Spectroscopic and Chemical Imaging Analysis of Lithium Iron

J. Phys. Chem. C 2007, 111, 1049-1054

1049

Spectroscopic and Chemical Imaging Analysis of Lithium Iron Triphosphate C. V. Ramana,*,† A. Ait-Salah,‡ S. Utsunomiya,† J.-F. Morhange,‡ A. Mauger,§ F. Gendron,‡ and C. M. Julien‡ Nanoscience and Surface Chemistry Laboratory, Department of Geological Sciences, UniVersity of Michigan, Ann Arbor, Michigan 48109; Institut des Nano-Sciences de Paris (INSP), UniVersite´ Pierre et Marie Curie, CNRS-UMR 7588, Campus Boucicaut, 140 rue de Lourmel, 75015 Paris, France; and De´ partement MPPU, CNRS, Campus Boucicaut, 140 rue de Lourmel, 75015 Paris, France ReceiVed: August 7, 2006; In Final Form: October 19, 2006

A novel material, namely, Fe-containing lithium-metal triphosphate (LiM2P3O10; MdFe), has been synthesized by wet-chemical method. The phase shows the monoclinic structure (P21/m≡C22h). The crystal chemistry and chemical structural characteristics have been analyzed by X-ray-diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Surface chemical composition, valence state, and chemical bonding and local structure analysis have been performed by X-ray photoelectron (XPS) and Raman scattering (RS) spectroscopy. EDS and HAADF-STEM indicate the chemical homogeneity and purity of LiFe2P3O10 samples. Both XPS and RS experiments show the existence of an amorphous carbon deposit at the surface of the LiFe2P3O10 particles. Comparison between XPS results for LiFePO4 olivine and LiFe2P3O10 revealed the change in the binding energy of Fe(II) ions. Studies of the local structure characterized by Raman scattering show the evolution of the chemical bonding with the increased number of interconnected PO4 oxo-anionic groups. The electrochemical characteristics of LiFe2P3O10 demonstrate their applicability in lithium batteries.

I. Introduction Development of high-energy density rechargeable lithiumion batteries (RLBs) demands suitable active electrode materials exhibiting high potential versus Li+/Li0, good charge/discharge cycleability, and high thermal stability. Lithium iron phosphates form an important group of ceramic compounds that are now being investigated extensively as prospective high-capacity positive-electrode materials for RLBs.1-7 Phospo-olivine LiFePO4 was the first to be proposed and demonstrated as an Li intercalation electrode by Padhi et al.2 However, this compound presents the obvious considerations of low-cost and environmental benignity. The master advantage of phosphate materials built by polyanions such as (PO4)3-, (P2O7)4-, and (P3O10)5- is their thermal stability and improved safety.1-7 One of the key aspects of this class of materials is the major structural anomalies, such as anisotropic deformation, significant loss of long-range order, local phase segregation, and clustering impurities, which are strongly dependent on the synthesis conditions.8 The search for iron-phosphate materials having Fe2+ ions has attracted our particular interest toward a framework built with (P3O10)5- oxoanions. It is expected that the strong inductive effect of the assembly of interconnected (PO4)3- polyanions moderates the Fe2+/Fe3+ redox couple to generate a high-operating potential. Most of the materials containing the (P3O10)5- groups are hydrated systems in which hydrogen-bonding interactions play a role in determining the packing.9-10 Two anhydrous tripolyphosphates (LiM2P3O10) with MdCo, Ni have been identified,11-12 * Author to whom correspondence should be addressed. Tel: 734-7635344; fax: 734-763-4690; e-mail: [email protected]. † University of Michigan. ‡ Universite ´ Pierre et Marie Curie. § De ´ partement MPPU, CNRS.

