Key Features in the Manufacturing Process of Ultra-Thick Electrodes for High Energy Lithium Ion Batteries

Energy density is one of the key challenges in the development of advanced lithium ion batteries for many applications. To address this issue, a considerable number of studies have been conducted on high capacity cathode materials1–3. Another approach, covered in this contribution, is reducing the amount of passive material in the cell via increasing the mass loading of the electrodes4,5. These ultra-thick electrodes, however, suffer from certain drawbacks like a low rate capability4–7, reduced adhesion and flexibility or crack formation8. Numerous investigations on the development of electrodes for lithium ion batteries have been performed using preparation techniques typical for lab scale. In common studies, the electrochemical and mechanical properties of an electrode are attributed to mass loading, porosity and the distribution of passive materials in the binder-network9. This study considers manufacturing procedures for NMC cathodes highly relevant to industrial production. A strong influence of the manufacturing process especially on the morphology and electrochemical performance of ultra-thick electrodes was observed. As an example, ultra-thick electrodes with an areal capacity of 8 mAhcm-2 were prepared in pilot scale, yielding a significantly higher capacity than state of the art cathodes7,10. It is demonstrated that the mixing strategy used for slurry preparation significantly alters the distribution of materials and the morphology of the electrode, consequently also influencing adhesion strength and flexibility. These properties are likewise strongly affected by the applied drying procedure. By an optimized mixing and drying procedure, crack-free, ultra-thick electrodes were obtained even in pilot scale. These electrodes, manufactured by the optimized process, show significantly higher flexibility, which is very important for roll-to-roll processing. Scanning electron microscopy revealed the strong effect of the manufacturing process on the morphology of the material distribution within the electrode composite. Electrochemical measurements and impedance spectroscopy furthermore showed the decisive influence of electrode morphology on rate capability and Li+ transport properties. As a result, only by applying an improved mixing process, the areal discharge capacity of ultra-thick cathodes at 1 C was increased by more than 50% compared to a common procedure. Figure 1