Marcel Heidbuechel

Aqueous processing of conventional Ni‐rich layered oxide cathodes (here LiNi _0.83 Co _0.12 Mn _0.05 O ₂ , NCM) can be a sustainable alternative to conventional organic solvent‐based processing. However, the LiOH‐based native surface layer of NCM dissolves in water and increases pH of the electrode paste, which can cause Al corrosion after slurry coating on the current collector. In contrast, a decreased pH via, e.g., acid addition increases risk of NCM dissolution. Here, this dilemma is partly circumvented via a decreased acid exposure, i.e., by adding H ₂ SO ₄ directly before the slurry coating, which also yields the literature‐known Li ₂ SO ₄ surface layer on final NCM. The mechanism of this beneficial measure is further investigated by systematically varying the cations of the sulfates as slurry additives. Without these processing additives, the recrystallization of LiOH in course of drying is concluded to deposit as inhomogeneous agglomerates, which increases content of detrimental crystal water. The addition of Li ₂ SO ₄ , Na ₂ SO ₄ , or MgSO ₄ as slurry additives—contrary to sulfates with, e.g., Ni ²⁺ , Co ²⁺ , Mn ²⁺ , Al ³⁺ —are shown to suppress the detrimental LiOH agglomerates as these cations are better soluble with OH ⁻ and can effectively interfere with the detrimental LiOH recrystallization process in the final, i.e., almost solvent‐free, drying stage within the cathode composite.

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 state‐of‐the‐art (often toxic and costly) organic processing solvents. However, an aqueous environment remains challenging due to the high reactivity of Ni‐rich layered oxides towards moisture, leading to lithium leaching and Al current collector corrosion because of the resulting high pH value of the aqueous electrode paste. Herein, a facile method was developed to enable aqueous processing of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) by the addition of lithium sulfate (Li₂SO₄) during electrode paste dispersion. The aqueously processed electrodes retained 80 % of their initial capacity after 400 cycles in NCM811||graphite full cells, while electrodes processed without the addition of Li₂SO₄ reached 80 % of their capacity after only 200 cycles. Furthermore, with regard to electrochemical performance, aqueously processed electrodes using carbon‐coated Al current collector outperformed reference electrodes based on state‐of‐the‐art production processes involving N‐methyl‐2‐pyrrolidone as processing solvent and fluorinated binders. The positive impact on cycle life by the addition of Li₂SO₄ stemmed from a formed sulfate coating as well as different surface species, 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.