Hard carbon derived from coconut shells, walnut shells, and corn silk biomass waste exhibiting high capacity for Na-ion batteries

In recent years, hard carbon materials have gained significant interest as anode materials for Na-ion batteries. Biomass waste is considered one of the most interesting, renewable, available, and cost-effective precursor to obtain hard carbon (HC); however, HC properties must be finely tuned to achieve performance comparable to those provided by Li-ion batteries. In this work, three biomass wastes (coconut shells, walnut shells, and corn silk) were evaluated as potential precursors for HC preparation involving a pyrolysis process and subsequent acid washing to remove the inorganic impurities. All obtained materials exhibited low and similar specific surface areas (<10 m²⋅g⁻¹), but they presented different structures and surface functionalities. The walnut shell HC possessed a lower amount of inorganic impurities and oxygen-based functional groups compared to the coconut shell and corn silk HCs, leading to higher initial coulombic efficiency (iCE). The structural organization was higher in the case of the walnut shell HC, while the corn silk HC revealed a heterogeneous structure combining both highly disordered carbon and localized graphitized domains. All HCs delivered high initial reversible capacities between 293 and 315 mAh g⁻¹ at 50 mA g⁻¹ current rate, which remained rather stable during long-term cycling. The best capacity (293 mAh g⁻¹ after 100 charge/discharge cycles) and highest capacity retention (93%) was achieved in walnut HCs in half-cells, which could be associated with its higher sp² C content, better organized structure, and fewer impurities. An “adsorption-insertion” Na storage mechanism is suggested based on several techniques. The walnut HCs exhibited an attractive energy density of 279 Wh/kg when tested in full cells.