Unlocking High-Energy Sodium-Ion Batteries via Chemically Engineered Closed Pores in Hard Carbon

The low initial Coulombic efficiency (ICE) and limited plateau capacity hinder the enhancement of the energy density for hard carbon anodes. To address the above critical issue, this review focuses on chemically mediated engineering strategies for closed-pore construction to promote electrochemical performance through microstructure customization of hard carbon at the molecular scale. Specifically, a clear structure-property relationship linking microstructure to macroscopic electrochemical behavior is established. This reveals that ideal closed pores serve as the structural basis for efficient sodium storage via the "pore-filling" mechanism while simultaneously suppressing irreversible sodium consumption. Significantly, three key chemically mediated closed-pore strategies are highlighted: heteroatom doping, functional group guidance, and radical-mediated modulation. Several closed-pore parameters can be successfully optimized through the above approaches synergistically; hence, the plateau capacity, ICE, and rate performance can be comprehensively improved. In addition, the synergistic design of the closed-pore structure is investigated to enhance the energy density of the battery. Finally, future research directions integrating in situ spectroscopy, multiscale simulations, and machine learning are further outlined, aiming at predictable design and controllable regulation of the closed pores, thereby facilitating the transition of advanced hard carbon anodes from laboratory research to practical application.