P. Das †, J.X. Ma †, Z.-S. Wu * and H.-M. Cheng *

Chem. Soc. Rev., 2026, accepted.

The importance of mechanistic studies in the field of energy storage cannot be understated.  Electrochemical characterizations alone cannot provide a comprehensive picture of processes affecting performance, longevity and safety of electrochemical energy storage devices (ESDs). In-situ/operando (I-S/O) characterization techniques have contributed significantly to the understanding of such elusive processes. Here, we present a comprehensive review of I-S/O techniques for studying ESDs, especially focusing on specific materials and phenomena using different characterization tools. We provide in-depth discussions regarding various charge storage and degradation mechanisms in batteries and supercapacitors by systematic analysis of three aspects, including structure, morphology and chemistry. We show how studying these three aspects of ESDs via I-S/O characterizations can reveal the origin of high performance of certain materials and pinpoint the reasons for performance degradation. Such mechanisms vary across materials and may often be counterintuitive. Therefore, understanding these phenomena through I-S/O characterizations can aid researchers in developing strategies like composition tuning, doping, compositing, coating etc. that improve the performance while suppressing detrimental processes like phase segregation, cracking, volume expansion, etc. crucial for achieving higher voltage and charge-discharge rates. Finally, we discuss the challenges and opportunities for enhancing I-S/O characterizations by combining multiple techniques in one platform, leveraging the recent growth in machine learning and employing planar ESD architectures. I-S/O characterizations can accelerate the development of new-concept, high performance and long-cycling materials for ESDs.