Ultrasonic Spray Deposition of Metal Oxide Films on High Aspect Ratio Microstructures for 3D All-solid-state Li-ion Batteries

Abstract

Deposition of functional materials on non-planar surfaces remains a challenge for various applications, among which 3D all-solid-state Li-ion batteries. In this letter we present a new process to deposit functional oxide materials on high aspect ratio microstructures, without the use of vacuum-based deposition methods. Using ultrasonic spray deposition in combination with metal citrate chemistry, we were able to deposit high quality coatings on Si micro-cylinders with an aspect ratio of 10. These results were achieved by controlling the precursor chemistry, wetting properties, gel mobility and precursor decomposition. The versatility of the process was shown by depositing titanium oxide (TiO2), lithium lanthanum titanate (Li0.35La0.55TiO3) and tungsten oxide (WO3) on Si micro-cylinders of 50 μm length, with an inter-cylinder distance of 5 μm. Finally, a proof of the 3D battery concept was achieved by coating of TiN / Si micro-cylinders with WO3 using a minimized thermal budget to preserve the (oxidative) TiN current collector. This led to an almost threefold electrode capacity enhancement per footprint area, due to the high surface to bulk ratio of the 3D coating. Therefore, these results form a breakthrough in the field of solution-processing of non-planar microstructures. In addition, the flexibility, low-cost character and high up-scaling potential of this approach is very promising for various applications requiring coated 3D microstructures.