Upcycling Agricultural Waste: Gossypium Herbaceum Shell-Derived Hard Carbon Sodium-Ion Battery Anodes

Cotton shells derived hard carbon is synthesized by carbonization in Ar atmosphere at 800°C for 2 h. XRD analysis proves the amorphous nature of the hard carbon. The Raman spectroscopy shows the presence of a higher degree of defects in the Cotton-shell-derived hard carbon (CC). The Phosphoric acid-treated cotton shell-derived hard carbon (PCC) shows a higher I_d/I_g ratio than the CC implying the generation of more active defect sites, which is supported from BET, FESEM, and TEM analysis. The CV results indicate that the PCC favors better sodium ion diffusion into the carbon matrix than CC electrode. Electrochemical Impedance Spectroscopy (EIS) shows a low charge transfer resistance of only 25 Ω for PCC electrode which is rather high (110 Ω) in the case of CC electrode. PCC offers capacity 350 mAhg⁻¹ in the initial cycles, and stabilising at ≈300 mAhg⁻¹ after 300 cycles. The CC electrode displays capacities of 310 mAhg⁻¹ with a retention of 91%, while the PCC electrode gives capacities of 350 mAhg⁻¹ with a retention of 96% at 0.03 Ag⁻¹. The phosphoric acid treatment generates more active defect sites to accommodate sodium ions into the carbon matrix, which facilitates electrochemical performance.