Lattice vibrations of materials for lithium rechargeable batteries. VI: Ordered spinels

Materials Science and Engineering B 130 (2006) 41–48

Lattice vibrations of materials for lithium rechargeable batteries. VI: Ordered spinels C.M. Julien a,∗ , F. Gendron a , A. Amdouni b , M. Massot c a

Institut des Nano-Sciences de Paris, CNRS-UMR 7588, Universit´e Pierre et Marie Curie, Campus Boucicaut, 140 rue de Lourmel, 75015 Paris, France b Unit´ e de Recherche Physico-Chimie des Mat´eriaux, Facult´e des Sciences de Tunis, Campus Universitaire, 1060 Tunis, Tunisia c Institut de Min´ eralogie et Physique de la Mati`ere Condens´ee, CNRS-UMR 7590 Universit´e Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France Received 13 January 2006; received in revised form 31 January 2006; accepted 7 February 2006

Abstract Raman scattering spectra have been investigated to evaluate the local structure of lithiated oxides used as electrode materials for lithium-ion ¯ batteries. We report the analysis of the vibrational spectra of ordered spinel phases including the partially delithiated ␭-Li0.5 Mn2 O4 (F 43m SG), the partial charge-ordered LiMn2 O4 orthorhombic form (Fddd SG) and the LiNi0.5 Mn1.5 O4 substituted oxide (P41 32 SG). Analysis of spectroscopic data is performed using the classical factor group theory and the vibration features are compared with those of the ordered lithium ferrite ␣-LiFe5 O8 and the normal spinels LiMn2 O4 and LiNi0.5 Mn1.5 O4 (Fd3m SG), and the inverse spinel LiNiVO4 . © 2006 Elsevier B.V. All rights reserved. Keywords: Ordered spinels; Lattice dynamics; Raman spectroscopy; Lithium batteries

1. Introduction With the fast development of advanced electrochemical secondary cells such as the lithium-ion batteries, accurate characterization for the optimization of intercalation materials used as positive electrodes appear very crucial. [1]. The spinel phase LiMn2 O4 is well known as a potential positive electrode. In spite of the fact that LiMn2 O4 has apparently a simple cubic structure (Fd3m symmetry) with a normal cationic distribution (A[B2 ]O4 in spinel notation), it presents several peculiarities in its crystal chemistry as follows: (i) it is a mixed-valence compound (Mn3+ , Mn4+ ) with an average oxidation state of +3.5, (ii) the occurrence of a phase transition below room temperature (at ca. 280 K) from cubic to orthorhombic, attributed to a partial charge ordering [2], (iii) a complex superstructure is observed for the delithiated phase Li0.5 Mn2 O4 [3], and (iv) cation ordering has been predicted from ab initio calculations for most dopants [4]. Raman and infrared spectroscopy are currently used as powerful tools for the characterization of crystalline solids such as oxides for electrode materials in lithium-ion batteries [5–8]. These techniques offer a complementary method of charac-

∗

Corresponding author. Tel.: +33 144274561; fax: +33 144273882. E-mail address: [email protected] (C.M. Julien).

0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.02.003

terization because spinel-type oxides show distinct modifications in the recorded spectra with changing the composition, i.e. lithium content, cation substitution, over-stoichiometry, etc. [9–12]. Changes in either space group or primitive cell are necessarily accompanied by a modification of the vibrational spectra. White and DeAngelis [13] and Allen and Paul [14] have given a brief summary of vibrational features for spinel structures which indicates that they are intermediate between well-defined internal modes and purely lattice modes. Thus, the general trends in vibrational spectra of transition-metal oxides are as follows. In crystal structure built of MO6 , the stretching vibrations of octahedral groups observed in the wavenumber range 500–700 cm−1 are mainly due to displacement of oxide ions since the heavy central cation, at least five times the mass of oxygen neighbors, does not play any significant role in the stretching frequency. In the previous paper of this series, the lattice dynamics of manganese dioxides and lithium-manganese oxides having the spinel-related structure were presented [15–18]. The objective of this paper is to extend the vibrational studies concerning ordered spinel phases used as electrode materials in lithium-ion batteries. The local structure is investigated for the delithiated Li0.5 Mn2 O4 , the charge-ordered form of LiMn2 O4 , the lithium ferrite LiFe5 O8 , the substituted LiNi0.5 Mn1.5 O4 and LiCo0.5 Mn1.5 O4 oxides. Their vibrational features are compared to those of the normal spinel networks.

