Stabilization of a mixed iron vanadium based hexagonal tungsten bronze hydroxyfluoride HTB-(Fe0.55V0.45)F2.67(OH)0.33as a positive electrode for lithium-ion batteries

Article abstract of DOI:10.1039/d0dt01310b

In our search for novel insertion compounds for Li-based batteries, we have identified a new mixed iron vanadium based Hexagonal Tungsten Bronze (HTB) type phase. Its synthesis involves two steps which consist first of preparing mixed metal hydrated fluoride Fe1.64V1.36F8(H2O)2 by a microwave assisted thermal process, followed by thermal treatment under air to obtain metastable HTB-(Fe0.55V0.45)F2.67(OH)0.33 hydroxyfluoride. 57Fe M?ssbauer spectrometry demonstrates the presence of oxidation states Fe2+ and Fe3+ in Fe1.64V1.36F8(H2O)2 as opposed to only Fe3+ in HTB-(Fe0.55V0.47)F2.67(OH)0.33. Moreover, the M?ssbauer spectra recorded at 77 K reveal that none of the compounds shows magnetic ordering owing to the presence of V3+ distributed over the crystallographic sites of Fe3+. Complementary X-ray spectroscopy and Rietveld refinement further confirm the successful synthesis of HTB-(Fe0.55V0.45)F2.67(OH)0.33. Electrochemically, the new HTB-(Fe0.55V0.45)F2.67(OH)0.33 shows a first discharge capacity of 181 mA h g-1 with 67percent of this capacity remaining upon cycling. Unlike HTB-FeF2.66(OH)0.34, the structure remains stable after the first discharge confirming the positive effect of vanadium in the HTB network. This journal is

Full text of DOI:10.1039/d0dt01310b

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Moury, J. Lhoste, A. Ribaud, M. Leblanc, J. Grenèche, J. Tarascon and V. Maisonneuve, Dalton Trans.,
2020, DOI: 10.1039/D0DT01310B.
Volume 46
Number 10
14 March 2017
Pages 3073-3412
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DOI: 10.1039/D0DT01310B
Stabilization of a mixed iron vanadium based Hexagonal Tungsten
Bronze hydroxyfluoride HTB-(Fe0.55V0.45)F2.67(OH)0.33 as positive
electrode for lithium-ion batteries
Kévin Lemoine,a Romain Moury,a Jérôme Lhoste,a Annie Hémon-Ribaud,a Marc
Leblanc,a Jean-Marc Grenèche,a Jean-Marie Tarascon,b,c Vincent Maisonneuvea*
a
Institut des Molécules et des Matériaux du Mans, UMR 6283 CNRS, Le Mans
Université, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France
b
Collège de France, Chaire de Chimie du Solide et de l’Energie, UMR 8260 CNRS,
11 Place Marcelin Berthelot, 75231 Paris, France
c
Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459,
80039 Amiens, France
Kévin Lemoine present address: Department of Chemistry, Faculty of Science, Gakushuin
University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan
Abstract
In our search for novel insertion compounds for Li-based batteries, we have identified
a new mixed iron vanadium based Hexagonal Tungsten Bronze (HTB) type phase.
Its synthesis involves two steps which consist first of preparing the mixed metal
hydrate fluoride Fe1.64V1.36F8(H2O)2 by a microwave assisted thermal process,
followed by a thermal treatment under air to obtain the metastable HTB-
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(Fe0.55V0.45)F2.67(OH)0.33 hydroxyfluoride. 57Fe Mössbauer spectrometry demDO
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the presence of oxidation states Fe2+ and Fe3+ in Fe1.64V1.36F8(H2O)2 as opposed to
only Fe3+ in HTB-(Fe0.55V0.47)F2.67(OH)0.33. Moreover Mössbauer spectra recorded at
77 K reveal that none of the compounds shows magnetic ordering owing to the
presence of V3+ distributed over the crystallographic sites of Fe3+. Complementary X-
ray spectroscopy and Rietveld refinement further confirm the successful synthesis of
HTB-(Fe0.55V0.45)F2.67(OH)0.33
.
