Advanced characterization of thermally sprayed thin-film all-solid-state batteries

Abstract

The global push toward electrification, renewable integration, and safer energy storage has intensified the demand for next-generation battery technologies. Thin-film all-solid-state batteries (TFASSBs) have emerged as promising candidates due to their intrinsic safety, long cycle life, and compatibility with miniaturized and flexible devices. However, their widespread adoption is hindered by slow, complex, and expensive fabrication methods that limit scalability and industrial uptake. Addressing this manufacturing challenge requires alternative deposition routes capable of producing high-quality ceramic layers with controlled microstructure and interfaces-at scale.

The transition toward safer and high-performance TFASSBs requires scalable manufacturing routes capable of producing high-quality ceramic layers with well-controlled microstructure, interfacial integrity, and phase stability. This thesis explores thermal spraying, specifically high velocity oxy fuel (HVOF), atmospheric plasma spray (APS), and suspension plasma spray (SPS), as an industrially viable approach for depositing Lithium Titanate (LTO) anodes and Lithium Lanthanum Zirconium Oxide (LLZO) solid electrolytes, and investigates how these processes govern microstructural evolution, chemical stability, and lithium retention in thin-film architectures. Owing to the inherent heterogeneity of thermally sprayed ceramic thin-films, conventional laboratory techniques (SEM/EDS, XRD, XPS) provide only bulk-averaged insight, lacking required accuracy. To address this limitation, synchrotron µXRD/µXRF mapping is employed to obtain quantitative, micron-scale structural and elemental information across thin-film cross-sections and buried internal interfaces.

The results reveal strong process–structure relationships: HVOF and APS thin-films better preserve the LTO spinel phase, while SPS thin-films exhibit severe lithium volatilization and form secondary phases such as Li2TiO3 and TiO2. Synchrotron measurements uncover localized decomposition zones, through-thickness phase gradients, and interfacial diffusion effects that remain invisible to lab-scale XRD, SEM and EDS. XPS further highlights distinct surface chemistries, with HVOF and APS forming Li2CO3-rich surface layers, whereas SPS experiences substantial lithium depletion.

Laser post-processing (LPP) was introduced as a complementary, localized treatment for refining thermally sprayed thin-films. LPP enables rapid surfaceremelting, densification, and smoothening without globally heating the substrate, thereby restoring crystallinity, suppressing plasma-induced secondary phases, reducing surface roughness, and improving interfacial uniformity in both LTO and LLZO layers. These improvements provide a promising pathway for engineering high-quality solid–solid interfaces in TFASSB configurations.

Electrochemical testing correlates these structural and chemical findings with battery performance: APS- derived LTO electrodes exhibit the highest initial capacity, lowest polarization, and most stable cycling behavior, while SPS thin-films demonstrate poor performance due to extensive phase degradation. Together, the integration of thermal spraying, laser post-processing, and synchrotron micro-characterization establishes a robust, multi-scale framework linking processing conditions to microstructure, chemistry, and electrochemical behavior.

Overall, this work demonstrates how advanced synchrotron-based micro-characterization provides essential, spatially resolved insight into the process–structure–chemistry relationships governing thermally sprayed thin-film battery materials, while also highlighting thermal spray and laser post-processing as fast and scalable manufacturing routes for next-generation thin-film solid-state batteries.