Fundamental understanding of electrochemical energy storage devices <i>via in situ</i> or <i>operando</i> characterization

, which is crucial for achieving higher voltage and charge-discharge rates. Finally, we discuss the challenges and opportunities for enhancing I-S/O characterization by combining multiple techniques in one platform, leveraging the recent growth in machine learning and employing planar ESD architectures. I-S/O characterization can accelerate the development of new-concept, high-performance and long-cycling materials for ESDs.