Optimized LiNbO3 Sol-Gel Coatings for NCM622 in Sulfide-Based All-Solid-State Batteries: Insights into Synthesis, Uniformity, and Electrochemical Performance

Surface coating of electrode active materials has become a crucial step in stabilizing the interface (electro-)chemical reactivity with the electrolytes in rechargeable batteries. Thin and effective coatings are typically obtained through wet chemical processes such as sol-gel synthesis. In this study, we introduce a simple and effective method to quickly assess coating uniformity using SEM-EDX and Gaussian Kernel density mapping. By applying this approach, along with cyclic voltammetry and electrochemical impedance spectroscopy, we evaluated the quality of LiNbO3 coatings on LiNi0.6Co0.2Mn0.2O2 (NCM622) high-voltage cathode in Li6PS5Cl (LPSC)-based all-solid-state batteries. We compared two synthesis methods: non-hydrolytic synthesis and a chemically activated hydrolytic sol-gel process using H2O2. Additionally, we investigated the effects of different dispersion techniques on the surface’s native Li2CO3 content and on the coating uniformity. Our results suggest that the best interfacial stability between NCM622 and LPSC at 4.3 V vs. Li+/Li is achieved by sonication using the chemically activated sol-gel process under an inert atmosphere, outperforming the stirring methods. While a higher carbonate content may improve the coating homogeneity, it does not effectively mitigate the evolution of interface resistance between the NCM622 and LPSC. Finally, we discuss the underlying mechanism for the optimized LiNbO3 coating in terms of a classical understanding of the sol-gel reaction theory. Our findings support that despite the improved interface stability with LiNbO3 coating and enhanced capacity at 1C cycling rate, NCM622’s long-term specific capacity fading at C/10 is mainly linked to mechanical degradation. Both coated and uncoated NCM622 exhibit similar capacity retention of 90% after 75 cycles, highlighting the dominant role of structural degradation in long-term performance.