Sodium ion diffusion in Al2O3: a distinct perspective compared with lithium ion diffusion

Area of Science:

• Materials Science
• Electrochemistry
• Computational Chemistry

Background:

• Surface coatings on battery active materials enhance performance by reducing side reactions.
• Aluminum oxide (Al2O3) is a common coating material due to its affordability and ease of synthesis.
• Understanding Al2O3 coating effects on ion diffusion is crucial for lithium-ion batteries, but less so for sodium-ion batteries.

Purpose of the Study:

• Investigate the thermodynamic and kinetic aspects of sodium ion (Na+) diffusion in Al2O3 coatings for sodium-ion batteries.
• Compare Na+ diffusion in Al2O3 with lithium ion (Li+) diffusion.
• Elucidate the diffusion mechanism and its underlying causes.

Main Methods:

• Utilized ab initio molecular dynamics calculations.
• Analyzed the thermodynamics of sodiation in Al2O3.
• Simulated Na+ and Li+ ion diffusion within Al2O3 structures.

Main Results:

• Sodiation of Al2O3 is thermodynamically unfavorable, unlike lithiation.
• A threshold concentration of Na+ exists before diffusion into the active material.
• Na+ diffusivity in NaxAl2O3 is significantly higher than Li+ diffusivity in LixAl2O3.
• Na+ diffusion occurs via hopping between oxygen-rich sites, facilitated by weaker Na-O bonds.

Conclusions:

• Both thermodynamics and kinetics govern ion diffusion in material coatings.
• The conventional understanding based on atomic radii does not apply to Na+ vs. Li+ diffusion in Al2O3.
• Weaker Na-O bonds explain the superior Na+ diffusivity, offering new insights for sodium-ion battery design.