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Unraveling the structure of Iron(III) oxalate tetrahydrate and its reversible Li insertion capability Hania Ahouari, Gwenaelle Rousse, Juan Jose Rodriguez-Carvajal, Moulay Tahar Sougrati, Matthieu Saubanère, Matthieu Courty, Nadir Recham, and Jean-Marie Tarascon Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5043149 • Publication Date (Web): 12 Feb 2015 Downloaded from http://pubs.acs.org on February 17, 2015

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Chemistry of Materials

Unraveling the structure of Iron(III) oxalate tetrahydrate and its reversible Li insertion capability Hania Ahouaria,b, Gwenaëlle Rousseb,c,d,e*, Juan Rodríguez-Carvajalf, Moulay-Tahar Sougratib,e,g, Matthieu Saubanèreb,e,g, Matthieu Courtya,e, Nadir Rechama,b,e* and Jean-Marie Tarasconb,c,d,e a

Laboratoire de Réactivité et Chimie des Solides, UMR CNRS 7314, 33 Rue Saint Leu 80039 Amiens Cedex, France b ALISTORE - European Research Institute, FR CNRS 3104, 80039 Amiens, France c Collège de France, Chimie du Solide et de l’Energie, FRE 3677, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France d Sorbonne Universités - UPMC Univ Paris 06, 4 Place Jussieu, 75005 Paris, France. e Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, France f Institut Laue-Langevin, Avenue des Martyrs CS 20156 - 38042 Grenoble Cedex 9, France g Institut Charles Gerhardt - UMR 5253, 34095 Montpellier Cedex 5, France *Corresponding authors: Gwenaëlle Rousse [email protected] Nadir Recham [email protected]

Abstract In the search for better batteries, the quest for new positive electrode materials, which are derived from earth-abundant elements, and synthesized through low energy demanding processes, has become an area of interest to the scientific community. Many Liiron based polyanionic compounds have already been considered for Li ion batteries, but there still remain a large variety of Fe-based polyanionic carboxylate compounds (carbonates, oxalates, malonates...) to be explored, among which Fe2(C2O4)3·nH2O is commercially available, but for which structure and electrochemical reactivity towards Li are lacking. By means of combined X-ray and neutron powder diffraction, we unravel for the first time the structure of Fe2(C2O4)3·4H2O. It adopts a triclinic unit cell with iron atoms being octahedrally coordinated by one water molecule and three oxalate groups, two of them being tetradentate. This arrangement translates into zig-zag chains linked one to each other through the third oxalate molecule to lead to an opened layered structure into which two additional water molecules are located. Moreover, we demonstrate that it reacts with lithium at an average potential of 3.35 V vs. Li leading to a sustainable capacity of 98 mAh/g. The insertionextraction of lithium process follows a biphasic process and involves the Fe3+/Fe2+ redox couple as deduced from in situ X-ray powder diffraction and operando Mössbauer spectroscopy measurements. 1

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Introduction

The massive use of fossil fuels (petroleum, coal and gas) is now at the origin of growing economical and political concerns because of limited reserves while the world’s need for energy is foreseen to double within the next 20 years. Great hopes are placed on the use of renewable energies such as wind, wave and solar which can generate huge amounts of electricity. However, because of their intermittency, the use of such energies has placed energy storage as one of the greatest challenges of the 21st century1. Rechargeable Li-ion batteries, which have already conquered the portable market, are emerging as the technology of choice for powering electric vehicles (EV)2, 3, and show great promise for grid applications. Aside from safety, cost and sustainability are the most overriding factors for such applications, the reason why LiFePO4 has become the most praised material for the next generation of Li-ion batteries for EV’s4, 5. This phase is also used in the Li-metal polymer batteries6, 7 which are presently powering the blue EV’s cars cruising through Paris every day although in that case having a Li-based polyanionic compound is not mandatory as Li metal is used for the negative electrode. Searching for additional sustainable Fe-based polyanionic compounds, we have uncovered i) a new family of Li-based fluorosulfates8, 9 among which LiFeSO4F shows the highest redox voltage (3.9 V vs. Li+/Li) ever reported for any inorganic compound and ii) more recently a new Li-free iron oxysulfate having a reversible capacity of 125 mAh/g centered at 3 V10. Aside from the inorganic polyanionic compounds, iron(II) oxalate has already been explored as negative electrode material via a conversion rather than an intercalation mechanism11-14. More recently, we have shown the cost-wise attractiveness of some Fe-based phases having organic polyanions such as malonates as positive electrode.15 Pursuing this survey, we decided to study the electrochemical performance of the iron(III) oxalate. Fe2(C2O4)3·6H2O is commercialized by Sigma-Aldrich and has been so far widely used as a catalyst for the photo-reduction/oxidation reactions of dyes, organics wastes, etc…16-18 but never as an electrode to the best of our knowledge. Let’s first recall the controversy in literature regarding the exact amount of structural water present in iron(III) oxalate, which is sometimes

referred

as

Fe2(C2O4)3·6H2O

(Sigma-Aldrich),

Fe2(C2O4)3·5H2O19

or

Fe2(C2O4)3·4.6H2O20-22, while recent chemical analysis on the commercialized product are consistent a chemical formulae Fe2(C2O4)3·4H2O23. In this paper, we confirm that the 2


