Visualizing Lithiation of Graphite Composite Anodes in All-Solid-State Batteries Using Time-of-Flight Secondary Ion Mass Spectrometry.

A fundamental understanding of lithiation dynamics in composite electrodes is essential for the development of all-solid-state batteries with high capacity and rate capability. However, a comprehensive methodology to monitor the time evolution of lithiation in a composite electrode has not been fully established because of the limitations of the characterization techniques currently employed. In this study, we demonstrate the visualization of the transient distribution of Li in composite graphite anodes during cell operation using time-of-flight secondary ion mass spectrometry. Selective detection of Li in the graphite electrode was successfully achieved by mapping the Li fragment rather than the conventional mapping of positive Li-containing fragments. The results revealed the formation of a Li concentration gradient during charging, indicating that the reaction of the composite electrode is limited by the diffusion of Li ions inside the graphite. The demonstrated technique is expected to facilitate the investigation of the underlying lithiation mechanisms of composite electrodes in operating cells.