Revealing an Intercalation Nature of High-Capacity Conversion Cathode Materials for Fluoride-Ion Batteries by...

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• High-energy-density electrode materials are crucial for advancing all-solid-state fluoride-ion batteries (ASSFIBs).
• Understanding the structural evolution and phase transitions during battery operation is vital for mitigating capacity fading.
• Bismuth trifluoride (BiF₃) is a promising cathode material, but its operational mechanisms require detailed investigation.

Purpose of the Study:

• To investigate the real-time structural changes and phase evolution of bismuth trifluoride (BiF₃) cathodes during operation in ASSFIBs.
• To identify the degradation mechanisms of the ionic conductor BaSnF₄ under negative potentials.
• To correlate structural dynamics with battery performance and capacity fading.

Main Methods:

• Operando X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) were employed to monitor structural changes in situ at 100 °C.
• Ex-situ XRD analysis complemented the operando studies.
• Rietveld refinement was used to quantify phase fractions and structural transitions.

Main Results:

• A multi-step defluorination process for BiF₃ was observed, transitioning through orthorhombic, cubic, and distorted orthorhombic phases to metallic bismuth (Bi).
• Bismuth oxidefluoride (BiOF) formation was identified, attributed to oxygen impurities transported by the BaSnF₄ ionic conductor.
• Degradation of BaSnF₄ occurred below -200 mV, indicating a narrower electrochemical stability window than anticipated.

Conclusions:

• The study elucidates the complex defluorination mechanism of BiF₃ cathodes, revealing partial intercalation-type behavior.
• Oxygen impurity transport via the BaSnF₄ electrolyte contributes to cathode degradation (BiOF formation).
• The findings provide critical insights into the operational limitations and degradation pathways of BiF₃-based ASSFIBs, guiding future material design.