Article

New paper from EMPA on XPS/HAXPES analysis of solid-state interface in batteries

In the recent paper published in Advanced Energy Materials, our customer used a PHI Quantes instrument for analyzing the solid electrolyte/metal interface in cubic Li7La3Zr2O12  (LLZO) garnet-type solid electrolytes based solid-state battery. The Li/Sb-coated LLZO interface was studied by soft and hard photoelectron spectroscopy (XPS/HAXPES).

XPS has shown a formation of Li₂CO₃  overlayer due to the reaction of Li from LLZO with moist air and/or carbon residues during the polishing step. A heat-treatment of the polished LLZO surface has been shown to reduce surface contamination overlayer,  thus creating a much cleaner surface for successive deposition of Sb and/or Li. XPS provides very important information on the chemistry of the top surface layer; however, the depth-dependent chemical composition remains unclear. Using sputtering is undesirable as it may introduce artifacts into the composition and morphology of the sample.

Quoting the paper: “Therefore, in this study, a more detailed analysis of the surface overlayer by lab-based  hard X-ray spectroscopy XPS (HAXPES) was performed, which opens unique opportunities to investigate the unperturbed chemical  constitution  of the  reaction  layer  up  to  much  larger probing depths.”

Figure 4 in the paper shows HAXPRES spectra obtained using Cr X-ray, which provides a signal about 3 times deeper than Al X-Rays. The calculated probing depth for elements of interest is on the order of 17-20 nm.

- The first benefit of using Cr X-ray is the absence of C 1s signal at the surface due to increased probing depth. Li oxide and hydroxide species were detected at the as-received surface and after sputter cleaning, indicating that these species form along the grain boundaries and open pores.

The second significant benefit of a larger sampling depth of HAXPES is probing unperturbed chemistry of the interface deeper than the 5 nm damage zone induced by 1kV Ar ions.

Using XPS/HAXPES, authors revealed  that  “the  major  factor  governing  an  efficient  plating/ stripping of Li at the Sb-coated LLZO surface is the formation of a Li-Sb alloy, which enables an efficient Li-ion and electronic percolation at the Li/LLZO interface and effectively mitigates the formation  of cavities and  Li whiskers upon  plating/stripping  of  Li.”

You can read more details in the paper.