Valorization of Cassia fistula Pods Waste into Hard Carbon Anodes via Impregnation with NaCl and H3PO4 for Sodium-Ion Batteries

The use of biomass waste as a precursor to biochar represents a sustainable avenue for the development of materials for energy storage. In this research, added value is given to the residual biomass of the Cassia fistula tree pod to manufacture anodes in sodium-ion batteries. The biochar was prepared using impregnation agents (NaCl and H3PO4) at different temperatures. Samples were characterized by X-ray diffraction, Raman spectroscopy, nitrogen physisorption, impedance spectroscopy, cyclic voltammetry, and galvanostatic cycling. This allowed identification of structural changes, the degree of disorder and the porosity of the resulting material, in addition to evaluating the electrochemical behavior as an anode in a semi-cell. The yield of the pyrolysis process with NaCl was 25.97–30.46%, and with H3PO4, the yield was 35.07–49.30%. According to the X-ray technique, biochar made with NaCl impregnation has interlayer spacings (d) in the range of 0.38–0.40 nm, higher than the spacing in the structure of crystalline graphite (0.34 nm). In the case of H3PO4 impregnation, the interlayer spacing range was 0.37–0.38 nm. The results of Raman spectroscopy indicate that the synthesized samples correspond to hard carbons with short-range graphitic domains, characterized by a sp2/sp3 hybrid structure, with a high density of defects and amorphous regions. This type of microstructure is especially favorable for energy storage applications, as defects, edges, and disordered regions often act as active sites for ion adsorption and storage processes. Initial galvanostatic cycling tests showed stabilized capacities from desodiation of ~140 mAh/g for NaCl-impregnated anodes and ~60 mAh/g for H3PO4-impregnated anodes; these results correspond to anodes based on biochar derived from Cassia fistula pods pyrolyzed at 900 °C.