In vacuo XPS study: controlled ALD growth of Al2O3 on metallic lithium enabled by plasma pretreatment

Abstract

Direct atomic layer deposition (ALD) of protective coatings on metallic lithium is a promising strategy to suppress unwanted side reactions in next-generation batteries. Although several studies report ALD coatings on lithium, with the majority depositing Al2O3, inconsistencies in growth behavior are present. Also in this work, the deposition of Al2O3 using trimethylaluminium (TMA) and O2 plasma on as-received metallic lithium results in irreproducible and non-ALD-like growth. This highlights the need for a fundamental understanding of how the initial surface state governs subsequent ALD reactions. In this work, plasma pretreatments are used to systematically control the lithium surface and in vacuo X-ray photoelectron spectroscopy (XPS) is used during the TMA/O2 plasma ALD process in order to understand its growth behavior on metallic lithium. The as-received lithium surface consists of Li2CO3, LiOH and Li2O, with composition evolving over time, making it an unreliable starting surface for ALD. Argon plasma pretreatment efficiently removes carbon contamination, producing a Li2O-terminated surface, while O2 plasma results in a mixed LiOH/Li2O surface. Self-limiting, ALD-like growth of (lithiated) Al2O3 is observed only on the LiOH/Li2O surface, enabled by reactive hydroxyl groups that facilitate TMA adsorption. In contrast, TMA exposure on the Li2O surface results in immediate precursor decomposition, likely due to a porous nature of the Li2O layer allowing direct interaction between TMA and the bulk lithium underneath. These results show that controlling and clearly defining the initial surface state is essential for achieving reproducible ALD growth on metallic lithium and may help explain the inconsistencies reported in earlier studies.