(Invited) Insights from Computational Modeling and Experiments on the Li-Ion Dynamics and Electrochemical Stability of Garnet-Based Solid Electrolytes
Authors: Jalem Randy 
Published: February 2020

Solid electrolytes are crucial for the development of next-generation Li-ion batteries; they can drastically improve safety during work operation as they are essentially non-flammable unlike conventional liquid- or organic-based electrolytes. In this regard, garnet-type Li-based oxide compounds are amongst the most promising ones and they are now being intensively optimized to push their conductivity to levels that would make them practical for use. However, technical challenges such as poor scattering due to Li (which makes diffraction techniques measurements tedious) and inherent variability in synthesis, material processing, and powder handling have led to ionic conductivity values differing by several factors even for the same nominal composition (thus, the scatter of data in literature), thereby making it difficult to study the exact nature of the Li ion dynamics and distribution at the Li sub-lattice in these materials, especially when considering dopant incorporation. Another important property is electrochemical stability which can have profound impacts on battery performance, this is in relation to Li ion transport across electrolyte-electrode interface. Interfacial impedance can change significantly depending on the various decomposition products formed at the interface, this is also technically challenging to study by direct approaches.
In this presentation, several fundamental issues related to garnet-type compounds will be tackled that are critical towards achieving high-performance all-solid state Li ion batteries: Li ion dynamics,[1,2,3] electrochemical stability behavior,[4] and Li and dopant distribution[1,2,3]. Results from various computational techniques will be discussed (DFT, augmented plane-wave approach, classical force-field method, and molecular dynamics). Specifically, special attention will be given to the study of the distribution, dynamics, and ion transport of Li, vacancy, and dopants in the Li sub-lattice of the garnet structure. Two dopants of interest will be highlighted: Ga and proton. Doping by Ga has been reported to show one of the highest conductivity so far (on order of 10-3 S/cm) for garnet solid electrolytes while proton incorporation via exchange reaction with Li is associated to interfacial resistance increase (due to formation of resistive phases) and variability in ionic conductivity. On the other hand, experimental data from XRD, SEM, FTIR, and cyclic voltammetry will also be presented in relation to the capacity fading phenomena at the first-cycle charge process (related to interfacial impedance increase) of a garnet solid electrolyte in a solid-state battery cell.

Acknowledgements

The authors acknowledge the financial support from JST PRESTO program (JST PRESTO: Japan Science and Technology, “Promoting Individual Research to Nurture the Seeds of Future Innovation and Organizing Unique, Innovative Network”) and National Institute for Materials Science (NIMS) for the financial support.

References

[1] Jalem et al., Chem. Mater. 2015, 27, 2821-2831.
[2] Jalem et al., J. Phys. Chem. C 2015, 119, 20783-20791.
[3] Jalem et al., Chem. Mater., 2013, 25, pp 425-430.
[4] Jalem et al., J. Mater. Chem. A 2016, 4, 14371-14379.