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compound contains four structural water molecules, therefore from now on, it will be referred to as Fe2(C2O4)3·4H2O. Despite all these studies, and the fact that iron oxalate is commercialized by Sigma-Aldrich for decades; its synthesis is not widely reported. Moreover, to the best of our knowledge, the structure of this crystalline material remains unknown as stated by d’Antonio in 200923: “The corresponding FeIII compound [Fe2(C2O4)3·4H2O] is so far no so well characterized, and information on its structural features is not available”. Herein, we report a new synthesis route to prepare Fe2(C2O4)3·4H2O and determine its crystal structure through X-ray powder diffraction coupled with neutron powder diffraction. We also show for the first time that iron(III) oxalate compound is electrochemical active versus lithium as it can reversibly insert 1.6 Li atom per formula unit at 3.35 V versus Li+/Li0. Experimental Section a) X-ray powder diffraction

X-ray diffraction patterns (XRD) were collected in Bragg-Brentano geometry using a Bruker D8 advance diffractometer with a Cu-Kα radiation source (λ1=1.54056 Å, λ2=1.54439 Å) equipped with a Lynxeye XE detector operating at 40 kV and 40 mA.

b) Mössbauer Spectroscopy Mössbauer spectra were collected at room temperature using a 0.55 Gbq source of 57

CoRh in constant acceleration mode. Absorbers were made by mixing 30 mg of the

compounds with 60 mg of boron nitride. The hyperfine parameters isomer shift (IS), linewidth (LW) and quadrupole splitting (QS) were determined by fitting lorentzian lines to the experimental data, using the MOSFIT program.

c) Neutron powder diffraction Neutron powder diffraction was performed on the D20 powder diffractometer at ILL (Institut Laue Langevin, Grenoble) in its high resolution mode (take-off angle 120°) with a wavelength of λ=1.35821 Å obtained with a germanium (117) monochromator. The powder was placed in a 6 mm diameter vanadium cylindrical container.

d) Electrochemical characterization The electrochemical tests were carried out using Swagelok type cells. The positive electrode composite was prepared by mixing 70 wt% of active material with 30 wt% of 3



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Ketjenblack carbon (as electronic conductor) using a Spex-800 unit for 8 min under argon atmosphere. The cells were assembled in an argon-filled glove box by using 4-6 mg/cm2 of the previous composite as positive electrode separated from the negative electrode (lithium disk) by 2 sheets of glass fiber disks (Whattman GF/D borosilicate), the whole set-up being soaked in a LiPF6 (1M) solution of ethylene carbonate (EC)/dimethyl carbonate (DMC) mixture (1/1 w/w). Galvanostatic discharge-charge tests were conducted with a MacPile (Claix, France) controller at a constant temperature of 25°C. The cells were cycled between 2.5 to 3.9 V vs. Li+/Li0 at a C/10 rate (1 Li+ exchanged per 10 h). e) DFT calculations Periodic DFT+U calculations were performed using both the VASP code24,25 and the projector-augmented-wave (PAW) method of Blöchl26. The exchange-correlation energy was described by the functional of Perdew, Burke and Ernzerhof (PBE)27 and the Hubbard corrections28 were made using a Ueff for Fe equal to 4.0 eV. Long-range dispersion corrections have been taken into account within a DFT-D2+U approach of Grimme29, DFT-D3+U30, 31 as implemented in Vasp32. Gibbs energy of bulk phases at finite temperature are obtained following the quasiharmonic approximation33 while for the gas phases there are obtained by adding the DFT energy, the zero point energy, the mean rotational and translational contribution and the entropic term taken from the JANAF reference.

Results The earlier report on the synthesis of greenish powdered Fe2(C2O4)3·4H2O dated back from 1993. Galway et al.21 have obtained this compound (whose chemical assignment was “Fe2(C2O4)3·4.6 H2O”) by precipitation of a solution containing a mixture of ferric hydroxide and oxalic acid. Herein we deviated from this process by using a nitrate rather that an iron hydroxide precursor. More specifically the phase was prepared by complete evaporation in an oil bath at 80°C under a continuous stirring of 10 ml of a concentrated water solution of H2C2O4·2H2O and Fe(NO3)3·9H2O precursors with 3:2 molar ratio. The glass vial containing the solution was covered with an aluminum foil to prevent the photoreduction in water of iron(III) oxalate complex into iron(II) oxalate. Its X-ray powder diffraction (XRD) pattern is analog to the one produced by commercial Fe2(C2O4)3·4H2O powders (Sigma-Aldrich) with however a higher degree of crystallinity. Mössbauer spectra (Figure 1a) indicates that iron is present in a unique environment with an isomer shift of 0.38 mm/s and a quadrupole splitting 4


