X-ray and neutron diffraction, 57Fe Mössbauer spectroscopy and X-ray absorption spectroscopy studies of iron-substituted lithium cobaltate

PERGAMON

Solid State Communications 115 (2000) 1–6 www.elsevier.com/locate/ssc

X-ray and neutron diffraction, 57Fe Mo¨ssbauer spectroscopy and X-ray absorption spectroscopy studies of iron-substituted lithium cobaltate R. Alca´ntara a,*, P. Lavela a, C. Pe´rez Vicente a, J.L. Tirado a, J. Olivier-Fourcade b, J.C. Jumas b a b

Laboratorio de Quı´mica Inorga´nica, Edificio C3, Primera Planta. Campus de Rabanales, Universidad de Co´rdoba, 14071 Co´rdoba, Spain Laboratoire des Agre´gats Mole´culaires et Mate´riaux Inorganiques UPR ESA 5072, Universite´ Montpellier II, Place Euge`ne Bataillon, 34095 Montpellier, France Received 15 February 2000; received in revised form 20 March 2000; accepted 28 March 2000 by M. Cardona

Abstract Powdered solids with LiFe0.1Co0.9O2, LiFe0.2Co0.8O2 and LiFe0.2Co0.6Ni0.2O2 compositions have been prepared by ceramic procedures in the presence of excess lithium. The combined refinement of X-ray and neutron diffraction data reveals that cation substitution takes place in the octahedral sites with an almost complete absence of tetrahedrally coordinated iron. The 57Fe Mo¨ssbauer spectra of these solids agree with this model and show a non-random distribution of Fe and Co ions in octahedral sites. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: B. Chemical synthesis; C. Crystal structure and symmetry; C. X-ray scattering; D. Order–disorder effects; E. Neutron scattering

1. Introduction The ion-transport properties through solid lattices are directly dependent on the distribution and oxidation state of the lattice-forming ions. Ion mobility is of prime interest in the study of superionic conductors (SICON) and intercalation compounds. During the last decades, particular attention has been paid to the lithium intercalation properties of numerous oxides with a view to their potential—and unfortunately less frequently—practical application as active electrode materials in advanced energy storage systems. The layered solid with a LiCoO2 stoichiometry was successfully used as active electrode material in the first commercial battery product of “Li-ion” technology [1]. Initially, LiNiO2 showed a poorer performance than LiCoO2, mostly due to two possible effects: mixing of atomic layers and partial reduction of nickel. These effects lead to (Li1⫺xNix)3b(Ni1⫺xLix)3aO2 and (Li1⫺xNix)3b(Ni)3aO2  cation distributions in the equivalent sites of the R3m * Corresponding author. Tel.: ⫹34-957-218-637; fax: ⫹34-957218-606. E-mail address: [email protected] (R. Alca´ntara).

space group, respectively [2,3]. The presence of nickel atoms in the 3b lithium layers puts obstacles to lithium diffusion and has negative consequences on cell performance by inducing polarization effects. Nevertheless, the use of LiCoO2 has important limitations emerging from the higher price and lower availability of cobalt as referred to structurally similar oxides with nickel or other first-row transition metal in their composition. This has prompted several authors to study different mixed compounds with similar rhombohedral structures, such as Li(Co,Ni)O2 [4,5], Li(Al,Ni)O2 [6,7], Li(Fe,Co)O2, [8,9] and Li(Fe,Ni)O2 [10,11]. Most of these systems have been described as statistical cation-distributions of the tripositive metal ions in 3a sites. However, recent studies carried out on Li(Al,Co)O2 evidenced that a single structural model may not always be followed [12]. Although the X-ray diffraction patterns of this new material revealed a single-phase product  structure for y ⬍ 0:7; 27Al MAS NMR data with R3m evidenced the coexistence of tetrahedral and octahedrally coordinated aluminum. For Fe-doped LiCoO2 the hydrothermal synthesis has been reported [8], yielding a partial Fe/Co substitution in  space group. Recently [9], we reported 3a sites of the R3m

0038-1098/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(00)00138-1

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Fig. 1. Simultaneous Rietveld refinements of neutron and XPD obtained at room-temperature (upper: neutron data, lower: X-ray data) for two selected compositions: LiFe0.1Co0.9O2 and LiFe0.2Co0.8O2. 57

