Tortuosity values of porous battery electrodes determined using electrochemical impedance spectroscopy in symmetric cells with a non-intercalating electrolyte are typically higher than those values based on numerical analysis of 3D tomographic reconstructions. The electrochemical approach assumes that the electronic resistance in the porous coating is negligible and that the tortuosity of the porous electrode can be calculated from the ionic resistance determined by fitting a transmission line equivalent circuit model to the experimental data. In this work, we validate the assumptions behind the electrochemical approach. First, we experimentally and theoretically investigate the influence of the electronic resistance of the porous electrode on the extracted ionic resistances using a general transmission line model, and provide a convenient method to determine whether the electronic resistance is sufficiently low for the model to be correctly applied. Second, using a macroscopic setup with known tortuosity, we prove that the ionic resistance quantified by the transmission line model indeed yields the true tortuosity of a porous medium. Based on our findings, we analyze the tortuosities of porous electrodes using both X-ray tomography and electrochemical impedance spectroscopy on electrodes from the same coating and conclude that the distribution of the polymeric binder phase, which is not imaged in most tomographic experiments, is a key reason for the underestimated tortuosity values calculated from 3D reconstructions of electrode microstructures. In commercially relevant lithium ion battery cells operating at high currents or low temperatures and/or cells with thick and low porosity electrodes (i.e., electrodes with high areal capacity and high volumetric energy density), the ionic transport in the electrolyte throughout the thickness of the porous electrode becomes limiting, leading to the buildup of excessive electrolyte concentration gradients across the thickness of the electrode. Concentration gradients not only lead to increased overpotentials and thus lower accessible capacities, but also play an important role in battery aging caused by lithium plating reactions at the graphite anode/separator interface.1 Along with the intrinsic transport parameters of the liquid electrolyte, the morphological properties of a porous electrode, quantified by the parameters porosity and tortuosity, are key to understanding the buildup of concentration gradients across the electrode thickness and the resulting performance limitations of porous electrodes.In the battery community, there are currently two commonly used approaches to obtain values for the tortuosity of porous electrodes; however, they yield different results. One is based on numerical diffusion simulations on 3D reconstructions of the electrode obtained using X-ray (XTM) or focused ion beam scanning electron microscopy (FIB SEM) tomography.2,3 The other approach is based on electrochemical impedance spectroscopy (EIS) measurements of the electrodes in a symme...

The electrochemical performance of porous graphite anodes in lithium ion battery applications is limited by the lithium ion concentration gradients in the liquid electrolyte, especially at high current densities and for thick coatings during battery charging. Beside the electrolyte transport parameters, the porosity and the tortuosity of the coating are key parameters that determine the electrode's suitability for high power applications. Here, we investigate the tortuosity of graphite anodes using two water as well as three n-methyl-2-pyrrolidone based binder systems by analysis of symmetric cell impedance measurements, demonstrating that tortuosities ranging from ∼3-10 are obtained for graphite anodes of similar thickness (∼100 μm), porosities (∼50%) and areal capacity (∼3.4 mAh/cm 2 ). Furthermore, selected electrodes with tortuosities of 3.1, 4.3, and 10.2 were cycled in cells with reference electrode at charging C-rates from 0. Understanding and predicting rate limitations in lithium ion batteries with porous electrodes requires profound knowledge of not only the electrolyte transport parameters (i.e., transference number, diffusion coefficient, conductivity) and the thermodynamic factor, but also the porosity and the tortuosity of the electrodes. The tortuosity of the electrode is particularly critical because the effective electrolyte conductivity and the effective diffusion coefficient in the electrolyte directly scale with the inverse of the tortuosity, so that care has to be taken to develop electrodes with minimized tortuosity. Using experimental approaches 1-4 or 3D tomography, 5,6 it was shown that the shape of active material particles distinctly influences the electrode tortuosity. For a given active material, however, electrode tortuosity and rate capability can also be improved by the design of the electrode layer such that short diffusion distances can be obtained across the electrode. For example, improved performance was demonstrated for graphite anodes when the platelet graphite particles were aligned normal to the current collector surface by means of a magnetic field 7 or when silicon/graphite anode electrodes were laser structured. 8 In this study we focus on the role of the electrode composition, specifically the role of the binder, on its tortuosity as well as on its implication for battery performance. While in the literature the link between binder and electrochemistry is frequently studied empirically using rate capability tests 9 and long-term cycling experiments, 10-13 we focus on the correlation of electrode tortuosity with binder content/type and its effect on rate capability.In the following, the tortuosity of graphite anodes with different binders, different binder contents, and different amounts of conductive carbon additive will be determined by electrochemical impedance spectroscopy (EIS) of symmetric cells using the transmission line model approach. 3,14 In the first part of our analysis we will demonstrate the effect of binder and conductive carbon additives on electrode t...

The tortuosity of a porous electrode is one of the critical parameters that governs the effective transport properties such as effective conductivity and effective diffusivity of ions in the electrolyte phase. While there exist several methods to determine the through-plane tortuosity for lithium-ion battery electrodes, methods to determine in-plane tortuosity are scarcely explored. In this paper, we present a method that restricts ion transport to only the in-plane direction, which facilitates the determination of transport properties in the in-plane direction. The here proposed cell design is analyzed using a blocking condition transmission line model to measure the in-plane tortuosity, followed by validation experiments to establish its validity and robustness. The measurement of the in-plane tortuosity may act as an indispensable tool for the design, characterization, and modeling of 3-dimensional battery architectures (e.g., laser structured electrodes) and help to estimate the distribution of various properties in the through-plane direction such as porosity, tortuosity, binder content, etc.