Paper TF+EN-MoM4
Engineering Lithium-Containing Ionic Conductive Thin Films by Atomic Layer Deposition for Lithium-ion Battery Applications

Monday, October 28, 2013, 9:20 am, Room 104 A

Lithium (Li)-ion batteries have drawn much attention for their outstanding performance in portable electronic applications with the potentials to function as a power source for further miniaturized devices including micro-systems through the utilization of 3-dimensional electrodes based on high aspect ratio pillars. To fully utilize such potential, however, an ultra-thin and highly conformal electrolyte layer is required to coat the 3D electrode array. The solid oxide Li-ion conductors lithium aluminosilicate (LASO) synthesized by atomic layer deposition (ALD) are promising electrolyte materials for 3D battery applications not only due to their adequate ionic conductivities for electrolyte applications in Li-ion microbatteries and improve the cell cycling stability upon coating these ultra-thin metal-oxide films synthesized via ALD directly on electrodes.

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 Li₂O, Al₂O₃, and SiO₂ with the deposition rates in the range of 0.8~2Å/cycles. The deposition rate of stoichiometric LiAlSiO₄ was ~20Å/cycle at a temperature of 290°C. The concentration of each metal element in Li_xAl_ySi_zO (LASO) was found to correlate closely to ALD cycles of the constituent oxides. Based on this class of material and the change in the Li to Al(Si), thin films of LAO, LSO and LASO are synthesized to assess the effect of materials’ composition and structure. The as-deposited materials were amorphous and the crystallinity of the LASO/LAO/LSO films after post-deposition rapid thermal annealing (RTA) was found to be a function of cation atomic percentage. Li-ionic conductivities and the activation energy of as-deposited LASO/LAO/LSO films with respect to lithium contents and the film thickness were studied.

The detection and characterization of pinholes on the deposited LASO/LAO/LSO films were investigated by cyclic voltammetry. Lithiation cycling tests of thin LASO/LAO/LSO films were found to be both functions of composition and thickness. The reversibility and kinetics of insertion as well as cycling stability enhancement effects from the direct deposition of LASO/LAO/LSO on 2D and 3D electrode materials such as carbon and porous silicon anodes were investigated as a half-cell.