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

Abstract

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 (TiO2), lithium lanthanum titanate (Li0.35La0.55TiO3), and tungsten oxide (WO3) 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 WO3 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.

Wet-Chemical Synthesis of 3D Stacked Thin Film Metal-Oxides for All-Solid-State Li-Ion Batteries

“…To study this extraordinary deposition requirement, Si substrates with trenches were used which were approximately 30 μm deep and 30 μm wide, i.e., aspect ratio 1 [ 8 , 11 ]. Upon deposition of the W-precursor at the optimized deposition temperature of 180 °C [ 21 ], the heavy tungsten is clearly visible all over the trench using backscattered imaging ( Figure 6 a), i.e., a fully covering WO 3 film is formed. The coating is thicker in the corners, which can be explained by a capillary pressure gradient, leading to enhanced flow towards the corners of the trench geometry [ 56 ].…”

“…Despite the presence of residual organic fragments and incomplete crystallization after only 10 min of isothermal heating at 500 °C, active WO 3 present on the micro-cylinders could be cycled as shown in Figure 8 . To study the effect of 3D deposition more thoroughly than earlier reports by our labs [ 21 ], the micro-cylinder sample was compared with a planar substrate exhibiting the same amount of material per footprint area (i.e., 2 deposition cycles for 2D, compared to 16 deposition cycles for 3D with an area enhancement factor of 8). In addition, the current applied to the 3D samples is eight fold the current passed through the 2D samples ( Figure 8 ).…”

“…The final pH was 8, with a concentration of 0.35 mol·L −1 , as was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Optima 3300, PerkinElmer, Zaventem, Belgium). Before ultrasonic spray deposition, the precursor was diluted and mixed with ethanol (10:9 water/ethanol volume ratio) to yield a 25 mM concentration [ 20 , 21 ].…”

“…In this study, an alternative is presented: conformal coatings of oxide materials for Li-ion batteries using ultrasonic spray deposition. Based on a breakthrough reported previously regarding non-planar deposition using wet-chemical methods [ 20 , 21 ], 3D all-solid-state Li-ion batteries could be within reach at much lower costs and a wider choice of oxide materials. Based on previous results using this approach [ 21 ], this study aims to investigate the stacking of tungsten oxide (WO 3 ) as a negative electrode in combination with amorphous lithium lanthanum titanium oxide (Li 3x La (2/3)−x TiO 3 , referred to as LLT) solid-electrolyte to compile a 3D all-solid-state half-cell.…”

Ultrasonically spray coated silver layers from designed precursor inks for flexible electronics

“…Moreover, patterning and coating on virtually any substrate is possible [48], including 3D structures. [49] Finally, USSC has the additional benefit of being capable to effectively deposit inks with multifarious rheological properties [50]. Due to these inherent benefits of the USSC technique, the deposition of silver nanoparticle inks was already reported in various studies.…”