Manufacturing of high-loading aqueous cathodes for Li-ion batteries

Abstract

Increasing the energy density of lithium-ion traction batteries is one of many challenges in electromobility. One way of increasing the energy density lies in fabricating electrodes with high active material loadings and correspondingly increased coating thicknesses.Unfortunately, producing such thick electrodes involves a high degree of complexity and requires innovative methods in electrode production. Additionally, there is a need for more sustainable and cost-effective ways of producing these electrodes.In this work, a special coating technique was used, which ensures defect-free electrodes with coating thicknesses of over 200 μm, by subsequent coating of material layers ontop of each other. This multi-layer coating technique was used in particular to process cathode materials with a high nickel content (LiNi0.8Mn0.1Co0.1O2, NMC811). With thismethod, an areal loading of >8.6 mAh/cm2 was realised. Great improvements in bothmechanical properties and electrochemical performance were achieved by switching to this innovative coating method. For example, the adhesion to the current collector film was increased by 40%. Moreover, the specific discharge capacity was improved for all tested current rates in rate capability tests. Especially for low current rates (0.1C), an increase of more than 20% was achieved. Furthermore, multi-layer coating was used to create a binder gradient in the thickness direction of the coated layer. Here, the quantity of polymethyl acrylate (PMA) in the top layer was reduced in three steps (50%, 25% and 0% of the initial proportion of PMA). The electrodes produced were again tested in coin cells and, particularly at a C-rate of 1C, improvements in the specific discharge capacity of up to 39% were achieved compared to single-layer coatings. Electrochemical impedance spectroscopy, Raman spectroscopy and scanning electron microscopy provide additional information about the importance of a distinct domain and distribution of conductive carbon black and binder in the coating.As the coating thickness increases, the importance of optimal transport of Li+-ions through the electrode layer coating also rises. The tortuosity can be used as a characteristic parameter. especially for porous material layers. Using electrochemical impedance spectroscopy of symmetrical cells, the dependence of the tortuosity on the layer thickness was determined. A drastic increase in tortuosity was detected, for electrodes exceeding a certain thickness threshold of the coating (150 μm). This indicates a change in the material composition in the coating during the fabrication process. Subsequently,cathodes, produced using the multi-layer coating technique were examined. The results show that merely switching to this coating method can improve the tortuosity by approximately 55 %. Introducing a binder gradient further enhances this effect and enables minimum tortuosity values of τ ≈ 2, which correspond to a reduction of more than 80 % compared to thick single-coated layers. Finally, a strong negative correlation between tortuosity values and specific discharge capacities was determined.