but no reports are available for Fe-bearing material. We report our most recent work on the synthesis and characterization of lithium iron tripolyphosphate (LiFe2P3O10) in this paper. LiFe2P3O10 materials were synthesized using a wet-chemical method, and characterization was performed using a variety of spectroscopic and chemical imaging analytical techniques to probe the crystal chemistry, chemical composition, local chemical environment, and electrochemistry. The results obtained are presented and discussed to demonstrate their application in batteries. II. Experimental Details A. Synthesis. Solid-state synthesis approach has relied on the use of Fe2+ precursor compound, typically, iron(II) acetate. LiFe2P3O10 powders were synthesized by a sol-gel method at a moderate temperature using metal acetates, Li(OOCCH3)2H2O, Fe(OOCCH3)2‚H2O, and phosphoric(V) acid H3PO4 (Fluka purum grade), as starting materials. Raw materials were mixed in a molar ratio of Li:Fe:P ) 1.05:1:1 in N,Ndimethylformamide. The solvent was evaporated at 120 °C in air. The solid dry residue, obtained after careful grinding, was preheated at 350 °C for 2 h, and successive grinding operations were performed before the final heat treatment at 600 °C for 24 h. The product was heated in a flow of Ar/H2 gas (85:15) to prevent any oxidation of Fe to form Fe3+-based impurities.8 A second series of LiFe2P3O10 powders were synthesized using metal nitrates as raw materials following the same procedure. B. Characterization. X-ray diffraction (XRD) patterns were recorded in a Philips X’Pert PRO MRD (PW3050) diffractometer equipped with a Cu anticathode (CuKR radiation λ ) 1.54056 Å) at room temperature. The patterns were recorded under Bragg-Brentano geometry at 2θ with a step of 0.05° in the range 10-80°. A Kratos Axis Ultra XPS system, with

10.1021/jp065072c CCC: $37.00 © 2007 American Chemical Society Published on Web 12/22/2006

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Figure 1. XRD pattern of LiFe2P3O10 synthesized from sol-gel method using metal acetates as raw materials. Microcrystalline powders were obtained after heating at 600 °C for 24 h in Ar/H2 (85:15) ambient. The insert shows the typical SEM image of lithium iron triphosphate powders.

monochromatic Al KR X-rays (hν ) 1486.6 eV), was employed for X-ray photoelectron spectroscopy (XPS) analysis. The pass energy and step size were 160 and 1 eV for survey scans and 10 and 0.1 eV for detailed core scans, respectively. The charging effects were accounted with reference to the binding energy of C 1s core level at 284.6 eV. Energy-dispersive X-ray spectrometry (EDS) and high-angle angular dark field scanning transmission electron microscopy (HAADF-STEM) experiments were performed in a Jeol TEM2010F at a 200 kV acceleration voltage. EDAX Genesis software was used for chemical analysis. Samples were prepared by dispersing LiFe2P3O10 on 3-mm Cu grid with a hole size of 1 × 2 mm. Elemental mapping was made using Emispec ES Vision version 4.0 of the STEM-EDX mapping system. To minimize the effect of sample drift, a drift-correcting mode was used during the acquisition of EDX maps. Specification of STEM is listed as follows: Cs is 1.0 mm, probe size is 0.5 nm for analysis and 0.2 for high-resolution imaging, the collection angle of HAADF detector is 50-110 mrad, the objective aperture size is 30 µm for analysis and 20 µm for high-resolution imaging, and the defocus condition is approximately -55 nm. Gatan Digital Micrograph 3.4 was used for image analysis. Raman spectra (RS) were recorded between 100 and 1800 cm-1 in a quasi-backscattering configuration. A double monochromator (Jobin-Yvon model U1000) with holographic gratings and a computer-controlled photon-counting system was used. The source was 514.5-nm line of an Ar+ ion laser at a low power level of 10 mW (to avoid photodecomposition, if any). Each RS spectrum is the average of 12 successive scans (resolution: 2 cm-1) to ensure the high signal-noise ratio. Raman spectra were fitted using the GRAM/386 software from Galatic Industries Co. Electrochemical studies were made in a cell using 1 M LiPF6-EC-DEC as nonaqueous electrolyte at an operating temperature of 25 °C. Cycling was made galvanostatically at a constant current density of 0.5 mA/cm2 between 2.7 and 3.9 V. III. Results and Discussion A. Crystal Chemistry and Surface Morphology. The XRD pattern of LiFe2P3O10, as shown in Figure 1, was indexed in the monoclinic structure with space group P21/m≡C22h. The

Ramana et al.

