Mechanistic study of Al₂O₃ coating effects on lithium deposition and dissolution reaction

Lithium metal anodes show great promise for use in next-generation secondary batteries, but they suffer from lithium dendrite growth, as well as other issues, which cause safety problems and result in a loss of capacity with time. The use of artificial inorganic solid electrolyte interphase (SEI) layers, such as those comprising Al₂O₃, is a promising way to mitigate these disadvantages, but the mechanism behind these observed improvements remains poorly understood. Therefore, in this study, using pulsed laser deposition (PLD), the surface of a Cu electrode was coated with a physicochemically stable and mechanically strong Al₂O₃ thin film, and the effects of the film coating on the lithium deposition and dissolution behaviour were investigated. When the morphology of the deposits was evaluated by scanning electron microscopy, small lithium nuclei (approximately 0.2 μm in diameter) were observed to be deposited uniformly over the entire surface of the uncoated Cu electrode in the initial electrodeposition, and these grew into needle-like crystals from the nuclei. After 60 min of electrodeposition, the needle-like precipitates had aggregated and grown into three-dimensional structures with dendritic form. In contrast, on the surface of the Cu electrode modified with Al₂O₃ by PLD for 1 h, lithium clusters of about 50 μm in diameter were found to be aggregated and precipitated in the initial stages of electrodeposition. Notably, this is the first report of lithium deposition on Al₂O₃ thin films. With further cycling, the precipitates grew into two-dimensional flat plates. Analysis of the SEI film formed during the first deposition reaction revealed that the Al₂O₃ coating reduced the thickness of the SEI compared to that of the uncoated electrode. Therefore, the Al₂O₃ coating suppressed the decomposition of the electrolyte with the Cu electrode. The use of Al₂O₃ coatings results in (i) the growth of two-dimensional lithium clusters with an island shape on the Al₂O₃ thin film, and these could ensure a uniform electron conduction path to the electrode; in addition, (ii) the inhibited electrolyte decomposition caused by the low-surface-area lithium clusters and the low electronic conductivity of the Al₂O₃ thin film. These improve the coulombic efficiency and cycling behaviour.