Lithium Insertion Mechanism in Iron-Based Oxyfluorides with Anionic Vacancies Probed by PDF Analysis

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Iron oxyfluoride (FeOF) materials with hexagonal-tungsten-bronze (HTB) structures are explored for electrochemical applications.
• Understanding the mechanism of lithium insertion and extraction is crucial for optimizing battery performance.

Purpose of the Study:

• To elucidate the structural and chemical transformations of HTB-type iron oxyfluoride during lithium insertion.
• To investigate the role of anionic vacancies in the electrochemical activity of these materials.

Main Methods:

• Pair distribution function (PDF) analysis was employed to study the structural evolution at the atomic level.
• Electrochemical cycling was performed to assess the material's performance and reversibility.

Main Results:

• Lithiation induces HTB framework collapse, forming disordered rutile and rock salt phases, followed by conversion to lithium fluoride and nanometer-sized metallic iron.
• The presence of anionic vacancies in the pristine FeOF framework significantly impacts electrochemical activity, with reversible capacity directly scaling with vacancy content.
• De-lithiation results in the formation of a disordered rutile phase, indicating that anionic chemistry governs the atomic arrangement of the re-oxidized phase.

Conclusions:

• The conversion reaction and subsequent nanoscaling of iron oxyfluoride materials lead to in situ formation of new electrode materials with improved electrochemical properties.
• Anionic vacancy concentration is a key factor in determining the electrochemical performance of HTB-type iron oxyfluoride electrodes.
• The study highlights the importance of anionic chemistry in dictating structural rearrangements and electrochemical behavior in conversion-type electrode materials.