Non-Invasive Monitoring of Lithium-Solid State Electrolyte Interface Morphology From In Situ Thermal Resistance Measurement

Void-formation and interface morphology change at the lithium metal solid-state electrolyte interface has been identified as a significant challenge in the development of solid-state batteries. However, operando monitoring of the buried interface morphology is challenging. Several techniques that require isolating/exposing the interface for characterization inadvertently modify the interface and cannot be used for monitoring. While non-destructive methods such as Electrochemical Impedance Spectroscopy (EIS) and X-ray Tomography (XRT) have been employed to study the interface, XRT is not easy to use, whereas EIS cannot resolve other interface effects with the change in the morphology. In this work, we introduce thermal interface resistance measured using modified 3-omega sensors as a reliable method to monitor the interface morphology evolution in lithium metal-solid state cells. We first relate the thermal interface resistance at the lithium-LLZO interface with the physical morphology of the interface and then use the measured thermal interface resistance to extract morphological parameters such as the average contact radius and the number density of contacts. In this work, we study the evolution of the morphology as a function of stack pressure and the number of cell cycles in symmetric lithium-LLZO cells and verify our observations through ex-situ optical characterization.