Interfacial dynamics of carbon interlayers in anode-free solid-state batteries

Abstract

Carbon interlayers have been shown to improve the uniformity and reversibility of lithium (Li) plating in anode-free solid-state batteries (SSBs). However, there remains a lack of fundamental understanding of the dynamic mechanisms that control Li transport and nucleation at these interfaces. In this study, we utilize a combination of electrochemical analysis and operando microscopy to examine the lithiation of carbon interlayers and the subsequent transition to Li plating. The current density during charging was varied to examine the corresponding changes in carbon lithiation, lithium nucleation, and the subsequent relaxation dynamics. A combination of electrochemical impedance spectroscopy, current-interrupt measurements, operando video microscopy, and post-mortem microscopy were performed. The results demonstrate that a transition in reaction pathways from lithiation of carbon to Li plating occurs, which is dependent on the applied current density during charging. As a result, a gradient in the state of charge of the carbon interlayer is observed, which subsequently relaxes during open-circuit rest periods. Finally, the influence of concentration gradients in the carbon interlayer on Li metal nucleation and subsequent solid-state lithiation is shown, which illustrates the importance of the charging protocol on the establishment of a stable Li metal anode interface. These fundamental electrochemical studies will further our understanding of the design requirements for interlayers that can enable Li metal anodes in SSB systems.