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2. Experimental Samples were prepared using the melt-impregnation method as described elsewhere [5]. Single-phase samples were obtained by mixing fine powders of LiOH, manganese dioxide and appropriate oxides of substituting elements (NiO, CoO) provided from Alpha Ventron. The partially delithiated spinel Li0.5 Mn2 O4 and Li0.1 Mn2 O4 samples were obtained from electrochemical extraction of Li+ ions from ␭-LiMn2 O4 [16]. Structural properties (long-range and local environment) and morphology were investigated by X-ray diffractometry (XRD), Raman scattering (RS), Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM), respectively. The XRD diagrams were obtained on a Philips X’Pert PRO MRD (PW3050) diffractometer equipped with a Cu anticathode ˚ at room temperature. The mea(Cu K␣ radiation λ = 1.54056 A) surements have been recorded under Bragg–Brentano geometry at 2θ with step of 0.05◦ in the range 10–80◦ . The micromorphology of the samples was examined by SEM experiments with an electron microscope (IDI-DS 130C dual stage). RS spectra of lithium-containing samples were recorded on a JobinYvon U1000 double monochromator using the 514.5 nm line from a Spectra-Physics 2020 argon-ion laser. Standard photoncounting techniques were used for detection. In a typical spectral acquisition, six RS spectra each recorded with a resolution of 2 cm−1 were averaged to increase the signal-to-noise ratio. To avoid sample photodecomposition or denaturation, RS spectra were recorded using a low excitation power of 1 mW.

3. Structure and symmetry Spinel-type oxides of the transitions metals can be divided into two categories, normal and inverse. Both normal and inverse spinels have a cubic close-packed (ccp) oxygen lattice. Although LiMn2 O4 spinel and related compounds are not molecular solids, the treatment of vibrational modes of spinels in terms of a molecular model, as proposed by Waldron [19], makes it easy to visualise the eigenmodes of different Raman active modes. Spinel have been indicated to have vibrations intermediate between well-defined internal modes (e.g. CaCO3 ) and purely lattice mode (e.g. MgO) [13].

The structure of normal spinel, which is cubic with the space group (SG) Fd3m (O7h ) containing eight AB2 O4 units per unit cell, has the Li+ ions occupying the tetrahedral 8a sites, and a 1:1 mixture of Mn3+ and Mn4+ ions randomly distributed over the octahedral 16d Wyckoff positions. From this structure and cation distribution, there are four IR- and five Raman-active modes (Table 1). It is worth noting that Tarte [20] suggested that the tetrahedral vibration of the LiO4 unit occurs in the lowwavenumber region because Li+ is monovalent and the force constant in oxide materials, being manly coulomb forces, are related to the ionic charge. The low charge of Li+ compared with Mn3.5+ counteracts the effect of the shorter Li–O distances and lowers the frequency. Theoretically, all AB2 O4 transition-metal spinel-type oxides have four infrared-active bands. These vibrations were found in the following infrared spectral regions: ν1 (630–560 cm−1 ), ν2 (525–390 cm−1 ), ν3 (380–335 cm−1 ), and ν4 (300–200 cm−1 ). In practice, however, two of the bands found in the far-infrared region were sometimes difficult to record. The ν1 and ν2 bands are currently assigned to the vibration of the BO6 octahedron for the trivalent cation. The ν3 and ν4 bands are affected to complex vibrations involving both octahedral and tetrahedral sites. The situation is further complicated by the possibility of ordering of the cations on tetrahedral and octahedral sites, thereby lowering the crystal symmetry to increase significantly the number of IR- and Raman-active modes. There are a number of types of ordering which may take place in the spinel structure. These have been rather nicely summarized by Haas [21]. Lattice dynamics of some of the ordering types have been predicted by White and DeAngelis [13]. No change occurs in ¯ SG) and the same number of IR and the inverse spinel (Fd 3m Raman bands have been confirmed experimentally [22]. The total irreducible representation for the vibrational modes of nor¯ SG) spinel lattice is mal (Fd3m SG)spinel and inverse (Fd 3m Γ(AB2 O4 ) = A1g (R) + Eg (R) + F1g (in) + 3F2g (R) +2A2u (in) + 2Eu (in) + 4F1u (ir) + 2F2u (in),

