Chemical and beam instability of Na3.4Zr2Si2.4P0.6O12 NaSICON electrolyte for all-solid-state sodium batteries

Development of solid-state electrolytes (SSE) is the basis to establish safe and reliable sodium batteries on the market. Despite their advantages, including non-flammable components and reduced reactivity with metallic sodium, many crystalline SSEs suffer from increased charge transfer resistance, sodium dendrite formation, and lower sodium ion conductivity compared to liquid electrolytes. To optimize SSE properties, comprehensive studies of the SSE surface in its pristine state and after contact with metallic sodium during battery operation are desirable. In this work, the surface of pellets with the NaSICON-type Na 3.4 Zr 2 Si 2.4 P 0.6 O 12 composition, which is considered as a promising SSE for sodium batteries, was investigated in detail using conventional surface analytical techniques. Grazing-incidence X-ray diffraction revealed that the subsurface region of the as-prepared pellets consists of several crystalline NaSICON phases and amorphous ternary oxides, extending to a depth of approximately 1 μm. In contrast, after grinding the pellet, only one NaSICON phase could be detected using the transmission XRD technique. The chemical composition of the surface appeared to change after contact with metallic sodium, particularly for the Si-, P- and O-species. Furthermore, we observed surface instability under electron beam exposure during electron microscopy and spectroscopy measurements over 30–60 min, leading to the formation of metallic sodium on the irradiated surface area. The irradiation with X-rays during photoelectron spectroscopy measurements for several hours induced sodium depletion within the irradiated spot while depositing metallic sodium radially around the irradiated area. These findings demonstrate the restricted application of surface analytical techniques for studying ternary oxides in the Na-Zr-Si-P-O system, especially when using NaSICON as a solid electrolyte. • NaSICON pellets show multiple phases in the subsurface region of about 1 μm • In the bulk, they represent a single-phase material • Contact with metallic sodium irreversibly changes the surface chemistry • Electron-beam exposure leads to formation of sodium on the irradiated surface • XPS irradiation causes metallic sodium development as well