Elucidating the Surface Chemistry of Atomic Layer Deposition on Complex Metal Oxide Cathode Materials

Abstract

Current research in the field of lithium-ion batteries (LIBs) is geared towards stabilizing the electrode-electrolyte interfaces to improve the LIB cycling stability and storage capacity. In particular, irreversible reactions between non-aqueous electrolytes and metal oxide cathode surfaces can corrode the active material and reduce LIB performance. Protective coatings on the cathode surface can mitigate these issues, and atomic layer deposition (ALD) is well-suited to prepare ultrathin, conformal, corrosion-resistant barrier layers on the surface of the electrodes. While there are reports that some metal oxide and metal fluoride barrier coatings on select cathodes can improve LIB performance, there is no general understanding of why certain ALD coating/cathode combinations are beneficial while others are not. This problem is complicated by the move towards complex LIB cathodes such as lithium nickel manganese cobalt oxide (NMC) materials that may have multiple phases present on the surface including hydroxides and carbonates. Understanding how the ALD precursors react on these materials to create protective coatings is challenging and requires a systematic study to elucidate the surface chemistry of the ALD precursors with each component of the complex oxide surface. Therefore, this work aims at understanding the effects of ALD precursor exposures and ALD coatings on the surface electronic structure of LIB cathode materials.Al2O3 and AlF3 were selected as prototypical metal oxide and metal fluoride surface coatings as they are widely reported in literature due to their ability to act as barrier coatings on cathodes. Additionally, the ALD precursor vapors used for these coatings, i.e., trimethyl aluminium (TMA) and hydrogen fluoride pyridine (HFpy), were adsorbed on the surfaces of these cathode materials under typical ALD conditions to observe their effect on the surface composition and electronic state. The cathode materials chosen for this study included all of the component oxides for NMC cathodes (i.e. Li, Ni, Mn, and Co) in all common oxidation states (e.g., MnO, MnO2, and Mn2O3) as well as selected ternary, quaternary, and quinary forms. In each case, the substrate surface was characterized using X-ray photoelectron spectroscopy (XPS) before and after surface treatment to elucidate changes in the surface composition and charge state induced by the individual ALD precursors and the ultrathin coating. Overall, this fundamental study will assist in the design and synthesis of novel ALD barrier coatings that demonstrate the highest sensitivity on multi-element cathode materials, ultimately leading to robust electrode-electrolyte interfaces for high-performance LIBs.