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An Operando Mössbauer Spectroscopy Investigation of the Electrochemical Reaction with Lithium in Bronze-type FeF3·0.33H2O Donato Ercole Conte, Lidia Di Carlo, Moulay Tahar Sougrati, Bernard Fraisse, Lorenzo Stievano, and Nicola Pinna J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06711 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on October 1, 2016
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An Operando Mössbauer Spectroscopy Investigation of the Electrochemical Reaction with Lithium in Bronze-type FeF3·0.33H2O Donato Ercole Contea*, Lidia Di Carloa, Moulay Tahar Sougratib, Bernard Fraisseb, Lorenzo Stievanob, Nicola Pinnaa a Humboldt-Universität zu Berlin, Institut für Chemie, Brook-Taylor Str., 2, 12489 Berlin, Germany b Institut Charles Gerhardt Montpellier, UMR CNRS 5253, Université de Montpellier, 2 Pl. Eugène Bataillon, 34095 Montpellier, France
Abstract The electrochemical reaction of bronze-type FeF3·0.33H2O, synthesized via a simple room temperature solution route, with lithium was investigated by operando Mössbauer spectroscopy and X-ray diffraction. The two techniques revealed a complex electrochemical mechanism where the pristine crystalline compound is gradually transformed into an amorphous material containing nanodomains of a FeF2-like rutile structure mixed to iron nanoparticles. Upon charge, two steps are dominating the electrochemical behavior: the partial reformation of the initial bronze structure and the oxidation to Fe(III). This reaction mechanism however, is not constant and noticeable variation can be observed during galvanostatic cycling (up to 55 cycles) until eventually an amorphous material containing rutile nanodomains composes the final active electrode. The material performance, under the form of a fluoride/graphene oxide composite, is also assessed with respect to the long term effect of the depth of first discharge.
Introduction Iron trifluoride has recently attracted attention as both a heterogeneous catalyst and catalyst support (due to its surface acidity, high surface areas and porosity)
1,2
, and as energy storage
material (due to the possibility of a high-voltage multielectron reaction with lithium) 3. Kemnitz et al. developed a non-hydrolytic sol-gel synthesis yielding a biacidic (from both Lewis and Brønsted point of view) iron trifluoride hydrate 4 as well as an electrochemically composite material active vs Li+ reaction 5. While the mechanism of the electrochemical reaction with Li+ has been thoroughly studied and is widely accepted nowadays for anhydrous FeF3
6–9
, there is still uncertainty concerning the mechanisms of the main electrochemically
active hydrated phases, i.e. FeF3·0.33, 0.5 and 3 H2O. Since the synthesis conditions for the 1 ACS Paragon Plus Environment
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anhydrous trifluoride can be particularly constraining
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10,11
, the hydrated phases, and, in
particular hexagonal tungsten bronze FeF3·0.33 H2O, represent the bulk of the recent efforts in optimization of these ionic compounds capable of high voltage electrochemical reactions. Most of the efforts, however, were devoted to the optimization of the electrochemical performance and not to the understanding of the underlying mechanisms 3. FeF3·0.33 H2O crystallizes in the hexagonal tungsten bronze (HTB) type lattice (orthorhombic, space group Cmcm, Figure S1) where six FeF6 corner-sharing octahedra are arranged around a hexagonal cavity running along the z direction. Maier's group 12 proved experimentally that, in the region 1.6-4.4 V vs Li+/Li, the insertion of lithium into the hexagonal channels takes place only via a solid-solution mechanism without any conversion reaction occurring. In a later publication however, the same group varied slightly the synthetic procedure of FeF3·0.33 H2O and, whilst obtaining the same crystalline product, the authors were forced to impose a discharge cutoff potential of 1.7 V in order to avoid the appearance of the conversion pseudoplateaus, usually considered as poorly reversible
13
. Increasing cutoff potential is not exceptional and many
other authors follow this direction usually inverting current flow in the region of 2 V 3,14–16. Only very recently, the studies by Duttine et. al.16,17 evidenced a complex interaction between the iron fluoride matrix and the molecular water present in the crystal structure. The electrochemical reactions in HTB iron trifluoride obtained by soft chemistry approaches, containing structural hydroxyl groups in the crystal lattice, occur at potentials halfway between those observed for anhydrous FeF3 (around 3.3 V for insertion and around 2.6 V for conversion, at RT) 18 and those observed for FeOF (around 2.3 V for insertion and around 1.7 V for conversion, at RT) 19. Reaction potentials in the two compounds describe essentially the same kind of chemical reaction with lithium ions, namely an insertion reaction accounting for 1e- followed by a conversion reaction accounting for the remaining 2e-, but the reaction pathways and phase transformations are distinctly different. While anhydrous FeF3 (hexagonal lattice) transforms into a LiF/Fe mixture during discharge and reverts to a lithiated rutile phase during charge 8, the mechanism for FeOF is complicated by the presence of oxygen. The reaction with Li+ distorts the pristine FeOF rutile structure until the formation of an oxygen-rich intermediate, characterized by a poorly ordered rock salt structure similar to αLiFeO2. This compound still coexists with LiF, Li2O and Fe at the end of discharge. Upon charge, a poorly ordered, fluoride rich rutile phase (similar to FeF2) is reformed with about 30% of the rock salt intermediate still present at the end of the process 19. This separation of oxygen and fluorine into two distinct phases, even if very poorly crystalline, has been labelled as “anionic partitioning” and the mechanism was also recently confirmed for HTB2 ACS Paragon Plus Environment
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FeF2.2(OH)0.8-xOx/2x/2
17
. In particular for the bronze structure, few differences arise with
respect to FeOF, namely the absence of Li2O at the end of the first discharge and the presence of two different disordered rutile phases at the end of charge. Moreover, a very recent paper relates that it is possible to induce the hexagonal-to-bronze transformation by mechanical treatment of anhydrous FeF3, transforming its electrochemical behavior into the one typical for bronze phases (lowered insertion and conversion potentials) but hybridizing the reaction pathway between that of pure FeF3 (LiF/Fe mixture at the end of discharge) and that of FeOF (amorphous structures upon charge) 20. In conclusion, the existing mechanistic studies on electrochemical reactions of iron fluorides with lithium point to complex behaviors which are highly dependent on the structure type, hydration and hydroxylation of the compounds and are not yet completely understood. Therefore, novel studies, especially based on operando techniques, have a high potential in terms of capacity to draw a clearer picture of the structural transformations and redox reactions taking place at the electrodes during the reaction with Li+. In this manuscript, we report a combined operando
57
Fe Mössbauer, operando X-ray diffraction and prolonged
galvanostatic cycling study conducted on FeF3·0.33 H2O. The combination of these techniques allows us to show that the electrochemical reaction with Li+ follows a complex path which involves a protracted and continuous transformation of the active material and is not fully in agreement with the above reported studies, adding another layer of complexity to the electrochemical reaction mechanisms in hydrated trifluorides.
