Fundamental understanding of electrochemical energy storage devices via in situ or operando characterization

The importance of mechanistic studies in the field of energy storage cannot be understated. Electrochemical characterization alone cannot provide a comprehensive picture of processes affecting the 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 an in-depth understanding of critical ESD mechanisms by analyzing over a decade of I-S/O characterization results. Wherever available we provide cross-technique analysis of various charge storage and degradation mechanisms in key ESD materials by considering structural, morphological and chemical aspects. We show how studying these three aspects of ESDs via multiple I-S/O characterization techniques 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 characterization can aid researchers in developing strategies for improving the performance while suppressing detrimental processes like phase segregation, cracking, volume expansion, etc., 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.