Advanced Characterization Techniques for Sodium‐Ion Battery Studies

Abstract Sodium (Na)‐ion batteries (NIBs) are considered promising alternative candidates to the well‐commercialized lithium‐ion batteries, especially for applications in large‐scale energy storage systems. The electrochemical performance of NIBs such as the cyclability, rate capability, and voltage profiles are strongly dependent on the structural and morphological evolution, phase transformation, sodium‐ion diffusion, and electrode/electrolyte interface reconstruction during charge–discharge cycling. Therefore, in‐depth understanding of the structure and kinetics of electrode materials and the electrode/electrolyte interfaces is essential for optimizing current NIB systems and exploring new materials for NIBs. Recently, rapid progress and development in spectroscopic, microscopic, and scattering techniques have provided extensive insight into the nature of structural evolution, morphological changes of electrode materials, and electrode/electrolyte interface in NIBs. In this review, a comprehensive overview of both static (ex situ) and real‐time (in situ or in operando) techniques for studying the NIBs is provided. Special focus is placed on how these techniques are applied to the fundamental investigation of NIB systems and what important results are obtained.