Electrochemically-wise,
the
new
HTB-
(Fe0.55V0.45)F2.67(OH)0.33 shows a first discharge capacity of 181 mAh.g-1 with 67% of
this capacity remaining upon cycling. Unlike HTB-FeF2.66(OH)0.34 the structure
remains stable after the first discharge confirming the positive effect of the vanadium
in the HTB network.
1. Introduction
Improving both the energy density and power rate of present Li-ion batteries together
with their sustainability while preserving safety is nowadays a major research
challenge.1,2 To achieve this goal, hope is placed on all-solid-state batteries, which by
virtue of using a Li metal negative electrode and a solid electrolyte could lead to both
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energy density and safety improvements, respectively. However, interfaceDO
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remain to be solved prior such an option could become a practical reality. Meanwhile,
the LiFePO4-based lithium-metal-polymer battery technology presently on the market
provides a midway solution since it enhances safety owing to the use of a solvent-
free PEO polymeric membrane.3-5 In contrast, this technology does not fully capitalize
on the use of Li negative electrode, which must be in large excess because of some
dead mossy Li forming upon cycling. Furthermore, it is penalized by its mandatory
operation at 55°C to reach sufficient ion conduction in the electrolyte for ensuring
good performances. As a whole, besides enhancing the coulombic plating-stripping
efficiency of the Li-electrode another way to enhance the energy of Li-polymer
batteries is to design better positive electrodes bearing in mind that they can be Li-
free content, as opposed to the Li-ion technology, since they use a Li negative
electrode. Let us simply recall that Li-free V2O5 was the positive electrode material
early used in the development of Li-polymer batteries, thus a greater opening of the
materials field to explore.
Within this context, FeF3 with a theoretical capacity of 237 mAh.g-1 and an
anticipated high operational voltage due to the presence of the very electronegative
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fluorine element has been intensively explored.6-11 Three polymorphs exis
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The most thermodynamically stable one adopts the rhombohedral (
푅3푐
) ReO type
3
structure12 and simultaneously shows the closest packing, hence being the most
prone to limited Li+ diffusivity. In contrast, the two other polymorphs, namely
13
pyrochlore (Pyr-FeF3,
Bronze (HTB-FeF ,
)
with 3D open network and Hexagonal Tungsten
퐹푑3푚
)14 with 1D channels along the c axis, are metastable and
퐶푚푐푚
3
display open frameworks.10 These metastable structures can be synthesized with
water molecules within the framework.15 Recently, we have succeeded in preparing a
water-free (HTB-FeF2.67(OH)0.33) by thermal dehydration of Fe3F8(H2O)2 which
exhibits an experimental capacity of 170 mAh.g-1 vs. Li+/Li.16 Following this strategy,
we have also explored the effect of mixing 3d transition metals (TM) on the
electrochemical properties of new amorphous oxyfluorides obtained by thermal
treatment of the hydrated M2+Fe3+ F8(H2O)2 (M2+ = Mn, Co, Ni, Cu) and found
2
attractive electrochemical properties.17,18 The iron-vanadium hydrated fluoride
Fe1.64V1.36F8(H2O)2 was studied as well and interestingly, we showed that during its
thermal decomposition an intermediate phase is formed with HTB structure type. The
hydrate adopts the monoclinic C2/m Fe3F8(H2O)2 structure type which is built up from
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HTB sheets (Figure 1a). Its 3D structure can be described by perovs
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∞[M3+F4] connected to each other by isolated M2+ F4(H2O)2 octahedra (Figure 1b).19,20
2
At this stage, we should recall that vanadium trifluorides were only found in ReO3 and
pyrochlore packing12,21 and that were tested as a cathode material in LIB and NIB
batteries.6,22 Herein we report on the microwave assisted solvothermal synthesis of
Fe1.64V1.36F8(H2O)2 followed by a proper thermal treatment to unravel the first
example of mixed iron vanadium based fluoride (Fe0.55V0.45)F2.67(OH)0.33 with a
framework HTB. We precisely defined its formulation and structure from combined
XRD Rietveld refinements, thermogravimetric measurements (TGA), Energy
Dispersive X-ray spectroscopy (EDX), and Mössbauer spectrometry, while its
electrochemical activity towards Li+ (181 mAh.g-1) was deduced by assembling HTB-
(Fe0.55V0.45)F2.67(OH)0.33/Li cells.