(1)

in which (R), (IR) and (in) represent Raman- and infrared-active vibrations and inactive modes, respectively. The 1:1 ordering on the tetrahedral sites of spinel frameworks ¯ produces a change of SG from Fd3m to F 43m (O7h to T2d ). The

Table 1 Correlation for the room-temperature phase (cubic Fd3m SG), the low-temperature phase (orthorhombic Fddd SG) of LiMn2 O4 and the partially delithiated phase ¯ SG) Li0.5 Mn2 O4 (tetragonal F 43m Phase

Fd3m (z = 8) (O7h )

Fddd (z = 32) (D24 2h )

¯ F 43m (z = 8) T2d

Atom occupancy

1(Li): 8a; 1(Mn): 16d; 1(O): 32e

4(Li): 8a, 16e, 16f, 32h; 1(Mn): 16d; 1(Mn): 32h; 9(O): 32h

0.5(Li): 8a; 1(Mn): 16d; 1(O): 32e

Vibrational modes

A1g Eg F1g 3F2g 2A2u 2Eu 5F1u 2F2u

Ag Ag + B1g B1g + B2g + B3g 3(Ag + B2g + B3g ) 2B1u 2(Au + B1u ) 5(B1u + B2u + B3u ) 2(Au + B2u + B3u )

A1 E F1 3F2 2A1 2E 3F2 2F1

C.M. Julien et al. / Materials Science and Engineering B 130 (2006) 41–48

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Fig. 1. Schematic representation of the structure of AB2 O4 spinel lattices. (a) The smallest (primitive) cubic unit cell of normal spinel (Fd3m SG) and (b) the unit cell of the 1:3 ordered spinel (P41 32 SG). The structure is composed of alternating octants of AO4 tetrahedra and B4 O4 cubes to build the fcc unit cell.

new symmetry removes the high-frequency restrictions from the internal vibrations and the modes of both tetrahedral ions appear to be infrared-active modes. The 1:1 ordering on the octahedral sites reduces the symmetry from Fd3m to P31 21 (O7h to D43 ). The primitive unit cell is tetragonal with a volume half of the unit cell and the reduction of symmetry greatly increases the number of allowed vibrations. The 1:3 ordering on the octahedral sites reduces the SG from Fd3m to P42 32 (O7h to O7 ). This ordered unit cell is primitive cubic with the same number of atoms as the normal spinel cell. A great increase in the number of allowed modes is due to the enlargement of the smallest Bravais cell by a factor of four. Fig. 1 shows a comparison between the normal and the ordered spinel structure. Ordered structures can be obtained in LiMn2 O4 spinel from different manners: by partial chargeorder in the orthorhombic form at 280 K, by partial delithiation or by replacement of Mn by Ni cations. The results of the allowed modes and selection rules for spinel lattices are listed in Table 1. Upon lithium extraction from LiMn2 O4 , the Li0.5 Mn2 O4 ¯ spinel lattice can be modeled in the F 43m SG, which is a lower symmetry subgroup of Fd3m. In this structure, every second lithium tetrahedral site is vacant, producing an ordered Li configuration which has been experimentally observed [23]. The calculation shows phonons for Li0.5 Mn2 O4 which derived from vibrational modes of LiMn2 O4 and ␭-MnO2 phase. The total irreducible representation for the vibrations of the Li0.5 Mn2 O4 phase contains seven F2 IR-active modes and thirteen Ramanactive modes (3A1 + 3E + 7F2 ). The orthorhombic form (Fddd SG), which is attributed to a charge ordering transition, is a superstructure of the cubic phase including 72 Li, 144 Mn and 288 O atoms distributed onto 18 different crystallographic sites [24]. The existence of a large elementary cell suggest the lowering of the spectroscopic symmetry from O7h to D24 2h . As this latter lattice is considered such as a lower symmetry, we expect an increasing number of phonons. The vibrational modes assignment of the orthorhombic phase can be derived from those of the spinel phase ␭-LiMn2 O4 using the correlation of the irreducible representation Oh → D2h . A huge number of vibrational modes is expected to be 15 Ramanand 20 IR-active bands (Table 1).

