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J. Phys. Chem. C 2010, 114, 10034–10044
A Layered Iron(III) Phosphate Phase, Na3Fe3(PO4)4: Synthesis, Structure, and Electrochemical Properties as Positive Electrode in Sodium Batteries Khiem Trad,†,‡ Dany Carlier,*,† Laurence Croguennec,† Alain Wattiaux,† Besma Lajmi,‡ Mongi Ben Amara,‡ and Claude Delmas† CNRS, UniVersite´ de Bordeaux, ICMCB site de l’ENSCBP, 87 aVenue du Dr. A. Schweitzer, Pessac F-33608, France, and UR:Mate´riaux Inorganiques, Faculte´ des Sciences de Monastir, AVenue de l’enVironnement, 5019 Monastir, Tunisia ReceiVed: January 26, 2010; ReVised Manuscript ReceiVed: April 15, 2010
For the first time, a powder of Na3Fe3(PO4)4 was obtained by solid state reaction. It crystallizes in monoclinic space group C2/c in good agreement with previous studies of a single crystal. The Rietveld refinement of the XRD pattern showed line broadening of some diffraction lines associated with size and strain effects. Its layered structure can be described by complex layers of corner-sharing FeO6 octahedra connected by PO4 tetrahedra through corner and edge sharing. The Na+ ions are located in the interslab space. The local environments of Fe, Na, and P were characterized by 57Fe Mo¨ssbauer spectroscopy and 23Na and 31P MAS NMR. A Second ORder Graphic Extrapolation (SORGE) diagram as introduced by Massiot et al. allowed us to fully interpret the 23Na MAS NMR spectrum that exhibits three signals for two crystallographic Na sites. The electrochemical properties of Na3Fe3(PO4)4 were tested in sodium cells. Ex situ and in situ X-ray diffraction data and Mo¨ssbauer spectroscopy measurements indicate that the intercalation-deintercalation process of Na+ ions in Na3Fe3(PO4)4 is reversible and that the structural framework is maintained during cycling. Some degree of disorder is, however, observed for a large intercalated Na+ amount. 1. Introduction Iron phosphates have a rich and complex structural chemistry. Synthesis and structural characterization of these compounds are of great interest in terms of fundamental research and for numerous practical applications such as heterogeneous catalysis and1 corrosion inhibition2 and more recently as a positive electrode for lithium ion batteries. Since the discovery of highly interesting electrochemical properties for LiFePO4,3,4 the search for novel polyanion-based insertion hosts is intense.5-11 Few phosphate materials were tested as positive electrodes in sodium cells.12-15 In this study, we were interested in a new sodium iron phosphate phase, Na3Fe3(PO4)4, recently reported by some of us.16 The structure of this compound was first determined from single crystals16 and appeared to be similar to those of Na3Cr3(PO4)4 and K3Fe3(PO4)4 · H2O.17,18 It exhibits a layered structure with layers perpendicular to the a direction built of corner-sharing FeO6 octahedra and PO4 tetrahedra that make a complex framework with Na+ ions inserted between these layers. From a structural point of view, Na3Fe3(PO4)4 can allow both ionic and electronic conductivities and was therefore interesting as intercalation positive electrode material for sodium and lithium batteries. In this paper, we report the synthesis of Na3Fe3(PO4)4 as powder, the study of its structure by XRD, and the characterization of the local environments of iron, sodium, and phosphorus ions by Mo¨ssbauer and NMR spectroscopies. The cycling properties of sodium cells with Na3Fe3(PO4)4 used as a positive electrode will be first presented, followed by the study of the * To whom correspondence should be addressed. Phone: +33 (0) 5 40 00 31 75. Fax: +33 (0) 5 40 00 27 61. E-mail:
[email protected]. † Universite´ de Bordeaux. ‡ UR:Mate´riaux Inorganiques.
