Investigating the Impact of Various Binder Contents and Compression on Graphite Electrode Thickness Change Measurements
Authors: Kuecher Simon , Spingler Franz B. , Durdel Axel , Reuter Lennart , Jossen Andreas 
Published: December 2024

Volume change of active materials during the insertion of Li+-ions into the electrode’s host structure in lithium-ion batteries (LiBs) has become an increasingly researched topic. Reversible and irreversible thickness changes occur during operation because of active material volume change upon lithiation/delithiation and SEI growth. In LiBs, particularly the volume change of the negative active material can cause mechanical disintegration of the composite coating [1], causing loss of active material. LiBs are usually operated in a compressed state maintained by geometrical boundary conditions to ensure uniform contact between electrode layers. Therefore, the thickness changes can cause increased stack pressure and, subsequently, microstructural changes [2] and thus affect the performance and lifetime of the battery [3]. To better understand the aforementioned phenomena, accurate thickness measurements are required.

Measuring the thickness change of a cell stack does not provide accurate information about the individual thickness change of the two electrodes. Therefore, single-electrode electrochemical dilatometry (ECD) can help to better understand the impact of the individual electrodes on cell stack thickness change. Due to the common use of a commercially available setup (EL-Cell, ECD-3-nano, Germany) in literature, most ECD experiments were conducted at a pressure of ~ 16 kPa onto a Ø10 mm working electrode (WE). Only in some publications higher pressures > 0.1 MPa were applied during single electrode measurements [4]. The applied pressure in lab-scale coin cell setups and state-of-the-art LiBs is usually significantly higher (0.1 – 0.5 MPa) than that of single-electrode ECD investigations. Therefore, the question arises whether the pressure has an impact on the measured thickness change.

Thus, this work integrates a wave spring washer into a commercially available single-electrode ECD Setup (ECD-3-nano, El-Cell GmbH, Germany) to apply additional pressure onto the WE, without requiring geometrical modifications of the setup. The integration of the wave spring washer increases the applied pressure from 16 kPa to > 100 kPa. Graphite electrodes with different binder types and contents are investigated since distinct measurement artifacts were observed for electrodes with certain binder compositions in preliminary experiments. Electrodes without additional pressure show a first-cycle lithiation thickness increase of up to 30% (see Figure 1), which is much higher than the active material volume change of graphite obtained from XRD data [5] would suggest. For all examined electrodes at lower pressure the first and tenth cycle lithiation thickness change is more than 50 % higher compared to the setup at high pressure. The mechanism causing the discrepancy between measurements at the two pressure levels is identified as the curvature of the electrode edges at low pressures present in commercially available ECDs. Therefore, the setup at increased pressure enables a meaningful investigation of the influence of the binder content on irreversible and reversible thickness change for the electrodes under investigation using more realistic mechanical boundary conditions. Measurements show that irreversible thickness change decreases with increasing binder content. Possibly the higher binder content helps to prevent restructuring of the particles during cycling due to higher mechanical strength of the electrode. Delithiation thickness change is similar among electrodes with normal to high binder contents. Only electrodes with a very low binder content have a significantly lower reversible thickness change.

Overall, this study shows that the influence of pressure onto the WE should be considered for future ECD-experiments and setups. This work presents an easy to implement solution to apply additional pressure onto the WE for a common commercially available ECD-setup.

References

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[2] Reuter, L., Dickmanns, J.L.S., Kücher, S., Friedrich, S., Morasch, R., Jossen, A., and Gasteiger, H.A.: State-of-Charge Dependent Change in the Microporous Structure of Graphite Electrodes. In: ECS Meeting Abstracts MA2023-02.8 (2023), p. 3392. doi: 10.1149/MA2023-0283392mtgabs.

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[4] Hahn, M., Buqa, H., Ruch, P.W., Goers, D., Spahr, M.E., Ufheil, J., Novák, P., and Kötz, R.: A Dilatometric Study of Lithium Intercalation into Powder-Type Graphite Electrodes. In: Electrochemical and Solid-State Letters 11.9 (2008), A151. doi: 10.1149/1.2940573.

[5] Schweidler, S., Biasi, L. de, Schiele, A., Hartmann, P., Brezesinski, T., and Janek, J.: Volume Changes of Graphite Anodes Revisited: A Combined Operando X-ray Diffraction and In Situ Pressure Analysis Study. In: The Journal of Physical Chemistry C 122.16 (2018), pp. 8829–8835. doi: 10.1021/acs.jpcc.8b01873.

Figure 1