Lithium-ion batteries have drawn much attention for their outstanding performance in portable electronics applications. They also have the potential to function as a power source for micro-systems through engineering of electrodes into 3D architectures based on high aspect ratio pillars. However, to realize this potential, an ultra-thin and highly conformal electrolyte layer is needed to coat the 3D electrode array. The solid electrolyte lithium aluminosilicate (LiAlSiO4), is a promising candidate for this application due to high ionic conductivity along its c-axis resulting from channels formed by the alternating tetrahedra of aluminum-oxygen (Al-O) and silicon-oxygen (Si-O).
Atomic layer deposition (ALD) was employed in this work to synthesize thin film lithium aluminosilicate as an electrolyte. The self-limiting characteristic of ALD allows for precise control of thickness and composition of complex oxides and results in a highly conformal and pinhole-free coating suitable in 3D micro-battery applications. The metal precursors used in this work are tetraethyl orthosilicate (TEOS), trimethylaluminum (TMA) and lithium t-butoxide (LTB). These precursors, along with water vapor as the oxidant, were used to deposit SiO2, Al2O3 and Li2O, with the deposition rates in the range of 0.8~2Å/cycle. The deposition rate of stoichiometric LiAlSiO4 was ~5Å/cycle. The concentration of each metal element in LixAlySizO thin films is found to correlate closely to ALD cycles and the associated incubation times. The crystalline structures as well as the local environment of the Li-conducting channels are also affected by the ALD cycles and sequences, as indicated by ultraviolet photoelectron spectroscopy (UPS), transmission electron microscopy (TEM) imaging and nuclear magnetic resonance (NMR) analyses. The Li-ion conductivities of ALD LixAlySizO thin films were determined by impedance measurements using a four-point probe setup with contacts made to the film surface. The films have high ion conductivity and low electronic conductivity, the values of which are strongly inf luenced by the lithium content and distribution in the synthesized thin films.