Element-Resolved Imaging of Electrochemical Materials with X-ray Fluorescence Microscopy

High Resolution Image Download MS PowerPoint Slide Conspectus Electrochemical systems feature complex and spatially heterogeneous elemental distributions that evolve during operation. Revealing these elemental dynamics is critical to understanding operation and degradation mechanisms, and thus to improving energy storage and conversion systems. X-ray fluorescence microscopy (XFM) provides a powerful, nondestructive, and element-specific probe for mapping elemental distributions with spatial resolution across relevant length scales. These capabilities enable XFM to address key questions in electrochemical materials research, including materials synthesis and compositional engineering, post-mortem analysis of aged materials, and operando visualization of elemental redistribution during electrochemical processes. In this Account, we first introduce the mechanics and experimental setup of XFM, followed by its major capabilities and applications in electrochemical systems. We highlight its element specificity, high sensitivity, multielement detection, penetration power, and compatibility with operando and nondestructive measurements, which together make it suitable for probing complex and spatially heterogeneous electrochemical materials. We also compare XFM with other elemental analysis techniques to clarify its distinctive advantages. We then summarize ex situ XFM studies of electrochemical materials. In materials design and compositional engineering, XFM can directly verify whether intended elemental architectures are successfully realized, including surface coatings, elemental doping, and concentration-gradient designs in battery electrodes. This capability is important because the performance of engineered materials often depends not only on the presence of specific elements but also on their spatial distribution. In postcycling analysis, XFM reveals unintended elemental redistribution associated with degradation, such as transition-metal dissolution and deposition, polysulfide migration, electrolyte heterogeneity, and the formation of spatially localized reaction products or byproducts. These examples show how XFM connects elemental heterogeneity with electrochemical function and failure mechanisms. We further highlight operando and in situ XFM studies that directly visualize dynamic elemental evolution in working electrochemical systems. These studies are discussed in two categories: elemental evolution within bulk electrochemical materials and interfacial processes at electrode–electrolyte interfaces. By repeatedly probing the same region during operation, XFM can track dynamic electrochemical phenomena such as dissolution–redeposition, reversible and irreversible phase evolution, and elemental diffusion layers, which are difficult to reconstruct from post-mortem analysis alone. Dynamic behavior captured by XFM enables the correlation of spatial, temporal, and compositional information with electrochemical performance. Following representative scientific case studies, we finally discuss the current limitations and future opportunities of XFM in electrochemical research. Although XFM directly links elemental distributions with electrochemical function and degradation, its broader application is still limited by trade-offs among spatial resolution, temporal resolution, sensitivity, and field of view, especially for high-resolution, three-dimensional, and operando measurements. Future advances in higher-brilliance synchrotron sources, faster scanning schemes, improved energy-dispersive detectors, focusing optics, positioning systems, multimodal integration, real-time data processing, quantitative reconstruction, and AI/ML-assisted analysis are expected to improve throughput, spatial and temporal resolution, quantitative interpretation, and the analysis of large XFM data sets. Looking forward, XFM is expected to be applied to broader electrochemical systems and to become an increasingly powerful tool for guiding the design and mechanistic understanding of next-generation electrochemical materials and devices.