Structure-Performance Relations in Multi-Layer Solid-State Li-Ion Electrolytes Using FIB-Tomography
Authors: Hamann Tanner R. , Zhang Lei , Godbey Griffin L. , Gritton Jack E. , Gong Yunhui , Xu Shaomao , McOwen Dennis W. , Hitz Gregory T. , Hu Liangbing , Wachsman Eric D. 
Published: February 2020

As demands on battery technology approach the limits of standard organic liquid electrolytes, a multitude of new materials are being developed as potential replacements, including solid-state Li-conducting ceramics. However, the major obstacles to integrating solid electrolytes into Li-ion batteries originate primarily from the relatively low ionic conductivity of the electrolyte itself (compared to organic liquid electrolytes) and from the high interfacial impedance associated with poor electrolyte-electrode contact. Several solid electrolytes, including the garnet-like ceramic Li7La3Zr2O12 (LLZ), have partially addressed the first issue by achieving ionic conductivities of ~1 mS/cm, approaching that of organic liquid electrolytes. With regards to high interfacial impedance, surface chemistry modifications at the electrolyte-electrode interface have greatly improved solid-solid contact with lithium metal anodes, though cathode interfaces remain a significant challenge for solid-state batteries.
An especially promising development replaces the typical pellet geometry with a variable-porosity multi-layer structure, resulting in dramatically increased electrolyte-electrode interfacial area, thus decreasing interfacial impedance. Such an approach opens the possibility of tailoring the architecture of the electrolyte microstructure to improve the electrolyte-electrode interface and all-solid battery performance. However, what constitutes the ideal microstructure is currently unknown. At the same time, adopting such a complex structure may introduce unforeseen effects that impact overall electrolyte properties, positively or negatively. Li-ion migration through tortuous electrolyte pathways, cycling of electrode material at varying distances from the electrolyte dense layer, and effective pore filling by the electrodes are some of the new variables that must be accounted for when designing and evaluating such structures. While research on non-planar solid electrolytes exists, the focus has been limited to thin film batteries, with little work undertaken for macro-scale systems.
Here, we have varied the porosity of high dimension (>100 um thick) LLZ electrolyte porous-dense-porous multi-layers, characterized the physical microstructure of the layers, and investigated how key structural parameters (e.g., tortuosity, LLZ/pore volume fractions, MAZO angle) related to the electrochemical properties. To study the LLZ microstructure, we utilized focused ion beam (FIB) tomography to reconstruct 3D representations of regions in the electrolyte, from which structural parameters were calculated. Samples to be analyzed were epoxy infiltrated and mounted in a Xenon-plasma source FIB/SEM, where we deposited platinum on the area of interest and milled a trench around the area to expose a cross-section of the LLZ (Figure 1, top). We then moved the sample to a Gallium-ion source FIB/SEM to finish polishing the cross-section (Figure 1, bottom), followed by serial milling-and-imaging to produce the set of images necessary for 3D reconstruction. To evaluate electrolyte performance, we relied on electrochemical impedance spectroscopy (EIS) to determine Li-ion conductivities for the different LLZ multi-layer microstructures.

References:

1. Han, X. et al. Negating Interfacial Impedance in Garnet-based Solid-State Li Metal Batteries. Nat. Mater.
1, (2016).

Figure 1