Figure 2. Schematic representation of the polyhedral structure of LiFe2P3O10. The triphosphate network is built by FeO6 octahedra and P3O10 groups forming tunnels within which the Li+ ions are located.

pattern is dominated by the intense (120) line (d ) 3.11 Å) and six lines with moderate intensity, which are indexed as (101), (111), (1h02), (1h21), (112), and (1h03), respectively. The lattice parameters evaluated from XRD data are a ) 4.597(7) Å, b ) 8.566(4) Å, c ) 9.051(4) Å, β ) 97.47(2)°, and V ) 353.5 Å.3 The (P3O10)5- groups are formed by the linear chain consisting of three shared corner (PO4)3- units (Figure 2). The main interest in the atomic arrangement in LiFe2P3O10 is the new type of configuration for (P3O10)5- groups. The three P atoms and the two O oxygen atoms are located on the mirror plane of the internal symmetry of the oxo-anions with successive PO3 moieties displaying both staggered and eclipsed conformations.13 Thus, the average value of the P-O-P bridging angle is 134°, which is close to that for Na5P3O10.14 Fe atoms display distorted octahedral coordination. Each octahedron shares common edges with the two adjacent ones in FeO6 chain. Li ions are located on an inversion center with four O atoms as near neighbors with average Li-O bonds of 2.09(5) Å. This structure is characterized by channels parallel to the b-axis where Li ions are located in rectangular coordination in the ab plane. The typical SEM image (insert of Figure 1) reveals the grain morphology of LiFe2P3O10 with a particle size in the range of 0.3-0.5 µm and homogeneous distribution. Monoclinic grains have smooth surfaces with rounded edges. Decreasing the annealing temperature led to a further lowering of the crystallinity of powders. B. Elemental Analysis and Distribution Characteristics. The EDS and HAADF-STEM measurements indicate that the grown materials are stoichiometric and homogeneous with a uniform distribution. The characteristic peaks of Fe, P, and O are evident in EDS of LiFe2P3O10 shown in Figure 3. It is not possible to detect Li for the obvious reason that the X-ray fluorescence yield is extremely low for elements Li and Be. X-ray energy is characteristic of generating atom and, therefore, detection of X-rays emitted provides the signature of the atoms present.15,16 Therefore, EDS measurements can be used to qualitatively discuss the chemical quality of LiFe2P3O10. The lines identified are O KR1, P KR, Fe KR, Fe Kβ, Fe LR, and Fe Lβ at their respective energy positions, indicating that the X-rays are only due to Fe, P, and O. X-ray lines from the Cu grid can also be seen (Figure 3), which is hard to eliminate but can be used as a reference. Quantitative analysis of EDS results indicates that the P/Fe atomic ratio is close to the expected value

Analysis of Lithium Iron Triphosphate

J. Phys. Chem. C, Vol. 111, No. 2, 2007 1051 TABLE 1: Binding Energies of the Core-Level Peaks of Elements in LiFe2P3O10 Compared with Data of LiFePO4 Olivine and Other Compounds binding energy (eV) material LiFe2P3O10 LiFePO4 LiFePO4 Li (metal) LiF LiBr LiOH Li2O Li2CO3 LiMn2O4 Na3PO4 Na2HPO4

Figure 3. EDS spectrum of LiFe2P3O10 powders grown by sol-gel method using acetate raw materials. The spectrum exhibits the characteristic peaks of Fe, P, and O present in the sample. The X-ray lines are as indicated at their respective energy positions.

Figure 4. HAADF-STEM image of LiFe2P3O10 particle grown by solgel method using acetate raw materials. The images labeled Fe, P, and O represent the elemental mapping of the respective elements.

