Binders can have a significant impact on the performance of lithium-ion batteries (LIBs), even though they constitute only a small portion of the battery electrodes. Among others, binders influence mechanical properties of the electrode, like adhesion and cohesion, as well as its electrochemical properties, which include the ionic and electronic conductivity.¹ The most commonly used binder system for water-based graphite anodes is a combination of CMC and SBR. CMC ensures good rheological properties and is used as a thickening agent, whereas SBR provides an increased flexibility of the electrode.^2–4 However, there are also disadvantages associated with the use of this binder system. CMC is very brittle and provides only a limited Li-ion conductivity, which is especially problematic for thick electrodes and high current densities.^{5, 6} In addition, CMC/SBR systems show poor electrical conductivity.⁷

Since fast-charging of LIBs is a key factor to ensure the economic success of battery electric vehicles, in this work a tailored binder system for water-based graphite anodes is developed by combining specific rheological, mechanical and electrochemical properties to enable an improvement in fast-charging capability. The fast-charging capability of the water-based graphite anodes prepared in this work is characterized in pouch cells using a NCM622 cathode. Since the fast-charging capability of anodes is closely related to the ionic resistance within the electrode`s porous structure, as well as its porosity and pore size distribution, electrochemical impedance measurements of symmetrical coin cells are performed to determine the ionic pore resistance. Hg-intrusion porosity measurements also provide information about the pore size distribution of the graphite anodes containing the used binder systems.

It is shown that compared to the CMC/SBR binder system, a selective combination of different alternative binders for graphite anodes, based on their rheological, mechanical, as well as electrochemical properties, can improve the fast-charging capability of full cells at 4C (~ 10 mAh cm^-2) in the constant-current step by more than 20% with regard to the theoretical capacity. In addition, a direct correlation between the ionic pore resistance and the pore size distribution of the graphite anodes containing the different binder systems with their fast-charging capability could be revealed.

References:

1. A. Cholewinski, P. Si, M. Uceda, M. Pope and B. Zhao, Polymers, 13(4) (2021).
2. S. Lim, S. Kim, K. H. Ahn and S. J. Lee, Journal of Power Sources, 299, 221–230 (2015).
3. D. Bresser, D. Buchholz, A. Moretti, A. Varzi and S. Passerini, Energy Environ. Sci., 11(11), 3096–3127 (2018).
4. B. Lestriez, Comptes Rendus Chimie, 13(11), 1341–1350 (2010).
5. Y.-M. Zhao, F.-S. Yue, S.-C. Li, Y. Zhang, Z.-R. Tian, Q. Xu, S. Xin and Y.-G. Guo, InfoMat, 3(5), 460–501 (2021).
6. L. Ibing, T. Gallasch, P. Schneider, P. Niehoff, A. Hintennach, M. Winter and F. M. Schappacher, Journal of Power Sources, 423, 183–191 (2019).
7. D. Shin, H. Park and U. Paik, Electrochemistry Communications, 77, 103–106 (2017).