Density-dependent sodium-storage mechanisms in hard carbon materials

Area of Science:

• Materials Science
• Electrochemistry
• Computational Chemistry

Background:

• Sodium-ion battery (SIB) technology relies on understanding sodium storage mechanisms in hard carbon (HC) anodes.
• The complex interplay between HC microstructure and sodium remains poorly understood, hindering SIB advancement.

Purpose of the Study:

• To investigate sodium insertion mechanisms in hard carbons with varying porosity and densities.
• To elucidate the correlation between hard carbon morphology and sodium storage behavior at an atomic scale.

Main Methods:

• A multiscale methodology integrating grand-canonical Monte Carlo (GCMC) simulations with machine-learning interatomic potentials (Gaussian approximation potential framework).
• Simulation of structural models with densities ranging from 0.7 to 1.9 g cm^-3 to represent different hard carbon porosities.

Main Results:

• Low-density carbons primarily utilize pore-filling for high capacity at near-zero voltages.
• High-density carbons store sodium via adsorption and intercalation, offering lower but more stable capacities.
• Intermediate-density carbons (1.3-1.6 g cm^-3) exhibit a balance of moderate capacity (480 and 310 mAh g^-1), safe voltages, and minimal volume expansion (<10%).

Conclusions:

• Hard carbon density directly correlates with electrochemical behavior and sodium storage mechanisms (pore-filling vs. adsorption/intercalation).
• The study provides atomic-scale insights into optimizing HC morphology for SIB anodes.
• The proposed framework enables rational design principles for high energy density and stable SIBs.