4. Results and discussion 4.1. Vibrational features of normal spinel Fig. 2 shows the Raman spectra of the ␭-LiMn2 O4 spinel and its delithiated product Li0.5 Mn2 O4 in the spectral region 100–800 cm−1 . A common feature of these spectra is the presence of a strong band around 600 cm−1 a group of bands in the range 200–500 cm−1 with weaker intensity. In spinel oxides and in other manganese oxides energies of ∼600–650 cm−1 are characteristic of vibrations involving motion of oxygen atoms inside the octahedral unit MnO6 [8]. The assignment of modes observed in the RS spectrum of ␭-LiMn2 O4 spinel has been reported previously [16–18]. The Raman band located

Fig. 2. The Raman spectra of the ␭-LiMn2 O4 spinel and its delithiated product Li0.5 Mn2 O4 .

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at ca. 625 cm−1 is viewed as the symmetric Mn–O stretching vibration of MnO6 groups. This high-wavenumber band is assigned to the A1g species in the O7h spectroscopic symmetry. Its broadness is related with the cation–anion bond lengths and polyhedral distortion occurring in LiMn2 O4 . As the manganese ions of the spinel structure exhibit a charge disproportionation such as LiMn3+ Mn4+ O4 , there are isotropic Mn4+ O6 octahedra and locally-distorted Mn3+ O6 octahedra due to the Jahn–Teller effect. Thus, we expect to observe stretching vibrations of MnO6 9− and MnO6 8− octahedra which provide the broadness of the A1g mode. The shoulder peak at 580 cm−1 (1) of the F2g mode in this region is not well separated because of its low intensity. In the localised vibration approach, it is speculated that the intensity of the Raman shoulder is closely related to the manganese average oxidation state in the spinel phase. As the intensity of this shoulder is very sensitive to the lithium stoichiometry, it is suggested that its character originates mainly from the vibration of MnIV O bonding. The RS (2) peak with medium intensity located at 483 cm−1 has the F2g symmetry, while the weak bands located at 426 and 382 cm−1 (3) (3) have the Eg and F2g symmetry, respectively. The F2g mode is related to the Li–O motion, i.e. connected to the tetrahedral cation movements. The weak band at 300 cm−1 is a unexpected mode, which could be Raman active due to the cationic disorder that induces a breakdown of the translation symmetry. It can be stated that in the ideal cubic spinel LiMn2 O4 , the Mn3+ and Mn4+ cations are considered as crystallographically equivalent (16d sites) in agreement with XRD data; therefore, occupation probabilities of 0.5 must be affected for each cation in 16d position. Hence, a loss of translation invariance certainly occurs, due to local lattice distortions around the Mn3+ cations which exhibit a Jahn–Teller effect and have higher ionic radii than Mn4+ ions. As a result, a breakdown in the Raman and IR selection rules is expected, which may explain the observation of broad bands (somewhat disorder) and the fact that more vibrational modes than expected are observed in cubic LiMn2 O4 . 4.2. Li0.5 Mn2 O4 delithiated spinel The low-wavenumber Raman features of the delithiated Li0.5 Mn2 O4 phase (Fig. 2) display almost the same shape as the ␭-LiMn2 O4 spinel. However, in the high-wavenumber region, above 500 cm−1 , the bands at 563, 597, 611, and 648 cm−1 are diagnostic of the T2d spectroscopic symmetry. For intermediate compositions such as the particular Li0.5 Mn2 O4 every second lithium tetrahedral site is vacant, producing an ordered Li configuration. Thus, the Li0.5 Mn2 O4 spinel lattice can be modeled in ¯ the F 43m space group, which is a lower symmetry subgroup of Fd3m. The calculation for Li0.5 Mn2 O4 shows phonons which derived from Raman-active modes of LiMn2 O4 and ␭-MnO2 phase. The total irreducible representation for the vibrational modes of the Li0.5 Mn2 O4 phase is Γ(Li0.5 Mn2 O4 ) = 3A1 (R) + 3E(R) + 3F1 (R) + 6F2 (R, ir).

