Aqueous Processing of Ni-Rich Layered Oxides: Impact of Native Surface Layer on Crystal Water and Feasible Sulfate-Based Processing Additive Strategies

Abstract

Aqueous processing of conventional Ni-rich layered oxide cathodes (here LiNi_0.83Co_0.12Mn_0.05O₂, 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 H₂SO₄ directly before the slurry coating, which also yields the literature-known Li₂SO₄ 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 Li₂SO₄, Na₂SO₄, or MgSO₄ as slurry additives—contrary to sulfates with, e.g., Ni²⁺, Co²⁺, Mn²⁺, Al³⁺—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.