Advances in the Aqueous Processing of Ni-Rich Positive Electrodes

Abstract

A sustainable and environmental friendly production process of the battery components is a crucial factor for a successful contribution of Li-ion batteries to a more sustainable energy supply. For carbon-based anodes the eco-friendly, water-based processing is already widely established, whereas the same procedure still poses several challenges for cathode materials, especially those with high Nickel content. One major challenge is the detrimental side-reaction of Ni-rich cathode active materials (CAM) with water, creating hydroxide and carbonate species which increase the pH of the paste to above 12. Consequently, the Al foil is corroded during the coating process. Therefore, it is essential to control the paste’s pH before casting the paste on the substrate Al foil. This step becomes more challenging the longer the electrode mixing and coating procedure takes. On top of this, a promising way of improving the aqueous-based processing is to identify a suitable water-based binder system to improve the electrochemical cycling stability.We present the application of a variety of different water-based binders, all used in combination with carboxymethyl cellulose (CMC) as thickening agent. The paste pH is controlled by adding H3PO4. Electrodes with industry relevant areal capacities of up to 3 mAh/cm2 are electrochemically evaluated in full pouch cells. The long-term cycling stability determined with 1 C charge/discharge was highly dependent on the binder system. The best electrodes reached an excellent capacity retention of up to 88% after 1000 cycles. The NMP-based reference electrode reached 91%.The investigation of the aged cells via post mortem analysis suggests that the main source of cell aging is the cathode electrode. According to scanning electron microscopy data, no stronger cracking of the secondary CAM particles and no enhanced dissolution of transition metals were observed for the aqueous cathodes. However, a stronger rise in charge-transfer impedance was observed for the aged, water-based cathodes. This suggests that the formation of a blocking surface layer is the major reason for a stronger performance decay with increasing cycle number.AcknowledgmentThe presented work was financially supported by the project DigiBattPro4.0 BW (3–4332.62-IPA/69) by the Ministry of Economic Affairs, Labour and Housing Baden-Württemberg and by the project DigiBattPro4.0 BMBF funded by the German federal ministry of education and research Baden-Württemberg (03XP0374D).