Engineering Pore Architecture in Hard Carbon for High-Performance Sodium-Ion Batteries: Distinguishing the Contributions of Ultramicropores and Closed Pores

Hard carbon (HC) stands as a leading anode candidate for sodium-ion batteries, yet the specific functions of its multiscale pore structure, especially ultramicropores (< 0.7 nm), have remained contentious. Herein, the distinct functions of different pores (open pores, sub-0.7 nm ultramicropores, and closed pores) are elucidated through the rational design of model HC materials derived from a homologous zinc gluconate. It is demonstrated that closed pores (2–3 nm) serve as the dominant contributor to the low-voltage plateau capacity via quasi-metallic sodium cluster formation. Ultramicropores significantly enhance the initial Coulombic efficiency (ICE, up to 93.5%) by enabling selective desolvated Na + access while restricting electrolyte ingress, though they contribute only secondarily to the plateau capacity via adsorption. In contrast, open pores exacerbate irreversible electrolyte decomposition, impairing ICE. The optimized material, ZHC600-1300, delivers a high reversible capacity of 414.8 mAh g −1 , an exceptional ICE of 93.1%, a superior plateau capacity (306 mAh g −1 ), and remarkable cycling stability. When configured in full cells, it delivers a high energy density of 321.5 Wh kg −1 , demonstrating great potential for practical applications. This work provides fundamental insights into pore-dependent sodium storage mechanisms and establishes a design principle for high-performance HC anodes.