Fe Mo¨ssbauer spectroscopy evidence that some pseudotetrahedral 6c iron was present in LiFexCo1⫺xO2 …y ⱕ 0:2† single phase solids obtained by a conventional ceramic method. The aim of this work is to study the products with LiFe0.1Co0.9O2, LiFe0.2Co0.8O2 and LiFe0.2Co0.6Ni0.2O2 compositions of a new ceramic synthesis in which the amount of tetrahedral ion has been minimized. Attempts to resolve cation distributions in these mixed oxides by X-ray diffraction procedures is complicated by the similar X-ray scattering factors of Co 3⫹ and Fe 3⫹ ions and the low scattering factor of Li ⫹ ions. Thus, the use of neutrons is of great value, as the bound coherent scattering cross-sections (s c) of cobalt and iron nuclei differ significantly (s c ˆ 12:64 and 0.79 b for 56Fe and 59Co, respectively), and the value of s c for 7Li is also of the same order of magnitude (0.619 b) [13]. Moreover, the information that can be extracted from neutron diffraction data on these layered materials may include the unequivocal identification of atoms in non-octahedral coordination, ordering of Co and Fe nuclei, and/or the presence of magnetic ordering. Complementary information was also obtained by XAS and Mo¨ssbauer spectroscopy.

2. Experimental Lithium-transition metal mixed oxides LiFexCo1⫺xO2 …0 ⱕ y ⱕ 1† with different nominal compositions were prepared from Fe2O3 and Co3O4 in a 3x=‰2…1 ⫺ x†Š molar ratio. The powdered solids were intimately mixed with LiOH in both stoichiometric proportion and slight excess, by prolonged grinding, pressed into pellets and heat treated at 800⬚C for 24 h in an air atmosphere. A powder sample

with LiFe0.2Ni0.2Co0.6O2 stoichiometry was obtained by adding NiO in appropriate proportion, which was obtained by the in situ thermal decomposition of Ni(NO3)2, and a small excess in lithium hydroxide. The first reaction step is the decomposition of nickel nitrate to NiO with the release of nitrous fumes, followed by the reaction with lithium hydroxide and the formation of the solid product. After quenching from 800⬚C, the solid was ground in an agate mortar and stored in an air atmosphere. X-ray powder diffraction data (XPD) were obtained with a Siemens D5000 apparatus provided with CuKa radiation and graphite monochromator. Step-scan recordings for structure refinement by the Rietveld method were carried out by using 0.04⬚2u steps of 12 s duration. A computer program of the series DBWS9000 was used in the calculations [14]. Neutron powder diffraction data were collected at the Institute Laue Langevin (ILL) at Grenoble (France) by using the D1A high-resolution 2-axis diffractometer with a ˚ neutron take-off angle of 122⬚ and provided with 1.911 A radiation monocromatized by the (115) reflection of an anisotropically squashed germanium monochromator. The detectors were 25 3He counters at 5 atm with 90% efficiency ˚ . A cryostat ILL system was used to record the at 1.5 A spectra under constant sample temperatures of 1.4 and 300 K cryofurnace. The 4 h scans were recorded from 0 to 160⬚2u in steps of 0.05⬚. The XND program [15] for crystal structure refinement of powder data was used in order to take into account the correlation between data obtained by X-ray and neutron diffraction. 57 Fe Mo¨ssbauer spectra of the samples were obtained at room temperature with a classical constant acceleration spectrometer. The source was 57Co in a Rh matrix. The

R. Alca´ntara et al. / Solid State Communications 115 (2000) 1–6 Table 1 Results of the simultaneous refinement of X-ray and neutron diffrac space group tion data in the R3m Parameter

LiFe0.1Co0.9O2

LiFe0.2Co0.8O2

˚ a/A ˚ c/A x6c

2.8284(2) 14.129(1) 0.26025(8)

2.8397(7) 14.195(4) 0.25951(9)

Li (3b) b 11 b 33

0.039(1) 0.0014(3)

0.094(2) 0.0005(3)

Fe, Co (3a) b 11 b 33

0.025(5) 0.0002(1)

0.01(4) 0.0007(1)

O (6c) b 11 b 33

0.03(3) 0.00051(9)

0.036(5) 0.00040(8)

Rwp RB RF GoF

9.03 2.37 3.62 1.14

8.36 3.20 4.05 1.13

velocity scale was calibrated by using the magnetic sextet spectrum of a high purity iron foil absorber. Recorded spectra were fitted to Lorentzian profiles by least square method [16] and the fit quality was controlled by the classical x 2. The origin of the isomer shift scale was determined from the center of the a-Fe spectra, also recorded at room temperature. X-ray absorption experiments at the O–K edge (520– 575 eV) were carried out at room temperature in the total electron yield mode using the SA72 beam line monochromator with an energy resolution of 0.2 eV at the LURE Super-ACO (Orsay, France). The samples were finely ground and dispersed in acetone. The mixture was spread on a Ta sample holder and dried. The spectra were recorded in the range 510–575 eV.

