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

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.