Investigation of Lithium Transport in 3D Porous Solid-State Li-Ion Electrolytes
Authors: Neumann Anton , Becker-Steinberger Katharina , Hein Simon , Danner Timo , Hamann Tanner , Wachsman Eric , Latz Arnulf 
Published: July 2021

All-solid-state batteries (ASSB) are candidates for the next generation of high power- and energy storage systems. To reach their full potential, limiting factors like high interfacial resistances or decomposition of the electrolyte require further understanding and optimization. Porous 3D electrolyte networks, like the oxidic trilayer structures prepared by the authors, provide a much higher specific contact area between the electrolyte and the active materials [1]. The fabrication via a tape casting process allows for control of the porosity and sample thickness and is scalable to industrial production levels [2]. It was demonstrated that such a system allows for much higher current densities compared to a planar setup. Nevertheless, the critical influences of grain and grain boundaries on the cell performance and the lithium transport in these porous oxidic systems are still under investigation and not fully resolved.
In this contribution, we investigate the lithium transport in porous garnet-type 3D electrolyte network consisting of Li7La2.75Ca0.25Zr1.75Nb0.25O12. The focus of our study is on the influence of grain and grain boundary effects. Our multi-scale approach consists of 3D microstructure-resolved simulations, which are supplemented by a thermodynamically consistent modeling framework [3,4]. The modeling framework enables us to provide additional microscopic interface- and bulk properties such as space charge layer effects, which are transferred as theory-based input parameters to our 3D microstructure-resolved simulations.
The 3D simulations are performed directly on virtual microstructures reconstructed from FIB-SEM tomography measurements. In our study, we investigate samples with different porosity, thereby, our approach inherently incorporates the morphological features of the porous solid electrolyte. Moreover, we perform impedance simulations on the porous polycrystalline 3D electrolyte network. The simulations reproduce the experimental data and allow us to identify features in the corresponding impedance spectra. This approach enables us to analyze the correlation between grain size and garnet porosity on the cell impedance.
In the final step, we virtually fill the porous electrolyte networks with metallic lithium corresponding to different state of charges during operation. Based on the impedance analysis, we evaluate the area specific resistance in our simulations and validate the results against experimental data. The current density distribution analysis allows identifying flux hot spots, which can serve as an indicator for dendrite growth [5]. The comparison between simulations and experiments demonstrates the importance of interfacial processes at the grain boundaries and gives directions for optimizing future oxide-based ASSBs.

Acknowledgment

This work was conducted as part of the US-German joint collaboration on "Interfaces and Interphases In Rechargeable Li-metal based Batteries” supported by the US Department of Energy (DOE) and German Federal Ministry of Education and Research (BMBF). Financial support was provided by BMBF under grant number 03XP0223E and DOE under grant number DEEE0008858.

Reference
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[1] T. Hamann et al., Adv. Funct. Mater. 30 (2020) 1910362.
[2] T. Hitz et al., Materials Today 22 (2019) 50-57.
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[4] S. Braun et al., J. Phys. Chem. C 119 (2015) 22281-22288.
[5] C.Tsai et al., ACS Appl. Mater. Interfaces 8 (2016) 10617-10626.