Ultrasonic Spray Deposition of Metal Oxide Films on High Aspect Ratio Microstructures for Three-Dimensional All-Solid-State Li-ion Batteries

Deposition of functional materials on nonplanar surfaces remains a challenge for various applications, including three-dimensional (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 microcylinders 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 (TiO 2 ), lithium lanthanum titanate (Li 0.35 La 0.55 TiO 3 ), and tungsten oxide (WO 3 ) on Si microcylinders of 50 μm length with an intercylinder distance of 5 μm. Finally, a proof of the 3D battery concept was achieved by coating of TiN/Si microcylinders with WO 3 using a minimized thermal budget to preserve the (oxidative) TiN current collector. This led to an almost 3-fold electrode capacity enhancement per footprint area, due to the high surface-to-bulk ratio of the 3D coating. Therefore, these results represent a breakthrough in the field of solution-processing of nonplanar microstructures. In addition, the flexibility, low cost, and high scale-up potential of this approach are very promising for various applications requiring coated 3D microstructures.