This study developed a real-time X-ray photoelectron spectroscopy (XPS) analysis technique and utilized it to understand the lithium-ion behavior at metal interface layers in all-solid-state secondary batteries. Notably, the result of this research was published as a cover article in Advanced Energy Materials, one of the leading journals in the field, highlighting its significance.
Research Background and Goals
All-solid-state batteries (ASSBs) are gaining attention as next-generation batteries, offering higher energy density and enhanced safety than conventional liquid lithium-ion batteries (LIBs). In particular, lithium lanthanum zirconium oxide (LLZO)-based electrolytes are considered a key material for next-generation batteries because they exhibit excellent properties, including high ionic conductivity, chemical stability, and a wide bandgap. However, to ensure the long-term stability of batteries, it is necessary to understand their role at metal interfaces (Au, Ag) within the battery. Conventional XPS analyses have the strength of accurately measuring chemical property changes. However, they have limitations in analyzing under real conditions, as lithium compounds can be degraded due to their high reactivity in air during batteries’ charge-discharge. To solve this problem, the research team developed a real-time XPS analysis technique that can compare the reactions of Au and Ag metal interfaces with the previous analyses to elucidate the lithium-metal interaction mechanisms.
Methods
The research team performed real-time charge-discharge analysis using Ag and Au battery cells deposited onto the interface layers between LLZO solid electrolytes and current collectors. Then, Li-ion behavior was analyzed for high spatial resolution using operando XPS and scanning photoelectron microscopy (SPEM). This analysis was used to examine the spatial distribution of Li ions at a high resolution. These methods provided deeper insights into Li-ion migration mechanisms.
Results and Discussion
This study optimized a reliable real-time (operando) XPS analysis technique to determine the factors determining the ASSB performance. While conventional analysis methods are limited in making real-time observations of material changes at metal interfaces during the charge-discharge process, the newly developed real-time XPS technique enables analyzing the precise chemical and electronic structures of metal interface layers at each stage. The research team thoroughly examined the impact of metal interface layers, such as Ag, Au, and Cu, on the ASSB interfacial properties through this approach. As a result, it was confirmed that an increase or decrease in Li⁰ content serves as a critical metric for assessing the efficiency and reversibility of Li plating/stripping processes. Additionally, this research discovered that oxygen bonding within the metal interface layers reacts with Li⁺ ions to form Li₂O, which influences the chemical stability of interfaces. Furthermore, while comparative analysis of core-level electrons showed no significant changes, the formation of Li-metal alloys could be judged by changes in valence-band structure. Based on these analyses, this research identified the key factors that make Ag interface layers superior to other metal interface layers in terms of interface stability and ASSB performance.
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