Understanding the Key Role and Evaluation Methods of Sodium Storage Behavior/Mechanisms for Hard Carbon Anode

ABSTRACT Hard carbon anodes for sodium‐ion batteries (SIBs) show promising application potential because of their wide raw material availability and high sodium storage capacity. However, their complex microstructure leads to unclear and controversial sodium storage mechanisms, which significantly hinders further improvement in electrochemical performance. Although the sodium storage mechanisms are similar across different carbon materials, their contributions vary considerably due to differences in microstructure, resulting in diverse sodium storage behaviors. Compared with previous articles that focused on a single mechanism or characterization method, this review takes the sodium storage mechanism as the core bridge and key descriptor that links the microstructure and macroscopic properties. By systematically integrating physical characterization, in situ characterization, and electrochemical methods, this review aims to clarify the necessity of in‐depth research on the sodium storage mechanism of hard carbon. Furthermore, through a comparative analysis of the sodium storage behavior of special carbon materials, we further reveal how research on the sodium storage mechanism can, in turn, provide the guidance for material structure design. By employing the sodium storage mechanism as a key descriptor, we analyze and construct the intrinsic relationship between material structure and performance, thereby providing a new perspective and theoretical framework for the controllable preparation of hard carbon materials.