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DOI: 10.1039/D0DT01310B
a)
b)
Figure 1. View of the HTB sheet (a) and projection along b of the structure of
Fe1.6V1.4F8(H2O)2 (b).
2. Methodology
2.1 Experimental Methods
The mixed metal fluoride Fe1.64V1.36F8(H2O)2 is synthesized by solvothermal reaction
using a MARS-5 microwave Digestion System (CEM Corp.) from the starting
reactants FeCl2∙4H2O (98%, Alfa Aesar), VCl3 (Alfa Aesar), absolute methanol
(99.8%, Sigma Aldrich), deionized water and hydrofluoric acid solution “HF” (27.6
mol.L-1, 48%, Riedel De Haen). The safe use of highly hazardous concentrated HF
solutions requires wearing gloves (neoprene) and handling inside a fume hood. The
concentration [Fe2+] + [V3+] was fixed equal to 0.1 mol.L-1 and the Fe2+/V3+/HF/MeOH
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ratio equal to 1/2/44/699. The mixture was inserted in Teflon autoclaves
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for 30 min then heated at 160°C for 1 h with stirring. After cooling, the light brown
powder product was filtered, washed with 2 mL of ethanol and dried overnight under
air at 100°C. A partial oxidation of Fe2+ into Fe3+ ions due to the presence of water
3+
leads to the formulation Fe2+Fe3+
V
1.36F8(H2O)2. This product was then inserted in
0.64
a furnace under ambient air and heated at 280°C, during 1 h (heating/cooling rate of
2°C/min). After cooling to room temperature, a dark brown hydroxyfluoride resulted
with the formulation (Fe0.55V0.45)F2.67(OH)0.33. It is also worth to emphasize that after
the thermal treatment; all the phases are immediately outgassed at ambient
temperature under secondary vacuum (P < 10-5 bar) and stored in a glove box prior
being characterized.
2.2 Characterization Methods
2.2.1 Structural and chemical compositions
X-ray diffraction patterns were collected in the range 8° ≤ 2θ ≤ 150° on a Panalytical
MPD-PRO diffractometer equipped with a linear X’celerator detector with CuKα
(1.541 Å). In some cases, the CoKα radiation (1.789 Å) was used to avoid the X-ray
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fluorescence. Rietveld refinements were performed by using the FullpDOroI:
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refinement program.23,24 For the temperature dependant diffraction the Anton Parr
reactor chamber XRK 900 heating device was used with an alumina sample holder,
the temperature was allowed to equilibrate for 15 min prior to the measurement.
57Fe Mössbauer measurements were performed in transmission geometry with a 925
MBq -source of 57Co/Rh mounted on a conventional constant acceleration drive. The
velocity of the source was calibrated using -Fe as the standard at RT. The samples
with 5 mg Fe/cm2 were prepared from a softly milled powder. Data were fitted using
the MOSFIT program25 involving quadrupolar components with lorentzian lines. The
isomer shift values are referred to that of -Fe at RT.
SEM images of the powders were obtained using a JEOL microscope (JSM 6510
LV). Acceleration voltages varied between 20 and 30 kV as a function of the
analyzed samples. Elementary quantitative microanalyses were performed using an
Energy Dispersive X-ray (EDX) OXFORD detector with Aztec software.
2.2.2 Thermal analyses
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The thermogravimetric (TGA) experiments were carried out with a thermDO
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SETARAM TGA 92 with a heating rate of 2°C.min-1 from room temperature up to
900°C under ambient air and dry air (Alphagaz, mixture of oxygen (20%) with
nitrogen (80%), H2O < 3 ppm).
X-ray thermodiffraction (HT-XRD) experiments were performed under dry air flow in
an Anton Parr XRK 900 high temperature furnace with the diffractometer already
described. The samples were heated from 40°C to 600°C at a heating rate of 10°C
min-1. X-ray diffraction patterns were recorded in the 5-60° range with a scan time of
10 min at 10°C intervals from room temperature to 400°C and 50°C intervals from
400°C to 600°C.
