Biomass-derived hard carbon with tunable microstructures for sustainable and high-rate sodium-ion batteries†

Biomass-derived hard carbon (HC) is considered one of the most promising anode materials for sodium-ion batteries (SIBs) owing to its eco-friendly nature, abundant sources, and excellent electrochemical properties. However, the relatively poor rate performance and cycling stability still hinder the further development of HC. Here, red yeast rice powder (RP), an inexpensive natural food pigment, was identified as an optimal precursor to synthesize HC materials through direct pyrolysis. By controlling the annealing temperature, a series of RP-derived HCs with tunable interlayer spacing, defects, and degrees of graphitization were obtained. Electrochemical characterization showed that the RP-derived HC obtained at 1200 °C (denoted as HC-1200) exhibits the best Na⁺ charge storage properties due to its large interlayer spacing (0.389 nm), suitable degree of graphitization, and low specific surface area (38.9 m² g⁻¹). Specifically, HC-1200 yields a high specific capacity (322.6 mA h g⁻¹ at 0.02 A g⁻¹), good rate performance (110.8 mA h g⁻¹ at 2 A g⁻¹), and long-term cycling stability (capacity persisting at 298.6 mA h g⁻¹ after 300 cycles at 0.02 A g⁻¹). Furthermore, an assembled Na⁺ full cell (HC-1200 || Na₃V₂(PO₄)₃) yields a high capacity of 98 mA h g⁻¹ and good cycling performance over 350 cycles.