(P/Fe ) 1.5). The absence of either dopants or impurities in EDS is an indication of the chemical quality of the samples. The HAADF-STEM elemental mapping images of LiFe2P3O10 are shown in Figure 4. The images labeled Fe, P, and O are those obtained for respective elements in the particle. These images are uniform over the particle examined. X-rays emit from Fe, P, and O at the same location and region indicating the chemical homogeneity. C. Chemical Composition and Valence-State Analysis. XPS has been used to characterize various lithium-containing compounds,17-20 but data on lithium iron phosphates are rather scarce (see data for LiFePO4 in Table 1).21,22 The core-level scans of each and every individual element in LiFe2P3O10 are shown in Figure 5. All the spectra can be deconvoluted mainly into one or two peaks with Gaussian shape. The Li 1s core level scan of LiFe2P3O10 has a low intensity with an asymmetric shape (Figure 5a). The deconvolution of Li 1s gives two peaks for its best fit indicating the existence of Li in different forms. The binding energy (BE) of the main peak is at 55.5 eV, which is higher than that of Li metal (54.8 eV).23-25 Comparison of this Li 1s BE with that reported for various compounds (55-56 eV; Table 1)21-32 confirms the existence of monovalent Li+ ions in LiFe2P3O10. The shape of the P 2p core level is Gaussian-like with a BE of 133.3 eV and a full width at half-maximum (fwhm) of 1.6 eV (Figure 5b). This typical value of BE represents P bonded to O.27-29 Comparing the reported BE values of different compounds (132.2-132.9 eV; Table 1),21,33,34 the P 2p core level at 133.3 eV in this work is attributed to P5+ state resulting from PO43- group.

Li 1s

P 2p

O 1s

55.5 133.4 530.9 55.0 133.2 531.2 55.3 54.8 55.5-56.8 55.6 54.7 531.1 55.0 529 55.0 531.5 55.0 530.1 132.7 530.7 132.9

Fe 2p3/2 ∆E(Fe 2p) 711.6 711.0 710.6

13.5 13.0 13.4

reference this work 21 22 23-25 26-29 27 30 27, 31, 32 32 19 33, 34 33

The shape of O 1s core level is an overlap of two peaks at about 530.9 and 532.5 eV with a shift ∆EO1s ) 1.6 eV (Figure 5c). The latter component is a shoulder at higher BE energy side of the main peak. The two-peak structure of O 1s, which is common in transition-metal oxides, is usually ascribed to impurity Li-oxides on the surface.17-20,35 The surface admixtures of lithium carbonate and hydroxide brings the O 1s BE close to 531 eV (Table 1).30,32 Sherwood and co-workers36-40 have performed extensive studies, using valence-band and core-level XPS measurements, on several P-O containing compounds and have attributed the overlapping two-peak structure of O 1s to terminal (lower BE component) and bridging (higher BE component) oxygens. Therefore, the two possibilities for the observed O 1s features in the present work are either the surface admixtures or different types of oxygen. Following the arguments of Sherwood and co-workers ,36-39 we attribute the peak at 530.9 eV to the terminal oxygens while the component at 532.5 eV to the bridging oxygens. The reason for this assignment is based on two facts. The observed BE (532.5 eV) of the higher component peak is in good agreement with the reports of Sherwood and co-workers rather than that of surface admixtures (531.2 eV). The second and important fact is that it is consistent with the crystal chemistry of LiFe2P3O10 in which PO43- tetrahedral joined to give a P3O105- oxo-anion with two types of oxygens, P-O oxygen atoms attached to each of the three P atoms and two P-O-P bridging oxygen atoms. Furthermore, surface admixtures and water contamination are unlikely because of the sintering temperature (600 °C) and high vacuum during XPS analysis. However, the possibility of surface admixtures cannot be ruled out completely as the sample was grown by a wet-chemical method. The components of Fe 2p doublet (Fe 2p3/2 and Fe 2p1/2), because of spin-orbit splitting, are observed at 711.6 and 725.1 eV, respectively, with an energy separation (∆EFe) of 13.5 eV (Figure 5d). The data are in excellent agreement with those reported for LiFePO4.21,22 XPS confirms the presence of Fe as Fe2+ in LiFe2P3O10. The peak observed at 284.3 eV (Figure 5e) is attributed to the C 1s core level that originates from carbon coated on LiFe2P3O10. Our XPS data are consistent with what is expected for LiFe2P3O10 on the basis of the structure and chemistry. D. Chemical Bonding and Local Structure. Raman spectra of LiFe2P3O10 synthesized from acetates and nitrates are shown in Figure 6. We first consider the case of LiFe2P3O10 grown using nitrates (curve a). The vibrational spectra (150-1200 cm-1) can be divided into two groups corresponding to the internal and external modes of the tripolyphosphate structure. The vibrational response of phosphate-based materials usually exhibits dominant features (150-1200 cm-1) because of

