Microstructure of the Li–Al–O Second Phases in Garnet Solid Electrolytes

Abstract

The microstructure of the Li7La3Zr2O12 (LLZO) garnet solid electrolyte is critical for its performance in all-solid-state lithium-ion battery. During conventional high-temperature sintering, second phases are generated at the grain boundaries due to the reaction between sintering aids and LLZO, which have an enormous effect on the performances of LLZO. However, a detailed structure study of the second phases and their impact on physical properties is lacking. Here, crystal structures of the second phases in LLZO pellets are studied in detail by transmission electron microscopy. Three different crystal structures of Li–Al–O second phases, γ-LiAlO2, α-Li5AlO4, and β-Li5AlO4 were identified, and atomic-scale lattice information was obtained by applying low-dose high-resolution imaging for these electron-beam-sensitive second phases. On this basis, the structure–property relationship of these structures was explored. It was found that sintering aids with a higher Li/Al ratio are beneficial to form Li-rich second phases, which result in more highly ionic conductive LLZO.

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.…”

“…3,47−49 This can be due to changes in the material-specific properties, such as lithium loss within the structure or the formation of different phases at grain boundaries, but it may also originate solely from its microstructure. 24,29,30 To elucidate the effect of the sintering protocol on the electrochemical properties of LLZO, we perform a combinatorial analysis of experiments and simulations. First, we experimentally study the impedance behavior of the different LLZO pellets described in the previous sections and derive their microscopic transport quantities using the standard 1D brick layer model analysis approach.…”

“…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.…”

“…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). 29,30 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. One might infer whether it is more reasonable to use the term "mesostructure" instead, but this is not yet an established term in the field of ceramics.…”

“…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. One might infer whether it is more reasonable to use the term “mesostructure” instead, but this is not yet an established term in the field of ceramics …”