(2)

The two most intense bands at 597 and 611 cm−1 are assigned to the modes of A1 and F2 symmetry derived from the Raman-

Fig. 3. Raman scattering spectra of LiMn2 O4 as a function of the temperature. (a) 300 K and (b) 165 K. (3)

active modes A1g and F2g modes under O7h spectroscopic group [17]. The band at 483 cm−1 is attributed to the F2 mode which (2) is related to the F2g mode under O7h spectroscopic group. 4.3. Partial charge-ordered orthorhombic phase We now consider the temperature behavior of the observed Raman spectrum of LiMn2 O4 recorded at 165 K. As is seen from Fig. 3, the number and the positions of the RS bands are temperature dependent. The net effect of the lowering temperature of ␭-LiMn2 O4 is the first-order phase transition from cubic to orthorhombic structure, which occurs around 290 K [24]. The orthorhombic form (Fddd SG), which is attributed to a charge ordering transition, is a superstructure of the cubic phase including 72 Li, 144 Mn and 288 O atoms distributed onto 18 different crystallographic sites. The existence of a large elementary cell suggest the lowering of the spectroscopic symmetry from O7h to 2 . As this latter lattice is considered such as a lower symmeD2h try, we expect an increasing number of phonons. The vibrational modes assignment of the orthorhombic phase can be derived from those of the spinel phase ␭-LiMn2 O4 using the correlation of the irreducible representation Oh → D2h as shown in Table 1. The total irreducible representation for the orthorhombic phase is given by Γ3 = 5Ag (R) + 2B1g (R) + 4B2g (R) + 4B3g (R) +4Au (in) + 8B1u (ir) + 6B2u (ir) + 6B3u (ir),

(3)

for which a huge number of vibrations is expected to be 15 Raman- and 20 IR-active modes. Consequently, the phonon spectrum should become much more structured below the transition temperature, confirming that the crystal structure is more distorted.

C.M. Julien et al. / Materials Science and Engineering B 130 (2006) 41–48

Fig. 4. Raman scattering spectra of LiNiVO4 and LiCoVO4 inverse spinels ¯ SG). (Fd 3m

As expected, the low-temperature RS spectrum (Fig. 3) displays a global band shift toward the high-wavenumber side due to the lattice freezing at 165 K that shrinks the MnO6 skeleton. In addition, we observe several changes in the spectral features as follows. (i) The Raman scattering efficiency increases due to the lowering of the electrical conductivity, i.e. the electron hop freezing below 300 K. (ii) The phonon spectrum appears to be much more structured than the room-temperature spectrum due to the orthorhombic D24 2h symmetry at temperature lower than 280 K. (iii) The bandwidth of the phonons lines becomes narrow. (iv) The high-wavenumber phonon lines at 481 and 631 cm−1 split and new phonons appear in the high-wavenumber side at 499 and 655 cm−1 , respectively. (v) The F2g mode at 586 cm−1 (as a shoulder at 300 K) has changed giving rise to well-resolved band. (vi) A broad band is recorded in the low-wavenumber region which is attributed to external modes. 4.4. Vibrational features of inverse spinels LiMVO4 (M = Ni, Co) are inverse spinels where Li and (Ni, Co) atoms are thought to occupy the octahedrally coordinated interstices equally and randomly and the V atoms are thought to occupy the tetrahedrally coordinated interstices [25], so the cation arrangement is (V)IV (LiM)VI O4 . In Li(Ni,Co)VO4 , the oxidation state of the cations is believed to be Li1+ , (Ni, Co)2+ , and V5+ .The cubic inverse-spinel structure possesses prototypic symmetry O7h . The pentavalent vanadium is located on the tetrahedral 8a sites (1/4, 1/4, 1/4), Li and Ni are distributed on the octahedral 16d sites (1/2, 1/2, 1/2), the distribution being disordered in the spinel structure, and the positions of oxygen are in 32e sites (z, z, z). Fig. 4 shows the Raman scattering spectra of LiNiVO4 and LiCoVO4 inverse spinels. These compounds crystallize in the ¯ SG group theory predicts, based on the cubic system with Fd 3m factor group (or unit cell) approximation, that LiMVO4 oxides exhibit the same number of allowed modes that for normal spinel lattice, so we expect five Raman-active vibrations for LiNiVO4 and LiCoVO4 [22]. The Raman bands are located at 820, 790,

