Thermal conversion of banyan aerial roots into hard carbon: Carbonization temperature-regulated microstructure and sodium-ion storage performance

Banyan aerial roots (BAR) are underutilized lignocellulosic residues that are commonly removed during landscape management, yet their conversion into value-added carbon materials remains insufficiently explored. In this work, banyan aerial root-derived hard carbon (BARHC) was obtained through a two-step carbonization process, and the effect of final carbonization temperature from 1000 to 1600°C on carbon microstructure and sodium-storage behavior was systematically investigated. Increasing the carbonization temperature progressively promoted local structural ordering while modifying the interlayer spacing, pore characteristics, and surface oxygen chemistry of the resulting carbon materials. However, further increasing the temperature to 1600°C led to a smaller interlayer spacing and a higher charge-transfer resistance, indicating that excessive heat treatment was unfavorable for overall sodium-storage performance. BARHC-1400 achieved the most favorable balance among structural ordering, accessible porosity, interlayer spacing, and surface functionalities, delivering a reversible capacity of 316.56 mAh g −1 at 0.02 A g −1 with an initial Coulombic efficiency of 64.7%. It maintained 197.6 mAh g −1 after 200 cycles at 0.2 A g −1 and retained 73.81% of its capacity after 1000 cycles at 1 A g −1 . Kinetic analyses and phenolphthalein visualization further supported a combined adsorption-intercalation-pore filling mechanism. These results demonstrate a value-added route for converting banyan aerial root residues into functional hard carbon materials for sodium-ion energy storage.