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Figure 5. XPS core-level spectra of Li 1s, P 2p, O 1s, and Fe 2p core level of LiFe2P3O10 sample grown by sol-gel method using metal acetate as raw materials. The spectrum of the C 1s core level is due to the carbon coating at the surface of particles.

strongly covalent PO4 groups.16,41 Therefore, RS features can be explained using the intermolecular model involving PO4 and FeO6 units and additional bands because of Li-O bonds. The free phosphate ions are regular tetrahedral (Td symmetry) with internal modes: (a) the asymmetric stretching ν3 modes as a multiplet in the range 1000-1150 cm-1, (b) the symmetric stretching ν1 mode as a singlet at 960 cm-1, (c) the asymmetric bending ν4 modes as doublet at 650 and 635 cm-1, and (d) the symmetric bending ν2 modes below 600 cm-1, which are all Raman active. Incorporation of PO43- oxo-anions in LiFe2P3O10 crystal decreases symmetry and induce changes in the RS spectrum (Figure 6) when compared to those of orthophosphates or pyrophosphates.41,42 However, features in the low-frequency region are quite similar to those recorded for olivine LiFePO4.41 Nevertheless, the RS features of LiFe2P3O10 are in good agreement with the molecular model. The symmetric (ν1)

stretching vibration appears at 995 cm-1 and the dominant bands due to asymmetric (ν3) stretching vibrations appear as multiple components located at 1066, 1086, 1119, and 1177 cm-1. The low-frequency region involves modes primarily due to the translation and vibrational motion of PO4 groups and translation motion of the Fe2+ ions. These modes are sensitive to the presence of impurities and polyhedral distortion.5 Two specific spectral features that appear in RS spectra that provide fingerprints of LiFe2P3O10 are the following. The bands at 1212 and 1246 cm-1, in the high-frequency region, which are assigned to the terminal-stretching mode of P3O105- are the first. These modes represent the Raman-active species (B2g + B3g) and are reported at 1181 and 1208 cm-1 for Li3Fe2(PO4)3.41 Second is the presence of additional bands in “gap region” that separate the stretching and bending vibrations, which are located at 700 and 736 cm-1. They cannot arise from a factor group splitting

Analysis of Lithium Iron Triphosphate

J. Phys. Chem. C, Vol. 111, No. 2, 2007 1053

Figure 7. The electrochemical charge-discharge profile of Li// LiFe2P3O10 cell. The insert shows the plot of the capacity retention against cycle number for LiFe2P3O10 prepared using acetates. Figure 6. Raman scattering spectra of LiFe2P3O10 sample synthesized by sol-gel method using (a) metal nitrates and (b) metal acetate as raw materials. The presence of carbon deposit at the particle surface is clearly evidenced by the appearance of G- and D-band of carbon for the sample prepared with (COOCH3) groups.

