Regulating closed-pore concentration of hard carbon for ultrahigh plateau capacity sodium-ion batteries

Constructing closed-pore structures provides an effective strategy to enhance the plateau capacity of hard carbon (HC) anodes in sodium-ion batteries. However, the ambiguous role of closed-pore size, volume, and particularly the less-reported closed-pore concentration in governing plateau capacity hinders the rational design of closed-pore structures. Herein, we develop a hydrothermal zinc oxide template approach that precisely regulates closed-pore size, volume, and concentration through precursor zinc content modulation, thereby investigating the critical determinants of plateau capacity. Electrochemical analysis reveals a lack of positive correlation between individual closed-pore size/volume metrics and plateau capacity. We establish a quantitative formula for calculating closed-pore concentration, confirming this parameter delivers a significant linear correlation with plateau capacity. The optimized HC-2 sample with highest closed-pore concentration (2.00 × 10²¹ g⁻¹) exhibits a high-reversible capacity of 466.7 mAh g⁻¹ and exceptional plateau capacity of 371 mAh g⁻¹. This work establishes a closed-pore engineering paradigm from precursor design to structural control, providing fundamental insights for developing high-concentration closed-pore HC anodes.