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478, 414 and 335 cm−1 for LiNiVO4 and at 807, 785, 470, 400 and 312 cm−1 for LiCoVO4 . A detailed interpretation of the Raman spectra is difficult because the molecular model does not work properly due to the different types of vibrations that exist in the spectral domain, involving simultaneously significant displacement of octahedral and tetrahedral cations [26]. Nevertheless, we can consider in the first approximation that the bands observed in the 700–850 cm−1 region can be attributed to the stretching vibrations of VO4 tetrahedron. It is interesting to make comparison between vibrational frequencies of isolated [VO4 3− ] units in solution and those of VO4 tetrahedron in LiMVO4 (Fig. 4). The presence of the light Li+ cation must contribute to enhancing the coupling effect in LiMVO4 but, interestingly, its high polarizing power apparently has a negligible effect on V O bond strengths. Thus, in LiNiVO4 the high-frequency band located at 820 cm−1 corresponds to the stretching mode of VO4 tetrahedron which has the A1 -type symmetry, whereas the band situated at 335 cm−1 corresponds to the bending mode of VO4 tetrahedron with an E-type symmetry [27]. The broadness of the high frequency band could be tentatively explained in terms of asymmetrical bonding of VO4 tetrahedron. Two types of cations, namely Li and Ni, may be bonded with each oxygen atom of a VO4 tetrahedron. This introduces some asymmetry in the VO4 unit without disturbing the overall cubic symmetry of the elementary unit cell. A close inspection of the spectra in Fig. 4 also shows little change across the series for the vibrational frequency ν3 at 310–340 cm−1 . This band may also be associated with the octahedral MO6 groups. 4.5. Vibrational features of ordered LiFe5 O8 Lithium ferrite spinel LiFe5 O8 possesses two crystalline forms. The disordered ␤-LiFe5 O8 is obtained by the rapid quenching of the samples from high temperatures above 800 ◦ C to room temperature. It has a disordered face centered cubic ˚ in which the Fe3+ and Li+ structure (Fd3m SG, a = 8.333 A), ions are randomly distributed in the octahedral interstices. The ␣-LiFe5 O8 lithium ferrite has an ordered spinel structure belonging to the X-ray P41 32 space group (this type of order reduces the spectroscopic group from O7h to O7 ) with the unit cell for˚ White and DeAngelis [14] mula Fe8 [Li4 Fe12 ]O32 (a = 8.337 A). described the selection rules for this particular spinel for which the enlargement of the smallest Bravais cell by a factor of four results in great increase in the number of allowed modes. There are as many as 62 allowed modes for ordered LiFe5 O8 which are represented by Γ4 = 6A1 (R) + 14E(R) + 22F2 (R) + 20F1 (IR).

(4)

Fig. 5 shows the RS spectra of the ordered and disordered phases of LiFe5 O8 . It is shown that two intense bands located at 709 and 492 cm−1 are the dominating RS features of the ordered phase. The complex spectral features in the region below 700 cm−1 and a empirical vibrational assignment has created an ongoing discussion about interpretation of the spinel spectra [11,12]. The proper approach is to carry out complete vibrational

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Fig. 5. Raman scattering spectrum of the lithium ferrite LiFe5 O8 . (a) Disordered sample (Fd3m SG) and (b) ordered sample (P41 32 SG).

study supported by lattice dynamics calculations. The RS spectrum collected at 514.5 nm displays strong bands at 492 and 709 cm−1 . Aroca et al. [28] have reported a resonance effect on the Raman features when samples are excited with the 785 nm laser line. The ␣-LiFe5 O8 compound is strongly colored and an absorption in the visible wavelength brings about the resonance Raman effect. When samples are excited at higher energy than 4 mW, transformations induced by the laser highly focused are observed. The net effect of the energy density on LiFe5 O8 is an optically visible change which produces the spectrum of ␥-Fe2 O3 , a spinel structure with octahedral vacancies [28]. It is obvious that the RS spectrum of the ␤-LiFe5 O8 phase displays, as expected, less number of active modes. Three bands appears at 603, 405 and 293 cm−1 with medium intensity in the spectrum of the disordered phase. All the bands have their frequency corresponding to those of the ␣-LiFe5 O8 phase with a slight shift toward the low-wavenumber side. Thus, it is worth noting the spectral difference observed between these phases, while a distinction between them has not been possible on the basis of X-ray powder diffraction data. 4.6. Vibrational features of ordered LiNi0.5 Mn1.5 O4 Recent investigations have shown that, among the Nisubstituted LiMn2 O4 spinels, the composition LiNi0.5 Mn1.5 O4 possesses specific electrochemical characteristics such as a high capacity of 130–160 mA h/g associated to a high-voltage plateau in the 5 V range [29]. The early work by Gryffroy and Vandenberghe [30], infrared spectra of slowly cooled samples LiNi0.5 Mn1.5 O4 have shown features of the 1:3 octahedral order between Ni2+ and Mn4+ cations. This superstructure corresponds to the space group P43 32 (or P41 32). Fig. 6 shows the XRD patterns of the normal and ordered spinel phase for LiNi0.5 Mn1.5 O4 . The diffraction patterns of LiNi0.5 Mn1.5 O4 grown by the sol-gel method can be indexed in the cubic system with Fd3m symmetry with the lattice parame˚ while the diffraction patterns of LiNi0.5 Mn1.5 O4 ter a = 8.182 A,

