Evidence for Anionic Vacancy and Electrochemical Perfor - American

Article pubs.acs.org/cm

Tailoring the Composition of a Mixed Anion Iron-Based Fluoride Compound: Evidence for Anionic Vacancy and Electrochemical Performance in Lithium Cells Mathieu Duttine,†,‡,§ Damien Dambournet,‡,§ Nicolas Penin,† Dany Carlier,† Lydie Bourgeois,∥ Alain Wattiaux,† Karena W. Chapman,⊥ Peter J. Chupas,⊥ Henri Groult,‡,§ Etienne Durand,† and Alain Demourgues*,† †

CNRS, Univiversité Bordeaux, ICMCB, UPR 9048, F-33600 Pessac, France Sorbonne Universités, UPMC Université Paris 06, UMR 8234, PHENIX, F-75005, Paris, France § CNRS, UMR 8234, PHENIX, F-75005, Paris, France ∥ Groupe de Spectroscopie Moléculaire, Université de Bordeaux, ISM, 351 cours de la Libération, 33405 Talence Cedex, France ⊥ X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States ‡

S Supporting Information *

ABSTRACT: Microwave-assisted synthesis allows stabilizing Febased fluoride compounds with hexagonal tungsten bronze (HTB) network. The determination of the chemical composition, i.e., FeF2.2(OH)0.8·(H2O)0.33, revealed a significant deviation from the pure fluoride composition, with a high content of OH groups substituting fluoride ions. Rietveld refinement of the Xray diffraction data and Mössbauer spectroscopy showed that the partial OH/F substitution impact on the structure (interatomic distances, angles, and so on) and the local environment of iron (isomer shift and quadrupole splitting distribution). The thermal behavior of the hydroxyfluoride compound has been thoroughly investigated. From room temperature to 350 °C under Ar flow, the HTB-type structure remains stable without any fluorine loss and only water departure. At T > 350 °C, the structure started to collapse with a partitioning of anions leading to α-FeF3 and α-Fe2O3. Within 200 °C ≤ T ≤ 350 °C, the chemical composition can be tuned with different contents of OH−/O2− and structural water. By an adequate thermal treatment, it has been shown that anionic vacancies formed by dehydroxylation reaction could be stabilized within the HTB network yielding a compound containing three different anions, i.e., FeF2.2(OH)0.8−xOx/2□x/2. XRD Rietveld analysis, atomic pair distribution function, and Mössbauer spectroscopy confirmed the formation of under-coordinated iron FeX5□1 (X = O2−, F−, and OH−) atoms. Different compositions have been prepared by thermal treatment at T ≤ 350 °C and their electrochemical properties evaluated in lithium cell. Structural water seems to block the diffusion of lithium within the hexagonal cavities. Increasing the content of anionic vacancies significantly improves the reversible capacity emphasizing a peculiar role on electrochemical properties. Pair distribution functions obtained on lithiated and delithiated samples indicated that the HTB network was maintained (in the 2− 4.2 V voltage range) during the intercalation processes.

■

INTRODUCTION Within the frame of the development of electric vehicles, lithium−polymer batteries have been considered over several years as suitable batteries for transportation applications.1 A lithium−polymer battery used a lithium metal negative electrode, a solid polymer electrolyte, and a positive electrode such as the olivine LiFePO4.2 In spite of the use of metallic lithium, this electrochemical storage device is suggested to provide enhanced safety features due to the absence of flammable organic electrolyte along with a stable positive electrode material LiFePO4 phase. However, the use of a lithiated positive material required the cell to be charged prior to operation. A Li-free positive electrode would allow suppression of this step, minimizing the lithium content of the cell. Alternative candidates © 2014 American Chemical Society

to LiFePO4 should provide similar/higher energy density, stable operation upon electrochemical reaction, and low cost. Based on the preceding criteria, iron fluoride appeared as a potential material for positive electrode. Indeed, a multiple electron transfer can occur with transition metal-based fluorides; i.e., the metal fluoride can be either partially or fully reduced involving the exchange of one or three electrons per 3d-metal, respectively. Thus, in the high-potential region (down to about 2 V vs Li+/Li), the insertion/extraction of Li+ into the host lattice and the reduction of Fe3+ into Fe2+ occur. One electron is exchanged Received: April 17, 2014 Revised: May 23, 2014 Published: May 30, 2014 4190

dx.doi.org/10.1021/cm501396n | Chem. Mater. 2014, 26, 4190−4199

Chemistry of Materials

Article

F−. Recently, the preparation of well crystallized iron-based fluoride compound was briefly introduced.22 Herein, we report a detailed investigation of the structural features of such a compound having the HTB-type structure. The impact of the OH groups/F substitution on the structural features and thermal behavior are thoroughly investigated by combining X-ray diffraction, PDF, thermogravimetric analysis, Mössbauer spectroscopy, and electrochemical properties vs Li+ insertion/ extraction. For the first time, it is shown that anionic vacancies could be stabilized by a controlled thermal treatment. Finally, this allows correlation of the lithium intercalation properties with the thermal behavior of this compound.