evolution of the structure during the first electrochemical cycle. Electrochemical performances in lithium cells will be published elsewhere. 2. Experimental Section The Na3Fe3(PO4)4 powder sample has been synthesized through a solid state reaction conducted under an oxygen atmosphere. Stoichiometric quantities of Na2CO3, FeC2O4 · 2H2O, and (NH4)H2PO4 were intimately mixed for 2 h with a tridimensional mixer. The XRD characterization of the resulting mixture shows that it consists of an ammonium iron phosphate hydrate (NH4)FePO4 · H2O and Na2CO3, which was then ball milled for 1 h in a planetary mill using an agate vessel and agate balls. These precursors were thermally treated under an oxygen flow successively for 6 h at 400 °C, 6 h at 600 °C, and then 3 days at 750 °C, with intermediate grinding between each thermal treatment. A light yellow powder was finally obtained when the product was quenched in air. Note that the stoichiometry of the mixture and the intimate mixing of the precursors had to be absolutely respected to obtain a pure phase. Otherwise, NaFeP2O7 and/or Na3Fe2(PO4)3 impurities were observed. The XRD pattern of the initial Na3Fe3(PO4)4 phase was collected using a Panalytical X’pert Pro diffractometer (Cu KR1 radiation, antiscatter slit of 1/2°, and divergence slit of 1° on the incident beam path). The diffraction pattern was recorded in the 5-120° (2θ) angular range using a 0.0167° (2θ) step and a constant counting time of 10 s. The Rietveld refinement of the XRD data was performed using Fullprof.19 The grain size and morphology characteristics of the synthesized powder were obtained by means of scanning electron microscopy (SEM) analysis using a Hitachi S-450 apparatus. The powder was metallized by gold-palladium plasma. 23 Na magic angle spinning (MAS) NMR spectra was recorded at 79.403 and 132.302 MHz with, respectively, a 7.05 T
10.1021/jp100751b 2010 American Chemical Society Published on Web 05/06/2010
A Layered Iron(III) Phosphate Phase, Na3Fe3(PO4)4 magnetic field on a Bruker 300 Avance spectrometer and with a 11.75 T magnetic field on a Bruker 500 Avance spectrometer. The powder was placed in a zirconia rotor. A rotor spinning rate of 30 kHz was used. As 23Na is a quadrupolar nucleus with an I of 3/2, a short pulse length of 1 µs corresponding to a selective π/12 pulse determined using an aqueous 0.1 mol/L NaCl solution was employed in a single pulse sequence. The spectral width was set to 1 MHz, and the recycle time (D0 ) 0.5 s) is sufficiently long to prevent T1 saturation effects with 1600 scans per spectrum. The baseline distortions resulting from the spectrometer dead time (5-10 µs) were computationally removed using a polynomial baseline correction routine. The external reference was a 0.1 mol/L NaCl aqueous solution. 31 P magic angle spinning (MAS) NMR spectra with a 30 kHz spinning rate were recorded at 40.834 MHz on a Bruker 300 Avance spectrometer with a magnetic field that was lowered to 2.35 T. A rotor-synchronized Hahn echo sequence was used with a 1.2 µs pulse length corresponding to a π/2 pulse. The spectral width was set to 1 MHz, and the recycle time (D0 ) 1 s) is sufficiently long to prevent T1 saturation effects with 56000 scans per spectrum. The external reference was the 31P signal of AlPO4. For all fits of the NMR spectra, DMfit was used.20 The positive electrode was made of 80% active material, 15% carbon black conductive additive, and 5% PTFE binder. The mixture was homogenized by ball milling for 15 min and then dried under vacuum at 100 °C overnight. In an argon-filled glovebox, the mixture was then crushed into a thin sheet and finally cut into a disk (14 mm diameter), and Swagelok type cells were assembled. Metallic sodium was used as a negative electrode, and a 1 M NaClO4 solution in propylene carbonate (PC) was used as an electrolyte with glass microfiber filters (Whatman) as separators. The electrochemical tests were conducted in the galvanostatic mode with a homemade apparatus or with a Biologic VMP1 system. The open circuit voltage (OCV) measurements consisted of 1 h discharges at a C/50 rate followed by open circuit periods achieved when a 1 mV/h stability criterion is reached for the end of the relaxation period. In situ and ex situ X-ray diffraction measurements were performed on a PANalytical X’pert Pro diffractometer with Co KR radiation. For in situ X-ray diffraction studies, a special cell was created homemade with a beryllium window. The diffraction patterns were recorded operando at room temperature in the angular range of 9-58° (2θ) in steps of 0.0167° (2θ) every 2 h (∼0.03 Na+ inserted). 57 Fe Mo¨ssbauer spectroscopy was also used to identify the nature of iron ions present in the material (environment and oxidation state). These measurements were performed at room temperature in transmission geometry with a constant acceleration spectrometer using a 57Co source in a Rh matrix equipped with a cryostat. The velocity was calibrated by using pure iron metal as the standard material at room temperature. All isomer shifts reported in this work refer to the natural R-Fe at 293 K. The spectra were fitted to Lorentzian profiles using the leastsquares method. The Mo¨ssbauer spectrum of Na3Fe3(PO4)4 was recorded over a large velocity range (from -10 to 10 mm/s) to ensure that Fe2O3 which is magnetic at room temperature was not present as an impurity in the sample. Ex situ 57Fe Mo¨ssbauer spectra of the electrodes at the end of the first discharge and charge were recorded. As the electrodes made at low potentials could be easily oxidized, they were handled with caution and specific tied sample holders were used for analysis. The electrodes were sealed under an argon atmosphere in a sample holder which was transparent to the γ-rays. To ensure that our
J. Phys. Chem. C, Vol. 114, No. 21, 2010 10035
Figure 1. SEM micrographs of Na3Fe3(PO4)4 powder.
analysis was quantitative, it was necessary to evaluate the value of the Lamb-Mo¨ssbauer factor (f) (the recoil-free fraction of iron atoms that contributes to the Mo¨ssbauer effect) of each type of iron ion. Some measurements were therefore conducted at a low temperature (T ) 40.5 K) and confirmed that all Fe ion types have similar f factors. 3. Results and Discussion (a) Characterization of Na3Fe3(PO4)4 as a Powder. Na3Fe3(PO4)4 crystallizes as a fine-grain light yellow powder. As expected from results already obtained for single crystals, all the diffraction peaks of the X-ray diffraction pattern could be indexed using a monoclinic cell described in the C2/c space group.16 The powder sample was pure and well-crystallized with rather small full width at half-maximum values [