Experimental Synthesis of FeF3·0.33 H2O and of FeF3·0.33 H2O/Graphene oxide composite The synthesis of the iron trifluoride hydrate was reported in detail elsewhere4. Briefly, 4 g of Fe(NO3)3·9 H2O (Aldrich, 99 %) were dehydrated for 2 h under vacuum at 65 °C and then dissolved in 25 mL of methanol. Fluorination was carried out by addition of 1.25 mL of a concentrated methanolic HF solution (24 M, equivalent to 30 mmol of HF) under Ar atmosphere. The obtained sol was aged for 18 h before methanol removal under vacuum. Further drying was carried out at 100 °C under vacuum for 2 h and a teal-colored powder was recovered. Part of the powder was immediately transferred into a argon-filled glovebox and stored for electrochemical tests, whereas part was kept under normal temperature and humidity conditions for further characterization. Graphene oxide (GO) was obtained following a modified Hummers method 21. 1 g of graphite powder (< 20 µm, Aldrich) was suspended in 20 mL of concentrated H2SO4 (J. T. Baker, 953 ACS Paragon Plus Environment
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97 %) and pre-oxidized with 2.5 g of K2S2O8 (Aldrich, ≥ 99%) and 2.5 g of P2O5 (Fluka, ˃ 97 %) under stirring. The resulting suspension was heated at 80 °C for 5 h, after which the solid was recovered by centrifugation, repeatedly washed with water to neutrality and dried. A second oxidation was performed starting from 1 g of pre-oxidized graphite in 46 mL of concentrated H2SO4. Addition of 1 g of NaNO3 (ABCR, 98 %) and 6 g of KMnO4 (ABCR, 98 %)) was performed under vigorous stirring at 0 °C. After 5 days at room temperature, water was added and the suspension was heated at 98 °C for 1 h prior to addition of 6 mL of H2O2 (VWR, 30 %). The warm suspension was finally filtered, washed with a 2 M HCl (Grussing, 37 %) solution and with deionised water, and finally dried under reduced pressure at 120 °C for 4 hours. In order to obtain a composite material, 100 mg of the as synthesized GO were suspended in 10 mL of MeOH, and 2.5 mL of the concentrated HF methanolic solution (24 M, equivalent to 60 mmol of HF) were added. The resulting mixture was sonicated for 1 h before addition to the 15 mL iron precursor solution obtained as described above. The reaction then took place normally without further modification of the described steps.
Characterization X-ray powder diffraction patterns were recorded on a STOE Stadi MP diffractometer equipped with a Dectris Mythen 1 K linear silicon strip detector and Ge(111) double crystal monochromator (Mo Kα1 radiation) in a Bragg-Brentano geometry. Data were collected in the range 3° < 2θ < 60° with constant steps of 0.015° and Rietveld refinement was performed using the FullProf software 22. Operando patterns were recorded using a specifically developed in situ cell
23
on a
PANAlytical X'pert (Cu Kα radiation) diffractometer in a Bragg-Brentano geometry in the range 12° < 2θ < 50° with constant steps of 0.033°. The diffractograms were recorded every 2 h with a galvanostatic cycling regime of C/20 (1 Li+ reacted in 20 h). In this way, every pattern roughly represents the reaction of 0.1 Li+. Transmission electron microscopy (TEM) images were recorded on a Philips CM 200 microscope equipped with a LaB6 cathode and operated at 200 kV. Fourier Transform Infrared (IR) spectra were collected on a Digilab FTIR 3000, Excalibur series in transmission mode configuration in the range 4000-400 cm-1. Thermogravimetric analysis (TGA) was performed using a Netzsch STA-409C Skimmer® coupled with a Balzers QMG-421 mass spectrometer. Samples were heated from room temperature to 600 °C under argon atmosphere (heating rate, 10 °C min-1). 4 ACS Paragon Plus Environment
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57
Fe Mössbauer spectra were measured in the transmission mode with a
57
Co:Rh source.