during the electrochemical reaction and the theoretical specific capacity corresponding to this reaction is 237 mAh/g whereas a theoretical capacity of only 170 mAh/g is obtained with LiFePO4. Below 2 V vs Li+/Li, FeF3 can support conversion reaction; i.e., Fe3+ can be reduced to the metal. Nevertheless, the structural modifications which usually occur during the conversion reaction explain why much attention has been paid to the use of metal fluorides mainly in the high-potential region, as in the present work. The use of iron fluorides as host electrodes for rechargeable Liion batteries was considered in the late 1990s.3 High band gaps of fluorides inducing low electronic conductivity as well as large voltage hysteresis impeded their utilization in commercial cells. However, the use of metal fluorides as electrode materials is more and more popular for two main reasons: the first one is due to the possibility to synthesize nanosized materials which enhanced the electric contact between grains and so decreased the resistivity; the second, to overcome these limitations, Badway et al. proposed the use of a carbon−metal fluoride nanocomposite prepared by high-energy ball milling4 and Di Carlo et al. FeF3 nanoparticles deposited onto partially reduced graphene oxide.5 Furthermore, the incorporation of oxygen within the framework lowered the polarization and improved the energy efficiency and cycle life of the cell.6 More recently, Yamakawa et al.7 investigated by solid state nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and pair distribution function (PDF) analysis the Li intercalation process within the ReO3-type network of a FeF3/C nanocomposite obtained by ball milling. After Li intercalation, the ReO3 network collapses and a new phase appears which contains local structural features similar to those of lithiated rutile structure with an iron oxidation state close to 2.5 after the first discharge. Iron fluoride compounds can adopt several networks including the VF3 (rhombohedral; space group, R3̅c), the pyrochlore (cubic; space group, Fd3m ̅ ), or the hexagonal-tungsten-bronze (denoted as HTB; orthorhombic; space group, Cmcm) type.8−10 The latter consists of a corner-shared FeF6 octahedra forming hexagonal section along the c-axis (Table 1). Structural waters are located within the tunnels and can be thermally removed at low temperature without any structural collapse.10 A reversible electrochemical intercalation of lithium has been demonstrated for nanosized FeF3·(H2O)0.33 prepared by using ionic liquids.11 The authors reported that the first discharge/charge curves for nanosized FeF3·(H2O)0.33 showed irreversible plateau regions highlighting subtle structural rearrangement. Subsequent cycles showed sloping curves which are ascribed to a solid-solution behavior with the accommodation of 0.66 Li+ (150 mAh/g) per formula unit.12 During the formation of transition metal fluorides, a competition between M−F and M−OH bond formation may occur eventually leading to a large content of OH groups substituting fluoride anions. Detailed characterizations should be performed in order to ensure the chemical composition of the metal fluoride.13,14 Importantly, the deviation from the ideal composition MF3 to MF3−x(OH)x impacted the structural features, the thermal behavior, and the targeted physicochemical properties. Therefore, dealing with metal fluorides and especially nanosized metal fluorides prepared by Chimie Douce required fine structural characterizations as slight changes within the composition might strongly affect the material’s properties.15 In our previous studies, several nanosized fluoro-compounds have been prepared by microwave-assisted synthesis.16−21 This technique often led to the stabilization of OH groups substituting

■

EXPERIMENTAL SECTION

In a typical procedure, a defined volume of hydrofluoric acid (4 mL of HF, Rectapur, 40%) was added to 46 mL of a molar aqueous solution of Fe3+ nitrates nonahydrate (Fe(NO3)3·9H2O) in order to have a F/Fe molar ratio equal to 2. The obtained clear solution was then poured in a closed vessel (XP-500 model) which was subsequently placed in a microwave-accelerated reaction system MARS5 (CEM Corp.) operating at 2.45 GHz with a power supply ranging from 300 to 1200 W. The multimode instrument was equipped with temperature and pressure monitoring devices (Pmax = 55 bar; Tmax = 220 °C). The microwaveassisted synthesis of iron fluorides consists of successive steps including activation and drying processes. For the former, the solution is heated to 90 °C within 5 min and kept at this temperature for 30 min. Thereafter, the solution was cooled to room temperature. The drying step, i.e., solvents removal, is performed at 90 °C under nitrogen gas stream and primary vacuum (∼500 mbar). The obtained powder is then washed by ultrafiltration using ethanol and nitrogen gas flow. Finally, the powder is outgassed at 110 °C under vacuum for 2 h. The samples have been annealed under Ar flow in a tubular furnace at 200, 300, or 350 °C during 12 h (heating/cooling rate of 2 °C/min) on the basis of thermogravimetric analysis. During the thermal treatment the color of the Fe-based oxyfluorides changes drastically from green (starting material) to red (sample annealed at 350 °C) through orange after annealing at intermediate temperature (200 °C). Fluorination of ironbased compound was achieved using pure F2 at 250 °C during 5 h. Thermogravimetric analysis (TGA) was performed using Setaram Setsys evolution that was coupled with a mass spectrometer (MS; Pfeiffer Omnistar) allowing the detection of HF and water departures. Samples were heated from room temperature up to 600 °C under helium or air atmosphere (heating rate, 5 °C/min). The departure of water and hydroxyl groups starts before the fluorine species evolution. Powder diffraction patterns were recorded on PANalytical X’Pert (Co Kα or Cu Kα1 radiation) diffractometers in a Bragg−Brentano geometry (θ−2θ) using hygroscopic sample holders. The investigated 2θ range was 10−110° with a step of 0.008°. The X-ray powder pattern was refined using Le Bail or Rietveld method as implemented in the FULLPROF software.23 In situ infrared spectroscopy measurements were performed using a FTIR Thermo Optek Nicolet 6700. The spectrometer was equipped with a DTGS detector and a Ge-coated KBr beamsplitter for getting access to the middle-IR range (400−4000 cm−1). The sampling technique chosen was the attenuated total reflection (ATR) which requires no specific sample preparation. A single reflection system with a diamond crystal was used to analyze the sample. It was loaded in a hightemperature cell under vacuum (P < 10−3 MPa) and gradually heated from 30 to 300 °C by steps of 10 or 20 °C with a 10 min stabilization period. High-energy X-ray scattering data, suitable for PDF analysis, were collected at the 11-ID-B station at the Advanced Photon Source (Argonne National Laboratory). After correcting for background and Compton scattering, PDFs, G(r), were extracted from the data using PDFgetX2.24 Refinements of the PDF data were performed using the PDFgui program.25 Refined parameters were the instrument parameters, the scale factor, the lattice parameters, the atomic displacement parameters, and the atomic positions. Lithiated and delithiated samples 4191



