At the Advanced Light Source (ALS), an ambient-pressure probe of a solid electrolyte revealed how surface electrochemical mechanisms lead to poor electrolyte performance and battery failure.
The results can help scientists engineer better coatings and interfaces, which are essential for building safer and better-performing batteries, particularly for use in vehicles.
A solid prospect for better batteries
Global efforts to electrify transportation and provide grid-level energy storage have driven demand for new battery technologies with improved safety, power density, and energy density. Ceramic solid electrolytes potentially offer significant advantages compared to the traditional liquid electrolytes used in lithium-ion batteries, including lower flammability and greater compatibility with high-energy electrode materials such as lithium metal. Among solid-electrolyte contenders, tantalum-doped lithium lanthanum zirconium oxide (LLZO) has garnered significant attention as a separator material because of its high bulk ionic conductivity and minimal chemical reactivity with lithium metal.
However, LLZO performance is limited by reactions that produce surface contaminants. Understanding the mechanisms behind these reactions is crucial for improving material processing. In this work, researchers used ambient-pressure x-ray photoelectron spectroscopy (APXPS) as part of a systematic investigation of the impacts of electrochemical reactions and contamination. The results will inform the design of safer and more-efficient batteries for electric vehicles or renewable energy storage.
Facing the interfacial challenges
It is well known that, in the presence of water vapor in air, LLZO undergoes Li+/H+ exchange, where protons (H+) can take up lithium-ion (Li+) sites without modifying the cubic crystal structure. This results in the formation of surface contaminants such as LiOH and Li2CO3 that contribute to poor interfacial contact and the constriction of current.
Numerous studies have explored different aspects of the surface contamination mechanisms on LLZO along with various processing techniques aimed at improving surface properties. However, most studies have focused on critical current density (CCD) tests, which provide limited mechanistic insight, or impedance analyses, with limited rationale behind their interpretation.
In this study, the researchers utilized a variety of surface-treatment processes on LLZO pellets to selectively induce proton exchange and contamination reactions in LLZO. The resulting bulk and surface chemistry was systematically characterized and correlated to changes in electrochemical properties.
ALS studies at ambient pressure
To observe the evolution of chemical species near the LLZO surface, ambient-pressure x-ray photoelectron spectroscopy (APXPS) was performed at ALS Beamline 9.3.2. The ability to tune the gas environment and temperature during measurement was crucial, as it allowed the researchers to optimize conditions (pressure, temperature, time) for removing surface contaminants. Also, the ability to vary the probe depth via beam energy was also essential for chemical speciation.
Read more on ALS website
Image: Illustrations of some of the surface treatments applied to a solid-state battery-electrolyte material (LLZO) as part of this study: glovebox polishing (Gb:Pol), heat treatment (HT), acid treatment (AT), water treatment (WT), and water treatment + heat treatment (WT:HT). Proton concentration (the result of H+ displacing Li+) is indicated by the color gradient, from low (orange) to high (blue). Pink and purple indicate surface contaminants.