Circumventing Self-Diffusion Enables High-Rate Hard Carbon Anodes

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Hard carbons (HCs) are promising anodes for sodium-ion batteries (SIBs).
• Sluggish kinetics at low potentials (<0.1 V) limit fast-charging capability in HCs.
• The origin of this kinetic limitation in HCs is not fully understood.

Purpose of the Study:

• To elucidate the fundamental barrier limiting fast-charging in hard carbons for SIBs.
• To design and synthesize an optimized hard carbon material with enhanced sodium-ion diffusion kinetics.
• To demonstrate the improved electrochemical performance of the designed hard carbon anodes.

Main Methods:

• First-principles calculations were employed to investigate sodium diffusion mechanisms.
• In situ and ex situ characterization techniques were utilized to analyze material structure and ion transport.
• A novel heterostructure of graphitic nanobelts embedded in an amorphous carbon matrix was designed.

Main Results:

• Slow sodium self-diffusion within metallic clusters was identified as the primary kinetic barrier in HCs.
• The designed heterostructure successfully redirected Na+ diffusion through rapid interlaminar pathways.
• Optimized HCs exhibited a high reversible capacity (386 mAh g-1 at 20 mA g-1) and excellent rate capability (312 mAh g-1 at 200 mA g-1).
• The material demonstrated robust cyclic stability (98% retention after 1000 cycles).

Conclusions:

• Slow sodium diffusion in metallic clusters is the fundamental limitation for fast-charging hard carbons.
• A rationally designed heterostructure can overcome this barrier by promoting rapid interlaminar diffusion.
• The developed hard carbons offer superior energy and power densities compared to graphite in lithium-ion batteries, paving the way for advanced SIBs.