Atomic Layer Deposition of Li2O–Al2O3 Thin Films

Energy and power densities of all-solid-state lithium ion batteries can be improved by moving from planar battery structures to three-dimensional structures. Atomic layer deposition (ALD) is a very suitable technique for fabrication of such three-dimensional battery structures. The solid-state electrolyte material lithium lanthanum titanate (LLT) that we have previously made by ALD is unstable in direct contact with many anode materials, and therefore, Li2O–Al2O3 is proposed as a barrier material between the anode and the LLT electrolyte. The deposition of Li2O–Al2O3 thin films by ALD has been accomplished by combining ALD processes for lithium oxide/hydroxide and aluminum oxide. The surface layer composition of the obtained films is Li2.2Al1.0Oz as analyzed by X-ray photoelectron spectroscopy (XPS) while the composition of the bulk film as dissolved in 10% HNO3 is Li1.6Al1.0Oz as analyzed by inductively coupled plasma mass spectroscopy (ICP-MS). The growth mechanisms of Li2O–Al2O3 films were studied by quartz crystal microbalance (QCM). The QCM data indicates that some absorbed water is left inside the films and that the absorbed water reacts with the metal precursor during subsequent pulses. The amount of absorbed water is reduced when the purge time after the water pulse is increased. Even if there is a contribution of absorbed water to the film growth, the growth saturates to about 2.8 Å/cycle and the film thickness increases linearly with increasing number of deposition cycles.