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of 0.40 mm/s, suggesting a high spin Fe3+ in octahedral coordination in agreement with the literature20, 23. TGA conducted under air (Figure 1b) confirmed that iron(III) oxalates contains only four water molecules (64% weight loss according to the reaction Fe2(C2O4)3·4H2O(s) + 3/2 O2(g) Fe2O3(s) + 6 CO2(g)+ 4 H2O(g)). Thus, being confident on the purity of the powder, an overnight XRD scan in high resolution configuration was collected in view of solving the crystal structure. The resulting XRD pattern presents many Bragg peaks, the first one appearing at low 2θ angle (2θ=9.76°, d=9.07 Å) is indicative of a low symmetry/large unit cell. The best way to solve a crystal structure is usually by recording single crystal X-ray diffraction but we never succeeded in growing single crystals. Moreover, our attempts to record Synchrotron X-ray powder diffraction patterns failed because the sample was not stable under beam. Therefore, we embarked into an ab initio structural determination from coupled X-ray and neutron powder diffraction data. The X-ray pattern (Figure 2, top) presents many sharp Bragg peaks proving that iron(III) oxalate is well crystallized, even though at high angles (above 50°-60°) the peaks are clearly broadened as a result of strain whose induced broadening varies as X*tanθ. The Bragg peaks were indexed with the Dicvol program34 in a triclinic unit cell with the following lattice parameters: a=5.306(1) Å, b=6.637(1) Å, c=9.138(1) Å, α=91.79(1)°, β=97.58(1)° and γ=93.32(1)°. This cell presents a volume of 318.2(1) Å3 which can accommodate one formula Fe2(C2O4)3·4H2O. The space group was naturally chosen as P –1, because the oxalate groups naturally present an inversion center. The structure was solved in two successive steps: first from X-ray diffraction to get the positions of the “heavy atoms”, then neutron diffraction to locate water molecules and complete the structural model. The first step (against X-ray diffraction data) was done using the EXPO software35 with two approaches considered: 1) phasing by Direct Methods 2) using the newly developed RAMM method36. The two approaches led to consistent structural models although incomplete and/or approximate (in particular relative to the oxalate groups), therefore the structure was finally completed using successive Rietveld refinements37 and Fourier difference maps performed using the FullProf suite of programs38. The second step was to measure the same sample on a neutron powder diffractometer, to take advantage of the different contrast neutrons diffract nuclei as compared to X-rays: bFe=9.45 fm, bC=6.646 fm, bO=5.803fm, bH=˗3.739 fm. A correction was applied to take into account the absorption and the "effective absorption" due to incoherent scattering of hydrogen atoms (µR= 0.3669). The water molecules were first treated as rigid bodies and their orientation was deduced from

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simulated annealing techniques39 using the FullProf program38, then each hydrogen atom was freely refined. The final structural model (Table 1) was obtained from a combined Xray/neutron Rietveld refinement without any constraints. Note that some preferred orientation in the [010] direction as well as anisotropic strain broadening was refined in the case of the Xray pattern. The final X-ray and neutron refinements are shown on Figure 2. All atoms are placed into the general Wyckoff position 2i, so that there is only one crystallographic site for iron atoms, in agreement with the Mössbauer spectrum (Figure 1a). Figure 3a shows the atomic positions within a unit cell. Iron atoms are located in the middle of an octahedron (Fe-O distances ranging from 1.92 to 2.09 Å) typical for Fe3+. Out of the six oxygen ligands, five are part of an oxalate molecule whereas the sixth one (O7) belongs to one water molecule. Each oxalate group is placed at the inversion centers of the unit cell, i.e. at (0, 0, 0), (½, ½, ½) and (½, ½, 0) for C1, C2 and C3 respectively so that only three crystallographic sites are necessary to describe them (one carbon and two oxygen sites); Lastly, O8, H81 and H82 constitute the second water molecule. The water molecules are oriented in such a way that hydrogen bonding between hydrogen and oxygen atoms is possible as highlighted in Figure 3b (O---H distances in the range 1.7˗2.1 Å). The structural model, which is consistent with the expected formula Fe2(C2O4)3·4H2O, is plotted on Figure 4 along different crystallographic directions. FeO6 are linked one to each other through two tetradentate oxalate molecules (built on C2 and C3) to form infinite zig-zag chains running along [001] (Figure 3c). These chains are connected through the third bidentate oxalate (built on C1) to form corrugated layers corresponding to the (1, -1, 0) planes, stacked along the [1 -1 0]* direction (Figure 4b). It results that Fe2(C2O4)3·4H2O presents an opened structure. This structural feature combined with the presence of Fe3+ was a motivation to embark into an electrochemical reactivity of this phase towards Li and eventually Na, as described next. The voltage versus composition profile for a Li/Fe2(C2O4)3·4H2O cell is shown in Figure 5. It reveals the presence of a plateau centered at around 3.35 V vs. Li+/Li0 as confirmed by the corresponding dx/dV curve (Figure 5, right panel). During the first discharge, about 1.8 Li+ per formula unit are inserted in the structure whereas 1.6 Li+ ions are removed from the structure upon the subsequent charge leading to a steady capacity retention of about 98 mAh/g which represents nearly 80% of the theoretical capacity (CTheo=122

