In situ optical imaging of sub-particle heterogeneity at electrochemical interfaces in batteries and (photo)electrocatalysis

Abstract

The performance and lifespan of electrochemical devices are fundamentally governed by complex and dynamic processes at their heterogeneous interfaces. Spatial, chemical, and structural variations evolve dynamically in situ at the sub-particle level. Traditional ex situ and ensemble-averaged techniques obscure these critical heterogeneities that limit in-depth mechanistic understanding. In situ optical imaging offers a divergent and non-invasive suite of techniques to probe these transient sub-particle phenomena in real-time. In this review, we critically discuss the basic principles, advancements, and applications of advanced optical imaging methods, including interferometric scattering microscopy (iSCAT), advanced fluorescence microscopy (including super-resolution variants), and electrochemiluminescence microscopy (ECLM). We highlight their unique capabilities in resolving dynamic ion transport, phase transitions, active site distribution, and chemical transformations at the sub-particle level in battery electrodes and catalytic materials. Furthermore, we evaluate persistent challenges, such as managing signal-to-noise, ensuring optical transparency, overcoming the diffraction limit, and comprehensive data interpretation. Finally, we outline promising future directions including multi-modal integration and next-generation smart probes. The capability to quantify and visualize sub-particle heterogeneity in situ is necessary for rational materials design and accelerating the development of high-performance and long-life electrochemical technologies.