Constriction Effect in All-Solid-State Cathodes
Authors: Bohn Moritz , Conforto Gioele , Kutsch Tobias , Schuster Robin , Gasteiger Hubert Andreas 
Published: December 2024

The demand for batteries with high gravimetric and volumetric energy density is driving research towards the development of new battery technologies. All-solid-state batteries (ASSBs) offer the potential to address current challenges faced by liquid electrolyte-based batteries, such as the implementation of Li-metal or fully utilized silicon as an anode material.1 To date, however, there are no commercially available ASSBs with an inorganic solid electrolyte on the market, because of new challenges when using solid electrolytes, such as interfacial stability, solid-solid contact problems, or the ability to operate at low pressures.2

Eckhardt et al.3 recently demonstrated that voids at the interface of a solid electrolyte based separator and a Li-metal anode cause an additional resistance called constriction resistance. This effect occurs when current lines are constricted by bottlenecks (voids) at the interface. In the absence of voids at the interface, the current lines are not constricted, and no additional resistance is observed.4 Due to its frequency-dependent nature, this additional resistance can generally only be detected by impedance spectroscopy. During Li stripping, voids may form at the interface, resulting in a higher current density at the remaining contact areas during plating.5 This ultimately leads to the formation of Li dendrites, making the constriction effect the primary driver of a critical failure mechanism of ASSBs.
In the present study, we present experimental evidence of a constriction effect in composite cathodes, composed of either single or polycrystalline LiNi0.6Mn0.2Co0.2O2, solid electrolyte (Li6PS5Cl), conductive carbon additive, and a polymeric binder. Voids can be caused by a poor microstructure or by the incorporation of a non-conductive polymer binder, which exhibits properties analogous to those of voids. However, large-scale roll-to-roll production of sulfidic ASSB components requires polymeric binders.6 The constriction effect can be observed through impedance spectroscopy of cathodes in blocking conditions as a semicircle that overlaps with the 45° line of the transmission line model. We investigate fitting with different equivalent circuits that model the constriction effect, and show the dependence of the constriction effect on densification pressure, binder content, and active material loading. Using a µ-reference electrode in ASSB single-layer pouch cells,7 the change of the constriction effect in the cathode is monitored during cycling. Additionally, LiNi0.6Mn0.2Co0.2O2/Li6PS5Cl core-shell particles are explored to examine whether this can reduce the constriction effect in sheet-type cathodes. Overall, the constriction of current lines in the cathode is a solid-state-related geometric effect that increases the overall cell resistance and is not observed in liquid electrolyte-based batteries. Our findings provide an overview of the constriction effect in all-solid-state cathodes and its impact on cell performance.

Acknowledgment

The authors wish to thank C. Sedlmeier and F. Friedrich from BMW as the project initiator and for the scientific support. This work is part of the BMW project “IPCEI EuBatIn” (16BZF205), which is funded by the German Federal Ministry for Economic Affairs and Climate Action and the Bavarian Ministry of Economic Affairs, Regional Development and Energy.

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