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mAh/g). Subsequent discharge/charge cycles superimpose well over more than 10 cycles as seen in Figure 5 inset. To grasp further insight into the mechanism of the Li insertion/extraction process in Fe2(C2O4)3·4H2O, in situ XRD measurements were carried out using an homemade electrochemical cell equipped with a beryllium window40. Figure 6 shows the XRD patterns collected for every change in ∆x of 0.1 as a Li/Fe2(C2O4)3·4H2O cell being discharged and charged at C/20 rate. On discharge, we observe the disappearance of the diffraction peaks of the pristine material at the expense of new peaks that becomes unique at the end of the discharge (Li1.8Fe2(C2O4)3·4H2O) which indicates a biphasic insertion process. Upon charge, the reverse phenomenon occurs. Moreover the XRD pattern of the fully charged material nicely superimposes with that of the pristine material, indicating a fully reversible Liinsertion-deinsertion process. Despite the bad quality of intensities induced by the beryllium window, we have been able to refine the pattern recorded at the end of the discharge with the same triclinic cell as the pristine one (whose refinement is shown Figure 8a), with lattice parameters a=5.400(1) Å, b=6.584(1) Å, c=10.011(1) Å, α=96.01(1)°, β=101.21(1)°, γ=94.58(1)° and a cell volume of 345.4(1) Å3 (Figure 8b). The result of the refinement suggests that the structure framework of the pristine triclinic Fe2(C2O4)3·4H2O phase is maintained upon lithiation, which is merely accompanied by a volume expansion of 8.4%, which is in same order of magnitude as LiFePO4 (7%). The reversibility of the Li uptake-removal process was further confirmed by operando Mössbauer measurements (Figure 7) collected during discharge and charge of the compound at C/10 rate using a stainless steel Swagelok-type cell40. The spectra recorded for 3 hours between ±4 mm/s during the cell discharge show a narrow doublet corresponding to Fe3+ which progressively disappears while a new doublet typical of Fe2+ (isomer shift of 1.17 mm/s and a quadrupole splitting of 1.93 mm/s) appears and becomes predominant at the end of discharge. During the subsequent charge up to 3.9 V, the reverse phenomenon was observed with almost all the Fe2+ returning to its +3 state upon oxidation. The intermediate Mössbauer spectra recorded during the first discharge and subsequent charge can all be fitted using a linear combination of the Fe2+ doublet and the Fe3+ doublet, as illustrated in Supporting Information, Figure S1. For sake of completion, we tested the electrochemical behavior of Fe2(C2O4)3·4H2O versus Na. The voltage-composition curve (not shown here) indicates that around 1.3 Na per formula can be inserted at around 3.0 V vs. Na, which is in agreement with the usual 300 mV

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difference observed between Na and Li. However, huge polarization and very poor cyclability prevent the use of this material as electrode for Na batteries.

Discussion We have reported the structural determination of iron(III) oxalate tetrahydrate, which constitutes a model case of what can be achieved in terms of structural determination from powder diffraction. Since the X-ray powder pattern of this phase is dominated by iron, we had to use neutron diffraction as a complementary technique to univocally locate oxalate groups and water molecules. Interestingly, Fe2(C2O4)3·4H2O which crystallizes in a triclinic unit cell presents some features already observed in the ferrous oxalate Fe(C2O4)·2H2O. The latter is a mineral called humboldtine, which exists in two forms, one monoclinic and one orthorhombic, both built on infinite chains made of Fe and tetradentate oxalate molecules. The main difference between humboldtine and Fe2(C2O4)3·4H2O is that in the former iron is coordinated to four oxygen atoms being part of two oxalate groups and two water molecules, while in the later only one water molecule coordinates iron and a third oxalate ligand bridges chains one to each other to form layers (Figure 9). Moreover, note that two of the four water molecules are linked only by hydrogen bonds within the Fe2(C2O4)3·4H2O phase. This motivated us to explore the feasibility of removing these two water molecules to form Fe2(C2O4)3·2H2O so as to decrease its weight and thereby increase its electrochemical capacity. Figure 1b shows the TGA profile collected under air with namely a major loss occurring in two steps between 150 and 250°C corresponding to the departure of water and carbon dioxide as deduced by mass spectroscopy measurements. This implies the occurrence of a simultaneous dehydration-decomposition and the concomitant formation of iron oxide Fe2O3 as confirmed by XRD measurements. When such measurements are performed under argon, we observe a similar weight loss at 400°C with however a spreading of the temperature range (150 - 380°C) over which the dehydration and a decomposition into CO2 and CO are taking place as determined by mass spectroscopy measurements. However, the formation of a certain amount of ferrous oxalate was observed during the first loss and which corresponds to the thermal decomposition of ferric oxalates in an inert atmosphere as confirmed by Mössbauer spectroscopy. This reduction of FeIII to FeII is ascribed to the electron transfer during oxidation of the oxalate group to CO2 (C2O42− 2 CO2 +2e−) yielding two electrons that consecutively reduce FeIII cations20. The carbon monoxide released during the second loss 8


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comes from the decomposition of FeC2O4 (FeC2O4(s)FeO(s) + CO(g)+ CO2(g)). At the end of the decomposition (400°C), a mixture of iron oxides (Fe3O4, FeO) were formed, which nearly accounts for the 65% weight loss measured. Whatever the details, it results that the preparation of the dehydrated oxalate phase is not possible in the described conditions and we believe that it is a direct consequence of the strong hydrogen bonding depicted previously. Lastly, we embarked into DFT calculations to i) obtain further insights about the Li positions in the discharged state ii) check whether the orientation of the water molecules determined from neutron diffraction was the most energetically stable, and iii) validate the non-stability of the partially or fully dehydrated compounds. The crystal structure of Fe2(C2O4)3·4H2O is perfectly reproduced by the DFT + U method provided that Van der Waals interactions are included (Supporting Information, Figure S2 and Table S1), and is shown to be dynamically stable (Supporting Information, Figure S3). By adding the Van der Waals interaction term, we could confirm that the solved structure with the hydrogen belonging to water molecules and pointing to oxygen atoms was the most stable configuration. Pursuing further into these calculations, we tried to position the Li ions into the fully lithiated Li2Fe2(C2O4)3·4H2O phase as we could not access this information from XRD or neutron techniques due to our inability to prepare sufficient powder amounts of Li2Fe2(C2O4)3·4H2O via chemical reducing agents. Because strongly electrostatic interactions are expected between the Li+ ions and the oxygen ligands, calculations have been performed with and without dispersion corrections. The most probable Li-sites, from structural considerations (e.g, tetrahedral sites), have been guessed prior to fully relax the structure based i) on the unit cell parameters starting from those obtained by the refinement of the in situ XRD data and ii) the pre-assigned Li atomic positions for Li2Fe2(C2O4)3⋅4H2O. Whatever the dispersion considerations (DFT+U, DFTD2+U, DFT-D3+U) we have tried, we obtained similar structures (Figure 10 and Supporting Information, Table S2), providing confidence about the robustness of our results. It turns out that the inserted Li-ions occupy tetrahedral sites at (0.176, 0.796, 0.201) which are built from three oxygen atoms of the oxalate ligands and one water molecule. More descriptively one oxalate ligand bridges Li in a tetradentate way. Interestingly, as compared to the delithiated pristine phase, water molecules undergo a rotation to coordinate to the Li-centered tetrahedra, therefore losing their weak hydrogen-bonds with the oxalate network, while new ones are created (Figure 10c). Such a Li-driven water molecule rotation enlisting the breaking and formation of Van der Waals hydrogen interactions is of weak energy and this is consistent 9



