Starch-derived N-doped hard carbons for sodium-ion storage: Preparation and enhanced electrochemical performance

Starch is a promising carbon precursor for sodium-ion battery anodes; however, its low thermal conductivity often leads to molten adhesion during pyrolysis, resulting in microstructural collapse and degraded electrochemical performance. While conventional crosslinking agents can address this issue, removing excess crosslinkers is tedious. This study employs graphite-phase carbon nitride (CN) as both a crosslinking agent and dopant to encapsulate starch particles, preventing adhesion and preserving their spherical morphology. Pyrolysis of starch creates a porous structure, while carbonization incorporates CN into the hard carbon matrix, resulting in nitrogen-doped hard carbons (CNHCs). The high surface area and abundant defect sites enhance sodium storage performance. Systematic investigation of pyrolysis temperatures reveals that CNHC-1400 exhibits excellent rate capability and cycling stability, attributed to its optimized interlayer spacing, rich porosity graphitic nitrogen, which improves Na⁺ diffusion kinetics. CNHC-1400 could maintains a reversible capacity of 254.9 mAh g⁻¹ and after 500 cycles at 1000 mA g⁻¹, it retains 213.9 mAh g⁻¹, corresponding to 79.8 % capacity retention. Electrochemical analysis confirms that the sodium storage mechanism of CNHCs involves a three-stage process: adsorption, intercalation, and pore filling. This study provides insights into the sodium storage mechanism of hard carbons and offers a new strategy for optimizing electrochemical performance.