Hybrid battery cells that combine a garnet-type Li7La3Zr2O12 (LLZO) solid electrolyte
with
other solid, polymer or liquid electrolytes are increasingly investigated.
In such cells with layered electrolytes, ensuring a low-resistive
heteroionic interface between neighboring electrolytes is crucial
for preventing major additional overpotentials during operation. Electrochemical
impedance spectroscopy is frequently used to extract such parameters,
usually on multilayer symmetrical model cells that contain the different
electrolytes stacked in series. Unfortunately, the impedance contributions
of the heteroionic interfaces often overlap with those of the electrolyte|electrode
interfaces, necessitating the use of sophisticated four-point cells
that probe the electrochemical potential away from the polarization
source. In this work, an alternative solution to this problem is demonstrated
by taking advantage of the inherent fast charge transfer kinetics
of LLZO with its parent metal electrode. The “resistance-free”
nature of a reversible Li|LLZO interface enables a precise evaluation
of the heteroionic interface impedance in symmetric two-point cells
of the type Li|LLZO|electrolyte|LLZO|Li with negligible electrode
contribution. This is exemplified for symmetric multilayer cells
containing tantalum-doped LLZO and a poly(ethylene oxide) (PEO)-based
dry polymer electrolyte. Validation and comparison of impedance data
with results from symmetric four-point cells and two-point cells with
ion-blocking electrodes demonstrate the advantage of the proposed
method. Overall, this study presents a simple and reliable method
for studying heteroionic interface impedances in LLZO-containing multilayer
cells.