During the measurements, both the source and the absorber were kept at ambient temperature. The spectrometer was operated in the constant acceleration mode with a triangular velocity waveform. The velocity scale was calibrated with the magnetically split sextet spectrum of a high-purity α-Fe foil at room temperature. For operando experiments, the same in situ cell employed for X-ray diffraction was used, as its design also allows for measurements in the transmission geometry. The spectra were recorded every 4 h with a galvanostatic cycling regime of C/50 (1 Li+ reacted in 50 h). In this way, every spectrum represents the average reaction of 0.08 Li+. The spectra were fitted to appropriate combinations of Lorentzian profiles representing quadrupole doublets by least-squares methods using the program PCMos II.24 This software is implemented with a specific fitting routine that allows the simultaneous refinement of selected fitting parameters on the whole series of spectra. In this case, only the relative resonance areas were left free to vary independently in each spectrum. In this way, unique spectral parameters such as the quadrupole splitting (∆QS), the isomer shift (δ) and the linewidth at half maximum (Γ) of the different spectral components were determined for the whole series. Isomer shifts are given relative to α-Fe. Galvanostatic cycling was performed using a Biologic® VMP3 potentiostat with potential limitations (GCPL). In the operando X-ray experiments, the potential window was set between 4.4 and 1.6 V vs Li+/Li and the positive electrode was built by thorough mixing of 80 wt.% active material powder (FeF3·0.33H2O/GO composite) and 20 wt.% Timcal super P® conductive carbon in an agate mortar. In the operando Mössbauer spectroscopy measurements, the first complete cycle was recorded between 4.5 and 1.5 V vs Li+/Li, and then down to 0.75 V for the second discharge. The positive electrode was again built by thorough mixing of 80 wt.% of active material powder (FeF3·0.33H2O) and 20 wt.% Timcal super P® conductive carbon in an agate mortar. Prolonged cycling was recorded in the potential window between 4.4 and 1.6 V vs Li+/Li except for one test in which the first discharge was protracted down to 1 V. For these experiments, the positive electrodes were prepared by suspending the fluoride powders, carbon and poly(vinylidendifluoride) (PVdF, Alfa Aesar) with a weight ratio of 70:10:20 in NMethylpyrrolidone (NMP, Alfa Aesar). The resulting slurry was then pasted on an aluminum foil, roll pressed and cut into discs. The latter were dried under vacuum at 120 °C overnight (Büchi glass oven) before being transferred into an argon-filled glove box (Mbraun). Twoelectrode cells (EL-Cell GmbH) were then assembled. In all cases, glass fiber separators (Whatman) and an electrolyte made of 1 M LiPF6 in 5 ACS Paragon Plus Environment
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Ethylene Carbonate : Propylene Carbonate : Dimethyl Carbonate (1:1:3) were used to build half cells vs. a lithium foil (Aldrich, ~500 µm thick) counter-electrode. All capacities were calculated with respect to the weighed amount of active material (FeF3·0.33H2O or FeF3·0.33H2O/GO).
Results X-ray diffraction powder pattern of the iron trifluoride hydrate is presented in Figure 1.
RBragg = 3.90 Rp = 7.97 Rwp = 8.27
5
10
15
20
25
30
35
40
45
50
55
60
2 θ / degrees Figure 1. Rietveld refinement of the X-ray diffractogram recorded for iron trifluoride hydrate under Mo Kα1 radiation (λ = 70.93 pm) at RT. Refined cell parameters and atomic positions are reported in Table 1. Experimental data: red circles; calculated fit: black line; residual: blue line and Bragg positions: green ticks.
The iron trifluoride sample can be indexed according to the orthorhombic symmetry of FeF3·0.33H2O as reported by Leblanc et. al.
25
. Despite relatively broadened reflections
typical of nanometric compounds (average calculated crystallite sizes of 19 nm, a TEM image is reported in Figure S2), crystallinity is good and both cell and atomic parameters (Table 1) are well in agreement with the above mentioned study. As a result of drying, water occupancy is almost halved and after Pohl et. al. it is possible to calculate a water content of 0.18 molecules per formula unit 20. While multiple refinement iterations took into account various possibilities such as the 6 ACS Paragon Plus Environment
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presence of fluorine vacancies
16
, the presence of rhombohedral FeF3
26
anhydrous HTB-type FeF3 , the presence of FeF3·0.5H2O
27
20
, the presence of
or the presence of oxides (such
as α-Fe2O3), none gave acceptable results leading to the conclusion that, save partial F-OH substitution (see below), the obtained trifluoride can be considered as a pure single phase. Table 1. Rietveld refinement parameters for FeF3·0.33H2O obtained by fluorolytic sol-gel. Space group Cmcm, lattice parameters: a = 742.3(1) pm, b = 1280.3(1) pm, c = 753.7(1) pm. Atom
Wyckoff
x
y
z
Biso (Å2)
Occ.
