Circumventing Self-Diffusion Enables High-Rate Hard Carbon Anodes.

Abstract

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.