Redox Staining of Metallic Lithium inside Batteries for Multimodal Visualization and Identification with Multiscale Spatial Resolution.

The dynamic behaviors of metallic lithium (Li0) in Li-based batteries including the formation of various morphologies, corrosion, isolation, and spatial distribution affect the cycle life and safety. However, analysis on formation mechanisms or governing factors of Li0 is largely limited by the lack of experimental techniques to chemically and spatially identify low-Z elements from nano- to macroscale. Here, we demonstrate that staining Li0 with high-Z elements via redox reactions effectively enhances its visibility under an electron beam or X-rays. We find that heavy metal ions (Mn+) can be reduced by Li0 and nucleate as nanoparticles (NPs) on surfaces, staining effectively for direct visualization. Our results demonstrate that inactive Li0 embedded within residual SEI can be visualized, thereby revealing correlations between its distribution and its electrochemical performance such as Coulombic efficiency (CE). Our new technique represents a new analytical characterization tool to spatially track previously "invisible" internal components in batteries.