Revisiting Conversion Reaction Mechanisms in Lithium Batteries: Lithiation-Driven Topotactic Transformation in FeF₂.

Area of Science:

• Materials Science
• Electrochemistry
• Solid-state Chemistry

Background:

• Intercalation electrodes offer reversible lithium storage with minimal structural change but have limited capacity.
• Conversion electrodes provide high lithium storage capacity but often suffer from poor cycling stability due to significant structural changes.
• Iron fluoride (FeF2) is an exception, exhibiting both high capacity and stable cycling as a conversion cathode.

Purpose of the Study:

• To investigate the lithiation-driven topotactic transformation mechanism in single-crystal iron fluoride (FeF2).
• To elucidate the spatial and crystallographic correlations between the parent FeF2 and converted phases during lithiation.
• To understand how FeF2 achieves high cycling stability despite being a conversion material.

Main Methods:

• In situ visualization techniques to observe the transformation process.
• Analysis of spatial and crystallographic relationships between parent and converted phases.
• Characterization of ion transport mechanisms during conversion.

Main Results:

• A lithiation-driven topotactic transformation was observed in FeF2.
• The conversion process involves the transport of both Li+ and Fe2+ ions within the stable F-anion framework.
• Fe formation occurs along specific crystallographic orientations, leading to a checkerboard-like structure that accommodates volume changes.
• The F-anion framework remains intact, ensuring high cyclability.

Conclusions:

• The unique transformation mechanism in FeF2, involving ion transport within a retained F-anion array, explains its high capacity and cycling stability.
• Understanding this mechanism provides insights into designing advanced conversion electrodes for high-energy lithium batteries.
• This study paves the way for developing next-generation lithium batteries with improved performance using conversion materials.