Fe1 Fe2 F1 F2 F3 F4 O (H2O)
4b 8d 8f 16h 4c 8g 4c
0 0.25 0 0.176(1) 0 0.200(2) 0
0.5 0.25 0.194(1) 0.394(1) 0.518(1) 0.224(1) -0.010(4)
0 0 0.535(2) 0.043(1) 0.25 0.25 0.25
0.58(2) 0.58(2) 0.96(8) 0.96(8) 0.96(8) 0.96(8) 27.7(8)
1.0 1.0 1.0 1.0 1.0 1.0 0.54(1)
IR spectra of the fluoride sample shows bending and stretching signatures of structural water and -OH groups (Figure S3). Signals centered at 1111 and 3627 cm-1, can be assigned to the bending and stretching modes of structural hydroxyl groups, respectively, while the band centered at 1617 cm-1 along with the wide feature in the region 2600-3800 cm-1 can be attributed to bending and stretching modes of structural water molecules. The presence of structural hydroxyl groups in FeF3·0.33H2O is not surprising and was already detected by other authors 16,28,29. Their presence is a direct consequence of the synthesis method used herein, which was firstly developed to induce the formation of both Lewis (surface Fe) and Brønsted (OH groups) acidic sites on the surface of the ferric fluoride particles. The partial hydroxylation of the fluoride is obtained because of (a) the pre-dehydration of the iron nitrate precursor (Fe(NO3)3·9H2O), which leads to an iron hydroxide-nitrate precursor of general formula Fe(OH)x(NO3)3-x·yH2O, in which OH groups are already present
30
; and (b) the competition
between M-F and M-OH bond formation during synthesis, which eventually leads to a partial substitution of fluoride anions by OH groups
15
. Moreover, iron fluorides are known to be
highly hygroscopic compounds, and can retain relatively high water contents. For our sample, a 19.5% weight loss related to water was recorded by TGA (Figure 2).
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TG
0.4 1.8
80
0.2
-2.9
DTG
0
m =18 60
-0.2
exo
-0.4 40
-0.6 m = 19
20
-0.8
m = 17
-1.0
0 100
200
300
400
500
-1.2 600
1.6 1.4 1.2 1.0 0.8 0.6
Ion current / pA
-16.6
DTG / wt%min-1
100
TG / wt%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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0.4 0.2 0
Temperature / °C Figure 2. Thermogravimetric analysis of the iron trifluoride hydrate. Mass spectrometry results are also reported.
This value is considerably higher than those reported by other authors13,15,16,31 and is of course much higher than the value of 5 % calculated for the simple extraction of the structural water molecules, therefore it can be associated to adsorbed water. Through mass spectrometry combined to the TG apparatus, the mass fragments m/z = +17, +18 and +19, characteristics of hydroxyls, water and HF, respectively, were detected. The main weight loss occurring in the region 60-240 °C partially overlaps with a second mass loss measured from 240 to 400 °C. Both water loss (adsorbed and structural) as well as dehydroxylation occur in this wide range, whereas HF evolution starts at around 300-320 °C. This temperature is higher than that reported in a previous study 28, where an increased thermal stability has been associated with a reduced amount of neighboring linked -OH groups in the bronze structure. If a structural OH group substituting a fluoride atom is surrounded by one or more other -OH groups, with increasing temperature these two neighboring hydroxides can create an oxo bridge (Fe-O-Fe) and release a water molecule. Iron immediate neighborhood is then destabilized via the weakening of the Fe-F bonds and an increasingly probable release of HF. The iron fluoride presented here should be thus more accurately described as an hydroxyfluoride of general formula FeF3-x(OH)x·0.18H2O. The evolution of the oxidation state and the chemical environment of iron in our sample are reported in Figure 3.
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4.5
4.5
4.0
4.0
UWE / V (vs Li /Li)
3.5
+
+
UWE / V (vs Li /Li)
3.0 2.5 2.0
3.5 3.0 2.5 2.0 1.5 1.0
1.5 0.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
Amount of reacted Li
2.4
0.0
0.5
+
1.0
100
100
90
90
80
80
70
70
70
60
60
60
50
50
50
40
40
40
30
30
30
20
20
20
10
10
10
0 0.0
1.0
1.5
2.0
2.5
2.0
2.5
3.0
3.5
4.0
+
100
Fe(III) Total Fe(II) Fe(0)
1.5 V
90 80
0 0.5
1.5
Amount of reacted Li 2° Discharge
Charge
Discharge
Relative %
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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0 0.0
0.5
1.0
1.5
2.0
Amount of reacted Li
0.0 0.6 1.2 1.8 2.4 3.0 3.6 +
Figure 3. Mößbauer spectra sequentially recorded during the operando experiment (circles) are located on the corresponding electrochemical curves. For every half cycle, relative Fe(III) (black line), total Fe(II) (red line) and Fe(0) (blue line) contributions are reported for each marked circle. Cycling rate C/50.
Every dot on the electrochemical curve is associated to a collected Mössbauer spectrum for a total of 106 spectra; each of them represents the average reaction of 0.08 Li+. Two sections are distinctly visible during the first discharge: the first one, ending around 2 V, represents the reaction of 1 Li+ in agreement with a solid solution insertion mechanism, while the second one proceeds for a further 1.4 Li+ and represents the conversion reaction. These two distinct features are barely recognizable during charge, and have been attributed to the phase transition from the rock salt phase (similar to α-LiFeO2) present at the end of discharge into one of the rutile phases predominant at the end of charge 17. However, the charge process has a tendency towards a featureless slope, as it will be discussed later for samples submitted to prolonged cycling. In the window 4.5-1.5 V, the electrochemical signature of the second discharge is similar to that of the initial lithiation, but with slightly shorter reaction pseudoplateaus, sign of the partial inactivation of the fluoride. Further conversion can be achieved 9 ACS Paragon Plus Environment
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when decreasing the potential to 0.75 V for a total reaction of 3.7 Li+ which would account for a partial irreversible reaction of lithium with the electrolyte, with the formation of a solid electrode interphase (SEI) layer 32. The evolution of the relative intensity of the spectral areas obtained from the fitting of the 57
Fe Mössbauer spectra are also reported for every recorded spectrum. Even though three
different spectral components were used to fit the contribution of divalent iron, only the overall Fe(II) content is shown in Fig. 3. Selected spectra collected during the first discharge are presented in Figure 4, while additional spectra for the charge and the second discharge are presented in the Supporting Information (Figure S4). 102 99 96 93 +
90
Relative transmission / %
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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+
0 Li
0.5 Li
87 102 99 96 93 90
+
1 Li
1.3 Li
+
87 102 99 96 93 +
90
+
2 Li
2.4 Li
87 -4
-3
-2
-1
0
1
2
3
4 -4
-3
-2
-1
0
1
2
3
4
-1
Velocity / mm⋅s
Figure 4. Selected Mössbauer spectra recorded during the first discharge. Subspectra are reported in black for Fe(III), in red for Fe(II) (all components) and in blue for Fe(0).
