Fast Solid-State Defluorination/Fluorination of FeF_x (x = 3-0) as Fluoride-Ion Battery Cathode

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Fluoride-ion batteries (FIBs) are emerging as a promising alternative to lithium-ion batteries due to their potential for higher energy density.
• Iron fluoride (FeFx) is a cathode material with a high theoretical capacity, but its electrochemical behavior and reaction mechanisms are not well understood.

Purpose of the Study:

• To investigate the discharge/charge capability and defluorination/fluorination mechanism of FeFx as a cathode material in all-solid-state FIBs.
• To evaluate the electrochemical performance of FeFx cathodes at room temperature and low temperatures.

Main Methods:

• Fabrication of a FeFx thin-film cathode for all-solid-state FIBs.
• Electrochemical testing including cyclic voltammetry and galvanostatic cycling at various rates and temperatures.
• Operando X-ray absorption spectroscopy (XAS) to elucidate reaction mechanisms.
• Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) for structural and surface analysis.

Main Results:

• The FeFx cathode achieved 88-72% of its theoretical capacity at 0.1-1C rates at room temperature.
• Excellent cyclability was observed with no significant overvoltage increase or capacity degradation.
• The cathode operated effectively even at -30 °C, indicating low activation energy for defluorination/fluorination.
• Operando XAS confirmed reversible defluorination/fluorination between FeF3, FeF2, and Fe.
• Nanostructured FeFx (approx. 10 nm grains) electrode structure was maintained during cycling, contributing to stability.
• Electrochemical analysis indicated that the FeF2/Fe reaction is limited by charge transfer, suggesting kinetic advantages.

Conclusions:

• FeFx is a viable cathode material for all-solid-state fluoride-ion batteries, demonstrating high capacity and excellent cycling stability.
• The nanostructure of the FeFx electrode plays a crucial role in its superior cyclability.
• The low-temperature performance and charge transfer kinetics highlight the potential for practical applications of FeFx in FIBs.