Exploring the influence of focused ion beam processing and scanning electron microscopy imaging on solid-state electrolytes

Performing reliable preparation of transmission electron microscopy (TEM) samples is the necessary basis for a meaningful investigation by ex situ and even more so by in situ TEM techniques, but it is challenging using materials that are sensitive to electron beam irradiation. Focused ion beam is currently the most commonly employed technique for a targeted preparation, but the structural modifications induced during focused ion beam preparation are not fully understood for a number of materials. Here, we have investigated the impact of both the electron and the Ga+ ion beam on insulating solid-state electrolytes (lithium phosphorus oxynitride, Na-β"-alumina solid electrolyte and Na3.4Si2.4Zr2P0.6O12 (NaSICON)) and observed significant lithium/sodium whisker growth induced by both the electron and ion beam already at fairly low dose, leading to a significant change in the chemical composition. The metal whisker growth is presumably mainly due to surface charging, which can be reduced by coating with a gold layer or preparation under cryogenic conditions as efficient approaches to stabilize the solid electrolyte for scanning electron microscopy imaging and TEM sample preparation. Details on the different preparation approaches, the acceleration voltage dependence and the induced chemical and morphological changes are reported.