3D Impedance Modeling of Metal Anodes in Solid-State Batteries–Incompatibility of Pore Formation and Constriction Effect in Physical-Based 1D Circuit Models

Abstract

A non-ideal contact at the electrode/solid electrolyte interface of a solid-state battery arising due to pores (voids) or inclusions results in a geometric constriction effect that severely deteriorates the electric transport properties of the battery cell. The lack of understanding of this phenomenon hinders the optimization process of novel components, such as reversible and high-rate metal anodes. Deeper insight into the constriction phenomenon is necessary to correctly monitor interface degradation and to accelerate the successful use of metal anodes in solid-state batteries. Here, we use a 3D electric network model to study the fundamentals of the constriction effect. Our findings suggest that dynamic constriction as a non-local effect cannot be captured by conventional 1D equivalent circuit models and that its electric behavior is not ad hoc predictable. It strongly depends on the interplay of the geometry of the interface causing the constriction and the microscopic transport processes in the adjacent phases. In the presence of constriction, the contribution from the non-ideal electrode/solid electrolyte interface to the impedance spectrum may exhibit two signals that cannot be explained when the porous interface is described by a physical-based (effective medium theory) 1D equivalent circuit model. In consequence, the widespread assumption of a single interface contribution to the experimental impedance spectrum may be entirely misleading and can cause serious misinterpretation.

Current‐Dependent Lithium Metal Growth Modes in “Anode‐Free” Solid‐State Batteries at the Cu|LLZO Interface

“…This cell configuration was deliberately chosen as the employed Li id counter electrode does not show any impedance contribution during the experiment, [9,26,27] which means that changes in resistance can solely be attributed to current constriction at the Cu film |LLZO interface. [28,29] LLZO was chosen because of its practical stability to lithium metal, which makes it well suited for this model system. [5,30,31] The metal current collector was a 100 nm thin copper film, which was deposited via thermal vapor deposition on the LLZO pellet.…”

Kinetics and Pore Formation of the Sodium Metal Anode on NASICON‐Type Na3.4Zr2Si2.4P0.6O12 for Sodium Solid‐State Batteries

“…C Int decreases with increasing dissolution time highlighting the contact loss at the interface and is consistent with the described CC effect. [34,35] After reaching the cut-off voltage of 60 mV after 700 min a total amount of 28 µmol sodium was dissolved, which corresponds to a thickness of about 16 µm normalized to the geometric electrode area.…”

“…The impedance contribution of current constriction is affected by the properties characterizing an interface, e.g., contact area, pore size, and distribution of contact points. [34,35] Thus, the observed equilibration and the measured impedance contribution result from a change of the interfacial properties after dissolution and cannot be assigned only to an improvement of the contact area. Reduction of the pore size can occur through diffusion of vacancies into the bulk of the metal.…”

“…It strongly depends on the interfacial morphology as recently systematically studied by Eckhardt et al. [ 34,35 ] According to their work, CC should be distinguished in “dynamic” and “static” constriction: The impact of dynamic constriction depends on the excitation frequency of an applied electric AC field, while the static constriction effect is independent of external influences, e.g., when the electrode area is smaller than that of the SE. [ 34,35 ] Especially dynamic CC is strongly affected by the interfacial morphology of pores formed during anodic dissolution.…”

“…[ 34,35 ] According to their work, CC should be distinguished in “dynamic” and “static” constriction: The impact of dynamic constriction depends on the excitation frequency of an applied electric AC field, while the static constriction effect is independent of external influences, e.g., when the electrode area is smaller than that of the SE. [ 34,35 ] Especially dynamic CC is strongly affected by the interfacial morphology of pores formed during anodic dissolution. The presence of microcontacts, however, does not require anodic dissolution to occur, since it can also originate from insufficient physical contact upon cell assembly or incomplete insulating interlayers like surface contaminations.…”