The shape of the initial Mössbauer spectrum for the iron trifluoride hydrate before the discharge (OCV conditions, 0 Li+, Figure 4) is in good agreement with many reported spectra for the HTB hydrated structure (see Table 2) with a broad and asymmetric doublet centered at around 0.4 mm·s-1. 10 ACS Paragon Plus Environment
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Table 2. Room temperature 57Fe Mössbauer hyperfine parameters for HTB hydroxyfluoride obtained during the operando measurements. The parameters of relevant reference compounds are also reported for comparison purposes. Note: NPs = nanoparticles; EOD = end of first discharge; EOC = end of charge. δ ∆QS Γ (mm·s-1) (mm·s-1) (mm·s-1)
Area (%)
Iron type
0.45(1)
0.51(3)
0.51(4)
86(4)
Fe(III), more symmetric environment
0.31(6)
1.1(1)
0.51(4)
14(4)
More asymmetric Fe environment
0.21(1)
0.64(2)
0.49(2)
-
Fe(III)
HTB-FeF3·0.33H2O 25
0.44(1)
0.64(1)
0.50(1)
-
Fe(III)
HTB-FeF3·0.33H2O
34
0.44(1)
0.64(1)
0.51(1)
-
Fe(III)
HTB-FeF3·0.33H2O
4
0.45(3)
0.58(3)
0.50(3)
-
Fe(III)
0.46
0.17
0.47
-
Fe(III)
Pristine electrode material (this work) “FeF3·H2O”
33
HTB-FeF3 anhydrous
10
HTB-FeF3 anhydrous/hexagonal FeF3 mixture 20
HTB-FeF2.2(OH)0.8-xOx/2x/2 16
α-LiFeO2 35
FeF2 nanoparticles 36 FeOF
37
FeII1-xFeIIIxOxF2-x
10
FeOF 38
+
Discharge to 0.5Li
+
Discharge to 1.3Li
0.47(1)
0.54(18) 0.67(10)
62
HTB-FeF3·0.33H2O
0.50(2)
0.22(2)
0.46(7)
28
HTB-FeF3 anhydrous
1.14(2)
2.56(3)
0.40
7
FeF2
0.46(1)
0.01(3)
0.41(5)
3
FeF3
0.44
0.61
0.30
-
Fe(III), reported values averaged from a QS distribution.
0.32(1)
0.60(1)
0.57(3)
-
Fe(III), rock salt structure, pristine sample
0.32(1)
0.70(1)
0.58(3)
-
sample discharged at 1.5 V
1.27
1.97
0.55
25
Fe(II), rutile structure, around 10 nm particles, NPs surface
1.29
2.73
0.35
75
NPs bulk
0.17
1.18
n/a
-
Fe(III), rutile structure
0.39
0.85
n/a
46
Fe(III), rutile structure, more asymmetric Fe environment
0.44
0.52
n/a
54
More symmetric Fe environment
0.45
0.76
n/a
-
Fe(III), amorphous
0.45(1)
0.51(3)
0.51(4)
0.31(6)
1.1(1)
0.51(4)
55
Fe(III)
1.20(1)
2.14(2)
0.62(2)
45
Fe(II)-1
0.45(1)
0.51(3)
0.51(4)
0.31(6)
1.1(1)
0.51(4)
10.6
Fe(III)
1.20(1)
2.14(2)
0.62(2)
35.6
Fe(II)-1
1.06(1)
1.78(2)
0.62(2)
43.6
Fe(II)-2
1.11(3)
0.70(4)
0.62(2)
3.1
Fe(II)-3
0.12(2)
0.73(4)
0.55(5)
0.00(1)
-
0.55(5)
7.1
Fe(0) NPs surface Fe(0) NPs bulk
11 ACS Paragon Plus Environment
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EOD
EOC
Page 12 of 25
1.20(1)
2.14(2)
0.62(2)
5.2
Fe(II) 1
1.06(1)
1.78(2)
0.62(2)
19
Fe(II) 2
1.11(3)
0.70(4)
0.62(2)
12.7
Fe(II) 3
0.12(2)
0.73(4)
0.55(5)
0.00(1)
-
0.55(5)
0.40(1)
0.86(2)
0.62(4)
93
Fe(III) cycled
1.20(1)
2.14(2)
0.62(2)
7
Fe(II)-1
63.1
Fe(0) NPs surface Fe(0) NPs bulk
The observed spectrum, fitted using only two spectral components with quadrupole splitting (∆QS) of 0.51 and 1.1 mm·s-1 and with relatively large linewidths, is the result of a distribution of local environments around the two crystallographically distinct Fe atoms which is produced by a non-homogeneous variation of the electric field gradient induced by F, OH and H2O ligands
4,16,34
. The two wide components thus account for two different distributions of
quadrupole splitting, in agreement with the results recently reported by Duttine et al.,16 who showed a major distribution centered at around 0.6 mm·s-1 (with a small contribution at 0.25 mm·s-1), followed by a second one centered at around 1.2 mm·s-1. If the quadrupole splitting is related to the symmetry of charges, the isomer shift is more related to the electronegativity of ligands. The difference in isomer shift of the two Fe components (0.45 and 0.31 mm·s-1 respectively) is an indication of the fact that if most of the iron (86%) is in a symmetrical environment surrounded by fluoride anions, some of the octahedrons (14%) bear –OH ligands which affect the symmetry of charges (higher quadrupole splitting) with a lower electronic pull on the central Fe atoms (the lower electronegativity of the –OH group results in a smaller isomer shift). The same effect has been shown to occur in a similarly F/OH substituted structure such as FeSO4F1-yOHy also employed as a cathode material for Li-ion cells
39
.
