Close pore engineering for biomass-derived hard carbon toward high-performance sodium-ion batteries

The closed pore architecture of hard carbon (HC) materials significantly influences the performance of sodium ion storage in sodium-ion batteries (SIBs). Nevertheless, the strategic design of closed pore structures in high-performance hard carbon anodes continues to pose a considerable challenge. In this study, we implement a magnesium oxide (MgO) template approach to generate a substantial number of appropriate-sized closed pores in hard carbon (HC) through the thermal treatment of a mixture of walnut green husk and magnesium gluconate (Mag-G) in an inert atmosphere. The closed pore can be finely-tuned by adjusting the size and concentration of the MgO template derived from Mag-G. As a result, the optimized HC anode exhibits a remarkable reversible specific capacity of 366 mAh g −1 at a current density of 20 mA g −1 , alongside a high initial Coulombic efficiency of 89.8 % and stable cycling performance (92.9 % after 300 cycles at 100 mA g −1 ). Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses reveal that the primary mechanism governing sodium storage behavior in the plateau region involves the accommodation of Na + within the closed pores. This study presents an important approach for closed pore design aimed at improving sodium ion storage in biomass-derived hard carbons, demonstrating considerable potential in practical applications.