Quantifying lithium enrichment at grain boundaries in Li7La3Zr2O12 solid electrolyte by correlative microscopy

One of the key materials for all-solid-state batteries based on oxide electrolytes is the polycrystalline Li7La3Zr2O12 (LLZO). It has been reported that metallic Li can accumulate at LLZO grain boundaries during operation, causing a short circuit and, hence, a failure of the battery. However, it is not known to date if Li under its ionic state is enriched at grain boundaries of LLZO already during processing. This is because probing and quantifying lithium reliably remains difficult or sometimes even impossible with most microscopy techniques. Therefore, in the present work the grain boundaries in LLZO are investigated by atom probe tomography and correlative microscopy (combination of atom probe tomography with scanning electron microscopy) at the subnanometer level. Interestingly, Li-based phases denoted as complexions (where Li is most probably found in an oxidized state) have been discovered at grain boundaries. High frequency impedance spectroscopy revealed that the grain boundaries exhibit a lower conductivity and thus affect the Li-transport in LLZO. Furthermore, these Li based complexions may facilitate the formation of dendrites and support short circuits in LLZO solid electrolytes.