The powder of Fe2+Fe3+0.64V1.36F8(H2O)2, was heated in a furnace (Nabertherm LT
3/12/P330) at a heating rate of 3°C.min-1 from room temperature to T ≤ 300°C under
ambient atmosphere and were cooled at room temperature. Then, IR spectra were
collected ex-situ (20°C intervals) between 40 cm-1 and 4000 cm-1 with a scan time of
2 min with an ALPHA FT-IR Spectrometer.
2.2.3 Electrochemistry characterizations
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The electrochemical activity against lithium was evaluated using a Swagel
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cell. Electrodes were composed of 80% active materials and 20% carbon-SP (super
pure) as the conductive agent; they were ball-milled using a SPEX Miller 800 at 875
cycles/min for 15 minutes. The area of the electrode was 1 cm2 with a loading mass
of 8 mg. The electrolyte was the commercial LP30 (1M LiPF6). The cell was cycled
between 2 and 4 V versus Li+/Li at C/20 current density.
2. Results and discussion
2.1. Synthesis and characterization of the hydrated fluoride
Our group recently developed a new solvothermal method assisted by microwave
heating to synthesize under mild conditions pure hydrated fluorides of general
formulae MFe2F8(H2O)2.17 Herein we implemented this strategy for the preparation of
the mixed 3d TM starting with a ratio Fe2+:V3+ of 1:2. The as-synthesized hydrate
fluoride was studied by 57Fe Mössbauer spectrometry to establish the Fe valence
states (Figure 2a). The spectrum collected at 300 K can be clearly interpreted as a
quadrupolar hyperfine structure with two paramagnetic components consisting of well
resolved doublets with Lorentzian profiles. The asymmetrical profile of the right line
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corresponding to the ferrous component can solely be fitted with two qDO
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doublets, resulting from disordered chemical atomic environments. The refined
(mean) values of the characteristic hyperfine parameters (Table 1) reveal clearly both
valence states Fe2+ and Fe3+, with a molar ratio Fe3+/Fe2+ = 0.64.
b)
a)
V (mm/s)
-4
-2
0
2
4
1.00
0.99
0.98 300 K
1.00
0.99
0.98
77 K
0.97
-4
-2
0
V (mm/s)
2
4
Figure 2. 57Fe Mössbauer spectra recorded at 300 K and 77 K on Fe1.64V1.36F8(H2O)2;
the red line corresponds to Fe2+ and the blue line to Fe3+ (a), Rietveld refinement of the
structural model with the diffraction pattern (b) of Fe1.64V1.36F8(H2O)2 at room
temperature.
Interestingly, the spectrum recorded at 77 K does not reflect the expected magnetic
order previously observed for Fe3+ below 157 K in M2+Fe3+ F8(H2O)2 phases, with the
2
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typical splitting in Zeeman sextet. Instead, it shows two paramagnetic co
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confirming the analysis made on the 300 K spectrum. A priori, the presence of V3+ on
Fe3+ crystallographic sites explains the magnetic order reduction, namely the
weakening of the Fe-Fe interactions by the presence of V3+ ions. Consequently,
instead
of
the
expected
formulation
Fe2+V3+ F8(H2O)2
we
obtained
2
Fe2+Fe3+
V
1.36F8(H2O)2 denoted Fe1.64V1.36F8(H2O)2 hereafter. In fact, during the
3+
0.64
synthesis, part of Fe2+ is oxidized to Fe3+ due to the water content in aqueous HF.
