The effects of aluminum concentration on the microstructural and electrochemical properties of lithium lanthanum zirconium oxide

Abstract

LLZO is a promising solid-state electrolyte for Li-metal batteries. It is known that Al stabilizes the high conductivity cubic phase. In this study, the effect of Al concentration on the microstructure and electrochemical behavior was investigated.

Influence of Microstructure on the Material Properties of LLZO Ceramics Derived by Impedance Spectroscopy and Brick Layer Model Analysis

“…This is consistent with the experimental results for the LLZO pellets with sintering times of up to 8 h. In contrast, the large changes in the time constant and activation energy of the grain boundary process for the sample sintered for 12 h cannot be explained by microstructural variations alone. In this case, some kind of material alteration, such as lithium loss from the sample or changes in the chemical composition of the grain boundaries, e.g., by segregation, cannot be ruled out. ,, A full clarification requires further detailed chemical analysis, which is beyond the scope of this work.…”

“…The observed trends could be explained by, e.g., lithium loss from the ceramics at elevated temperatures or changes in the chemical composition of the grain boundaries. For instance, there are several reports in the literature where different Li–Al–O phases have been identified at the grain boundaries. ,, It is not clear, however, whether this is the sole or the true reason for the underlying effect. The changes in the macroscopic bulk and grain boundary parameters ( R i , C i ) in the impedance spectrum shown in Figure a can also be explained (solely) by the microstructural changes during sintering, i.e., the collapse of the sintering necks and the reduction of the grain boundary to bulk volume ratio in the dominant transport path as observed in Figure .…”

“…In this case, some kind of material alteration, such as lithium loss from the sample or changes in the chemical composition of the grain boundaries, e.g., by segregation, cannot be ruled out. 24,29,30 A full clarification requires further detailed chemical analysis, which is beyond the scope of this work.…”

“…Reasons are very likely differences in the actual fabrication processes (e.g., sintering protocol or particle homogenization), which result in considerable variations of the polycrystalline microstructure of the ceramics obtained . It is well-known that LLZO exhibits abnormal grain growth when the sintering temperatures are too high, i.e., discontinuities in the grain growth rate lead to individual coarse grains. , Thus, the sintering protocol (e.g., processing temperatures and sintering time or pressing and annealing steps) strongly affects the grain size distribution and porosity in the microstructure. − In consequence, the comparison of electrochemical results of different research groups reported in the literature is challenging, in particular, as the effect of the actual microstructure is usually not properly considered in the data interpretation. This concerns the derived material properties and also the derived conclusions about material alterations (e.g., lithium loss) or the formation of different phases, e.g., at grain boundaries (GB). , At this point, we would like to note that we stick to the commonly used term “microstructure” in describing the polycrystalline grain pattern with typical dimensions in the order of a few micrometers.…”

Clarifying the Dopant Local Structure and Effect on Ionic Conductivity in Garnet Solid-State Electrolytes for Lithium-Ion Batteries

“…This is in line with a recent report wherein a reduction in bulk ionic conductivity of Al-LLZO was observed with increasing Al content in the precursors. 61 The existence of Al in γ-LiAlO 2 and the uneven distribution of Al between γ-LiAlO 2 and the LLZO lattice depending on synthesis conditions and grain sizes might be one of the major reasons for the wide-ranging total ionic conductivity values reported in the literature. The absence of tetragonal Al-LLZO in the Al-LLZO + 10% Li HP sample suggests that ∼0.19 Al per 7 Li atoms (quantified from MAS NMR spectra see Supporting Information) is enough Al to form cubic LLZO, and the highest conductivity is found at the transition point between tetragonal and cubic LLZO.…”