During the discharge, a progressive increase of a Fe(II) component is observed for the whole duration of the insertion reaction in the region between 3.4 and 2.3 V, with the consequent reduction of Fe(III), in agreement with the occurring of a simple insertion reaction as previously reported by Li et al.12. However, the reaction does proceed above the limit of 0.66 Li+, which corresponds to the maximum achievable content given the presence of 0.33 structural water molecules occluding the hexagonal channels the HTB structure possesses. This result is similar to that observed for the defective sample HTB-FeF2.2(OH)0.8-xOx/2x/2 reported by Duttine et al. after annealing at 300 °C
16
, with the notable exception that our
sample was not intensively heat treated, but only withstood a drying process at 100 °C under primary vacuum for 2h. Under these conditions, the pristine hydrated fluoride might have lost most of its structural water during the final drying phase as TGA experiments seem to suggest 12 ACS Paragon Plus Environment
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The Journal of Physical Chemistry
and also considering that water molecules have a higher mobility in HTB-Fe fluoride with respect to other HTB fluorides such as AlF3 28. In fact, a considerable ion current is recorded for m/z = +18 already at 100 °C (Figure 2). A further consideration needs to be done on the content of Fe(III), which is not fully consumed at the end of the insertion reaction (Figure 3). It is instead necessary to reach 1.8 Li+ (1.8 V) to achieve the complete reduction of Fe(III) to Fe(II) and Fe(0), again in agreement with the behavior observed for the defective HTBFeF2.2(OH)0.8-xOx/2x/2
17
. The Fe(0) component appears as soon as the conversion reaction
starts around 2 V, and its appearance is parallel to the progressive decrease of the overall amount of Fe(II). This reaction is not yet complete at 1.5 V, and a final 60/40 ratio is obtained for the two species respectively. The Mössbauer spectrum obtained at the end of discharge (2.4 Li+, Figure 4) is dominated by two paramagnetic components: a single line with an isomer shift of 0 mm·s-1 and a quadrupole doublet with a slightly more positive isomer shift, which are usually attributed to the formation of iron metal superparamagnetic nanoparticles. The quadrupole split component, in particular, was attributed to the atoms at the surface of iron metal nanoparticles, which are surrounded by a non-symmetric environment 40,41. Similar results were obtained also for iron metal nanoparticles embedded in silica gel 42, or after the reduction of nanometric FeF2 (cf. Table 2) 36. During charge (Figure 3), a non-negligible amount of Fe(III) percentage is immediately formed as soon as the cell potential starts to increase, and the spectral contributions of Fe(III) and Fe(II) grow together at the expenses of the components of zerovalent iron until a turning point is reached after 1.25 Li+ are extracted (3.1 V). As the potential continues to increase, Fe(II) then starts to be also oxidized to Fe(III), which finally accounts for 90 % of the spectral area at the end of the charge (4.5 V). Fe(0) is also not fully consumed until at least 1.5 Li+ are extracted at 3.4 V, which is a considerably advanced state of charge. Now, according to the published literature, charging of the oxyfluoride electrode results into the formation of highly distorted phases exhibiting low range order, which can be described by a rutile-type structure. In the case of FeOF, the end product would be a 70/30 mixture of a rutile phase similar to FeF2 and of a rock salt phase similar to α-LiFeO2 (with a possible partial substitution of fluoride anions with oxide in the two phases) xOx/2x/2
19
. On the other hand, for HTB-FeF2.2(OH)0.8-
two possible rutile-like phases were suggested for the charged electrode: one more
similar to FeF2, and a second possibly more similar to FeOF 17. Both studies suggest a higher amount of Fe(II) than of Fe(III) in the charged state, in disagreement with our observation which point to more than 90% of Fe(III). It is possible to observe that the Fe(III) component in the spectrum recorded at the last point of charge (Table 2 and 0.3 Li+, Figure S03) can be 13 ACS Paragon Plus Environment
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Page 14 of 25
fitted with just one doublet. The asymmetric character of the initial spectrum is lost, in line with a more homogeneous environment surrounding the Fe atoms; moreover, the average quadrupole splitting grows to 0.86 mm·s-1, a value very close to that of one of the spectral components observed by Louvain et al. for non-stoichiometric FeOF 10. There seems to be no apparent relation to FeF2, as it possesses much larger ∆QS values, nor to α-LiFeO2 as it seems not to have a sufficient splitting (see Table 2). As regards the isomer shift, its value is lowered to almost half value between those of the two components initially recorded for the pristine material: 0.40 mm·s-1 at the end of charge against 0.45/0.31 for the pristine fluoride. This is also an indication of a more widespread distribution of the –OH groups and of the loss of long-range structural order. Thus, at the end of the charge, the electroactive material would be mostly composed of a low-crystallinity phase bearing similarities to FeOF with the addition of a second phase (see further discussion for the attribution of Fe(II)-1 component). The electrochemical signal recorded during the second discharge (Figure 3) matches relatively well that recorded for the initial one, in spite of two main differences. The first and more evident one, is that a lower amount