Investigating Grain Boundary Contributions in Polycrystalline Solid Electrolytes
Authors: Dippell Jan , Danner Timo , Latz Arnulf 
Published: July 2021

All-solid-state batteries (ASSB) are candidates for the next-generation of battery electric vehicles. They potentially enable the use of lithium metal as an anode material thereby highly improving the energy density of the battery. Among the most promising candidates for solid electrolytes are polycrystalline materials such as LLZO, due to their high ionic conductivity and chemical stability. An accurate theoretical model for an ASSB therefore requires a proper understanding of the transport mechanisms at grain boundaries.

Irregularities in the solid electrolyte such as grain boundaries can theoretically lead to (1) additional current dependent resistances and (2) the formation of secondary phases i.e., lithium deposits.

Space charge layers (SCLs) can form at interfaces due to differences in material parameters and electric potential. The SCLs in turn affect the ionic conductivity of the solid electrolyte, which can give rise to nonlinear resistive behaviour.

It was found that lithium does not only deposit at the anode, but also nucleates inside solid electrolyte pellets. Short circuiting of the cell was observed at current densities above 0.6 mA/cm² in LLZO due to its non-negligible electronic conductivity [1]. Grain boundaries have been shown to be hotspots for lithium deposits [2,3], but the mechanisms driving this process are still unclear.

In our contribution we explore transport across grain boundaries using continuum scale models [4,5]. We simulate the spatial distribution of lithium ions at grain boundaries depending on material parameters and applied potential in order to help us predict the influence of grain boundaries on lithium deposits and cell resistance. The results are then correlated with electrochemical impedance spectroscopy and relaxation measurements.

The aim of our work is to get an understanding of the influence of material properties, manufacturing processes, and cycling conditions on the properties of grain boundaries. This information will give important directions for the development of future ASSBs.

Acknowledgements

This work contributes to the research performed at CELEST (Center for Electrochemical Energy Storage Ulm-Karlsruhe). The authors thank the German Ministry of Education and Research (BMBF) for funding of the project CatSE under grant number 03XP0223E.

References:

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[3] Cheng, E. J., Sharafi, A., & Sakamoto, J. (2017). Intergranular Li metal propagation through polycrystalline Li6. 25Al0. 25La3Zr2O12 ceramic electrolyte. Electrochimica Acta, 223, 85-91.

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