Understanding the Composition-Phase-Mechanical (in)Stability of Li-La-Zirconate Based Solid Electrolyte upon Exposure to Air and Addressing the Same Via Mg-Doping

Abstract

Al-doped Li-La-Zirconate (LLZO), crystallizing in the cubic garnet structure (Ia-3 d), is one of the most sought after solid electrolytes for solid-state Li-ion batteries due to the inherently good thermal stability, excellent chemical stability against Li metal, large electrochemical window (6V) and decent Li-ion conductivity (10-3-10-4 S/cm). The potential benefits of solid-state batteries include the possibilities of getting rid of ‘air-tight’ casings and humidity controlled environment/chamber for synthesis/fabrication; which render it necessary for the solid electrolyte materials to be very stable in ambient atmosphere. However, perhaps the major issue associated with the Al-doped LLZO, as solid electrolyte, is the spontaneous deleterious reactions of the same with moisture and CO2 present in the ambient atmosphere. In this context, we performed systematic sets of studies for better understanding of the causes for instability of sintered Al-doped LLZO upon exposure to ambient atmosphere; which, in turn, has thrown new insights pertaining to not only compositional instability, but also concomitant instabilities in the phase assemblage and mechanical integrity, even leading to spontaneous cracking/disintegration upon continued exposure [1]. In brief, sintered phase pure Li6.16Al0.28La3Zr2O12 cubic garnet pellets were exposed to air for up to a month’s duration, with various investigations and observations been made intermittently. For comparison, similar Al-doped LLZO pellets were kept inside Ar-filled glove-box (with moisture level below ~0.3 ppm) and subjected to the same studies. One of the very significant observations from a practical point of view was the fracture and disintegration of the samples that were kept in ambient air after about three weeks’ time, whereas absolutely no loss in integrity for those kept inside the glove box. Systematic sets of X-ray diffraction (XRD), Raman Spectroscopy and XPS studies (at different time intervals) with the samples kept in ambient air atmosphere indicated the formation of Li2CO3 layer on the surface within a few days’ time. Subsequently, the associated Li-loss from LLZO resulted in the formation of La2Zr2O7 in the bulk of the LLZO pellets kept in air for about three weeks’ time. By contrast, no change in the phase/composition could be detected for the ones kept inside the glove-box for the entire duration of a month. Interestingly, crack formation in LLZO coincided with the drastic increase in the volume fraction of La2Zr2O7; with analysis indicating that the internal stresses associated with the formation of La2Zr2O7 from LLZO (in the bulk) resulted in the cracking-cum-disintegration of the pellet. Impedance spectroscopy indicated that the Li-ion conductivity decreased by ~3 orders of magnitude during storage of LLZO in ambient air due to such phase instability and crack formation. Accordingly, it is not unlikely that such mechanisms are also partly responsible for the increasing in the impedance with time of solid-state Li-ion cells having Al-doped LLZO as the electrolyte. Research directed towards addressing the above composition-phase-mechanical instability of LLZO has indicated for the first time that replacement of Al-dopant with Mg-dopant can suppress the reaction of LLZO with atmospheric species (and, thus, the aforementioned instabilities), while still stabilizing the desired cubic garnet structure and bestowing LLZO with similar Li-ion conductivities at room temperature, at similar dopant levels [2]. Accordingly, formation of the deleterious La2Zr2O7 could not be detected in phase pure Mg-doped LLZO (viz., Li6.6Mg0.2La3Zr2O12) upon storing the same in ambient air. This also prevented the occurrence of spontaneous macroscopic, as well as sub-surface, cracking of sintered Mg-doped LLZO pellet during storage in ambient atmosphere, in contrast to Al-doped LLZO (as pictorially depicted in Fig. 1). XPS studies probing different depths from the surface of the pellets indicate considerably suppressed loss of Li from the sub-surface or bulk in the case of Mg-doped LLZO, as compared to Al-doped LLZO, after exposure to air for more than three weeks. The above led to stable retention of Li-ion conductivity of Mg-doped LLZO during storage in air for more than three weeks, unlike that for the Al-doped LLZO. Accordingly, the newly developed Mg-doped LLZO promises to be a stable and high performing solid electrolyte for solid-state Li-ion batteries. Keywords: solid electrolyte; garnet; structural instability; Mg-doping; Li-ion conductivity References (publications): S. Kobi, and A. Mukhopadhyay, J. Eur. Ceram. Soc., 38 , 4707 (2018). Amardeep, S. Kobi, and A. Mukhopadhyay, Scripta Mater., 162, 214 (2019). Figure 1