Studying Interfacial Effects in Solid-Electrolyte Batteries

SCIENTIFIC ACHIEVEMENT

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.

SIGNIFICANCE AND IMPACT

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 horizontal flow chart, with six circles in two rows, show the effects on LLZO of treatments as described in the caption. The top row flows from the initial state (air-exposed) to glovebox polishing to heat treatment. The second row has two circles branching down from glovebox polishing, with acid treatment on the left and water treatment on the right. From water treatment, an arrow points to water + heat treatment on the right. — 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.

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.

The results demonstrated that maintaining a contaminant-free LLZO surface is likely only possible above temperatures of 350-400 °C or under ultrahigh vacuum. Although LLZO’s susceptibility to moisture and LiOH formation was well known, the extent of this sensitivity was surprising, possibly indicating a fundamental limitation of the material. The persistent presence of ionically resistive contaminants such as LiOH also supports the study’s electrochemistry and modeling results, which suggest that that the constriction resistance commonly observed at the lithium/electrolyte interface is not solely due to voids but also to ionically resistive surface layers.

Overall, this work advances our understanding of surface contamination reactions on LLZO and establishes correlative insights into its electrochemical performance that are also broadly applicable to other solid electrolyte systems.

A series of spectra (seven rows, four columns), as described in the caption. — Select APXPS data for air-exposed LLZO samples heated from room temperature to 400 °C in 500 mTorr O₂. The main contaminants observed at 25 °C in vacuum are LiOH (pink) and Li₂CO₃ (purple). The combustion of surface organics at T ≥ 200 °C produces CO₂, which causes significant Li₂CO₃ formation. Further heating leads to the near-complete removal of LiOH and Li₂CO₃, with a notable decrease in contaminants from 350 to 400 °C; this decrease is accompanied by the appearance of photoemission peaks attributed to LLZO (red) and Zr (green). However, the contaminant-free LLZO surface is not preserved after cooling—significant quantities of LiOH and trace amounts of Li₂CO₃ are observed upon returning to 110 °C.

Contact: Sunny Wang

Researchers: S. Wang, E. Barks, X. Xu, C. Melamed, G. McConohy, and W.C. Chueh (Stanford University and SLAC); P.-T. Lin (Stanford University); and S. Nems̆ák (ALS and UC Davis).

Funding: Samsung Advanced Institute of Technology, National Science Foundation, Quad Fellowship, and US Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office, Advanced Battery Materials program. Operation of the ALS is supported by the US Department of Energy, Office of Science, Basic Energy Sciences program.

Publication: S. Wang, E. Barks, P.-T. Lin, X. Xu, C. Melamed, G. McConohy, S. Nems̆ák, and W.C. Chueh, “Effect of H⁺ Exchange and Surface Impurities on Bulk and Interfacial Electrochemistry of Garnet Solid Electrolytes,” Chem. Mater. 36, 6849 (2024), doi:10.1021/acs.chemmater.4c00738.

ALS SCIENCE HIGHLIGHT #511