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with the voltage-composition curve which shows a limited polarization between charge and discharge. Metal-oxygen bonds breaking in contrast leads to humongous polarization voltages as reported for conversion reactions. During the course of this investigation, we also computed using the DFT+U approach the average redox potential associated to the Li insertion reaction [Fe2(C2O4)3⋅4H2O + 2Li Li2Fe2(C2O4)3⋅4H2O] and found a value of 3.4 V and which is in good agreement with the experimental value (3.35 V). This redox potential value falls within the same range of values reported for numerous iron based polyanionic compounds (Figure 11) and is positioned between the potential of the sulfates and phosphates among which LiFePO4 (3.45 V vs. Li)1 is the most studied and attractive application-wise. The C2O42− species, being less electronegative than PO43− and SO42− polyanions, one would expect a lower potential. This reminds us that the inductive effect can literally be applied for materials solely sharing the same structure.

Lastly, turning back to the non-stability of the hypothetical di-hydrated “Fe2(C2O4)3⋅2H2O” phases, we calculated within the DFT-D3+U formalism the energy associated with the removal of the two molecules of water at T=0 K: ∆E(H2O)= E[Fe2(C2O4)3⋅4H2O] − (E[Fe2(C2O4)3⋅2H2O] + 2E[H2O]). We found this reaction to be endothermic (∆E(H2O)= −1.6 eV, which is about two times the vaporization energy of water: ~0.42 eV/water molecule), implying that the water molecules, although not covalently bonded to the network, strongly stabilize the structure through combination of electrostatic and Van der Waals interactions (Supporting Information, Table S3). The effect of temperature was then considered, with H2O coming out of the structure in a gaseous form so that an entropy term was added to our calculations42. The corresponding temperature-dependent phase diagram at ambient pressure ∆G(H2O[P,T]) in air plotted versus T is shown in Figure 12 with ∆G(H2O[P,T] is defined as : ∆G(H2O[P,T]) = G[Fe2(C2O4)3⋅4H2O] – {G[Fe2(C2O4)3⋅2H2O] + 2G[H2O]}, It indicates that the following hypothetical “Fe2(C2O4)3⋅2H2O + two water molecules in the gaseous state” system present a lower energy than Fe2(C2O4)3⋅4H2O when the temperature is increased above 250°C. However at this temperature Fe2(C2O4)3⋅4H2O is already decomposed; therefore these calculations explain our inability to remove two water molecules from the parent tetrahydate oxalate phase without destroying the structure.

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Conclusions We have solved from combined neutron/X-ray experiments the structure of Fe2(C2O4)3·4H2O and shown the ability of this phase to reversibly insert 1.6 Li per formulae unit with a plateau at around 3.35 V vs. Li, leading to a sustained capacity of 98 mAh/g. Complementary in situ XRD and operando Mössbauer spectroscopy measurements have confirmed the occurrence of an electrochemical activity via a biphasic insertion process, linked to the Fe3+/Fe2+ redox couple. Among the few reported 3d-metal based organic intercalation compounds, this oxalate phase compares favorably in terms of voltage and capacity to the metal organic framework phase MIL-53(Fe) (LixFeIIxFeIII1−x(OH)0.8F0.2L with L=O2CC6H4CO2, 75mAh/g at 3V)43. Iron oxalate presents a nearly identical energy density as the recently reported carbonophosphate Li3FePO4CO344,45 because its lower specific capacity (98 mAh/g versus 110 mAh/g) is compensated by its higher voltage (3.35 V versus 3 V). Moreover, Fe2(C2O4)3·4H2O is commercially available and there is room to improve its capacity to the theoretical one (122 mAh/g). This approach is an attractive path towards the quest for inexpensive, environmental friendly positive electrode materials for large volume applications such as grids where weight is not a penalty.

Supporting Information CIF file of Fe2(C2O4)3·4H2O. Fit of the Mössbauer spectra recorded on discharge and charge; Structure of the relaxed Fe2(C2O4)3·4H2O as obtained with DFT-D3+U. Structural parameters of relaxed Fe2(C2O4)3·4H2O and Li2Fe2(C2O4)3·4H2O using DFT+U, DFT-D2+U, DFT-D3+U, DFT-HS+U; Stability energy of water molecules ∆E(H2O) obtained using DFT+U, DFT-D2+U, DFT-D3+U and DFT-TS+U. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgments We are grateful to the French national computing center (CINES grant cmm6691) and to the computer center of the Institut Charles Gerhardt in Montpellier (ICG, France).The authors would like to thank all ALISTORE-ERI nanopositive team for the fruitful discussion. H.A. acknowledges ALISTORE-ERI for her Ph.D. grant.

