Interfacial relaxation time as a metric for hidden interfacial states in all-solid-state lithium metal batteries with an Ag–C interlayer: An X-ray computed tomography study

In all-solid-state lithium (Li) metal batteries, incomplete interfacial recovery after discharge triggers localized resistance and degradation. Although carbon interlayers stabilize the interface, conventional metrics, such as macroscopic thickness profiles and Coulombic efficiency, cannot capture the underlying state of interfacial recovery. Thus, new approaches are required to evaluate the hidden microscopic and dynamic interfacial states. This study examined poststripping voltage relaxation time as a diagnostic metric to determine microscopic interfacial health. A systematic analysis of Ag–C interlayer cells under various deposition conditions (temperature, current density, and stack pressure) revealed the fundamental limitations of static macroscopic metrics because recovery times significantly differed even under conditions exhibiting comparable efficiency. The extended voltage recovery under high stack pressures, low current densities, and elevated temperatures indicated a distinct interfacial state. Its uniquely slow kinetics suggested that this recovery is consistent with the dynamic rearrangement of Li and possible deeper redistribution into the carbon layer. Conversely, the rapid voltage relaxation suggests partial recovery stemming from superficial Li plating. By revealing these hidden interfacial dynamics, relaxation time serves as a metric for microscopic interfacial health that complements macroscopic thickness profiles and Coulombic efficiency and offers vital guiding principles for high-performance all-solid-state Li metal battery design.