In Li-Ion batteries, the graphite anode undergoes chromatic changes from grey (unlithiated graphite) to blue (LiC₁₈) to red (LiC₁₂) to gold (LiC₆) during lithiation.¹ When charged with high current rates, lithiation gradients are observable due to mass transport limitations within the anode.² Due to high lithiation degrees at the surface, the anode potential decreases locally and the possibility of Li deposition is increased.³

We introduce a new in situ optical microscopy set-up, which allows a direct observation of the cross-section of Li-ion full cells in combination with simultaneous recording of electrochemical data (Figure 1). Extensive analysis of the chromatic changes from the graphite electrode surface to the current collector gives insights into the lithiation processes on electrode and particle level. The set-up was validated by comparing the electrochemical results with data from coin full cells. The propagation of lithiation fronts for LiC₁₂ (~ 3300 µm² min^-1) and LiC₆ (~1260 µm² min^-1) at 1C through the graphite electrode coating were determined as well as the estimation of apparent solid-state diffusion coefficients in the order of 10^-10 cm² s^-1 from analyzing the phase propagation within single particles at C/10. Additionally, the expansion of all components of the whole cell can be observed individually. We found that the graphite contributes mainly to the cell expansion, both irreversibly (4%) and reversibly (4-13%). Directly observing the described phenomena in the full cell can give insights into aging mechanisms of the materials.

Figure 1 Exemplary image from the video at the end of charge of an in situ measurement (cycling at C/10) of a graphite-NMC 622 full cell cross-sections.

References

1. P. Maire, A. Evans, H. Kaiser, W. Scheifele and P. Novák, J. Power Sources, 155(11), A862 (2008).
2. M. Weiss, R. Ruess, J. Kasnatscheew, Y. Levartovsky, N. R. Levy, P. Minnmann, L. Stolz, T. Waldmann, M. Wohlfahrt-Mehrens, D. Aurbach, M. Winter, Y. Ein-Eli and J. Janek, Adv. Energy Mater., 11(n/a), 2101126 (2021).
3. T. Gao, Y. Han, D. Fraggedakis, S. Das, T. Zhou, C.-N. Yeh, S. Xu, W. C. Chueh, J. Li and M. Z. Bazant, Joule (2021).