of lithium is reacted during both the insertion and the conversion processes when considering a cut-off potential of 1.5 V. A less obvious difference can be spotted through the analysis of the Mössbauer relative spectral contributions. In fact, Fe(III) is reduced much less rapidly than during the first discharge, and its content barely reaches a null value at the cut-off voltage of 1.5 V. Moreover, while a Fe(0) to Fe(II) ratio of 60/40 was recorded at the end of the first discharge, the amount of Fe(II) exceeds that of Fe(0) at 1.5 V, and only a deeper discharge (down to 1 V and 2.7 Li+) allows reaching the same ratio of Fe(0) to Fe(II). Since an insertion step is still observed at the beginning of the discharge, the initial HTB structure should also be at least partly retained. The value of the potential plateau, however, is roughly 0.3 V higher than that of first discharge, in line with a possible lower amount of oxide anions around the iron atoms, as a result of the anionic partitioning occurring at the end of the first discharge. Indeed, a rearrangement to a more fluorinated environment would result in a larger number of Fe-F bonds with a strongly ionic character, raising the reaction potential. A possible alternative and more convincing explanation, however, is a decrease in polarization caused by the decrease of the size of crystalline domains (for example, the size of the coherence domains at the end of the charge for FeOF was reported to be around 2.5 nm for Fe(0), 5 nm for the rock salt α-LiFeO2-like phase and less than 1 nm for the fluorine rich rutile phase 19), allowing for a much easier ionic diffusion into the HTB channels. During the second discharge, moreover, the full reaction of Fe(III) should not involve more 14 ACS Paragon Plus Environment
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than three exchanged electrons, whereas at 0.75 V slightly more than 3.7 Li+ are exchanged. Even though some electrolyte decomposition leading to the formation of SEI should to be taken into account in the potential region below 1 V, it is unlikely that such process produces a consumption of lithium as large as 0.7 Li+. Other parasite reactions irreversibly trapping part of the lithium and iron, and thus inactivating the compound might thus be responsible for this extra-capacity observed during the conversion reaction. The relative resonance area of the three Fe(II) subspectra, consistently increasing and decreasing in intensity during the electrochemical reaction, are reported in Figure 5. 100
Relative %
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Discharge
Charge
100
100
Fe(II) - 1 Fe(II) - 2 Fe(II) - 3
90
90
80
80
70
70
70
60
60
60
50
50
50
40
40
40
30
30
30
20
20
20
10
10
10
0 0.0
90
1.0
1.5
2.0
2.5
1.5 V
80
0 0.5
2° Discharge
0 0.0
0.5
1.0
1.5
2.0
Amount of reacted Li
0.0 0.6 1.2 1.8 2.4 3.0 3.6 +
Figure 5. Relative Fe(II)-1 (orange line), Fe(II)-2 (green line) and Fe(II)-3 (dark blue line) contributions corresponding to spectra recorded during the operando experiment.
The subspectra, centered around 1.1 mm·s-1, are characterized by very different quadrupole splittings, reflecting marked changes in the coordination environment of the Fe atoms. The spectrum labelled Fe(II)-1, (Figure 5, orange curve), is probably related to the HTB structure as its intensity closely follows that of the main Fe(III) component. Fe(II)-1 constantly grows and it reaches a peak in the region 0.5-0.6 Li+ at the middle point of the first discharge pseudoplateau, according to the maximum intake by insertion of 0.66 Li ions. Fe(II)-1 can be thus attributed to Fe(II) in octahedral coordination with a rather symmetric environment (negligible contributions from the F/OH exchange), as its hyperfine parameters are comparable to those of pure FeF2 nanoparticles taking into account differences coming from bronze and rutile crystal structures (Table 2, bulk component). As the insertion reaction proceeds, the chemical environment changes so that a new component, Fe(II)-2, arises (figure 15 ACS Paragon Plus Environment
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Page 16 of 25
5, dark green curve),representing 30% of the spectral area at the end of the electrochemical plateau (1 Li+, Figure 4). In agreement with the literature
17
, this component can be closely
related to the rutile-like structure of FeF2 (Table 2, surface component) with the mismatch probably deriving from the very reduced crystalline periodicity (possibly less than 1 nm), as mentioned earlier. A third component, Fe(II)-3 (Figure 5, dark blue curve), arises closely following the evolution of Fe(0) during both the first discharge and the subsequent charge. The value for the quadrupole splitting of this component (0.70(4) mm·s-1) is satisfactorily close to that of a discharged α-LiFeO2 (0.70 mm·s-1), that of an amorphous FeOF (0.76 mm·s1
) and that of the formed Fe(0) nanoparticles (0.73 mm·s-1). While there can be a certain
