Li7La3Zr2O12 interface functionalization via boric acid treatment: A step to contact improvement

Abstract

Realization of a functional surface layer of lithium metaborate at the interface of garnet-type lithium lanthanum zirconium oxide (LLZO) electrolyte pellets is achieved via a mild, scalable solution-based route employing boric acid to transform and remove detrimental Li2CO3 typically formed on LLZO during exposure to humid air. The effects of this boric acid treatment are evaluated by systematic morphological and structural characterization of LLZO pellets that were subjected to different conditions. Surface-sensitive analyses via X-ray photoelectron spectroscopy, combined with in situ Ar-ion etching and reflection electron energy loss spectroscopy (REELS) measurements, are purposely employed to probe the extent and efficacy of resulting LiBO2 in removing Li2CO3 and protecting the modified LLZO surface from unwanted carbonation that disrupts its interfacial Li-ion conductivity. This key aspect has been corroborated by an aimed comparison of the different LLZO surface energy bandgap values extracted from the REELS measurements, which provides further insight into the actual impact of Li2CO3 and its replacement with a beneficial, ionically conducting LiBO2 layer. The latter substantially improves the surface adhesion of the modified LLZO pellets to molten lithium without introducing metallic-like species and provides an overall reduced impedance by unlocking the interfacial Li-ion conduction otherwise severely hampered by Li2CO3.