Air‐Stabilization Creates Non‐Crosslinked Starch Particles for High‐Performance Hard Carbon Anodes

Abstract Hard carbons (HCs) are technologically important anode materials for Na‐ion batteries. Here, the thermal conversion of starch using a comprehensive suite of characterization techniques and delineating the synthesis‐parameter space to simultaneously control both the particle morphology and microstructure of HC anodes is investigated. A simple and energy‐efficient preheating step (e.g., 210 °C for 9 h) that effectively preserves particle morphology, substantially reducing both energy consumption and chemical costs compared to previously reported methods, is introduced. Contrary to conventional beliefs, no evidence of cross‐linked structures in the low‐temperature‐stabilized starch samples is found, and a revised reaction mechanism for the conversion of starch to HCs is proposed. The critical factor in maintaining particle morphology is the controlled release of small molecules (e.g., H ₂ O, CO ₂ , and CO) from the starch particles prior to their melting. By fine‐tuning the interlayer spacing and closed‐pore volume of the starch‐derived HCs, a high tap density (∼1 g cm⁻ ³ ) is achieved, a large reversible capacity with high Coulombic efficiency (321 mAh g⁻¹ and 88% in the first cycle), and good cycle stability in 100 mAh‐scale pouch‐type full cells (>76% capacity retention after 1000 cycles).