Standardizing XPS and HAXPES Analyses of LLZO Solid-State Electrolytes and Their Reactive Compounds

Abstract

Surface contamination of Li₇La₃Zr₂O₁₂ (LLZO) is a significant challenge that impedes its use as a nonflammable and nontoxic solid-state electrolyte in high energy density, temperature-tolerant Li metal solid-state batteries. This work presents detailed dual-beam lab-based XPS/HAXPES analyses of the LLZO surface, complemented by studying reference samples such as Li, Li₂O, LiOH, Li₂CO₃, La₂O₃, ZrO₂, and La₂Zr₂O₇. The objective is to establish baseline reference data, binding energy (BE) positions and more robust chemical shifts, for unambiguously identifying potential surface contaminants and surface reaction layers, for example, as a function of the synthesis and surface treatment conditions. Furthermore, the established procedures for the calibration and charge correction of the XPS and HAXPES energy scales are proposed, as is essential for comparing results across different laboratories and for different incident X-ray sources and spectrometer setups. While lab-based HAXPES analysis of LLZO surfaces is still at its infancy, it is proven to be a very powerful tool in addition to conventional XPS for nondestructively resolving in-depth inhomogeneities in the composition of LLZO surfaces up to probing depths in the range of 20–30 nm.