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

Abstract

Variants of garnet-type Li 7 La 3 Zr 2 O 12 are being intensively studied as separator materials in solid-state battery research. The material-specific transport properties, such as bulk and grain boundary conductivity, are of prime interest and are mostly investigated by impedance spectroscopy. Data evaluation is usually based on the one-dimensional (1D) brick layer model, which assumes a homogeneous microstructure of identical grains. Real samples show microstructural inhomogeneities in grain size and porosity due to the complex behavior of grain growth in garnets that is very sensitive to the sintering protocol. However, the true microstructure is often omitted in impedance data analysis, hindering the interlaboratory reproducibility and comparability of results reported in the literature. Here, we use a combinatorial approach of structural analysis and three-dimensional (3D) transport modeling to explore the effects of microstructure on the derived material-specific properties of garnet-type ceramics. For this purpose, Al-doped Li 7 La 3 Zr 2 O 12 pellets with different microstructures are fabricated and electrochemically characterized. A machine learning-assisted image segmentation approach is used for statistical analysis and quantification of the microstructural changes during sintering. A detailed analysis of transport through statistically modeled twin microstructures demonstrates that the transport parameters derived from a 1D brick layer model approach show uncertainties up to 150%, only due to variations in grain size. These uncertainties can be even larger in the presence of porosity. This study helps to better understand the role of the microstructure of polycrystalline electroceramics and its influence on experimental results.

Heteroionic Interfaces in Hybrid Solid-State Batteries─Current Constriction at the Interface between Different Solid Electrolytes

“…The microstructure of garnet-type oxides, such as grain size distribution and (residual) porosity, is highly dependent on the sintering protocol. − Similarly, the manufacturing conditions (e.g., pelletizing pressure or time) of cold-pressed thiophosphate ceramics have a major influence on the formation of intergranular contacts . When two solid electrolytes are combined in a layered stack, the resulting heteroionic interface morphology introduces an additional geometric degree of freedom into the system.…”

“…Unless otherwise noted, we consider material parameters (i.e., conductivities σ i and permittivities ε i ) reported in the literature for LLZO (σ LLZO = 0.5 mS/cm, ε LLZO = 200•ε 0 ) and LPS (σ LPS = 1.0 mS/cm, ε LPS = 20•ε 0 ). 42,45,46,51 Here, ε 0 represents vacuum permittivity. The charge transfer reaction between the two solid electrolytes is assumed to be resistance-free.…”

“…In the following, the solid electrolytes are termed oxide and sulfide, but the results are general in nature and apply to any system involving a (hetero)­junction between two solids. Unless otherwise noted, we consider material parameters (i.e., conductivities σ i and permittivities ε i ) reported in the literature for LLZO (σ LLZO = 0.5 mS/cm, ε LLZO = 200·ε 0 ) and LPS (σ LPS = 1.0 mS/cm, ε LPS = 20·ε 0 ). ,,, Here, ε 0 represents vacuum permittivity. The charge transfer reaction between the two solid electrolytes is assumed to be resistance-free.…”

“…A detailed analysis of the microstructure can be found elsewhere. 46 Cell Assembly and Electrochemical Characterization. All preparation steps were performed in a glovebox under an argon atmosphere (MBraun LabMaster, p(H 2 O)/p < 1.0 ppm, p(O 2 )/p < 0.1 ppm).…”

“…59 The material-specific transport parameters rely on literature reports for common thiophosphate and garnet solid electrolytes. 42,45,46,51 Unless otherwise noted, the following transport parameters are taken into account: (1) The permittivity of the pores is set to vacuum permittivity, i.e., ε Pore = ε 0 . (2) The charge transfer reaction at the heteroionic interface is assumed to be resistance-free.…”