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

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.

Automated summary

This review provides a comprehensive overview of the Na-storage mechanism in hard carbons as anode materials for sodium-ion batteries, focusing on Na-storage active sites and related models, and offers guidance for designing high-performance anodes.