Enabling Aqueous Processing of Ni-Rich Layered Oxide Cathode Materials by Using Lithium Sulphate as Processing Additive
Authors: Heidbuechel Marcel , Schultz Thorsten , Koch Norbert , Schmuch Richard , Gomez Martin Aurora , Winter Martin 
Published: September 2023

Roughly 75% of the cost for Lithium ion batteries are attributed to material cost for electrodes, electrolyte and separator. Furthermore, the production cost of the cathode material is responsible for >50% of the overall material cost. Therefore, technological breakthroughs for an increased energy density and decreased production cost along the whole battery value chain are urgently needed. State of the art (SOTA) cathode active materials (CAMs) are LiFePO4 (LFP) and layered oxides such as LiNi1-x-yCoxMnyO2 (NCM). By increasing the Ni content within NCM materials, the discharge capacity and therefore the energy density on material level can be gradually increased. Since a higher Ni content (>80% Ni) in NCM´s implicitly entails several challenges with respect to the material synthesis procedure, stability during electrode processing as well as life time, the broad commercialization of these CAMs still needs further advances.

Aqueous processing of Ni-rich layered oxide cathode materials is a promising approach to simultaneously decrease electrode manufacturing costs, while bringing environmental benefits by substituting the SOTA, often toxic and expensive organic processing solvents. Furthermore, recycling of batteries and especially of the cathode material, will probably become an important topic in the coming years. The conversion of electrodes into black mass might be cheaper and easier for aqueously-processed cathodes (e.g., by using fluorine-free binders). However, an aqueous environment still remains challenging due to the high reactivity of Ni-rich layered oxides towards moisture, leading to surface reconstruction, lithium leaching and Al current collector corrosion due to the resulting high pH value of the aqueous electrode paste. Common approaches to suppress current collector corrosion are the protection of the Al current collector by a carbon coating or decreasing the pH value by using dilute acids. The latter approach, especially with phosphoric acid, might lead to formation of a phosphate coating at the surface of cathode particles, which is able to protect the NCM against further degradation.

Herein, we present a facile method to enable aqueous processing of LiNi0.8Co0.1Mn0.1O2 (NCM811) by the addition of lithium sulphate (Li2SO4) during electrode paste dispersion. The aqueously-processed electrodes retain 80% of their initial capacity after 400 cycles in NCM811 || graphite full-cells, while electrodes processed without the addition of Li2SO4 reach 80% of their capacity after only 200 cycles. Furthermore, with regard to electrochemical performance, aqueously-processed electrodes using carbon-coated Al current collector outperform reference electrodes, based on SOTA production processes involving N-methyl-2-pyrrolidone as processing solvent and fluorinated binders. The positive impact on cycle life by the addition of Li2SO4 stems from a formed sulphate coating, protecting the NCM811 surface against degradation. Results reported herein open a new avenue for the processing of Ni-rich NCM electrodes using more sustainable aqueous routes.

Figure 1