Understanding of Sodium Storage Mechanism in Hard Carbons: Ongoing Development under Debate

Abstract Hard carbons are promising anode candidates for sodium‐ion batteries due to their excellent Na‐storage performance, abundant resources, and low cost. Despite the recent advances in hard carbons, the interpretation of the Na‐storage mechanism in hard carbons remains unclear, with discrepancies over a general model describing the corresponding structure–property relationship. For the rational structural design of high‐performance hard carbon anodes, a thorough understanding of the charge storage mechanism and the relationship between microstructure and Na‐storage performance is critical. This review provides a comprehensive overview of the known models to describe the Na‐storage mechanism in hard carbons with a discussion focused on Na‐storage active sites such as interlayer space, pores, heteroatoms, and defects. The present models, including the “insertion‐filling”, “adsorption‐filling”, “adsorption‐insertion”, and “multistage mechanism” and the associated analytical characterization, are elaborately discussed with a view to the present challenges and the prospects for unveiling Na‐storage mechanism in hard carbons. The review serves to clarify the Na‐storage mechanism in hard carbons with guidance toward the design of high‐performance anodes for sodium‐ion batteries.