Detection of Li Deposition on Si/Graphite Anodes from Commercial Li-Ion Cells - a Post-Mortem GD-OES Depth Profiling Study

Fluegel, Marius

A new semi-quantitative method based on Post-Mortem glow discharge optical emission spectroscopy (GD-OES) depth profiling was developed to detect Li deposition on Si/graphite anodes. By means of the detected amounts of Li, Si and O in the GD-OES depth profiles, a corridor is determined, in which the minimum amount of Li deposition is found. Three different commercially available 18650 cell types containing Si/graphite as anode active material were acquired to evaluate the newly developed method. Those three cell types were initially characterized in detail by means of SEM/EDX, GD-OES, ICP-OES, and Hg porosimetry regarding their cell chemistry and electrode properties. One cell type was a high-power cell due to its high anode porosity and low anode coating thickness. The other two cell types were high-energy cells, which have thick anode coatings and low porosities.

We aged the cells at 45 °C until their SOH was 80% and analysed the aging behaviour using SEM, DVA, GD-OES, and in coin half-cells. GD-OES depth profiling revealed that no Li plating occurs during cycling aging at 45 °C. Contrary to this, Li plating was detected on the anodes of all three cell types by the new GD-OES method after the high-power cell was aged at -20 °C and the high-energy cells were aged to 0 °C.

Cells, which were previously aged at 45 °C until 80% SOH have afterwards been cycled under the same conditions, which led to Li plating in the fresh cells. Cell types exhibiting predominantly loss of anode active material (LAAM) as aging mechanism during the first aging at 45 °C, still suffered from Li plating during the second aging. However, if the main aging mechanism during the initial aging was loss of cyclable Li inventory (LLI), the cells did show a significantly reduced tendency for Li plating, even though the same conditions caused Li plating in the fresh cells.

Analysis of the aged anodes using coin half-cells revealed that the loss of anode material was caused by the deactivation of Si anode material. SEM images of the anode cross-section indicate that the deactivation is most likely mainly induced by the formation of a thick film surrounding the Si particles.

In combination with complementary methods like SEM, ICP-OES, and coin half-cell analysis, the newly developed GD-OES method yields in profound understanding of the aging behaviour of state-of-the-art Li-ion cells contain Si/graphite anodes.