3. Results and discussion 3.1. X-ray and neutron diffraction The new preparations carried out by using the excesslithium procedure led to single-phase rhombohedral solids. As described by Reimers et al. [17] and Arai et al. [18], the use of some lithium excess avoids the problems associated with lithium volatility, non-stoichiometry and defect interlayer ions in the ceramic oxides. In order to check the success of this procedure on the LiFexCo1⫺xO2 samples, a combined Rietveld analysis of X-ray and neutron powder diffraction data was carried out. Fig. 1 shows the experimental and calculated profiles obtained by Rietveld refinement of the full-patterns obtained at room temperature for two selected compositions: LiFe0.1Co0.9O2 and LiFe0.2Co0.8O2. The refined structural parameters obtained in the

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analysis are collected in Table 1. The main conclusions from these values are: • Neutron diffraction provides an interesting tool to discern cation distribution between tetrahedral and octahedral sites in solids containing two similar X-ray scattering atoms, such as Fe and Co. For LiFe0.1Co0.9O2, LiFe0.2Co0.8O2 and LiFe0.2Co0.6Ni0.2O2 prepared by the new ceramic procedure, the fitting of neutron diffraction data to a model based in a random distribution of both transition elements was achieved and low R factors were obtained. Moreover, no extra lines resulting from a possible ordering of Fe/Co within or between the layers could be observed. • The unit cell parameters increase with iron content in good agreement with previous reports, while the opposite is true for the x6c coordinate of the oxygen atoms in the rhombohedral solid. The best fit requires the use of anisotropic thermal factors in agreement with the trigonal crystal structure of the layered solid. • For some LiFe0.2Co0.8O2 preparations, the presence of small amounts of lithium carbonate impurity could be detected. These remained unresolved in the X-ray diffraction patterns. • Finally, neutron diffraction patterns were also recorded at 4.2 K for LiFexCo1⫺xO2 samples. The similarity with room-temperature data shows that magnetic ordering is not detectable by neutron scattering in these solids above liquid-helium temperature. 57

Fe Mo¨ssbauer spectroscopy and XAS data give complementary information about the local structure of the studied solids. 3.2. Fe Mo¨ssbauer effect As reported elsewhere [9], 57Fe Mo¨ssbauer spectroscopy shows that some pseudotetrahedral iron is present in LiFexCo1⫺xO2 …y ⱕ 0:2† powder samples obtained by a conventional ceramic method. For the samples studied here, the experimental and calculated spectra are shown in Fig. 2. The parameters emerging from the fitting which are also collected in the figure reveal traces of iron in a 4-coordination environment, which are close to the experimental limit of detection by the Mo¨ssbauer technique. This fact gives additional confirmation of the adequacy of the ceramic method to obtain substitutional solid solutions in the LiFexCo1⫺xO2 …y ⱕ 0:2† system, provided that some lithium excess is used. Another relevant conclusion from the RT spectra is the absence of measurable magnetic ordering, which agrees with the neutron diffraction measurements. Once the presence of tetrahedral iron is limited, the interpretation of the Mo¨ssbauer spectra could give additional information on the distribution of ions in octahedral sites. It should be noted that several alternative explanations have been given to the 57Fe Mo¨ssbauer spectra of LiCo1⫺xFexO2 and LiNi1⫺xFexO2 substitutional solid solutions. For

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Fig. 2. Experimental (circles) and calculated (filled lines) room temperature LiFe0.1Ni0.9O2.

LiCo1⫺xFexO2 up to x ˆ 0:25; Tabuchi et al. [8] found that the spectra could be fitted to at least two doublets with similar IS but different QS values, the intensity of the signal with higher QS being always lower. Moreover, they found that the relative intensity of the doublets changes with composition. These results were attributed to the presence of two or more local coordination geometries around Fe, resulting from the distribution of Fe/Co in octahedral sites. The different sites increase in number with iron content, leading to signal broadening. For LiNi1⫺xFexO2 up to y ˆ 0:3; Delmas et al. [11] also found a broad quadrupole split spectra and suggested a distribution of sites of different levels of QS. In order to check these models and their possi strucble relationships, we can consider a layer of the R3m ture where x Fe atoms and 1 ⫺ x M (Ni or Co) atoms are randomly distributed. For each iron atom the closest metal ions in octahedral sites define a regular hexagon of which 0

