Characterization of cycled cells to determine the degradation mechanisms of lithium ion battery cells is viable to both improve materials and cell design as well as predict cell performance. Quantification is important to evaluate the contribution of each aging mechanism to the degradation of cell performance also known as cell aging. Graphite is still a widely used anode material in lithium ion battery cells. Nevertheless, the performance of graphite electrodes is not fully understood. The volume expansion of graphite during intercalation can reach 13%, which might cause contact loss of the graphite (LAMGraphite) due to the induced mechanical stress. LAMGraphite was quantified by utilizing lithium metal cells with excessive lithium1-3, differential voltage plots4 and microscopy studies5. Nevertheless, at the moment the mechanism behind LAMGraphite is not fully understood. LAMGraphite might have a significant influence on aging, due to associated lithium loss and increased probability of lithium plating.
By quantitatively determining the correlation between graphite volume expansion and LAMGraphite a more profound understanding of the mechanism is reached. Therefore, this study identified the volume expansion of graphite over the state of charge (SoC) with in-situ XRD and in-situ dilatometry. With the detected high and low volume expansion regions, full cells were selectively cycled under three different SoC windows by coulomb counting. Aging mechanisms were quantified post-mortem by reassembling lithium metal cells with the harvested electrodes. During post-mortem analysis, Raman spectroscopy was conducted to investigate the effect of volume expansion on the graphite surface disorder.
In-situ XRD and in-situ dilatometry revealed low SoC (10%-30%) and high SoC (70%-90%) regions as high-volume expansion regions. Whereas, in the middle SoC region (40%-60%) volume expansion was of neglectable amount. The quantification results from the lithium metal cells show that LAMGraphite was not relevant for the aging. Although graphite volume expansion did not lead to LAMGraphite, the increased ID/IG ratios in the Raman spectra indicated an accelerated surface disorder of graphite selectively aged under high volume expansion regions.
Lithium loss was the key aging mechanism for the tested cells. Comparing the findings to the results described in literature LAMGraphite may be significant only at high C-Rates, which could be related to fast switches of mechanical stress or from plating. Hence, in a future study these factors will be investigated.

The authors acknowledge the BMBF for funding the project „KlemA“ (03XP0190D).

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