Fig. 6. XRD patterns of LiNi0.5 Mn1.5 O4 : (a) normal spinel and (b) ordered spinel.

grown by the pyrolysis method have been indexed in the cubic P41 32 symmetry rather than the cubic Fd3m SG due to additional weak lines located at 2θ = 15.3, 39.7, 45.7 and 57.5◦ which are absent from that of Fd3m structure. The cubic cell parame˚ is in agreement with previously reported value ter a = 8.1685 A which describes an octahedrally ordered spinel structure [31]. Therefore, the primitive unit cell in the cubic system with P41 32 SG results in the superstructure from the 1:3 cation ordering. We assume similar distribution of the atoms namely Li on 8c, Ni on 4b, Mn on 12d, O(1) on 24e and O(2) on 8c Wyckoff positions. The space group P41 32 allows placing the larger Ni2+ ˚ in bigger 4b site instead of 16d site ions (ionic radius 0.69 A) of normal spinel structure. The smaller unit cell dimension is primarily due to the change in Mn oxidation state. Fig. 7 presents the RS spectra of LiCo0.5 Mn1.5 O4 and LiNi0.5 Mn1.5 O4 normal spinel (Fd3m SG), and LiNi0.5 Mn1.5 O4 ordered spinel (P41 32 SG). For LiM0.5 Mn1.5 O4 (M = Ni, Co) substituted phase, the factor group method of classifying fundamental vibrational modes involves no change in the crystal symmetry, and therefore both spinels are expected to have the same number of active modes. Such a view disregards a consideration of a change in mass of the cation or cation–oxygen bond strength which could lead to a difference in vibrational frequency and hence extra Raman and/or IR bands. The situation is further complicated by the possibility of ordering of the cations on octahedral sites, thereby lowering the crystal symmetry to produce more Raman-active modes. The RS features of ordered LiNi0.5 Mn1.5 O4 powders display obvious differences to their spinel counterparts for disordered LiNi0.5 Mn1.5 O4 and LiCo0.5 Mn1.5 O4 . The introduction of Ni2+ in the spinel lattice has modified the Raman spectra in a complicated manner. The following observations can be made. (i) The band associated

C.M. Julien et al. / Materials Science and Engineering B 130 (2006) 41–48

Fig. 7. Raman scattering spectra of LiCo0.5 Mn1.5 O4 normal spinel (Fd3m SG), LiNi0.5 Mn1.5 O4 normal spinel (Fd3m SG), and LiNi0.5 Mn1.5 O4 ordered spinel (P41 32 SG).

to the symmetric Mn–O stretching vibration of MnO6 octahedra shifts slightly to 638 cm−1 . (ii) New features at 407 and 495 cm−1 became strong and thus can be unequivocally be assigned to the Ni2+ –O stretching mode in the structure [32]. (iii) (3) The F2g splits in three components at 583, 595, and 611 cm−1 . The frequency shift of the stretching vibrations is attributed to the increase of the average valence state of Mn ions and to the decrease in the unit cell volume. The small shift of the symmetric stretching vibration of MnO6 groups can be viewed as the shortness of Mn O bond lengths and polyhedral distortion occurring in LiNi0.5 Mn1.5 O4 . The intensity of the shoulder located at 583 cm−1 is enhanced upon nickel substitution. This may be due to the change of the Mn3+ /Mn4+ ratio versus Ni2+ in the material. The peak splitting of the F2g -type mode of the normal spinel is reduced between the bands at 407 and 387 cm−1 for LiNi0.5 Mn1.5 O4 due to the polyhedral distortion. For well-controlled synthesis of LiNi0.5 Mn1.5 O4 grown by glycine-assisted pyrolysis method, RS spectrum indicates characteristic peaks originating from the formation of a superlattice.

