Analysis of ALD-Applied Passivation Coatings on Ni-Rich Cathode Sheets for Lithium-Ion Batteries

Abstract

Applying a surface coating is among the most promising and fundamental approaches in improving the performance of existing electrode materials. This work examines four ultrathin coating materials (Al2O3, alucone, TiO2, and titanicone) applied on single-crystal LiNi0.83Mn0.06Co0.11O2 electrode sheets by atomic layer deposition (ALD). Although this approach has been repeatedly demonstrated in the literature to enhance cycle stability, the mechanism of the coating remains ambiguous. Surface-sensitive analysis suggests that the coating is concentrated near the outer regions of the electrode due to the limited penetration of the gaseous ALD coating precursors. Despite this, the coatings improved the cycle stability of the cathode, particularly at low C-rate, where active material loss from HF corrosion is predominant. Thus, the improvement mechanism cannot be ascribed to the passivating nature of the coating itself. Analysis of the chemical composition of the early formed cathode-electrolyte interface (CEI) layer reveals that the coatings accelerate the decomposition of LiPF6, forming a thin inorganic-rich layer comprising LiF, LixPFyOz, and Li3PO4. This rapidly passivates the cathode surface from HF corrosion, electrolyte solvent decomposition, and lattice oxygen release, suppressing both active material loss and impedance growth. The results indicate that (1) metal oxide or metalcone coatings can behave in a similar manner to electrolyte additives and (2) complete coverage of the active particle surface is not required. To support these conclusions, LiNi0.83Mn0.06Co0.11O2 was loosely “coated” with TiO2 nanoparticles either by mixing them in the electrode slurry or adding them to the electrolyte as a suspension. Both cases also resulted in an improvement in cycle retention, due to the favorable side reactions generated between TiO2 and F–/PF6–. Preliminary work was also conducted in analyzing the combined effect of passivating LiNi0.83Mn0.06Co0.11O2 both with a coating and common electrolyte additive (fluoroethylene carbonate, FEC). Adding FEC can further enhance the production of fluoride species on the CEI layer, resulting in the strongest cycle retention tested in this work.