and are attributed to the asymmetric νas′ stretching modes of P-O-P bridges. This feature is a fingerprint of connected PO4 tetrahedral groups to form P2O7 or P3O10 oxo-anions.27 It is obvious (Figure 6) that triphosphate groups are bent in LiFe2P3O10 structure as the symmetric and asymmetric bridge vibrations both are Raman active. The P-O-P bridges of P3O105- groups may be considered as an independently vibrating unit. Within the limits of this approximation, its stretching frequencies depend on the bridging angle θPOP (134°, from XRD) and also on the force constant of P-O bond of the bridge. The existence of carbon on LiFe2P3O10 grown using acetates is indicated by the presence of two broad Raman bands at 1585 and 1360 cm-1 (Figure 6, curve b). These bands, which are signatures of an amorphous carbon deposit, are completely absent for samples synthesized from nitrates. The band at 1585 cm-1 mainly corresponds to the G-line associated with the optical allowed E2g zone center mode of crystalline graphite.43 The band at 1345 cm-1 mainly corresponds to the D-line associated with disordered-allowed zone edge modes of graphite.43 These features are well documented in the literature.43,44 Knight and White43 made a comprehensive study to relate the D and G features to the structure of disordered graphitic film while Yoshikawa et al.44 reported their relation to physical properties. The differing disordered/graphene (D/G) ratio obtained for LiFe2P3O10 from the deconvolution of RS bands with four Gaussians components indicates that it is amorphous graphitic carbon, where the carbon atoms are essentially threecoordinated and are bound by sp2 type hybrid orbitals. The carbon film is hydrogenated (a-C:H), which is not surprising since processing involved organic precursor with postannealing in Ar/H2-atmosphere. The Raman spectra of hydrogen-free carbon films can be distinguished from those of hydrogenated films by an additional broad feature centered at 600 cm-1.45 The factors governing the structure of carbon produced by pyrolysis of organic compounds are poorly understood at present, but the low D/G ratio is due to conductive carbons. This explains the enhanced electrical conductivity 4 × 10-8 to 1 × 10-5 S/cm for LiFe2P3O10 upon carbon coating. E. Electrochemical Performance. Charge-discharge profile of Li//LiFe2P3O10 cell, which was cycled between 2.7 and 3.9 V at the C/10 rate, is shown in Figure 7. The insert shown in

Figure 7 is the plot of the capacity retention against cycle number for LiFe2P3O10 prepared using acetates. A reversible capacity of about 70 mAh/g is obtained when the cell was charged up to 3.9 V. For the first cycle, the resulting incremental capacity indicates a totally reversible oxidation-reduction process that corresponds to a stable structure. The potential of Fe2+/Fe3+ couple is slightly lower (3.15 V) in the LiFe2P3O10 structure than that for LiFePO4 (3.45 V) but is higher than that for LiFeP2O7 (2.90 V). The capacity of C-LiFe2P3O10 slightly decreases with respect to cycle number (0.24% per cycle) whereas the capacity of carbon-free LiFe2P3O10 falls off rapidly. IV. Summary and Conclusions The synthesis, structure, surface chemistry, and electrochemical properties of LiFe2P3O10 compounds synthesized by solgel method have been studied in detail. The chemical homogeneity and purity of the samples was confirmed by the absence of impurities in EDS and homogeneous elemental distribution characteristics in HAADF-STEM imaging. XPS data are consistent with what is expected for LiFe2P3O10 on the basis of the chemical structure and valence state and confirms the presence of Li+, Fe2+, and P5+ ions. The local structure and chemical bonding of LiFe2P3O10 probed by Raman spectroscopy has been correlated with chain of the PO43- oxo-anionic groups. The carbon coating at the surface of LiFe2P3O10 prepared using acetates has been evidenced by XPS and Raman spectroscopy. The differing disordered/graphene (D/G) ratio obtained on the C-LiFe2P3O10 particles from the deconvolution of RS bands with four Gaussians components indicates that the structure of the deposit is an amorphous graphitic carbon where the carbon atoms are essentially three-coordinated and are bound by sp2 type hybrid orbitals. The carbon film is hydrogenated because of the organic precursor and postannealing in partial H2 atmosphere. Electrochemical data of LiFe2P3O10 demonstrate their application in lithium batteries. Acknowledgment. Mr. M. Selmane is gratefully acknowledged for his assistance in XRD measurements. References and Notes (1) Nanjundaswamy, K. S.; Padhi, A. K.; Goodenough, J. B.; Okada, S.; Ohtsuka, H.; Arai, H.; Yamaki J. Solid State Ionics 1996, 92, 1. (2) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1188.

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