Article

were recovered after dissembling the cell. The electrode was washed with dimethyl carbonate (DMC) and the powder electrode was then scraped from the current collector and packed in a Kapton capillary which was sealed to prevent air contamination. Mössbauer measurements were performed at room temperature using a constant acceleration Halder-type spectrometer with a roomtemperature 57Co source (embedded in a Rhodium matrix) in transmission geometry. The absorber thickness was adjusted to have about 10 mg of Fe atoms/cm2. The velocity was calibrated using pure αFe as standard material. The refinement of the Mössbauer spectra was performed using WinNormos software.26 Electrochemical reaction with lithium was evaluated using a Swagelok Li metal cell. FeF3-based electrodes were composed of 75 wt % active materials, 15 wt % acetylene black as the conductive agent, and 10 wt % poly(vinylidene difluoride) as binder and were prepared by hand milling. Aluminum foils were used as the current collector. The area of the electrode was 1.0 cm2 with a typical mass loading around 3 mg. The electrolyte consisted of the commercially available LP30 (1 M LiPF6 dissolved in a mixture of ethylene carbonate and ethyl methyl carbonate (3:7 volume ratio). The cell was cycled between 2 and 4.2 V vs Li+/Li, and the current density used was 50 mA/g. All potentials will be referred to the Li+/Li reference, henceforth.

■

RESULTS AND DISCUSSION 1. Synthesis and Physicochemical Characterizations of Iron Hydroxyfluoride. The synthesis method used consists of a precipitation performed in an aqueous medium assisted by microwave heating (see Experimental Section). Starting from iron nitrate nonahydrate, a careful adjustment of the F/Fe molar ratio to 2 was required to isolate a single phase.22 The X-ray diffraction powder pattern (Figure 1) has been indexed with an

Figure 2. Thermogravimetric and mass spectrometer analyses performed under He of HTB-FeF3−x(OH)x·(H2O)0.33.

apparatus in order to identify the released gaseous species. Only the mass fragments m/z = +17, +18, which are characteristics of the water and OH groups released, were detected through two thermal events identified on both TGA and dTG (first derivative) curves. The measured weight loss thus allowed quantification of the water and OH contents. A weight loss occurring at 100−240 °C was ascribed to the departure of structural water molecules located within the hexagonal tunnels.10 The calculated number of structural water was close to 0.33 H2O similar to that for the reference compound FeF3· (H2O)0.33. The second weight loss was measured from 240 to 410 °C and has been ascribed to dehydroxylation reaction. XRD data recorded on the powder recovered after the TGA showed that the HTB compound has decomposed into α-FeF3 and α-Fe2O3 (Figure 3). The analysis of XRD data using the Rietveld method led to a molar percentage of 16% α-Fe2O3 and 84% α-FeF3, allowing the OH content to be estimated. Thus, the chemical composition FeF2.2(OH)0.8·(H2O)0.33 was determined. The thermal behavior of this hydroxyfluoride comprised three steps, summarized as follows:

Figure 1. Rietveld analysis of the (Cu Kα1) X-ray diffraction powder pattern of the HTB-FeF3−x(OH)x·(H2O)0.33 compound obtained by microwave-assisted synthesis. Refined cell parameters and atomic positions are reported in Table 1. Inset: Scanning electron micrograph.

≈ 240 ° C

FeF2.2(OH)0.8 ·(H 2O)0.33 ⎯⎯⎯⎯⎯⎯⎯⎯→ FeF2.2(OH)0.8 + 0.33H 2O (1) ≈ 410 ° C

orthorhombic symmetry referring to FeF3·(H2O)0.33.10 The structure of the pristine compound obtained after the microwave synthesis will be discussed in more detail in the following. The morphology of the prepared solid consists of agglomerate of hexagonal-shaped particles having submicrometer size (inset in Figure 1). TGA performed under He up to 600 °C (Figure 2) showed an overall weight loss of 11.5% which was significantly higher than expected for a single dehydration reaction (5% for FeF3· (H2O)0.33). Mass spectrometry was combined to the TG

FeF2.2(OH)0.8 ⎯⎯⎯⎯⎯⎯⎯⎯→ FeF2.2O0.4 □0.4 + 0.40H 2O

(2)

≈ 600 ° C

FeF2.2O0.4 □0.4 ⎯⎯⎯⎯⎯⎯⎯⎯→ 0.14Fe2O3 + 0.72FeF3

(3)

Reactions 1, 2, and 3 referred to dehydration, dehydroxylation reactions, and structural collapse, respectively. On the basis of these reactions, several points must be noticed: (i) the presence of a large content of OH groups substituting F− ions and (ii) the stabilization of anionic vacancies (denoted □) in the HTB network leading one to consider under-coordinated Fe3+ ions. In 4192



