Thermal Gradients Optimizing the Microstructure of Hard Carbon for Practical Sodium‐Ion Batteries

The thermal gradient effect induced by bamboo particle size significantly influences the microstructure and sodium storage performance of derived hard carbon anodes. This study systematically investigates bamboo powders with three distinct particle sizes carbonized at 1400°C. Characterization reveals that medium‐sized particles (~52.7 μm) optimize thermal gradients, yielding hard carbon (HCM) with balanced graphite‐like domains (interlayer spacing ~0.397 nm) and closed pores. HCM exhibits superior reversible capacity (310 mAh g −1 at 20 mA g −1 ) and cycling stability (93.4% retention after 100 cycles). In contrast, smaller particles form excessive defects, while larger particles develop heterogeneous structures due to pronounced thermal gradients. Coin full cells (HCM//Prussian blue) demonstrate practical viability with 86.05% capacity retention after 200 cycles. This work elucidates the “particle size‐thermal gradient‐microstructure‐performance” relationship, providing a design strategy for high‐performance sodium‐ion battery anodes.