The demand for Li-ion batteries with ever increasing energy and power density is not only for automotive applications. Inter alia, high energy density and mechanical stability together with small electrode and cell dimensions are required for medical technology systems or wearable devices. Thus, increasing the areal loading of electrodes is necessary to meet the demands. However, layer thickness and compaction of conventional electrodes cannot be increased arbitrarily since the underlying transport mechanisms as Li-ion and electron transport are limited during charge and discharge. This results in a strong fading of energy and power density at higher current rates. A 3D structured electrode design using a metal foam as current collector is regarded to be an alternative approach to overcome these issues.
In comparison to conventional electrodes using a layer of active mass on top of a current collector foil, 3D structured electrodes with an open-porous metal foam as current collector exhibit a 3D connected electronic network within the active material. This shortens the transport pathways of the electrons and contributes to lower intrinsic resistance of the electrode . Additionally, the high specific surface of the metal foam and consequently large contact area between current collector and active mass leads to an improved charge transfer and Li-ion diffusion [2, 3].
In this study, we used a 500 µm thick Al-foam with a porosity of 95%, which was loaded with an NMC111-based slurry by a vacuum supported infiltration process. This method enables a high active mass loading, so that cathodes with an aerial capacity of up to 8.0 mAh/cm² were fabricated. We present a detailed microstructure analysis of the foam-based electrodes depending on the degree of densification by viewing the cross-section and top-view SEM-images. The intrinsic resistances such as contact and pore resistance of the differently densified foam-type cathodes are determined by electrochemical impedance spectroscopy. Finally, we show the capacity and rate capability of foam-type cathodes and correlate them to the microstructure and electrochemical properties. We reveal that compaction is required to reduce shrinkage cavities and the inner porosity of the active mass as well as to reduce the closed porosity of the Al-foam. However, it becomes obvious that an excessively densification has a negative effect on the rate capability and an ideal process window will be proposed.
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