Table 1. Refined values of hyperfine parameters of Fe1.64V1.36F8(H2O)2 phase at
300 K and 77 K
Fen+
EQ


Temperature
± 0.01
± 0.01
± 0.01
Fe3+ 0.46
0.58
0.39
2
300 K
Fe2+ <1.33> <1.74> 0.61
Fe3+ 0.57
0.40 0.39
Fe2+ <1.45> <3.09> 0.61
doublets
2
77 K
doublets
 and EQ are given in mm.s-1
Diffraction pattern recorded on the as-synthesized phase reveals a structure isotypic
to Fe F (H O) that can be indexed with the
space group. Rietveld refinement
퐶2/푚
3
8
2
2
was performed using the atomic positions of Fe3F8(H2O)2 (ISCD-38366); the
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occupancy factor for the shared 4f Wyckoff position for Fe3+ and V3+ wa
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using the Mössbauer spectrometry results. 4f Wyckoff position for V3+ have been
chosen considering bond valence calculations (Table S3), and careful inspection of
M-(O/Ow/F) distances. A reliable fit with acceptable agreement factors (RB/Rf =
0.039/0.057) was obtained (Figure 2b). Structural parameters obtained after
refinement are gathered in Tables S1 and S2.
2.2. Thermal decomposition
The precursor Fe1.64V1.36F8(H2O)2 was then subjected to thermal treatment and its
structural and water content evolutions were followed by in situ XRD and TGA
analysis. Figure 3a displays the thermodiffractograms obtained on the hydrated
phase, it can be seen that this phase is stable up to 220 °C, and starts its
dehydration at 240°C giving the formulation stated now and discussed later HTB-
(Fe0.55V0.45)F2.67(OH)0.33. The temperature range of stability for this HTB
hydroxyfluoride is between 240 and 280°C. Above 280°C a mixture between HTB
and a new unknown phase up to 400°C is observed (Figure S1) and at 500°C only
hematite Fe2O3 is left. Owing to the absence of vanadium based crystalline phase
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above 500°C we can suggest the sublimation of a vanadium phase like
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sublimation temperature of 480°C, or VF3.26 The thermogravimetric measurement
(Figure 3b) shows a total weight loss of 63.8 % starting above 200°C. Combining the
previous thermal analysis with HT-XRD, the following decomposition reactions can
be suggested. Equation (1) is valid in the temperature range [220-280°C] and
Equation (2) in the range [280-500°C].
Fe1.64V1.36F8(H2O)2 + 1/4O2  3(Fe0.55V0.45)F2.67(OH)0.33 + 3/2H2O (1)
Fe0.55V0.45F2.67(OH)0.33 + 0.495H2O  0.45VF3 + 0.275Fe2O3 + 1.32HF (2)
a)
b)
Fe
V
F (H O)
1.46 2 2
1.6 1.4 8 2 2
Fe1.64V F8(H O)
: Fe2O3
0
-20
-40
-60
-80
T(
600
500
400
°
C)
380
360
340
320
300
280
260
Exp = 63.8 %
Theo = 62.7 %
240
220
200
180
160
140
120
80
0.8 Fe2O3
40
100
200
300
400
500
600
700
800
10
20
30
40
50
60
Temperature (°C)
 
Figure 3. Thermal evolution of X-ray diffraction patterns (Cu Kα) under dry air (a) and
thermogravimetric curve under dry air (b) of Fe1.6V1.4F8(H2O)2.
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Despite a narrow temperature window of stability, the HTB-phase was s
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isolated by heating the hydrated fluoride during 1 h at 280°C under ambient air. In
order to follow the dehydration process, ex situ FTIR experiment at different
temperatures were performed (Figure S2). The FTIR spectrum at 160°C presents two
bands: one broad, between 3000 and 3500 cm-1 corresponding to OH stretching
modes and one sharp at 1625 cm-1 characteristic of water bending mode. As the
temperature increases, these absorption bands decrease, confirming water removal,
while two new intense peaks at 1090 and 1170 cm-1 raise. These bands are
attributed to hydroxyl deformation bands that involve metal oxygen bonds. Vimont et
al. have demonstrated that these bands are a coupling of in-plane bending δ(OH)
and ν(M-O) vibration in HTB metal hydroxyfluorides.27 These results would evidence
the formation of a metal hydroxyfluoride.
