Atomistic insights into the conversion reaction in iron fluoride: a dynamically adaptive force field approach

Area of Science:

• Materials Science
• Electrochemistry
• Computational Chemistry

Background:

• Nanoscale metal fluorides are explored for high-capacity lithium ion batteries due to their conversion reaction mechanism.
• The molecular mechanisms and reaction pathways, particularly the need for nanoscale materials and the role of intercalation, remain unclear for iron difluoride (FeF2).

Purpose of the Study:

• To elucidate the atomistic mechanisms of the conversion reaction in FeF2 using molecular dynamics simulations.
• To address the controversy surrounding the role of lithium ion intercalation in FeF2 conversion.

Main Methods:

• Employed molecular dynamics simulations with a dynamically adaptive force field capable of simulating charge changes during reactions.
• Simulated lithium ion (Li+) exposure on low-energy FeF2 surfaces ((001) and (110)) to observe reaction initiation and pathways.

Main Results:

• Observed surface-initiated conversion forming nanocrystals of LiF and Fe(0), sometimes via amorphous Li-F intermediates.
• Identified Li+ intercalation as a function of surface orientation and Li+ exposure rate, influencing Fe(0) nanocluster formation.
• Provided atomistic insights into how intercalation contributes to the destabilization and subsequent formation of Fe(0) nanoclusters.

Conclusions:

• The study resolves the controversy between intercalation and conversion mechanisms in FeF2.
• The findings offer an atomistic explanation for why nanoscale FeF2 starting materials are necessary for efficient conversion reactions in battery cathodes.