A viable solid state inorganic Li-ion conductor, lithium aluminosilicate (LixAlySizO, LASO), was synthesized by atomic layer deposition (ALD). Investigations of the reaction mechanisms during ALD depositions of the constituent oxides of LASO, Al2O3, LiOH, and SiO2, were first conducted via in-situ Fourier transform infrared spectroscopy (FTIR). The deposition rate of ALD Al2O3 and LiOH were 1.1 and 1.2 Å/cycle respectively while it was not possible to deposit SiO2 by ALD at the conditions studied. The growth of three tertiary oxides, AlxSiyO, LixAlyO, and LixSiyO were confirmed by ALD, demonstrating that functionalization through hydroxylated LiOH and Al2O3 surfaces enabled the incorporation of ALD SiO2 at low temperatures. The in-situ FTIR study revealed that presence of electropositive metal atom in vicinity of surface Si-OCH2CH3* specie is essential for the incorporation of ALD SiO2 at low temperatures. No presence of incubation times were found for ALD deposition of each constituent oxide on the other, allowing ALD deposition LASO as a solid solution based on its constituent oxides. The growth rate of ALD LiAlSiO4 was found to be 20.6 Å/global cycle.
The as-deposited ALD LASO films were amorphous with desired physiochemical properties viable for solid electrolyte applications. The calculated values of ionic conductivity were in the range of 1.75 × 10-9 to 7.22 × 10-8 S/cm, which were tunable by adjusting the number of ALD sub-cycles of the constituent oxides. The activation energy of Li-ion conduction was found to be between 0.46-0.89 eV, which was comparable to that reported in literature. An epitaxial crystallization of the as-deposited amorphous films into β-LiAlSiO4 was achieved for selected compositions of LASO upon post-deposition rapid thermal annealing, with a relationship of LiAlSiO4 || Si (100) and β-LiAlSiO4 || Si (001) to Si substrate. An exceptional conformality of ALD LASO coating was demonstrated on prospective 3D architectures for electrodes, including high aspect ratio trenches and nanowires.
Integration of ALD LixAlySizO films with electrode materials including carbon electrode (both 2D and 3D), SnO2, and SiGe nanowires were investigated. ALD LixAlySizO coating on 2D carbon electrode improved the coulombic efficiency from 91% to 98%. Integration of 3D carbon array electrodes demonstrated a high areal capacity of 7.36 mAh/cm2, which was 6.75 times higher than the maximum areal capacity obtained from 2D carbon electrodes. In-situ HRTEM study on ALD coated SnO2 and Si0.6Ge0.4 nanowires demonstrated promising prospects of ALD LASO not only as a solid electrolyte but also as a stable artificial solid electrolyte interface layer which can improve both physiochemical and mechanical stability of electrode materials.