Mössbauer experiments were carried out at 300 K and 77 K on the as-obtained
57
phase to determine the valence states and the environment of Fe (Figure 4a) after
the thermal treatment. The spectra consist of a single broadened and asymmetrical
line which can be perfectly described by means of two main paramagnetic doublets
plus a minor doublet corresponding to FeF2 impurity (2 mol%). The refined values of
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the hyperfine parameters, summarized in Table 2, reveal the presence o
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valence state for Fe3+ in two different environments. Because the crystallographic
structure consists only of octahedral units, the significantly different values of the
isomer shift suggest different chemical environments: the first environment with the
smallest quadrupole splitting (0.24 mm.s-1) and the highest isomer shift (0.48 mm.s-1)
corresponds to a regular FeF6 octahedron whereas the second one, with the
quadrupole splitting of 0.70 mm.s-1 and a weaker isomer shift (0.39 mm.s-1) is
attributed to an asymmetric octahedron with either hydroxide or fluorine at the
vertexes.16,28,29
a)
b)
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Figure 4. 57Fe Mössbauer spectra recorded at 300 K and 77 K DO
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(Fe0.55V0.45)F2.67(OH)0.33
,
(a)
and
EDX
elemental
mapping
of
HTB-
(Fe0.55V0.45)F2.67(OH)0.33 (b).
The reported magnetic order temperature for fluorides having the HTB type structure
alike FeF3 is of ≈ 97 K whereas in our experiment, the spectrum at 77 K exhibits only
two paramagnetic components, meaning that the magnetic order temperature is
lowered.15,30 This feature would confirm the presence of V3+ on Fe3+ crystallographic
sites. To discard phase segregation which would corroborate the atomic distribution
between V3+ and Fe3+, EDX spectroscopy on the stabilized HTB hydroxyfluoride was
performed by SEM (Figure 4b). A homogenous dispersion of both metals without
phase segregation was noticed with Fe3+/V3+ ratio close to 1.
Table 2. Refined values of the hyperfine parameters of Fe0.55V0.45F2.67(OH)0.33 at
300 K and 77 K.


EQ
Temperature
Fen+
± 0.01
± 0.01
± 0.01
0.48
0.39
0.24
0.70
1.72
0.39
0.59
0.02
Fe3+
300 K
3 doublets
Fe2+ 1.15
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DOI: 10.1039/D0DT01310B
0.57
0.51
Fe2+ 1.25
0.26
0.71
2.92
0.39
0.59
0.02
Fe3+
77 K
3 doublets
 and EQ are given in mm.s-1
A diffraction pattern was recorded on the stabilized phase to confirm the mixed iron
vanadium based HTB type structure (Figure 5a). The pattern was indexed using
푃63/
instead of the original space group
for HTB-FeF3 (Table S3) in order to
퐶푚푐푚
푚
better estimate the Fe-F distances, as described by Leblanc et al. and further
confirmed by Lemoine et al.16,31 Rietveld refinement was performed with V3+ sharing
the 6g Wyckoff position of Fe3+ (Mössbauer result) and with a partial occupancy of
the 12i Wyckoff position of fluorine atoms F(2) by oxygen atoms fixed at 16.7% to
comply with the OH/F ratio established by TGA, structural parameters are gathered in
Table 3. The occupancy of V3+ and Fe3+ on 6g Wyckoff positions was fixed with
respect to Equation 1. As HTB structure easily absorbs water during sample
preparation, we added water molecules within the hexagonal channels on the
Wyckoff position 2a, giving satisfactory fit with agreement factors (RB/Rf =
0.086/0.099). A lack of intensity is observed on (002) (2 = 27.4°) reflection possibly
due to either preferred orientation or to the high degree of freedom of water
18

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Dalton Transactions
displacement within the cavity. Structural parameters after refinement are gDO
a
I
t
:
h
10
e
.1rView Article Online
0
e
39
d
/D0inDT01310B
Table 4. FTIR, Mössbauer spectrometry, EDXS and XRD confirm the successful
stabilization of the first vanadium based fluoride HTB type structure (Figure 5b).
a)
b)
Figure 5. Rietveld refinement of the structural model with the diffraction pattern (Co Kα1
Kα2) (a) and view of structure (b) of HTB-(Fe0.55V0.45)F2.67(OH)0.33∙nH2O.