11



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References 1. Rousse, G.; Tarascon, J. M. Chem. Mater. 2014, 26, 394. 2. Recham, N.; Armand, M.; Laffont, L.; Tarascon, J. M. Electrochem. and Solid-State Letters 2009, 12, A39. 3. Ati, M.; Sougrati, M. T.; Rousse, G.; Recham, N.; Doublet, M. L.; Jumas, J. C.; Tarascon, J. M. Chem. Mater. 2012, 24, 1472. 4. Masquelier, C.; Croguennec, L. Chem. Rev. 2013, 113, 6552. 5. Recham, N.; Oró-Solé, J.; Djellab, K.; Palacín, M. R.; Masquelier, C.; Tarascon, J. M. Solid State Ionics 2012, 220, 47. 6. Damen, L.; Hassoun, J.; Mastragostino, M.; Scrosati, B. J. Power Sources. 2010, 195, ( 6902. 7. Swiderska-Mocek, A. Electrochim. Acta 2014, 139, 337. 8. Ati, M.; Melot, B. C.; Chotard, J. N.; Rousse, G.; Reynaud, M.; Tarascon, J. M. Electrochem. Commun. 2011, 13, 1280. 9. Barpanda, P.; Recham, N.; Chotard, J.-N.; Djellab, K.; Walker, W.; Armand, M.; Tarascon, J.M. J. Mater. Chem. 2010, 20, 1659. 10. Sun, M.; Rousse, G.; Abakumov, A. M.; Van Tendeloo, G.; Sougrati, M. T.; Courty, M.; Doublet, M. L.; Tarascon, J. M. . J. Am. Chem.Soc. 2014, 136, 12658. 11. Aragón, M. J.; León, B.; Serrano, T.; Pérez Vicente, C.; Tirado, J. L. J. Mater. Chem. 2011, 21, 10102. 12. Aragón, M. J.; Pérez-Vicente, C.; Tirado, J. L. Electrochem. Commun. 2007, 9, 1744. 13. Aragón, M. J.; León, B.; Pérez Vicente, C.; Tirado, J. L. J. Power Sources. 2009, 189, 823. 14. López, M. C. T., J. L.Pérez Vicente, C. J. Power Sources. 2013, 227, 65. 15. H. Ahouari; G. Rousse; Y. Klein; J-N. Chotard; M. T.Sougrati; N. Recham; Tarascon, J.-M . Solid State Sc. SSSCIE-D-14-00564. 16. Zhou, D.; Wu, F.; Deng, N.; Xiang, W., P. Water. Res. 2004, 38, 4107. 17. Zhong, Y.; Su, L.; Yang, M.; Wei, J.; Zhou, Z. ACS Appl. Mater. Interfaces. 2013, 5, 11212. 18. Kadirova, Z. C.; Katsumata, K.-i.; Isobe, T.; Matsushita, N.; Nakajima, A.; Okada, K. Appl. Surf. Sci. 2013, 284, 72. 19. Suzdalev, I. P.; Yu. V. Maksimov; V. K. Imshennik; S. V. Novichikhin; V. V. Matveev; Yu. D. Tret´yakov; A. V. Lukashin; A. A. Eliseev; N. V. Avramenko; Malygin, A. A.; Sosnov, E. A. Russ. Chem. Bull, International Edition. 2006, 55, 1755. 20. Hermankova, P.; Hermanek, M.; Zboril, R. Eur. J Inorg. Chem. 2010, 1110. 21. Andrew K. Galwey; Mohamed, M. A. Thermochim. Acta 1993, 213, 279. 22. Mohamed, M. A.; Salem, M. A., T. J. Anal. Appl. Pyrolysis. 1993, 21, 275. 23. D’Antonio, M. C.; Wladimirsky, A.; Palacios, D.; González-Baró , A. C.; Baran, E. J.; Coggiola, L.; Mercaderd, R. C. J. Braz. Chem. Soc. 2009, 20, 445. 24. Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169. 25. Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47, 558. 26. Blöchl, P. E. Phys. Rev. B 1994, 50, 17953-17979. 27. Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. 28. Ben Yahia, M.; Lemoigno, F.; Rousse, G.; Boucher, F.; Tarascon, J.-M.; Doublet, M.-L. Energ. Environ. Sci. 2012, 5, 9584. 29. Grimme, S., J. Comput. Chem. 2006, 27, 1787. 30. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, S. J. Chem. Phys. 2010, 132, 154104. 31. Grimme, S.; Ehrlich, S.; Goerigk, L. J. Comput. Chem. 2011, 32, 1456. 32. Bucko, T.; Hafner J.; Lebegue S.; Angyan J. G. J. Phys. Chem. A 2010, 114, 1814. 33. Togo, A; Chaput, L.; Tanaka, I.; Hug, G. Phys. Rev. B., 2010, 81, 174301. 34. Boultif, A.; Louer, D. J. Appl. Crystallogr. 2004, 37, 724. 35. Altomare, A.; Cuocci, C.; Giacovazzo, C.; Moliterni, A.; Rizzi, R.; Corriero, N.; Falcicchio, A. J. Appl. Crystallogr. 2013, 46, 1231. 36. Altomare, A.; Cuocci, C.; Moliterni, A.; Rizzi, R. J. Appl. Crystallogr. 2013, 46, 476. 37. Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65. 38. Rodriguez-Carvajal, J. Physica. B 1993, 192, 55. 12


