Regulating open pores to engineer closed pores in ZIF-8-derived hard carbon for high-plateau-capacity sodium-ion batteries.

Hard carbon rich in closed pores is considered a highly promising anode candidate for advanced sodium-ion batteries (SIBs). Nevertheless, the precise regulation of closed pore structures is significantly impeded by the structural complexity of hard carbon. In this study, a hard carbon with tunable closed-pore architecture was constructed by using ZIF-8-derived porous carbon with uniform pore size (∼0.8 nm) and adjustable pore structure as the substrate. Carbonization temperature control and a chemical vapor deposition (CVD)-based pore-sealing strategy were combined to tailor the pore structure and optimize sodium storage performance. The results revealed that the size and number of closed pores are determined by the open-pore characteristics of the carbon substrate; specifically, substrates with larger micropores and higher specific surface areas promote the development of a greater number of closed pores of larger diameters. Benefiting from the optimized closed-pore architecture, the sample ZCHC-1200 exhibits outstanding electrochemical performance, delivering a high reversible capacity of 436.9 mAh g at 30 mA g with a plateau capacity of 265.0 mAh g, and retaining 86.5 % of its initial reversible capacity after 100 cycles. Even at a high current density of 2C, it provides 236.8 mAh g. In situ Raman and ex situ X-ray photoelectron spectroscopy (XPS) analyses revealed that the predominant contributor to the elevated plateau capacity is Na filling within closed pores. This study offers a novel strategy for designing closed pores through open-pore modulation, shedding light on the development of next-generation hard carbon anodes for SIBs.