Article

decrease of the absorbance and shifts toward lower and higher wavenumbers, respectively, is due to the dehydration reaction that is evidenced by the decrease of bending and stretching bands of water (Figure 5). The hydrogen interaction between water molecule and OH groups no longer occurred leading to “free” structural OH groups. At higher temperature (Figure 4), the δ(OH) and ν(OH) bands start to decrease at around 210 °C, highlighting the beginning of the dehydroxylation reaction that continued over the monitored temperature, up to 300 °C. The remaining peaks located at 1050 and 3585 cm−1 show the presence of OH groups within the HTB framework. 2. Structural Features of FeF2.2(OH)0.8·(H2O)0.33 Hydroxy Fluoride. The structural network of FeF2.2(OH)0.8· (H2O)0..33 was analyzed by the Rietveld refinement of the X-ray diffraction powder pattern (Cu, monochromator).23 The starting structural model, was based on the HTB-type FeF3·(H2O)0.33 phase (space group, Cmcm),10 yielding a good fit to the data. In Table 1 are gathered the crystallographic data extracted from the refinement. Iron atoms which occupied two sites (4b and 8d Wyckoff positions) showed heterogeneous Fe−F distances ranging from 1.90 to 1.98 Å. The impact of OH groups on the local environment of iron atoms was further investigated using Mössbauer spectroscopy (Figure 6). The reconstruction of the spectrum of the asprepared compound FeF2.2(OH)0.8·(H2O)0.33 evidenced one single Fe3+ signature. Isomer shift δ and quadrupole splitting ΔEQ values (Table 2) are consistent with trivalent iron ions in octahedral sites and close to those reported by LeBlanc et al.10 or Calage et al.32 for the hydroxyl-free HTB-FeF3·(H2O)0.33. Further analyses (not shown here) confirmed that the apparent dissymmetry of the spectrum is not due to textural effect but is the consequence of both isomer shift and quadrupole splitting distribution (Figure 6). The signal may thus be analyzed as the sum of quadrupole doublets with Lorentzian shape (line width, 0.30 mm/s), isomer shift ranging from 0.41 to 0.45 mm/s and different quadrupole splitting values. Considering that Fe3+ is in a high-spin state and that the 6S ground state has a spherically symmetric distribution of electronic charge, the quadrupole splitting arises mainly from the lattice contribution to the electric field gradient (EFG) through the crystal field produced by ligands at the 57Fe nucleus.33 The heterogeneous Fe−F/OH bonds revealed by XRD analysis and the occurrence of OH− groups substituted for F− ions in the first coordination sphere of Fe3+ ions induce slight variations of the EFG (at iron sites) which are reflected by the multimodal character of the observed ΔEQ distribution. Moreover, the Mössbauer spectrum of FeF2.2(OH)0.8·(H2O)0.33 fluorinated under 100% F2 at 250 °C was reconstructed using a sum of quadrupole doublets centered at 0.477 mm/s with slightly distributed values of quadrupole splitting (average value, ∼0.20 mm/s). The monomodal character of the quadrupole splitting distribution and the shift toward higher isomer shift values indicated a more fluorinated and less distorted environment for iron ions confirming that OH groups perturbed the iron local environment. XRD analysis (not shown here) confirmed that the fluorinated sample still has the HTB-type structure. 3. Structure of Partially Dehydroxylated Fe-Based Oxyfluoride: Stabilization of Anionic Vacancies. According to thermal analysis, dehydroxylation reaction might yield to the formation of anionic vacancies, i.e., under-coordinated iron atoms, within the HTB network. The mechanism of the HTBFeF2.2(OH)0.8·(H2O)0.33 thermal transformation was further

Figure 3. Rietveld analysis of the (Co Kα) X-ray diffraction powder pattern of the compound recovered after the TGA where HTBFeF2.2(OH)0.8·(H2O)0.33 was heated up to 600 °C (cf. Figure 2).

order to confirm the above statements, detailed structural and thermal analyses have been carried out in the following. FTIR spectroscopy recorded during an in situ thermal treatment performed under vacuum (P < 10−3 MPa) from room temperature (RT) to 300 °C was used to follow dehydration and dehydroxylation reactions (Figures 4 and 5). RT FTIR spectrum of HTB-FeF2.2(OH)0.8·(H2O)0.33 is characteristic of a material containing OH groups and water molecules.

Figure 4. Evolution of OH groups bending and stretching modes during the in situ thermal treatment of HTB-FeF2.2(OH)0.8·(H2O)0.33.

The asymmetric absorption band at around 1600−1650 cm−1 and the broad band located between 2800 and 3500 cm−1 are respectively attributed to bending δ(H2O) and stretching ν(H2O) modes of structural water molecules. The two additional sets of bands observed in the 900−1200 and 3500−3700 cm−1 regions are due to the bending δ(OH) and stretching ν(OH) modes of structural hydroxyl groups, respectively.27−29 At RT, these two last modes exhibit several components which reflect various possible environments for the hydroxyl groups (OH in hydrogen interaction with water, linked OH substituting F− ions, or OH located at different F sites in the HTB network).30,31 During the thermal treatment between 50 and 190 °C, the evolution of the δ(OH) and ν(OH) bands (Figure 4), i.e., 4193



Article

Figure 5. FTIR spectra recorded during the in situ thermal treatment of HTB-FeF2.2(OH)0.8·(H2O)0.33 up to 300 °C.

Table 1. Structural Parameters of FeF3−x(OH)x·(H2O)0.33 Hydroxy Fluoride and Representation of the HTB Structure along the aand c-Axes

diffraction technique, PDF analysis and Mössbauer spectroscopy were used on thermally treated samples. The PDF indeed provides local- and intermediate-range structural information independent of the long-range order35 while Mö ssbauer

investigated by selectively annealed the pristine material at 300 and 350 °C (in a tubular furnace for 2 h under Ar). Since the thermal degradation of metals hydroxyfluoride might involve an amorphized component34 undetectable by conventional X-ray 4194



Article

Figure 7. Refinements of the PDFs of HTB-FeF2.2(OH)0.8·(H2O)0.33 as synthesized and annealed at 300 and 350 °C (under Ar). The refinement was performed from 1 to 50 Å. For the sake of clarity, data have been shown from 1 to 20 Å.

Figure 6. Room-temperature 57Fe Mö ssbauer spectra of HTBFeF2.2(OH)0.8·(H2O)0.33 as synthesized and fluorinated at 250 °C (5 h) under F2. Right panels: Quadrupole splitting distributions.

Remarkably, the HTB network is shown to be stable even after a partial dehydroxylation of the framework. In agreement with thermal analysis, the formation of anionic vacancies is suggested to arise from the formation of an oxo-bridge Fe−O−Fe according to the following scheme:

Table 2. 57Fe Mössbauer Parameters Obtained from the Analysis of the Spectra Presented in Figure 6 sample FeF2.2(OH)0.8·(H2O)0.33 as-synthesized fluorinated at 250 °C a

δ (mm/s)

ΔEQ (mm/s)

Γ (mm/s)

0.435a 0.477(1)

0.61a 0.19a

0.30(−) 0.30(−)

Average value.