Table 3. Data collection and structural parameters of HTB-
(Fe0.55V0.45)F2.67(OH)0.33∙nH2O obtained by powder diffraction.
Distances (Å)
Formulation
System / Space Group
a (Å), c (Å)
(Fe0.55V0.45)F2.67(OH)0.33·0.1H2O
hexagonal / P63/m
7.391(1), 7.557(1)
357.5(1), 6
CoK
2x (Fe/V)-F(1) = 1.89(1)
2 x (Fe/V)-F(2)/O(2) = 1.91(9)
2 x (Fe/V)-F(2)/O(2) = 1.94(13)
Ow-F(1)/O(1) = 3.26(8)
V (Å3), Z
Wavelength (Å)
ρcalc. (g.cm-3)
2θ (°)
3.081
5 – 150
Valence
Unique refl.
366
19

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Page 20 of 28
View Article Online
Refined param.
Rp / Rwp
RB / Rf
15
DOI: 10.1039/D0DT01310B
0.436 / 0.251
0.084 / 0.081
2.13
ΣV(1)/Fe(1) = +3.31
2
Table 4. Atomic positions and isotropic displacement parameters of HTB-
(Fe0.55V0.45)F2.67(OH)0.33∙0.1H2O.
Beq (Å2)
1.7(1)
1.7(1)
4.6(3)
4.6(3)
4.6(3)
6.9(1)
Atome
Fe(1)
V(1)
F(1)
F(2)
O(2)
Ow(1)
 stands for occupancies
Site
6g
6g
x
½
½
y
0
0
z
0
0
¼

0.546
0.454
1
0.833
0.167
0.30
6h
0.505(5)
0.210(13)
0.210(13)
0
-0.017(4)
-0.206(13)
-0.206(13)
0
12i
12i
2a
-0.001(5)
-0.001(5)
¾
2.3. Electrochemical evaluation
Cylic voltammetry charge-discharge measurements were performed on Li-half cells
using either the hydrated fluoride or the HTB hydroxyfluoride as the positive
electrode. The cells were cycled between 2 and 4 V vs Li+/Li at a rate of C/20 (Figure
6). The low cutoff voltage (2 V) was chosen to avoid irreversible conversion into
metallic Fe that is known to happen in these types of phases below ≈ 1.9 V.22 The
HTB-(Fe0.55V0.45)F2.67(OH)0.33 phase can uptake during the first discharge more than
20

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Dalton Transactions
0.7 Li+ (e.g. ≈ 180 mAh.g-1) while Fe1.64V1.36F8(H2O)2 barely uptake 0.04
D
L +
O
i
I: 10
(
.
F10 View Article Online
i
3
g
9/
u
D0DT01310B
r
e
6a). This indicates that water molecules hinder Li+ diffusivity within the hydrated
fluoride, as already noticed.16 Turning back to the capacity of (Fe0.55V0.45)F2.67(OH)0.33
it drops to 123 mAh.g-1 and remains constant up to the 10th cycle (Figure 6b), the
maximum we have tried. The large capacity loss (≈ 30%) between the first discharge
and first charge exceeds what is usually found in intercalation compounds.
