Ultramicropores in hard carbon anodes for sodium-ion batteries: from pore structure to interfacial regulation

Ultramicropores are emerging as key structural units that regulate sodium storage behavior and interfacial reversibility in hard carbon (HC) anodes. Closed pores are commonly considered the dominant source of low potential plateau capacity in HC sodium storage. However, ultramicropores embedded in the complex carbon microstructure, including subnanometer surface pores and confined channels, differ essentially from closed pores in function and are closely involved in interfacial regulation. Their pore size and openness determine the entry of electrolyte solvent molecules into these pores, which further affects sodium ion desolvation and solid electrolyte interphase (SEI) formation. Therefore, rational HC design should move beyond simply enlarging the space for plateau sodium storage. Greater attention should be paid to suppressing electrolyte driven irreversible reactions while retaining reversible sodium storage sites. In this review, we discuss the structural characterization, sodium storage behavior, interfacial roles, and regulation strategies of ultramicropores in HC anodes, with the aim of elucidating how ultramicropore engineering can balance plateau capacity with interfacial stability.