Impact of Anion Vacancies on the Local and Electronic Structures of Iron-Based Oxyfluoride Electrodes

Abstract

The properties of crystalline solids can be significantly modified by deliberately introducing point defects. Understanding these effects, however, requires understanding the changes in geometry and electronic structure of the host material. Here we report the effect of forming anion vacancies, via dehydroxylation, in a hexagonal-tungsten-bronze-structured iron oxyfluoride, which has potential use as a lithium-ion battery cathode. Our combined pairdistribution function and density-functional-theory analysis indicates that oxygen vacancy formation is accompanied by a spontaneous rearrangement of fluorine anions and vacancies, producing dual pyramidal (FeF4)-O-(FeF4) structural units containing five-fold-coordinated Fe atoms. The addition of lattice oxygen introduces new electronic states above the top of the valence band, with a corresponding reduction in the optical band gap from 4.05 eV to 2.05 eV. This band gap reduction relative to the FeF3 parent material is correlated with a significant improvement in lithium insertion capability relative to defect-free compound.

Electrochemical properties of chromium oxyfluoride CrO2−xFx with 0 ≤ x ≤ 0.3

“…However, as x increases from x = 0.05, both Q dis and Q cha decrease to 150-300 mA h g −1 due to a low electrical conductivity, which has also been observed with other oxyfluoride materials. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Since the operating voltage of the discharge reaction was close to the discharge cut-off voltage ( = 0.02 V), the discharge reaction of the CrO 2−x F x compounds finished before storing a large amount of Li ions. Note that CrF 2 exhibits a Q recha of ∼100 mA h g −1 and a capacitor-like voltage profile.…”

“…Table 1 summarizes the crystal structures and electrochemical properties of the M a O b F c compounds reported thus far. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] NbO 2 F, 6,7 TiF 2 7,8,9,10 TaO 2 F, 11 and VO 2 F 12,13,14 have a regular or distorted ReO 3 -type structure with a rechargeable capacity (Q recha ) of 200-400 mA h g −1 . Overall, their operating voltage as a function of Q recha is featureless, suggesting the same conversion reaction that is typical for their parent MO compounds.…”

“…Overall, their operating voltage as a function of Q recha is featureless, suggesting the same conversion reaction that is typical for their parent MO compounds. 23,24 On the other hand, FeO x F 2−x with a rutile structure [15][16][17][18][19] and BiO x F 3−2x with a tysonite-like structure 20,21 exhibited a flat operating voltage at ∼2.4 and ∼3.0 V, respectively, indicating a two-phase reaction. The Q recha values of FeO x F 2−x [15][16][17][18] and BiO x F 3−2x 20,21 are ∼400 and ∼200 mA h g −1 ,…”

“…Moreover, M a O b F c compounds are usually synthesized via solution routes using highly toxic, corrosive fluorine-containing acids or by ball milling a mixture of transition metal oxides and LiF. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] These methods provide a bulky electrode material, lowering the volumetric energy density of LIBs. Hence, a denser M a O b F c compound with a Q recha > 330 mA h g −1 below ∼1.0 V is required, to surpass the electrochemical properties of graphite and further increase the energy density of LIBs.…”

Investigation of Amorphous Mixed-Metal (Oxy)Fluorides as a New Class of Water Oxidation Electrocatalysts

“…0.33H2O 32 lead to the successful preparation of a lacunar oxyfluoride with the formulation FeF2.2O0.4□0.4. 33 Following this strategy, the preparation of new iron-based hydrated fluorides M 2+ Fe 3+ 2F8(H2O)2 (M=Mn, Co, Ni, Cu) by microwave heating assisted solvothermal synthesis from metal salts, aqueous hydrofluoric acid and methanol as solvent was previously reported. 34 These hydrated crystallized phases were further calcinated under ambient air to obtain the corresponding amorphous oxyfluorides.…”