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Fe Mo¨ssbauer spectra for LiFe0.1Co0.9O2, LiFe0.2Co0.8O2 and

to 6 can also be iron atoms (see Fig. 3). The probability for each situation can be obtained from: ! 6 n Pn ˆ x …1 ⫺ x†6⫺n …1† n where n is the number of neighbor iron atoms. From Eq. (1), the number of isolated iron atoms, P0, can be obtained. P0 takes values decreasing from 73.51 to 17.80% on increasing x from 0.05 to 0.25. Moreover, the sum P0 ⫹ P1 ⫹ P2 ⫹ P3 accounts for more than 95% of the probability for compositions in the interval from x ˆ 0:05 (99.98%) to x ˆ 0:25 (96.24 %). The different distributions of iron atoms around a central iron for these n values are schematically shown in Fig. 3. It can be observed that the most symmetric distribution is found for n ˆ 0: This situation should lead to the signal with the lowest QS value,

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Fig. 3. Number of Fe neighbors of Fe atoms (black spots) in the FexM1⫺xO2 layer of LiFexM1⫺xO2 solids (M: Ni, Co).

which should be the most intense component below x ˆ 0:16: Nevertheless, the possible contribution to the low QS signal of other forms such as para-Fe(Fe2M4) and Fe(Fe3M3) cannot be discarded. On the contrary higher QS values are expected for less symmetric distributions leading to a non-uniform electric field around the 57Fe nuclei, particularly for n ˆ 1 and the ortho- forms for n ˆ 2 and 3. In this way, the interpretation of the LiCo1⫺xFexO2 spectra given by Tabuchi et al. [8] could be explained.

Fig. 2 shows the room-temperature Fe Mo¨ssbauer spectrum of LiNi1⫺xFexO2. It was fitted to two doublets by imposing a restriction over the FWHM values to be equal. The fitting led to contributions of 86(3) and 14(1)%, and a x 2 test value of 0.431. The isomer shift, quadrupole splitting (QS) and full width at half maximum (FWHM) of the more intense signal were 0.320(8), 0.34(1) and 0.28(2) mm/s respectively, while values of 0.30(4), 0.73(7) and 0.28 mm/s, respectively, were obtained for the weaker doublet. It should be noted that the relative contribution of the low QS signal to the spectrum falls below the data observed by Tabuchi et al. [8] for LiCo1⫺xFexO2. Thus, a similar interpretation cannot be given. Instead two different alternatives can be used: the first one is to admit a non random distribution of metal ions in 3a sites of the R3m structure that could account for the different contribution of the higher QS signal from LiCo1⫺xFexO2 (Tabuchi et al. [8]) to LiNi1⫺xFexO2b (this work). A second possibility is the presence of a different iron site in one of these structures. The different site could be either 3b octahedral site resulting from cation mixing in a partially ordered rock-salt structure, or a tetrahedral site. The latter interpretation was found by Alca´ntara et al. [9] for LiCo1⫺xFexO2 samples prepared by a ceramic procedure different to the hydrothermal method used by Tabuchi et al. [8]. Nevertheless, these subspectra (see Fig. 2 and Ref. [12]) are clearly distinguishable. 3.3. X-ray absorption spectroscopy The normalized X-ray absorption spectra are shown in Fig. 4. The spectra are characterized by the presence of two bands at ca. 529.3 and 532.9 eV, with variable relative intensity, followed by a more intense width band corresponding to two overlapped peaks. Precedent studies of transition metal oxides have shown that the first two bands are ascribable to interactions between the 2p orbital of oxygen and the d orbital of the transition metal, which are

Fig. 4. Normalized X-ray absorption spectra at the O–K edge. The various curves have been shifted vertically for the sake of clarity.

