A new in situ optical microscopy set-up is introduced which allows direct observation of cross-sections of Li-ion full cells in combination with simultaneous recording of electrochemical data. The method is validated by comparison of electrochemical data from coin full cells. Color changes give insights into processes on the electrode and particle level, such as lithiation behavior and electrode thickness changes. Our observations allow the evaluation of (i) the speed of lithiation fronts for LiC12 and LiC6 through anode coatings, (ii) estimation of apparent diffusion coefficients from analysis of the color distribution in single graphite particles, as well as (iii) electrical de-contacting and re-contacting of single graphite particles in connection with (iv) electrode thickness changes. Furthermore, our direct observations from the inside of full cells give indirect insights into aging phenomena such as Li plating and SEI growth.
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 P. Maire, A. Evans, H. Kaiser, W. Scheifele and P. Novák, J. Power Sources, 155(11), A862 (2008). 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). 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). Figure 1
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