Revisiting metal fluorides as lithium-ion battery cathodes

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Metal fluorides are promising cathode materials for lithium-ion batteries.
• They are typically classified as conversion materials due to presumed reconstructive phase transitions during lithiation.

Purpose of the Study:

• To challenge the conventional view of metal fluoride lithiation mechanisms.
• To elucidate the structural transformations occurring in iron trifluoride (FeF3) and other metal fluorides during lithiation.

Main Methods:

• X-ray total scattering and electron diffraction were used to analyze structure across multiple length scales.
• Density functional theory calculations provided theoretical insights.
• Prior experimental studies on FeF2 and CuF2 were revisited.

Main Results:

• Metal fluoride lithiation is dominated by diffusion-controlled displacement mechanisms, not reconstructive phase transitions.
• A topological relationship between metal fluoride and LiF sublattices was established.
• Specific intermediate phases (A-, B-, C-Li$_{x}$Fe$_{y}$F$_{3}$) and a buffer phase were identified during FeF3, FeF2, and CuF2 lithiation.

Conclusions:

• The lithiation mechanism of metal fluorides is primarily diffusion-controlled displacement.
• This understanding provides principles for developing advanced isomorphic metal fluoride cathode materials.