Electrolyte or electrode decomposition are usually responsible for such large capacity
losses, but the constancy of the capacity retention once the first cycle is achieved,
does not support this explanation.32 The diffraction pattern (Figure 6c) exhibits the
most intense HTB peaks at 13.8, 23.5 and 27.9° for HTB-(Fe0.55V0.45)F2.67(OH)0.33
,
still persisting after the discharge to 2 V with a visible shift in lower angle values
especially for the peaks corresponding to the (110), (002), (130), (112) and (220)
reflections attesting the lithium insertion in the structure. A broadening of these Bragg
peaks results probably from an amorphization and would explain the irreversible
trapping (0.2 Li+) occurring through the first cycle. Such trapping was also evidenced
in previous works on HTB-hydroxyfluoride through in situ XRD measurements16 and
PDF analyses by Dambournet et al. in which FeF2.2(OH)0.8Ox/2□x/2 collapse yielding
21

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Page 22 of 28
disordered rutile and rock salt structure types.33,34 The remaining HT
D
B
OI: 1
n
0
e
.10tView Article Online
3
w
9/
o
D0DT01310B
r
k
suggests the positive effect of vanadium in preventing the phase transition from the
HTB structure to the more thermodynamically stable allotropic varieties of FeF316 that
was observed for FeF2.67(OH)0.33 (Figure 6c). Such positive effect of vanadium within
the HTB structure was also studied and observed with other 3d elements like Ti,35,36
Co37 or Mn.38,39
c)
4.5
(Fe0.53V0.47)F2.67(OH)0.33
a)
4.0
Discharged to 2V
(002)
(220)
3.5
(110)
(130)
3.0
(112)
2.5
Fe1.6V1.4F3(H2O)2
HTB-(Fe0.53V0.47)F2.67(OH)0.33
2.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
10
15
20
25
30
35
40
2 (°)
x (Li)
4.5
4.0
3.5
3.0
2.5
2.0
FeF2.67(OH)0.33
(Fe0.53V0.47)F2.67(OH)0.33
d)
b)
Dischargedto 2V
2-1
10
ReO3-FeF3
HTB-FeF3
0
20
40
60
80
100 120 140 160 180
Capacity (mAh/g)
20
25
30
2 
Figure 6. First discharge and charge voltammetric curves vs Li+/Li for
Fe1.64V1.36F8(H2O)2 and HTB-(Fe0.55V0.45)F2.67(OH)0.33 (a) and cyclic voltammetry
curves of HTB-(Fe0.55V0.45)F2.67(OH)0.33 (b) at C/20, XRD patterns of HTB-
22

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Dalton Transactions
(Fe0.55V0.45)F2.67(OH)0.33 before and after the first discharge to 2 V with (h
D
k
O
l
I:
)
10
i
.
n
10
d
3V9iiec/wDArticle Online
e
0DT01310B
s
for 13.8°, 24.7° and 27.6° Bragg peaks, (c) and HTB-FeF2.67(OH)0.33 after the first
discharged to 2V compared to theoretical XRD pattern of ReO3-FeF3 and HTB-FeF3
(d).
3. Conclusion
We have reported the synthesis of the first vanadium based HTB fluoride
(Fe0.55V0.45)F2.67(OH)0.33 via a controlled dehydration reaction. It crystallizes, as
deduced by Rietveld refinement, in the
space group confirming the HTB type
푃63/푚
structure with hexagonal channels. The distribution of vanadium/iron at the
crystallographic sites belonging to the HTB structure ((Fe0.55V0.45)F2.67(OH)0.33) was
restrained from Mössbauer spectrometry. This phase can uptake 0.7 Li+ in discharge,
of which only 0.5 can be removed in charge, the irreversibility (≈ 0.2 Li+) having
probably been most associated with partial amorphization of the structure or a
reduction of the coherent diffraction domains. In addition, XRD analysis has
demonstrated the advantage of vanadium in stabilizing the metastable HTB network
which was partially preserved in the first discharge. An obvious extension of this work
23

Dalton Transactions
Page 24 of 28
will be to further manipulate the chemistry of this phase by selecting the aDO
p
I
p
: 1
r
0
o
.1p03View Article Online
r
9
i
/
a
D0DT01310B
t
e
cationic substituent and its correct content to stabilize HTB against its partial
amorphization during discharge to take advantage of its full capacity.
Conflicts of interest
There are no conflicts to declare.
Acknowledgments
The authors acknowledge the French Research Ministry for the doctoral grant to KL
and the platforms of IMMM “Diffusion et Diffraction des Rayons-X” and “Microscopy”.
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The first mixed iron vanadium hydroxyfluoride ((Fe0.53V0.47)F2.67(OH)0.33) with a Hexagonal
Tungsten Bronze (HTB) network was synthesized. The HTB structure possesses 1DDOIc: 1h0a.1n03n9e/Dls0DT01310B
allowing lithium insertion and recovering. Its electrochemical evaluation as positive electrode
reveals a capacity of 181 mAh.g-1. Unlike pure iron hydroxyfluoride (FeF2.66(OH)0.34) the
HTB network is partially maintained upon cycling.