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partially occupied [19]. Thus, the first peak at 529.3 eV is related to the contribution of the 2p-O states in the 5d-t2g conduction band, and then due to 5d-t2g(M)–2p(O) interactions (M ˆ Fe, Co, Ni). The second peak at 532.9 eV is due to the 5d(eg)(M)–2p(O) interactions, strongly affected by the 2p(O)–2s(Li) competitive interaction. The increase of the intensity peak suggests that the partial covalent interaction between oxygen and lithium ions increases as the cobalt content decreases [20]. The difference in energy between both bands allows estimate the gap t2g –eg: 3.6 eV 艑 29,000 cm ⫺1, or Dq ˆ 2900 cm⫺1 : Since no significant shift of peaks is observed, the value of Dq remains almost constant, independent of the nature of the M cation replacing cobalt in LiCoO2: Fe or Ni. Finally, the peaks at 538.4 and 540.8 eV are attributed to 2p(O)–4sp(M) interactions. In the case of FeO and NiO, these two peaks are observed at 6.0 and 9.0 eV and at 5.0 and 8.0 eV, respectively from the peak due to 5d(eg)(M)–2p(O) interactions [21]. These values are close to those observed in our case: 5.5 and 7.9 eV, respectively. 4. Conclusions LiFe0.1Co0.9O2, LiFe0.2Co0.8O2 and LiFe0.2Co0.6Ni0.2O2 compositions can be prepared by ceramic procedures, in which the presence of some excess lithium during the thermal treatment avoids almost completely the occurrence of tetrahedrally coordinated iron. The resulting solids show cation substitution in the octahedral sites of the alternate interlayer sites of a c.c.p. of oxygen atoms, in agreement with XAS data. However, the distribution of sites is not random, as revealed from the relative intensity of the components in the 57Fe Mo¨ssbauer spectra, and probability calculations. Acknowledgements The authors acknowledge the financial support of ILL-Grenoble (especially Dr J. Campo, local contact) for neutron diffraction measurements, the LURE-Orsay, France (especially Dr C. Laffon and Dr P. Parent) for SACO experiments and CICYT (contract MAT99-0741).

References [1] T. Nagaura, K. Tazawa, Prog. Batteries Sol. Cells 9 (1990) 20. [2] J.R. Dahn, U. von Sacken, C.A. Michal, Solid State Ionics 44 (1990) 87. [3] J. Morales, C. Pe´rez-Vicente, J.L. Tirado, Mater. Res. Bull. 25 (1990) 623. [4] T. Ohzuku, A. Ueda, N. Masatoshi, I. Yasunobu, H. Komori, Electrochim. Acta 38 (1993) 1159. [5] E. Zhecheva, R. Stoyanova, R. Alca´ntara, P. Lavela, J.L. Tirado, Solid State Commun. 102 (1997) 457. [6] Q. Zong, U. Von Sacken, J. Power Sources 54 (1995) 221. [7] T. Ohzuku, A. Ueda, M. Tomoguchi, J. Electrochem. Soc. 142 (1990) 4033. [8] M. Tabuchi, K. Ado, H. Kobayashi, H. Sakaebe, H. Kageyama, C. Masquelier, M. Yonemura, A. Hirano, R. Kano, J. Mater. Chem. 9 (1999) 199. [9] R. Alca´ntara, J.C. Jumas, P. Lavela, J. Olivier-Fourcade, C. Pe´rez Vicente, J.L. Tirado, J. Power Sources 81/82 (1999) 547. [10] J.N. Reimers, E. Rossen, C.D. Jones, J.R. Dahn, Solid State Ionics 61 (1993) 335. [11] C. Delmas, M. Menetrier, L. Croguennec, I. Saadoune, A. Rougier, C. Pouillerie, G. Prado, M. Grune, L. Fournes, Electrochim. Acta. 45 (1999) 243. [12] R. Alca´ntara, P. Lavela, P.L. Relan˜o, J.L. Tirado, E. Zhecheva, R. Stoyanova, Inorg. Chem. 37 (1998) 264. [13] V.F. Sears, in: A.J.C. Wilson (Ed.), International Tables for Crystallography, vol. C, Kluwer Academic Publishers, Dordrecht, 1992, pp. 384–385. [14] R.A. Young, D.B. Wiles, J. Appl. Crystallogr. 15 (1982) 430. [15] J.F. Berar, I.U.C. Sat. Meeting, Powder Diffractometry, Toulouse, 1990. [16] W. Ku¨nding, Nucl. Instrum. Methods 75 (1969) 336. [17] J.N. Reimers, J.R. Dahn, J.E. Greedan, C.V. Stager, G. Liu, I. Davidson, U. Von Sacken, J. Solid State Chem. 102 (1993) 542. [18] H. Arai, S. Okada, H. Ohtsuka, M. Ichimura, J. Yamaki, Solid State Ionics 80 (1995) 261. [19] F.M.F. De Groot, M. Grioni, J.C. Fuggle, J. Ghisjsen, A. Sawatski, H. Petersen, Phys. Rev. B 40 (1989) 5715. [20] J. Purans, A. Kuzmin, Ph. Parent, C. Laffon, Private communication. [21] L.A. Grunes, Phys. Rev. B 27 (1983) 2111.