Advances in Unraveling the Structure and Sodium‐Ion Storage Behaviors of Hard Carbons: From Basic to Extended Characterization Techniques

ABSTRACT Sodium‐ion batteries (SIBs) have garnered significant attention as promising alternatives for large‐scale energy storage devices, owing to abundant sodium resources and the similar chemical properties with lithium. However, the inability of graphite for Na storage presents a substantial barrier to commercial application of SIBs. While hard carbons (HCs) offer potential pathways for commercial SIBs, challenges such as low initial Coulombic efficiency, limited Na storage capacity, unclear structural understandings, and contentious energy storage mechanisms remain prominent issues. Driven by keen interests in elucidating the structure and Na storage mechanisms, numerous characterization techniques have been employed to monitor the structure information and structural evolution in sodiation‐desodiation process. Consequently, it is imperative to introduce these characterization techniques and their integrations followed with recent updates in a timely manner. This review provides a concise overview of HCs alongside their related Na storage mechanisms, addressing the development process, existing issues, and future perspectives. Furthermore, we comprehensively categorize and update various characterization techniques for HCs, while representative studies are selected to facilitate a deeper understanding of results and analyses. Finally, we clarify future directions for characterization techniques, highlighting potential avenues for further development that could significantly influence the design and fabrication of high‐performance HCs for SIBs.