Insights into the Dynamical Sodium Occupancy Evolution and Rate-Limiting Steps in Hard Carbon

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Hard carbon (HC) anodes are crucial for sodium-ion batteries.
• Controversies exist regarding sodiation mechanisms in HC anodes, impacting performance.
• Understanding these mechanisms is vital for developing advanced energy storage.

Purpose of the Study:

• To quantitatively elucidate the sodium storage mechanisms in hard carbon anodes.
• To resolve controversies surrounding the sloping and low-voltage plateau sodiation processes.
• To establish a foundation for designing high-performance HC materials.

Main Methods:

• Quantitative in situ Nuclear Magnetic Resonance (NMR) spectroscopy.
• Raman spectroscopy.
• Electrochemical analysis.

Main Results:

• Identified a transition of sodium ions (Na+) from intercalation/adsorption sites to quasi-metallic sodium clusters within closed pores.
• Observed that adsorbed Na+ exhibits higher mobility than intercalated Na+ during the transition.
• Demonstrated a strong correlation between this transition, decreasing diffusion coefficients, and rate performance.
• Achieved a high reversible capacity (413.2 mAh g-1) and exceptional rate capability (253.0 mAh g-1 at 1500 mA g-1) in a designed HC material.

Conclusions:

• The study clarifies the Na+ transition-storage mechanism in HC anodes.
• Defects, closed pores, and enlarged carbon layers facilitate fast Na+ transition pathways.
• These findings provide critical insights for the rational design of high-performance HC anodes.