Comparative Analysis of Doped Lithium Lanthanum Zirconium Oxide Thin Film Deposition Methods for Solid State Battery Applications

Abstract

Solid-state electrolytes used with lithium metal anodes are critical for next-generation lithium-based batteries, offering improved safety and performance over conventional liquid electro-lytes, and higher energy density compared to traditional graphite anodes. An intermediate dis-covery in this field could be the successful deposition of a solid-state buffer layer, which is essentially a thin film that helps improve interfacial properties of the solid electrolyte and lithi-um metal anode, addressing issues like dendrite growth and interfacial stability.

Lithium Lanthanum Zirconium Oxide (LLZO) has been found to be a promising buffer layer material due to its high ionic conductivity and chemical stability with lithium metal. Creating a buffer layer requires achieving a high-quality LLZO thin film, which in turn requires finding a suitable coating technique as well as optimizing deposition parameters to balance adhesion, electrical insulation, and desired microstructure.

This study compares cold spraying, suspension thermal spraying, tape casting, and physi-cal vapor deposition (PVD) for LLZO deposition on copper substrates. Copper substrates are used instead of lithium for various practical and safety reasons, as well as because in battery designs, a thin copper layer is typically used as a current collector beneath the lithium anode.

Coatings are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), adhesion tape tests, and sheet resistance measurements. Characterization results indi-cate that across all methods, further optimization is required, and the high cost of LLZO materi-als limited the number of samples, affecting statistical reliability. While resistivity and adhesion tests provided valuable comparative data, more advanced electrochemical and mechanical testing—including ionic conductivity and cycling performance in full cells—is needed to fully evaluate these coatings in solid-state battery applications.

Among the tested methods, cold spraying, explored as a novel method, successfully de-posited the cubic LLZO phase with good crystallinity and adhesion, but with moderate surface roughness and porosity. The required metallic phase in the feedstock resulted in better flow of the powder during deposition, but lower resistivity of the coating. Suspension thermal spraying produced coatings with strong adhesion and crystalline LLZO, but the film was non-uniform, with amorphous regions requiring further process optimization, such as increasing LLZO con-tent, multiple deposition layers, and post-spray annealing. Tape casting, based on an opti-mized slurry, resulted in a crystalline cubic LLZO coating, but the lack of sintering led to poor adhesion and cohesion and high resistivity. PVD (RF sputtering) was hindered by target insta-bility, as the LLZO pellet broke after cold pressing, suggesting that alternative fabrication tech-niques, such as slurry-based pressing or hot pressing, should be explored.

In this work, the aim is not to find a single “best” method. Instead, it is to highlight not only successful aspects of the coating methods, but also address the challenges, limitations and potential solutions for the problem, essential information that is either not published in scientif-ical journals or not shared in public domain. With this scientific approach and future works in mind, the findings of this work provide valuable insights into LLZO coating adhesion, micro-structure, and electrical properties, offering a foundation for further research and optimization for solid state battery development.