Garnet-structured solid-state Li-ion conductors attract attention owing to their wide electrochemical stability window, high room temperature Li-ion conductivity, and good thermal stability, rendering them as promising candidates to be integrated into a number of new technologies, including all-solid-state batteries¹ and very recently, electrochemical gas sensors.² In particular, research advancements have demonstrated the possibility of transferring Li-garnets to their thin-film forms with various crystalline and poly-amorphous states,³ giving room for researchers to redesign the structural and transport properties with alternatives of low-temperature film processing for grain-boundary-free poly-amorphous Li-garnets to compete with state-of-the-art electrolytes like LiPON.^3-4 Despite the promises, there is still a lack of understanding on the crystallization kinetics and phase stabilities and suited low temperature processing routes for Li-garnets and its effect on Li-ion transport properties requires attention. In this study, garnet-type Li₇La₃Zr₂O₁₂ (LLZO), is processed as thin films by spray pyrolysis. The crystallization and phase transitions of the LLZO thin films were studied by DSC together with Raman spectroscopy and in situ TEM. Johnson-Mehl-Avrami-Kolmogorov (JMAK) crystallization theory and Kissinger analysis are applied here, for the first time, to study the fundamentals of the crystallization involving chemical reactions and enthalpy changes during the phase formation process of LLZO. We develop Time-Temperature-Transformation (TTT) diagrams for Li-garnet films, and from here, the ionic transport properties of the LLZO films are added to the developed TTT diagrams as the third axis to Time-Temperature-Transformation-Conductivity (TTTC) diagrams. Role of network formers and builders is discussed for the poly-amorphous states of the LLZO and compared to the structure-transport characteristics of other Li-glass conductors. Our methodology and results highlight a new opportunity of precisely tuning the poly-amorphous states and thereby, tuning the transport properties of Li-garnet films by controlling the processing history with a cost-effective processing method. The insights from this work are expected to serve as fundamental guidelines for understanding the processing-structure-property relationships of poly-amorphous Li-garnet. Technologically, the work demonstrates new material opportunities for next-generation solid-state batteries.

Acknowledgments

This work was sponsored by Samsung Electronics.

References

[1] R. Pfenninger, et al., J.L.M. Rupp, Lithium Titanate Anode Thin Films for Li-Ion Solid State Battery Based on Garnets. Advanced Functional Materials 2018, 28 (21).

[2] M. Struzik, et al., J.L.M. Rupp, A Simple and Fast Electrochemical CO2 Sensor Based on Li7La3Zr2O12 for Environmental Monitoring. Advanced Materials 2018, 30 (44).

[3] I. Garbayo, et al., J.L.M. Rupp, Glass-Type Polyamorphism in Li-Garnet Thin Film Solid State Battery Conductors. Advanced Energy Materials 2018, 8 (12).

[4] R. Pfenninger, et al., J.L.M. Rupp, A Low Ride on Processing Temperature for a Fast Li Conduction in Garnet Solid State Battery Films. Nature Energy 2019, in press.