Reduction of Li2CO3 Surface Species from Garnet-Type Solid-State Electrolytes via Scalable Open-Air Plasma Treatment
Authors: Sahal Mohammed , Guo Jinzhao , Chan Candace K. , Rolston Nicholas 
Published: December 2024

Integrating garnet-type lithium lanthanum zirconium oxide (LLZO) solid electrolytes into all-solid-state or hybrid batteries presents numerous challenges. One significant obstacle is the spontaneous formation of lithium carbonate (Li2CO3) when surfaces of LLZO are exposed to moisture and carbon dioxide, which can lead to high interfacial impedance with electrodes and deleterious effects on the mechanical, electrochemical, and safety performance of the materials. Current methods for removing Li2CO3 include mechanical polishing and thermal treatment under inert gas, both of which are time-consuming and difficult to scale up or implement in practical applications.
We report on the use of atmospheric pressure (open-air) blown arc discharge plasma to remove Li2CO3 species from the surface of tantalum doped LLZO (LLZTO). Open-air plasma technology offers a unique opportunity for surface treatments and functionalization by driving reactions and changes to surfaces that are not possible with other plasma or modification approaches. The proccess can also be scaled and implemented in continuous, roll-to-roll processing. In this method, compressed N2 gas at room temperature is passed through a high voltage zone (1 kV) at a controllable rate (20 liters per minute in this study), where it is ionized. The gas blows the plasma discharge out of a nozzle, and the LLZTO sample is placed at approximately ~1 mm away from the afterglow. The uniqueness of the open-air plasma system is the combination of energy sources which are generated: electrons and reactive species (radicals, metastables, and photons) are produced in combination with convective heat to rapidly transfer energy to enable ultrafast precursor conversion or post-treatment.
The plasma treatment is carried out entirely in ambient conditions (RH ~40%) and is enabled by use of a custom-built metal shroud that is placed around the plasma nozzle to prevent moisture and carbon dioxide from reacting with the sample. After the plasma treatment, compressed gas is introduced through the shroud to mitigate reformation of the Li2CO3 as the sample is cooling. The surface chemistry of the LLZTO is characterized with X-ray photoelectron spectroscopy to quantify the rate at which Li2CO3 forms in ambient on a fresh LLZTO surface, as well as to assess the effect of plasma process parameters on removal of the Li2CO3 surface species. Specifically, we focus on 3 parameters: treatment time (5, 10, 20 or 40 minutes) and shroud gas chemistry (N2 or O2) used after treatment. The most substantial reduction in Li2CO3 contamination on the LLZTO surface is achieved after 20 minutes of exposure to open-air plasma with a N2 shroud gas.
To examine the impacts of the plasma treatment on the interfacial resistance, we prepared symmetric cells comprising LLZTO pellets contacted with nonblocking electrodes made from lithium-tin alloy. We find that the open-air plasma treatment can significantly reduce the interface impedance to levels comparable with those measured when using mechanical polishing to remove the Li2CO3. Additionally, we find that the plasma treatment may also be used to modify the LLZTO structure and microstructure properties, and can also be used on other types of garnets. The results show that this platform demonstrates a path towards open-air processed solid-state batteries with improved interfacial and microstructural properties.

Acknowledgments

This work was supported by the National Science Foundation (NSF) through award TT-2234636. We acknowledge resources and support from the Advanced Electronics and Photonics Core Facility at Arizona State University. We acknowledge the use of facilities within the Eyring Materials Center at Arizona State University supported in part by NNCI-ECCS-1542160.