Hydrothermal Carbonization-Assisted Synthesis of Starch-Derived N/P-Doped Hollow Hard Carbon Microspheres for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are emerging as a sustainable alternative to lithium-ion batteries for large-scale energy storage, but their development is hindered by the lack of cost-effective, high-performance anode materials. This study introduces an eco-friendly synthesis method for nitrogen/phosphorus (N/P) codoped hollow hard carbon microspheres derived from cornstarch, leveraging hydrothermal precarbonization with ammonium polyphosphate (APP) followed by high-temperature pyrolysis. The APP facilitates simultaneous N/P doping and gas evolution during pyrolysis to form hollow architectures. The combined effects of heteroatom doping and structural engineering significantly enhance ion transport kinetics and improve electrolyte accessibility, resulting in notably improved sodium-storage performances. The optimized hard carbon has a reversible capacity of 309.5 mAh g –1 at 0.03 Ag –1 and demonstrates excellent cycling stability with a capacity retention of 86.7% after 1000 cycles at 1 Ag –1 . The improvements can be attributed to the heteroatom doping that induces a large interlayer spacing (0.392 nm), high defect content, and a substantial hierarchical porous surface area (415.88 m 2 /g), which collectively enhance conductivity and sodium-ion kinetics. This work not only advances the design of biomass-derived hard carbon anodes but also provides a scalable strategy for sustainable energy-storage materials.