(604c) Engineering LixAlySizO Ionic Conductive Thin Films by Atomic Layer Deposition for Lithium-Ion Battery Applications

Lithium (Li)-ion batteries have drawn much attention for their outstanding performance in portable electronics applications. However, formation of the solid electrolyte interphase (SEI) layer on the surface of electrodes during the charge-discharge cycling can reduce battery capacity and the long-term reliability of current battery technology.  The use of solid electrolyte layers can effectively suppress formation of the SEI.  Another application for thin solid electrolytes is in microbatteries, especially those based on the engineering of electrodes into 3D architectures involving high aspect ratio pillars. To realize this potential, an ultra-thin and highly conformal electrolyte layer is needed to coat the 3D electrode array. The ionic conductor lithium aluminosilicate (LiAlSiO₄) synthesized by atomic layer deposition (ALD), is a promising candidate for these battery applications.  The material exhibits high ionic conductivity along its c-axis because of channels formed by the alternating tetrahedra of aluminum-oxygen (Al-O) and silicon-oxygen (Si-O).  

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 or electrolyte surface coatings. 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 deposition rates in the range of 0.8~2Å/cycle, respectively.  The deposition rate of stoichiometric LiAlSiO₄ was ~5Å/cycle at a temperature of 290°C. The concentration of each metal element in Li_xAl_ySi_zO (LASO) thin films is found to correlate closely to ALD cycles and the associated incubation times.  Complex impedance measurements show that the ionic conductivities of as-deposited LASO films are in the range of 10^-7 to 10^-8 S/cm and directly related to the lithium content in the film. The LASO ALD coating on 3D features, such as NWs and nanopaticles (NPs), were confirmed to be conformal and uniform by transmission electron microscopy (TEM) imaging. Further electrochemical tests to check pinhole-free coatings on 2D and 3D features were performed as well. The crystallinity of the films after post-deposition rapid thermal annealing (RTA) was a function of cation concentration.  The epitaxial relation of LASO to silicon, a potential anode material, was found to be beta-LiAlSiO4 (-1 2 -1 0) || Si (400) and beta-LiAlSiO4 (1 0 -1 0) || Si (004).