Although refinements indicated the presence of anionic vacancies, estimated standard deviations on F rate occupancy are high, preventing accurate quantitative analysis. On the other hand, Mössbauer spectroscopy is a sensitive probe to the local environment of iron atoms and was further used to detect and quantify the stabilization of 5-fold coordinated iron atoms within the HTB network. Mössbauer spectra for both samples annealed at 300 and 350 °C recorded at room temperature are displayed in Figure 8. The spectra of annealed iron hydroxyfluorides are mainly composed of three quadrupole

spectroscopy can provide information on anionic environment and coordination mode of iron atoms. PDF data of pristine and thermally treated compounds were refined using a structural model based on the orthorhombic HTB. The refinement of the pristine and the sample treated at 300 °C was successfully refined using a single phase. Note that, similarly to Rietveld analysis, structural parameters obtained from PDF analysis of the pristine showed deviation as compared to the hydroxyl-free compound, confirming the impact of OH groups on the structural features. On the other hand, the refinement of the PDF of the compound thermally treated at 350 °C led to a large reliability factor of Rw ∼ 20.7% with features in the residual curve (Figure 7). This suggested the presence of a second phase with only short-range order, i.e., amorphous/ nanoscale. The features in the residual, associated with this second poorly ordered phase, were closely related to iron oxide. This agrees with Mössbauer experiment performed at high velocity detecting a trace of Fe2O3. The introduction of a second phase based on α-Fe2O3-structural type (R3̅c) led to a significant improvement of the fit from Rw ∼ 20.7 to 13.7%. PDF analysis showed that the HTB-framework started to collapse between 300 and 350 °C. Structural parameters extracted from the refined data (see Supporting Information, Table S1) indicated that thermal treatment induced the stabilization of oxyfluoride−hydroxyfluoride compounds having the following general chemical formula: FeF2.2(OH)0.8−xOx/2□x/2 where □ referred to anionic vacancy. Upon dehydroxylation, the Fe−X chemical bonds become more covalent especially around the Fe1 site. It is worth mentioning that the content and the site location of the anionic vacancies depend on the thermal treatment emphasizing complex structural rearrangement upon annealing.

Figure 8. Room-temperature 57Fe Mö ssbauer spectra of HTBFeF2.2(OH)0.8·(H2O)0.33 annealed at 300 and 350 °C under Ar. Right panels: Quadrupole splitting distributions. 4195



Article

doublets Fe(a), Fe(b), and Fe(c) whose relative proportions are about 55:30:15. The Mössbauer parameters are reported in Table 3. Table 3. 57Fe Mössbauer Parameters Obtained from the Analysis of the Spectra Presented in Figures 8 and 9a δ (mm/s)

ΔEQ (mm/s)

Γ (mm/s)

area (%)

FeF2.2(OH)0.8·(H2O)0.33 Annealed at 300 °C under Ar Fe(a) 0.466(3) 0.17(1) 0.30(1) Fe(b) 0.402(3) 0.66a 0.30(−) Fe(c) 0.343(3) 1.16a 0.30(−) FeF2.2(OH)0.8·(H2O)0.33 Annealed at 350 °C under Ar Fe(a) 0.455(3) 0.17(1) 0.32(1) Fe(b) 0.398(3) 0.64a 0.30(−) Fe(c) 0.350(3) 1.19a 0.30(−) FeF2.2(OH)0.8·(H2O)0.33 after TGA under He (Tmax = 350 °C) Fe(a) 0.467(3) 0.16(1) 0.28(1) Fe(b) 0.408(3) 0.61a 0.30(−) Fe(c) 0.347(3) 1.01a 0.30(−) Fe3(PO4)O3 (Modaressi et al.36) 5-fold coord Fe3+ 0.34 1.13 0.28 a

58 30 12 52 33 15 58 12 30 100

Average value of quadrupole splitting distribution.

The Fe(a) signal with the higher isomer shift value (0.45−0.47 mm/s) and the lower quadrupole splitting value (0.17 mm/s) is similar to those observed for the pristine material fluorinated at 250 °C (Figure 6 and Table 2). It was associated with Fe3+ ions in a regular FeF6 octahedron. The second component, Fe(b), is characterized by an isomer shift of 0.40 mm/s and a monomodal distribution of quadrupole splitting (average value 0.65 mm/s) which may reflect the presence of oxygen in the vicinity of Fe3+ and also, more heterogeneous Fe−X distances (X = F or O). The Fe(c) signal, whose intensity increased slightly with the annealing temperature, is centered at δ = 0.35 mm/s and exhibits a quadrupole splitting distribution with an average value higher than 1 emphasizing a complex anionic environment and more covalent Fe−X chemical bonding. Based on the structural data and Mössbauer parameters, this signal was assigned to iron atoms in 5-fold coordination, thus having anionic vacancy in their first coordination shell, i.e., FeX5□1 with X = F−, OH−, and O2−. Similar Mössbauer parameters have been reported for Fe3(PO4)O336 where Fe3+ ions occupy trigonal bipyramids confirming the above assignment. Mössbauer spectrum of the 350 °C-annealed sample recorded at a high-velocity scale, ±12 mm/s (not shown here), revealed the presence of a small amount of amorphous or nanoscale Fe2O3 in good agreement with PDF analysis. To further confirm the assignment of the newly detected signal in Mö ssbauer spectroscopy, dehydroxylation reaction of FeF2.2(OH)0.8 was carefully monitored by the TG apparatus (see Supporting Information, Figure S1). A complete dehydroxylation was achieved at 350 °C. The treated sample was recovered and transferred in a sealed container inside a glovebox and further prepared for conventional XRD Rietveld analysis and Mössbauer spectroscopy. Figure 9a displayed the Mössbauer spectrum of the controlled dehydroxylated compound. The signal at 0.35 mm/s assigned to under-coordinated iron has significantly growth reaching almost 30%. This showed that the HTB network can support a high content of under-coordinated iron. The structure of Fe-based oxyfluoride obtained after TG experiment up to T = 350 °C under He of FeF2.2(OH)0.8·

Figure 9. (a) Room-temperature 57Fe Mössbauer spectra and (b) Rietveld analysis of the (Cu Kα1) X-ray diffraction powder pattern of the compound obtained after TGA (under He, Tmax = 350 °C) of HTBFeF2.2(OH)0.8·(H2O)0.33 (see Supporting Information, Figure S1). Inset: Mössbauer quadrupole splitting distributions.

