In situ characterizations of advanced electrode materials for sodium-ion batteries toward high electrochemical performances

Energy storage is an ever-growing global concern due to increased energy needs and resource exhaustion. Sodium-ion batteries (SIBs) have called increasing attention and achieved substantial progress in recent years owing to the abundance and even distribution of Na resources in the crust, and the predicted low cost of the technique. Nevertheless, SIBs still face challenges like lower energy density and inferior cycling stability compared to mature lithium-ion batteries (LIBs). Enhancing the electrochemical performance of SIBs requires an in-deep and comprehensive understanding of the improvement strategies and the underlying reaction mechanism elucidated by in situ techniques. In this review, commonly applied in situ techniques, for instance, transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), and X-ray absorption near-edge structure (XANES), and their applications on the representative cathode and anode materials with selected samples are summarized. We discuss the merits and demerits of each type of material, strategies to enhance their electrochemical performance, and the applications of in situ characterizations of them during the de/sodiation process to reveal the underlying reaction mechanism for performance improvement. We aim to elucidate the composition/structure-performance relationship to provide guidelines for rational design and preparation of electrode materials toward high electrochemical performance.