Handling the Carbonates: Interfacial Engineering of Garnet Electrolytes

Abstract

Since Murugan et al. [1] discovered the Li7La3Zr2O12 (LLZO) inorganic solid state electrolyte composition, extensive work has been devoted to understanding its behavior and optimizing its structural and compositional features. Although improvement of its bulk and composition has allowed reaching ionic conductivities roughly in the range of mS/cm, much more work is needed in terms of assembling actual all-solid-state batteries by using this material.In this work, we aim at a two fold improvement of the LLZO surface contact with the electrodes by not only creating a new layer on the surface of this ceramic electrolyte, but also by converting the non-conductive lithium carbonate layer [2] that forms on the LLZO surface upon air exposure. Through this rational approach, we expect a chemically relevant surface transformation, linked to a change in the electrochemical performance, via facilitation of the ionic transport through the surface of the ceramic pellet and an improved interface contact. LLZO pellets are treated with boric acid [3] in order to form borate species on their surface, substituting totally or partially the carbonate species that hinder ionic conductivity on the surface. The influence of both atmosphericexposure, treatment and stirring on the resulting pellets is tracked via Raman, XPS and XRD analyses to obtain a comparison and assess the impact of these factors. The final effect of the boric acid treatment is checked via SEM-EDX and electrochemical impedance spectroscopy (EIS) analyses, to determine both morphological and electrochemical effects of the surface modification, which will further influence the interfacial characteristics of the pellet with both electrodes, with a special focus on the lithium side. Our results show an improvement of the surface properties of the pellets, due to the morphological and chemical modification of the LLZO surface, withoutaffecting the bulk properties. [1] Ramaswamy Murugan, Venkataraman Thangadurai, and Werner Weppner. “Fast lithium ion conduction in garnet-type Li7La 3Zr2O12”. In: Angewandte Chemie - International Edition 46.41 (2007), pp. 7778–7781. issn: 14337851. doi: 10.1002/anie.200701144. [2] Asma Sharafi et al. “Impact of air exposure and surface chemistry on Li-Li7La3Zr2O12 interfacial resistance”. In: Journal of Materials Chemistry A 5.26 (2017), pp. 13475–13487. issn: 20507496. doi: 10.1039/c7ta03162a. [3] Yadong Ruan et al. “Acid induced conversion towards a robust and lithiophilic interface for Li-Li7La3Zr2O12 solid-state batteries”. In: Journal of Materials Chemistry A 7.24 (2019), pp. 14565–14574. issn: 20507496. doi: 10.1039/c9ta01911a. Figure 1