(H2O)0.33 hydroxyfluoride has been refined on the basis of powder XRD data (Figure 9b). As for the pristine phase, the Cmcm space group has been considered and the refinement of the structural data (Table 4) provides a good fit of experimental data with a R Bragg factor equal to 0.077. After annealing, the Fe2 sites became highly distorted with heterogeneous bond lengths and the F1 and F4 sites are partially occupied (about 90%) leading us to conclude, in good agreement with Mössbauer and PDF results, that Fe can be 5-fold coordinated to oxygen and fluorine. Moreover, in this compound, the Owater site is still halffilled despite TGA measurements showing the release of all structural water molecules. Nevertheless, one should note that the dehydroxylated sites are highly reactive, especially toward water. Unsaturated cationic sites indeed act as strong Lewis acids. A second TGA measurement performed after characterizations indeed showed that the sample has reabsorbed water (see Supporting Information, Figure S2). 4. Electrochemical Characterizations. As reported previously, the chemical compositions of Fe-based oxyfluorides and consequently the structural features of HTB framework can be monitored after annealing at T ≤ 350 °C. The stabilization of fluorine, hydroxyl groups, and oxygen,and anionic vacancies in the vicinity of Fe3+ demonstrate the high compositional and structural versatility of the HTB network. The impact of the structural features, i.e., composition, on the Li intercalation/ extraction properties was investigated considering four different compositions: (1) the pristine Fe-based hydroxyfluoride FeF 2.2 (OH) 0.8 ·(H 2 O) 0.33 , (2) a dehydrated compound FeF2.2(OH)0.8 obtained after treatment at 220 °C under vacuum, (3, 4) partially dehydroxylated samples FeF2.2(OH)0.8−xOx/2□x/2 obtained after treatment at 300 and 350 4196



Article

Table 4. Refined Structural Parameters of HTB-FeF2.2(OH)0.8·(H2O)0.33 Hydroxy Fluoride after TGA under He (Tmax = 350 °C) Atomic Coordinates: Space Group Cmcm atom

Wyckoff

x

Fe1 Fe2 F1 F2 F3 F4 Owater

4b 8d 8f 16h 4c 8g 4c

0 1/4 0 0.174(1) 0 0.210(2) 0

a = 7.3951(3) (Å)

y 1/2 1/4 0.178(1) 0.386(1) 0.509(2) 0.233(1) 0.047(1) Cell Parameters

b = 12.8077(5) (Å)

z

B (Å2)

occ.

0 0 0.538(1) 0.021(1) 1/4 1/4 1/4

0.2(1) 0.2(1) 0.3(1) 0.3(1) 0.3(1) 0.3(1) 0.5(2)

1.0 1.0 0.92(2) 1.0 1.0 0.90(2) 0.5(1)

c = 7.5683(2) (Å)

V = 716.82(5) (Å3)

Fe2−F1 Fe2−F2 Fe2−F4 ⟨Fe2−F⟩ bond valence

2.090(3) × 2 1.838(7) × 2 1.928(3) × 2 1.952 2.980

Distances (Å) Fe1−F2 Fe1−F3 ⟨Fe1−F⟩ bond valence

1.952(6) × 4 1.896(2) × 2 1.933 3.020

°C. The later sample contained the highest anionic vacancy content as well as a tiny amount of amorphous Fe2O3. The first cycles obtained within the 2−4.2 V vs Li+/Li voltage range are gathered in Figure 10 for the four types of HTB compounds. As

Table 5. Specific Capacities for the First and Second Discharge of Li/FeF3 Cells in a Nonaqueous Electrolyte. (Current Dnsity, 50 mA/g) capacity (mAh/g) positive active material FeF2.2(OH)0.8·(H2O)0.33 annealed at 220 °C (Ar) annealed at 300 °C (Ar) annealed at 350 °C (Ar)

1st discharge

2nd discharge

112 167 206 181

44 65 100 119

capacity of 110 mAh/g under a high current density of 50 mA/g. After the first cycle, the pseudoplateau region of the discharge curve changed to a slopping profile (Figure 11) with an average

Figure 10. First discharge and charge voltammetric curves for Li/FeF3 cells in a nonaqueous electrolyte: current density, 50 mA/g; positive active material, HTB-FeF2.2(OH)0.8·(H2O)0.33 outgassed at 110 °C under dynamic vacuum (pristine) and annealed under Ar flow at 220, 300, and 350 °C.

mentioned in the Introduction, the potential limit (2 V) allows use of the fluorinated materials as positive electrode and avoidance of irreversible structural changes which can occur usually at lower potential during conversion reaction. Intercalation of lithium within the pristine HTBFeF2.2(OH)0.8·(H2O)0.33 barely reached 0.5 Li+ for the first discharge. On the other hand, the removal of structural water greatly enhanced the discharged capacity up to 167 mAh/g (corresponding to 0.7 Li+) in FeF2.2(OH)0.8 (Table 5). This indicated that water molecules tend to block the lithium diffusion within the hexagonal tunnels. Upon deintercalation, 50 mAh/g was measured as the charged capacity showing that a large content of Li+ has remained trapped within the network. A significant improvement of the reversible capacity was achieved with nonstoichiometric FeF2.2(OH)0.8−xOx/2□x/2 compounds. Particularly, the sample treated at 350 °C showed a reversible

Figure 11. Cyclic voltammetry charge and discharge curves for Li/FeF3 cells in a nonaqueous electrolyte: current density, 50 mA/g; positive active material, HTB-FeF2.2(OH)0.8·(H2O)0.33 annealed at 350 °C under Ar flow.

voltage at 3.1−3.2 V. This indicates that an irreversible structural change has occurred during the first discharge which enabled the material to be electrochemically active. Finally this material exhibited a good cycling behavior for several cycles with unchanged slopping discharge/charge curves (Figure 11). It should also be noted that higher the temperature treatment and the anionic vacancies rate, the lower the polarization (Figure 10). 4197



