Impact of Coating Layer Parameters on Electrode Tortuosity
Authors: Neidhart Lukas , Froehlich Katja , Winter Franz , Jahn Marcus 
Published: February 2024

The performance of Li-ion batteries is highly influenced by the microstructure of the assembled electrodes. Typically, porosity is chosen as a standard characteristic parameter, describing the morphology of the material components in the coated layer. However, transport phenomena and concentration polarization resistance also depend on the tortuosity of the electrode coating [1]. Therefore, it is crucial to determine the relationship between coating parameters (such as coating speed and drying parameters) and their effect on the tortuosity. Especially for thick electrodes an elongation of ionic transport pathways can arise due to the inhomogeneous distribution of the materials. Dead ends, branches, and surface impurities can lead to an increase in effective tortuosity [2], which can be minimized by careful selection of the coating parameters. A high binder concentration at the layer surface, caused by binder particles migrating during the drying process, can lead to a blockage of Li-channels [3, 4]. This can not only diminish the mechanical stability of the electrode but also harm the cycling performance, due to limitations in mass and charge transport [5].
Electrochemical impedance spectroscopy can be used as a characterization technique to investigate the tortuosity of the electrode. Symmetric cells in combination with a non-intercalating electrolyte create a blocking condition of the cells. Extrapolating the spectrum's high and low-frequency regions, offsets in the real part of the impedance can be determined in the Nyquist plot (Figure 1a). The ionic resistance can be determined, and consequently, tortuosity can be calculated. A modified transmission line model can be used for in-depth analysis.
In the case of homogenously distributed material, tortuosity should be an intrinsic value. In this work, a threshold value of the electrode thickness was determined, above which the tortuosity increases drastically (Figure 1b). This leads to the conclusion that when increasing the thickness, changes in the layer composition can arise.Additionally, the correlation between tortuosity and porosity was investigated and associated Bruggeman coefficients were compared.
In a previous study, the multilayer coating technique has proven itself to be beneficial for increasing the electrochemical performance of high-loading electrodes containing cells [6]. Subsequent coating of multiple layers on top of each other enables the possibility to fine-tune material distributions along the layer thickness. Particularly, electrodes with an imposed binder gradient, of low concentration at the electrode separator interface, show a noteworthy decrease in tortuosity [7].
This study was done on cathode samples with NMC811 as active material. Processing was done with both water and NMP, to show the impact of the solvent on the electrode tortuosities.

Acknowledgments

This work was financially supported by the Austrian Federal Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology (bmk).
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Figure 1