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39. Kirkpatrick;, S.; Gelatt;, C. D.; Vecchi, M. P. New Series 1983, 220, 671. 40. Leriche, J. B.; Hamelet, S.; Shu, J.; Morcrette, M.; Masquelier, C.; Ouvrard, G.; Zerrouki, M.; Soudan, P.; Belin, S.; Elkaïm, E.; Baudelet, F. J. Electrochem. Soc. 2010, 157, A606. 41. Tarascon, J.-M.; Recham, N.; Armand, M.; Chotard, J.-N. l.; Barpanda, P.; Walker, W.; Dupont, L. Chem. Mater. 2010, 22, 724. 42. Lide, D. R., Handbook of Chemistry and Physics. CRC Press 2004, ISBN 0-8493-04857. 43. Ferey, G.; Millange, F.; Morcrette, M.; Serre, C.; Doublet, M. L.; Greneche, J. M.; Tarascon, J. M. Angew. Chem. Int. Ed. 2007, 46, 3259. 44. Chen, H.; Hautier, G.; Ceder, G. . J. Am. Chem.Soc. 2012, 134, 19619. 45. Chen, H.; Hautier, G.; Jain, A.; Moore, C.; Kang, B.; Doe, R.; Wu, L.; Zhu, Y.; Tang, Y.; Ceder, G. Chem. Mater. 2012, 24, 2009.

13



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Page 14 of 29

Table 1. Structural parameters for Fe2(C2O4)3·4H2O, deduced from the coupled Rietveld refinement of the X-ray and neutron diffraction patterns. Space group

P-1

a (Å) b (Å) c (Å) α(deg.) β (deg.) γ(deg.) V (Å3) Z Density (g/cm3)

5.3065(1) 6.6373(1) 9.1381(1) 91.793(1) 97.583(1) 93.323(1) 318.26(1) 1 2.337

x

y

z

B(Å2)

Fe

Wyckoff position 2i

0.4767(6)

0.2932(4)

0.2402(4)

0.95(7)

C1

2i

0.1217(11)

0.0709(9)

0.0116(7)

1.17(11)

C2

2i

0.3783(11)

0.4393(8)

0.5137(7)

0.67(9)

C3

2i

0.3823(10)

0.5561(8)

0.0121(6)

0.85(9)

O1

2i

0.2081(13)

0.1577(9)

0.9074(8)

1.68(13)

O2

2i

0.2318(12)

0.0869(8)

0.1491(6)

1.44(14)

O3

2i

0.2838(11)

0.3073(8)

0.4159(7)

0.75(9)

O4

2i

0.2906(12)

0.4803(9)

0.6345(7)

1.17(11)

O5

2i

0.2927(10)

0.5127(8)

0.1302(7)

0.87(10)

O6

2i

0.7050(12)

0.3227(9)

0.0780(8)

1.66(15)

O7

2i

0.6917(14)

0.0816(9)

0.3325(9)

2.00(15)

H71

2i

0.784(3)

0.113(2)

0.4312(19)

4.3(3)

H72

2i

0.786(3)

-0.008(2)

0.2640(16)

4.1(3)

O8

2i

0.1614(15)

0.7980(11)

0.3891(8)

2.23(16)

H81

2i

0.202(2)

0.8495(19)

0.3011(15)

2.8(3)

H82

2i

0.001(2)

0.698(2)

0.3548(16)

3.6(3)

Atom

14


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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


LIST OF FIGURES

Figure 1. a) Room temperature 57Fe Mössbauer spectra of Fe2(C2O4)3·4H2O. b) and c) TGA curve decomposition coupled with mass spectroscopy for Fe2(C2O4)3·4H2O carried under air (b) and under argon (c). Figure 2. Combined Rietveld refinement of X-ray powder diffraction pattern (RBragg=4.69%, %, Rp=11.4%, χ2=2.36) and neutron diffraction for Fe2(C2O4)3·4H2O (RBragg=2.25%, Rp=7.61%, χ2=2.36). The red circles, black continuous line and bottom dark grey line represent the observed, calculated, and difference patterns, respectively. Vertical blue tick bars are the Bragg positions allowed in the triclinic P-1 cell. Figure 3: a) Representation of a unit cell content: Fe is blue, C is grey, H is purple, oxygen atoms part of oxalate molecules and water molecules are red and pink, respectively. Labels for C are indicated for clarity. b) Illustration of the hydrogen bonding. c) FeO6 are linked one to each other through two tetradentate oxalate molecules to form infinite zig-zag chains running along [001]. Figure 4: Structure of Fe2(C2O4)3·4H2O viewed perpendicular to the [010], [1-10], [100] and [001] directions. Notice that in the [100] direction, we can see the zig-zag chain formed through FeO6 octahedra linked to two tetradendate oxalate molecules (denoted “Ox” in the figure). Fe is blue, C is grey, H in purple, oxygen atoms part of oxalate molecules and water molecules are red and pink, respectively. Figure 5. Voltage versus composition profile for Fe2(C2O4)3·4H2O cycled at C/10 together with the corresponding derivative δx/δV curve (right). Capacity vs. cycle number is given as inset. Figure 6. In situ XRD patterns obtained for Fe2(C2O4)3·4H2O cycled at C/20. Patterns have been collected every two hours. Figure 7. Operando room temperature