Article

the HTB network collapsed into α-FeF3 and Fe2O3 (demixion process). Within the temperature range of 200−350 °C, an oxolation reaction between two OH groups nearest neighbors occurs leading to the stabilization of O2− and anionic vacancies in the vicinity of Fe3+. The water departure and the variation of the O−H bonding (nature and rates) during annealing have been followed by FTIR spectroscopy. PDF, Rietveld analysis of XRD data, and Mössbauer investigation show various Fe environments. After dehydration and oxolation reaction, oxygen and anionic vacancies can be stabilized around Fe sites which can be 5- or 6fold-coordinated to anions. A maximum of 0.4 molar concentration of oxygen or anionic vacancies per iron atom can be stabilized into FeF2.2O0.4□0.4 oxyfluoride after annealing under Ar flow. Atomic pair distribution function (PDF) and Mössbauer study of these Fe-based oxyflouride−hydroxyfluoride pairs show that annealing temperature under Ar must be between T = 300 °C and T = 350 °C in order to avoid the formation of amorphous Fe oxide. This Fe-based oxyfluoride is highly reactive, and it is very difficult to keep a high concentration of anionic vacancies and to limit the structural water molecules trapped into the 1D tunnels of HTB framework as well as the hydroxyl groups substituting fluorine. Many works have been devoted to oxygen nonstoichiometry in transition metal oxides with perovskite structure. To our knowledge, considering another corner-sharing framework such as HTB, it is the first example of anionic nonstoichiometry in such a network with fluorine and oxygen as anions. Finally as the concentration of structural water and hydroxyl groups stabilized in HTB structure decrease, the cyclability and reversible capacity increase gradually. The distribution of various anions (F−, O2−, and OH−) and vacancies around Fe allow a better perturbation and polarization of the Fe−X chemical bonding during the redox process. The best electrochemical performance was obtained for the Fe-based oxyfluoride containing the higher rate of O2− and anionic vacancies as well as the lower content of structural water and hydroxyl groups.

The structural change occurring during the first cycle was probed by high-energy XRD. After being discharged to 2.0 V, the structure undergoes a drastic amorphization (see Supporting Information, Figure S3). Nevertheless, after charging to 4.2 V, Xray peaks belonging to the HTB structure are recovered suggesting that the HTB framework is maintained during redox processes. To gain further insights into the redox process, the pair distribution function was obtained on lithiated and delithiated samples (Figure 12). For a better visualization of the changes occurring, the PDF of the pristine compound was desummated into partial PDFs comprising the Fe−Fe, Fe−F, and F−F correlations.

Figure 12. PDFs of pristine compound (partial), after the first discharge to 2.0 V, and after the first charge to 4.2 V.

After lithiation, the first peak located at around 1.92 Å corresponding to the Fe3+−F bond shifted to 2.02 Å due to the partial reduction of Fe3+ into Fe2+. Additionally, a broadening of the peaks as well as apparent changes in intensity of certain features in the PDF are consistent with an amorphization likely due to structural disorder induced by lithium intercalation. Upon deintercalation, peaks characteristic of the pristine compound, i.e., Fe−Fe correlation located at 7.36 Å and characteristic of the HTB framework, reappeared confirming that the HTB network is maintained during the redox reaction. It seems clear that zeolite waters are unfavorable for lithium intercalation to proceed. Nevertheless, it has been shown that the water molecule can be easily removed. Furthermore, the occurrence of various Fe3+ sites with F−, OH−, and O2− or anionic vacancies in its vicinity (inducing deformations of Fe3+ polyhedra and changes in bonding ionicity) contribute to stabilize mixed valences Fe2+/Fe3+ during Li intercalation/ deintercalation. Then, the stabilization of oxygen and anionic vacancies substituting fluorine in the HTB network appears to be crucial to improve the cyclability and the electrochemical performance.

■

ASSOCIATED CONTENT

S Supporting Information *

Figure S1 showing thermogravimetric analysis (TGA) of HTBFeF2.2(OH)0.8·(H2O)0.33 performed under He up to 350 °C, Figure S2 showing TGA of the compound obtained by annealing at 350 °C under He (using TG apparatus) of HTBFeF2.2(OH)0.8·(H2O)0.33, Figure S3 showing high-energy XRD of pristine and charged, discharged materials, and Table S1 listing structural parameters obtained from PDF refinements. This material is available free of charge via the Internet at http://pubs. acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

■

Notes

The authors declare no competing financial interest.

CONCLUSION Microwave-assisted hydrothermal synthesis allows preparation of Fe-based hydroxyfluorides by adopting a HTB network with a general chemical formulas FeF3−x(OH)x·(H2O)0.33 (with x = 0.8). Thermal analyses revealed that the removal of zeolite water occurred before T = 240 °C. Thereafter, between 200 and 350 °C, dehydroxylation proceeded and no fluorine or HF departure was observed in this temperature range. At higher temperature,

■

ACKNOWLEDGMENTS Work done at Argonne and use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, were supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. 4198



■

Article

(33) Ledue, D.; Teillet, J. Phys. Status Solidi B 1993, 179, 201−205. (34) Dambournet, D.; Demourgues, A.; Martineau, C.; Majimel, J.; Feist, M.; Legein, C.; Buzare, J.-Y.; Fayon, F.; Tressaud, A. J. Phys. Chem. C 2008, 112 (32), 12374−12380. (35) Egami, T.; Billinge, S. J. L. Underneath the Bragg Peaks. Structural Analysis of Complex Materials; Cahn, R., Ed.; Pergamon Press: Oxford, U.K., 2004. (36) Modaressi, A.; Courtois, A.; Gerardin, R.; Malaman, B.; Gleitzer, C. J. Solid State Chem. 1983, 47 (3), 245−25.

