Improving the performance of Li-Ion batteries by enhancing their ionic accessibility through lower tortuosities is critical, especially when using high-loading electrodes with high areal capacities to increase cell energy density. Longer diffusion pathways and higher geometric current densities in this configuration can quickly lead to large concentration gradients across the electrode thickness, resulting in high diffusion overpotentials, which can lower cell capacities or lead to Li-plating. In addition, the extent of concentration gradients is affected by electrode tortuosity, which not only depends on particle geometry and electrode porosity but also on the binder content and its distribution, even though it amounts to only a few volume percent of the electrode.^1,2 Jaiser et al. ³ and Müller et al.⁴ have shown that short drying times cause binder migration towards the top of the electrode (i.e. the surface which will be adjacent to the separator after cell assembly), which has a detrimental effect on the cell performance.

Based on these findings, we will present an analysis on the effect of inhomogeneous ionic resistances, caused by an inhomogeneous binder distribution, on an electrode’s overall ionic resistance. While Jaiser et al. ³ and Müller et al.⁴ focus on the analysis of energy dispersive X-ray spectroscopy (EDS) cross sections and time consuming rate performance tests to obtain a qualitative understanding of the effect of binder gradients, we propose a simple method based on electrochemical impedance spectroscopy (EIS), allowing for a fast analysis of ionic resistance gradients in porous electrodes. As a first step, we model the blocking-condition electrochemical impedance spectra (bcEIS) response of ionic resistance gradients using COMSOL. We show how changes in the resistance distribution (i.e. electrode tortuosity gradients) affect the measured impedance response and thereby the electrode’s overall tortuosity, allowing for a facile detection of such detrimental binder gradients without the necessity for time-consuming full-cell testing. To validate our theoretical approach, we prepare graphite anodes with ionic resistance gradients by drying electrode slurries at different temperatures and show the change in binder distribution (i.e. ionic resistance) via EDS, as seen in Figure 1. bcEIS spectra of these electrodes allow us to directly correlate the experimentally measured binder distribution to the measured impedance response, verifying the analysis. Finally, we show by means of electrochemical rate tests the direct correlation between detrimental tortuosity gradients and the rate capability of graphite anodes.

Figure 1 a) Cross section scanning electron microscopy image of a graphite anode (Graphite:PVDF, 95:5 w:w), with the current collector / electrode interface at the bottom and the free electrode surface at the top (the surface which will be adjacent to the separator in the assembled cell). The electrode was dried at high temperature to form a binder gradient across the electrode thickness, then immersed in resin, hardened and polished to show the cross section. The white spots are from electrically insulating resin. b) EDS analysis of the cross section across the entire electrode thickness (~0.3 mm), showing the fluorine content from top (0 mm) to bottom measured cumulatively over a breadth of ~600 µm.

References:

(1) Landesfeind, J.; Eldiven, A.; Gasteiger, H. A. Submitted.

(2) Landesfeind, J.; Ebner, M.; Eldiven, A.; Wood, V.; Gasteiger, H. a. Tortuosity of Battery Electrodes: Validation of Impedance-Derived Values and Critical Comparison with 3D Tomography. J. Electrochem. Soc. 2018, 165 A469–A476.

(3) Jaiser, S.; Müller, M.; Baunach, M.; Bauer, W.; Scharfer, P.; Schabel, W. Investigation of Film Solidification and Binder Migration during Drying of Li-Ion Battery Anodes. J. Power Sources 2016, 318, 210–219.

(4) Müller, M.; Pfaffmann, L.; Jaiser, S.; Baunach, M.; Trouillet, V.; Scheiba, F.; Scharfer, P.; Schabel, W.; Bauer, W. Investigation of Binder Distribution in Graphite Anodes for Lithium-Ion Batteries. J. Power Sources 2017, 340, 1–5.

This work is supported by the BMBF (Federal Ministry of Education and Research, Germany) for its financial support under the auspices of the ExZellTUM II project (grant number 03XP0081) and the SurfaLib project (grant number 03ET6103F)