57

Fe fitted Mössbauer spectra of Fe2(C2O4)3·4H2O

recorded during the electrochemical process at C/10 rate together with the voltage composition curve. Figure 8. Portion of the Rietveld refinement of the XRD patterns collected in situ for pristine Fe2(C2O4)3·4H2O and at the end of discharge Li1.8Fe2(C2O4)3·4H2O. Deduced lattice parameters and unit cell volumes are indicated. Figure 9. Comparison between iron coordination in humboldtine (FeC2O4·2H2O) and Fe2(C2O4)3·4H2O. The color code used is the same as in Figure 2. 15



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

Figure 10. a) and b) Structure of the discharged phase Li2Fe2(C2O4)3·4H2O as deduced by DFT+U and DFT-D2+U calculations, viewed along the [010] and [100] directions. Fe octahedra and Li tetrahedra are in blue and green, respectively. c) Illustration of the hydrogen bonding using the same orientation as in Figure 3b for the pristine material. Figure 11. Comparison between the average potential of different polyanionic compounds used as positive electrode for Li batteries.1, 41, 43-45 LiFeSO4FTri and LiFeSO4FTavo refer to the triplite and tavorite forms of LiFeSO4F, respectively. Figure 12. Temperature and pressure dependent phase diagram computed with dispersion correction within the DFT+U formalism for water release from the Fe2(C2O4)3⋅4H2O to form the Fe2(C2O4)3⋅4H2O. The water molecules removed from the structure are those which are not directly bonded to iron.

16


Page 16 of 29

Page 17 of 29

Transmission

Figure 1

1.00 0.96 0.92 0.88

a)

-4

-2

0

2

4

Velocity (mm/s) Mass (%)

100

b)

80 60

H2O

64%

CO2

40 20 0

100

200

300

400

500

Temperature (°C) 100

Mass (%)

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


60

c)

H2 O

80

CO2

40

CO

20 0

100

200

300

400

Temperature (°C)

17


500


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

Page 18 of 29

Figure 2

Fe2(C2O4)34H2O X-ray diffraction

10

20

30

40 50 2θ (°) - λCu

60

70

Fe2(C2O4)34H2O neutron diffraction

20

40

60

80

100

2θ (°) - λ=1.35821 Å

18


120

Page 19 of 29

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


Figure 3

a)

b)

c)

1.91 Å

2.14 Å 1.71 Å

1.83 Å

c

a c

b a

b

19


c


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

Page 20 of 29

Figure 4

a)

(1, -1, 0) Layers

b)

c)

Fe-Ox-Fe chains

d)

[1 -1 0]*

c a

a

c

c

b a

b

b

20


Page 21 of 29

Figure 5

4.0

15

2.5

2.0 0.0

dx/dV

-1

)

3.5 3.0

3.4V

10

Capacity (mAh.g

Voltage (V vs. 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


100

5

3.35V

0

75

-5

50 25 0 0

-10 2

0.5

4

6 8 10 12 14 n° cycles

1.0

3.3V

-15 1.5

2.0

2.5

X in LixFe2(C2O4)34H2O

3.0

3.5

Voltage (V vs. Li)

21


4.0


Discharge

Figure 6

12

Charge

Intensity (a.u)

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

Page 22 of 29

14

16

18

20

2θ (°), λCu

22


22

24

Page 23 of 29

Figure 7

1.0

X=0

Discharge

X=0.4 X=0.7

0.9

X=1 X=1.5

0.8

X=1.8

Charge

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


0.7

0.6 -2

0

2

4

Velocity (mm/s)

3.5

3.0

2.5

Voltage (V vs. Li)

23



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

Figure 8

24


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Page 25 of 29

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


Figure 9

a) FeIII2(C2O4)3⋅4H2O

b) FeII(C2O4)⋅⋅2H2O

25



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

Page 26 of 29

Figure 10

a)

b)

c) 1.77 Å

1.70 Å

2.06 Å

1.87 Å a

c

c a

c b

26


b

Page 27 of 29

Figure 11

5 LiFeSO4FTavo

Potential (V vs. 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


4 LiFePO4

3

Fe2(C2O4)3⋅ 4H2O

LiFePO4F MIL-53

LiFeSO4FTri

Li2Fe(SO4)2 Fe2(SO4)3 FeSO4OH Fe2O(SO4)2

LiFePO4OH

2 1

FePO4⋅2H2O

Phosphates

Li3FeCO3PO4

Fe-organic compounds

Sulfates

0 Fe-based positive electrodes for Li Batteries

27



Figure 12

1200

∆G P,T] T] (meV) (meV) ∆EH2O H2O[P,

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

Page 28 of 29

900

Fe2(C2O4)32H2O + 2 H2O

600 300 0 -300

Fe2(C2O4)34H2O

-600 100

200

300

400

Temperature (°C)

28


500

600

Page 29 of 29

TOC graphics

4.0

FeIII2(C2O4)3⋅4H2O

2.5

2.0 0.0

dx/dV

-1

3.0

)

3.5

Capacity (mAh.g

Voltage (V vs. 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


100 75 50 25 0 0

2

0.5

4

6 8 10 12 14 n° cycles

1.0

1.5

2.0

X in LixFe2(C2O4)34H2O

29


Unraveling the Structure of Iron(III) - American Chemical Society

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