REFERENCES

(1) http://www.bluecar.fr/fr/pages-innovation/batterie-lmp.aspx. (2) Whittingham, M. S. Chem. Rev. 2004, 104, 4271−4301. (3) Arai, H.; Okada, S.; Sakurai, Y.; Yamaki, J. I. J. Power Sources 1997, 68 (2), 716−719. (4) Badway, F.; Cosandey, F.; Pereira, N.; Amatucci, G. G. J. Electrochem. Soc. 2003, 150 (10), A1318−A1327. (5) Di Carlo, L.; Conte, D. E.; Kemnitz, E.; Pinna, N. Chem. Commun. (Cambridge, U. K.) 2014, 50, 460−462. (6) Pereira, N.; Badway, F.; Wartelsky, M.; Gunn, S.; Amatucci, G. G. J. Electrochem. Soc. 2009, 156 (6), A407−A416. (7) Yamakawa, N.; Jiang, M.; Key, B.; Grey, C. P. J. Am. Chem. Soc. 2009, 131 (30), 10525−10535. (8) Leblanc, M.; Pannetier, J.; Férey, G.; De Pape, R. Rev. Chim. Miner. 1985, 22, 107−114. (9) De Pape, R.; Férey, G. Mater. Res. Bull. 1986, 21 (8), 971−978. (10) Leblanc, M.; Férey, G.; Chevalier, P.; Calage, Y.; De Pape, R. J. Solid State Chem. 1983, 47 (1), 53−58. (11) Li, C.; Gu, L.; Tsukimoto, S.; van Aken, P. A.; Maier, J. Adv. Mater. 2010, 22 (33), 3650−3654. (12) Li, C.; Gu, L.; Tong, J.; Tsukimoto, S.; Maier, J. Adv. Funct. Mater. 2011, 21 (8), 1391−1397. (13) Demourgues, A.; Francke, L.; Durand, E.; Tressaud, A. J. Fluorine Chem. 2002, 114 (2), 229−236. (14) Francke, L.; Durand, E.; Demourgues, A.; Vimont, A.; Daturi, M.; Tressaud, A. J. Mater. Chem. 2003, 13 (9), 2330−2340. (15) Yabuuchi, N.; Sugano, M.; Yamakawa, Y.; Nakai, I.; Sakamoto, K.; Muramatsu, H.; Komaba, S. J. Mater. Chem. 2011, 21, 10035−10041. (16) Penin, N.; Viadere, N.; Dambournet, D.; Tressaud, A.; Demourgues, A. Mater. Res. Soc. Proc. 2005, 891, No. EE07-04. (17) Dambournet, D.; Demourgues, A.; Martineau, C.; Pechev, S.; Lhoste, J.; Majimel, J.; Vimont, A.; Lavalley, J. C.; Legein, C.; Buzare, J. Y.; Fayon, F.; Tressaud, A. Chem. Mater. 2008, 20 (4), 1459−1469. (18) Dambournet, D.; Demourgues, A.; Martineau, C.; Durand, E.; Majimel, J.; Vimont, A.; Leclerc, H.; Lavalley, J. C.; Daturi, M.; Legein, C.; Buzare, J. Y.; Fayon, F.; Tressaud, A. J. Mater. Chem. 2008, 18 (21), 2483−2492. (19) Dambournet, D.; Eltanamy, G.; Vimont, A.; Lavalley, J. C.; Goupil, J. M.; Demourgues, A.; Durand, E.; Majimel, J.; Rudiger, S.; Kemnitz, E.; Winfield, J. M.; Tressaud, A. Chem.Eur. J. 2008, 14 (20), 6205−6212. (20) Dambournet, D.; Demourgues, A.; Martineau, C.; Durand, E.; Majimel, J.; Legein, C.; Buzare, J. Y.; Fayon, F.; Vimont, A.; Leclerc, H.; Tressaud, A. Chem. Mater. 2008, 20 (22), 7095−7106. (21) Demourgues, A.; Penin, N.; Durand, E.; Weill, F.; Dambournet, D.; Viadere, N.; Tressaud, A. Chem. Mater. 2009, 21 (7), 1275−1283. (22) Demourgues, A.; Penin, N.; Dambournet, D.; Clarenc, R.; Tressaud, A.; Durand, E. J. Fluorine Chem. 2012, 134, 35−43. (23) Rodriguez-Carvajal, J. IUCr Comm. Powder Diffr. Newsl. 2001, 26, 12−19, http://www.ill.eu/sites/fullprof. (24) Qiu, X.; Thompson, J. W.; Billinge, S. J. L. J. Appl. Crystallogr. 2004, 37 (4), No. 678. (25) Farrow, C. L.; Juhas, P.; Liu, J. W.; Bryndin, D.; Božin, E. S.; Bloch, J.; Proffen, Th.; Billinge, S. J. L. J. Phys.: Condens. Matter 2007, 19 (33), No. 335219. (26) Brand, R. A.: Universität Duisburg: Duisburg, Germany, 2008 (27) Serna, C.; VanScoyoc, G. E.; Ahlrichs, J. L. Am. Mineral. 1977, 62, 784−792. (28) Herzberg, G. Molecular Spectra and Molecular Structure. II. Infrared and Raman Spectra of Polyatomic Molecules; Krieger, Malabar, FL, USA, 1989; 676 pp. (29) Xu, W.; Johnston, C. T.; Parker, P.; Agnew, S. F. Clays Clay Miner. 2000, 48 (1), 120−131. (30) Burneau, A.; Barres, O.; Gallas, J. P.; Lavalley, J. C. Langmuir 1990, 6 (8), 1364−1372. (31) Vimont, A.; Lavalley, J. C.; Francke, L.; Demourgues, A.; Tressaud, A.; Daturi, M. J. Phys. Chem. B 2004, 108 (10), 3246−3255. (32) Calage, Y.; Leblanc, M.; Férey, G.; Varret, F. J. Magn. Magn. Mater. 1984, 43 (2), 195−203. 4199


Evidence for Anionic Vacancy and Electrochemical Perfor - American

Recommend Documents