2 research outputs found
Controlled Formation of Mixed Nanoscale Domains of High Capacity Fe<sub>2</sub>O<sub>3</sub>–FeF<sub>3</sub> Conversion Compounds by Direct Fluorination
We report a direct fluorination method under fluorine gas atmosphere using a fluidized bed reactor for converting nanophase iron oxide (n-Fe<sub>2</sub>O<sub>3</sub>) to an electrochemically stable and higher energy density iron oxyfluoride/fluoride phase. Interestingly, no noticeable bulk iron oxyfluoride phase (FeOF) phase was observed even at fluorination temperature close to 300 °C. Instead, at fluorination temperatures below 250 °C, scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) and X-ray photoelectron spectroscopy (XPS) analysis showed surface fluorination with nominal composition, Fe<sub>2</sub>O<sub>3‑<i>x</i></sub>F<sub>2<i>x</i></sub> (<i>x</i> < 1). At fluorination temperatures of 275 °C, STEM-EELS results showed porous interconnected nanodomains of FeF<sub>3</sub> and Fe<sub>2</sub>O<sub>3</sub> coexisting within the same particle, and overall the particles become less dense after fluorination. We performed potentiometric intermittent titration and electrochemical impedance spectroscopy studies to understand the lithium diffusion (or apparent diffusion) in both the oxyfluoride and mixed phase FeF<sub>3</sub> + Fe<sub>2</sub>O<sub>3</sub> composition, and correlate the results to their electrochemical performance. Further, we analyze from a thermodynamical perspective, the observed formation of the majority fluoride phase (77% FeF<sub>3</sub>) and the absence of the expected oxyfluoride phase based on the relative formation energies of oxide, fluoride, and oxyfluorides
Formation of Iron Oxyfluoride Phase on the Surface of Nano-Fe<sub>3</sub>O<sub>4</sub> Conversion Compound for Electrochemical Energy Storage
We have investigated a novel approach
wherein we undertake surface
fluorination of nanometer sized Fe<sub>3</sub>O<sub>4</sub> conversion
compound into corresponding oxyfluoride with the goal toward enhancing
their energy density as well electrochemical performance stability.
This is achieved by using direct fluorination of nano-Fe<sub>3</sub>O<sub>4</sub> in a fluidized bed reactor under controlled reaction
atmosphere and temperature. X-ray photoemission spectroscopy analysis
shows conclusive evidence of the surface fluorination of Fe<sub>3</sub>O<sub>4</sub> particles at a reaction temperature of 100 °C
and higher forming a surface oxyfluoride phase that can be nominally
described as FeOF. Formation of oxyfluoride phase is confirmed by
the appearance of a higher potential intercalation plateau during
the electrochemical charge–discharge cycling. Based on the
experimental results, various pathways are discussed for the formation
of oxyfluoride species on the surface