Circumventing Self‐Diffusion Enables High‐Rate Hard Carbon Anodes

Hard carbons (HCs) are promising anode materials for sodium‐ion batteries (SIBs), yet their application faces a critical challenge that sluggish kinetics in low‐potential regions (<0.1 V) severely limit fast‐charging capability, and the origin of this limitation remains unclear. Here, this study reveals slow sodium self‐diffusion within metallic clusters as the fundamental barrier of hard carbons, by combining first‐principles calculations and in/ex situ characterizations. By rationally designing a heterostructure where long‐ranged anisotropic graphitic nanobelts are in situ embedded into isotropic amorphous carbon matrix, Na + diffusion kinetics is redirected from the slow metallic‐cluster self‐diffusion to the rapid interlaminar pathways through the extended graphitic stacks, thereby significantly circumventing the sodium diffusion barrier at the low potential. The optimized HCs achieve a high reversible capacity (386 mAh g −1 at 20 mA g −1 ), exceptional rate capability (312 mAh g −1 at 200 mA g −1 ), and robust long‐term cyclic stability (98% after 1000 cycles) in a conventional ester electrolyte, with energy density and power density surpassing those of the state‐of‐the‐art graphite in lithium‐ion batteries. These findings provide fundamental insights into high‐rate hard carbons for advanced SIBs.