Ultra-thick cathodes based on Aluminium metal foams as current collector for high energy Li-ion batteries

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The demand for Li-ion batteries with ever increasing energy and power density applies 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 and 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 such 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. 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.

In this study, we used an Aluminium-foam with a porosity of 95%, which was infiltrated with an NMC-based slurry by a vacuum supported infiltration process. The ultra-thick foam-based cathodes exhibit a capacity of up to 8.0 mAh/cm². On the one hand, we demonstrate an improved multi-step infiltration process to further increase the active material loading. On the other hand, we present a detailed microstructure analysis focusing on (i) the homogeneity of the infiltration, (ii) the microstructural evolution during drying and subsequent compaction as well as (iii) the electrolyte accessibility depending on the degree of compaction. The electrochemical properties such as capacity and rate capability, which are correlated to the electrode’s microstructure, reveal that mechanical compaction of the infiltrated foam 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, a pronounced clogging of the surface pores is observed due to compaction – which in turn inhibits the Li-ion transport. To overcome this drawback we propose a selective surface treatment by means of IR-laser, which is already successfully applied to standard electrodes.

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