confidence in stating that Fe(II)-3 atoms are found in an environment which would be neighboring the metallic nanoparticles formed during the conversion reaction, it is less obvious to attribute this component to either the rock salt α-LiFeO2 or the rutile FeOF phases. Even if the reported literature describing the electrochemical behavior of iron oxy/hydroxyfluorides suggests that a rutile/rock salt mixture is formed at the end of the conversion reaction 17,19, the only two references we could find (cf. Table 2) cannot differentiate between them since both refer to Fe(III) compounds. Upon charge, Fe(II)-2 and Fe(II)-3 decrease in intensity to reform Fe(II)-1, thus confirming the (at least partial) reformation of the HTB structure still containing some lithium at the end of the process in order to stabilize iron in the Fe(II) state. However, this process is not linear as both Fe(II)-2 and Fe(II)-1 go through maxima before oxidation reduces their relative amounts. In particular, Fe(II)-2 reaches its maximum intensity at 2.6 V (1.8 Li+, Figure 3) at the beginning of the first oxidation plateau, whereas Fe(II)-1 peaks at 3.1 V (1.2 Li+, Figure 3) starting the second oxidation plateau. This latter point also marks the progressive increase of the content of Fe(III) which remains almost constant in the region 2.6-3.1 V (Figure 3). Such a behavior is a good indication of a preferential oxidation pathway where the reconversion of Fe(0) directly forms Fe(III) and Fe(II)-2 in the region 1.5-2.6 V. With the potential increase in the region 2.6-3.1 V, the preferential reaction is the formation of the Fe(II)-1 species, whereas the Fe(III) component starts increasing again in intensity only on the second oxidation plateau above 3.1 V, at the expenses of the Fe(II)-1 component. What is described here is of course a tendency, as the three processes largely overlap; yet, this tendency is not surprising since different lithiation/delithiation pathways were already predicted for the hexagonal FeF3 43. As described earlier, the redox processes observed during the first discharge are essentially replicated during the second discharge. However it is worth noting that the Fe(II)-2 component becomes dominant with respect to Fe(II)-1 and, as after prolonged cycling the 16 ACS Paragon Plus Environment
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electrochemical curve is a featureless slope (see description below), it is reasonable to assume that the short-order rutile analogue represents the main active species on the long term performances. In order to better analyze the electrochemistry of the fluoride material after prolonged cycling, a composite with graphene oxide was synthesized. The presence of an electron conducting agent greatly enhances the performance of the active material when compared to the pure fluoride powder, without introducing additional side reactions in the analyzed potential window (Figure S5). In Figure 6a, the first ten cycles recorded for a constant current measurement at a C/10 rate (1 Li+ reacted in 10 h) are reported. 4.5 10
4.0
th
450
(a)
st
1
(b)
st
1
375
+
UWE / V (vs Li /Li)
-dx/dV / V
-1
3.5 3.0 2.5 2.0 1.5 0
300 225 150 75 th
0 1.5
100 200 300 400 500 600 700 -1
10 2.0
2.5
3.0
3.5
4.0
4.5
+
Capacity / mAh⋅g
UWE / V (vs Li /Li) 4.5
(d)
4.0
+
(c)
UWE / V (vs Li /Li)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
OCV EOD EOC EO2°D
Li
3.5 3.0 2.5 2.0 1.5
12
16
20
24
28
32
36
40
0.0
2θ / degrees
0.1 0.2
0.3
0.4
0.5 0.6
0.7
+
Normalized Li content
Figure 6. First 10 galvanostatic cycles recorded for the iron trifluoride hydrate/graphene oxide composite at C/10 rate (a) with the derivative curves corresponding to charge half cycles (b). Derivative curve of the 55th charge is reported in red color. Selected operando XRD patterns for the composite material (c) and normalized Li+ content for the cycles 45 to 55 when the composite is cycled after a first discharge down to 1.6 V (red curve) or down to 1 V (black curve) (d).
The electrochemical activity related to the conversion part of the curve, rapidly decreases until no capacity is delivered in the region 1.8-1.6 V. Thus, discharge becomes mostly characterized by a single pseudo-plateau centered on 3 V. The same trend concerns the oxidation step occurring at around 2.6 V during charge, which 17 ACS Paragon Plus Environment
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Page 18 of 25
disappears completely at the 10th cycle leaving a pseudo-plateau centered on 3.2 V. This phenomenon is even more evident by observing the derivative curves (Figure 6b). As it was discussed for the Mössbauer experiments, the 2.6 V plateau marks the preferential transformation of Fe(II)-2 species into Fe(II)-1 or, in other words, the rearrangement of Fe octahedra from a poorly ordered rutile phase to the initial bronze phase. There is no more trace of this transformation after only 10 cycles and after 55 cycles only an enlarged peak centered at around 3.2 V can be recognized. The operando Mössbauer spectroscopy study showed that the conversion process is not complete at the end of the first discharge, that there is a partial reconversion to the initial bronze structure and also a partial inactivation of the material. With more prolonged cycling, the conversion plateau during discharge progressively disappears as well as the first charge pseudo plateau marking the transformation of Fe(II)-2 (rutile) into Fe(II)-1 (HTB). The initially crystalline ferric fluoride transforms into an essentially amorphous compound with iron octahedra loosely arranged into rutile structure. Even if a reaction is always visible at potentials above 2.8 V (Figure 6b), it is interesting to notice that there is no sensible potential evolution with cycling. This observation is also puzzling considering that there is a structural change associated to the cycling process: a mixture of amorphous (rutile)/semicristalline (HTB) evolves into fully amorphous state for the first ten cycles without the expected strong affection on the reaction potential. Operando XRD patterns measured during the first and a half cycle (Figure 6c and Figure S6) agree with the picture depicted above: a first insertion step results into a shift of the peaks relative to the HTB structure to lower angles, structural collapse follows until only a very broad reflection in the region 22 < 2θ
