 °C/0.75 C. i) Charging and j) relaxation of the aged cell with a silicate film in which Li deposition was not observed. The amounts of Li are intended to show the observed trend rather than quantitative values.…”

“…However, owing to its high volume expansion (up to 300 %), the resulting particle cracks, and its reaction with the electrolyte, one strategy is to combine the high gravimetric capacity of Si compounds with graphite . For example, the cells tested in this work contain 3.5 wt % Si, leading to a specific energy that is comparable with that of other state‐of‐the‐art cells . If 3.5 wt % Si is added and the anode coating thickness is kept constant at approximately 52 μm, the specific capacity increases from approximately 135 Wh kg −1 to 180 Wh kg −1 on a cell level.…”

“…Si is ap rominentc andidate for negative electrode (anode) materials, [2][3][4][5][6][7][8][9][10] owing to its abundance in the earth's crust [11] and its high theoretical capacity. [5] However,o wing to its high volumee xpansion (up to 300 %), the resulting particle cracks, and its reaction with the electrolyte, [5] one strategy is to combine the high gravimetric capacity of Si compounds with graphite.…”