47

Analytical results are in accordance with P41 32 SG in which Ni2+ ions are located at the 4a sites in the cubic close-packed oxygen array. Comparison with the disordered LiNi0.5 Mn1.5 O4 and LiCo0.5 Mn1.5 O4 (Fd3m SG) compound is made in Fig. 6. The main driving force for octahedral cation ordering is believed to be the charge difference between Mn and Ni atoms. For Ni substitution the RS patterns show a dramatic increase in the number of active modes explained by the lowering symmetry correlation O7h → O7 (Table 2). This prediction is well verified experimentally. The LiNi0.5 Mn1.5 O4 phase clearly shows an ordering of octahedral cations into 4b and 12d sites in space group P41 32 (O7 spectroscopic symmetry). Both contains small fractions of lithium on the 4b site. The sharpness of the Raman bands of LiMn3/2 Ni1/2 O4 sample is the fingerprint of well-separated Ni and Mn sites resulting from the symmetry lowering. As the integer valence distribution is (Li+ )(Ni2+ )0.5 (Mn4+ )1.5 O4 , the rather large broadness of the high-wavenumber band in normal spinel LiMn2 O4 becomes a well-resolved triplet in ordered spinel lattice. This lowering symmetry observed in the resonance spectroscopy (Raman) could not be detected by XRD due to the small contrast of XRD patterns. It is worth noting that no ordered spinel phase has been stabilized with the Co-based compounds. 4.7. Cation–oxygen bonding Assuming a simple harmonic oscillator model, the frequency of the F2g Raman band is given by the usual spring formula νIR = (1/2πc)(k/u)1/2 , where k is the spring constant related to the M O bond distance and u is the reduced mass of the MO6 octahedron. Thus, the decrease in the M O bond length in oxide compounds enhances the value of k to cause a shift toward the blue side, keeping an increase of the symmetric stretching mode. It is exactly the result obtained comparing the RS features of LiCo0.5 Mn1.5 O4 , LiNi0.5 Mn1.5 O4 and LiMn2 O4 . The shift of the A1g mode from 625 to 638 (639) cm−1 corresponds to the shorter Ni(Co) O bonds in substituted spinels. A relationship can be established between the F2g mode and the ionic radius of the transition-metal cation as shown in Fig. 8. A linear vari(2) ation of the frequency of the F2g mode versus the ionic radius is obtained for the spinel networks. Values of the ionic radius are those given by Shannon [33]. Similar features have been obtained for Li-rich spinel compounds, i.e. Li1+x Mn2−x O4 with

Table 2 The irreducible representation of the allowed modes of the 1:3-ordered spinel phase in O7 spectroscopic group Atom Li Mn Ni O1 O2 Total Acoustic Inactive Raman (42 modes) Infrared (20 modes)

Wyckoff position 8c 12d 4a 8c 24e

Point group

Irreducible representation (point group → O7 correlation)

C3 C2 D3 C3 C1

A1 + A2 + 2E + 3F1 + 3F2 A1 + A2 + 2E + 5F1 + 5F2 A2 + 2E + 3F1 + 3F2 A1 + A2 + 2E + 3F1 + 3F2 3A1 + 3A2 + 6E + 9F1 + 9F2

6A1 + 7A2 + 14E + 23F1 + 22F2 3F1 7A2 6A1 + 14E + 22F2 20F1

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C.M. Julien et al. / Materials Science and Engineering B 130 (2006) 41–48

Acknowledgments The authors would like to thank Dr. A. Mauger for valuable discussion and Mr. M. Selmane for careful work in performing the XRD measurements. References

(1)

Fig. 8. Frequency of the F2g mode vs. the ionic radius of the transition-metal cation in the normal (Fd3m SG) spinel network.

0 ≤ x ≤ 1/3 [34]. Thus in a very simplified scheme, we can admit that the vibrational spectrum of a solid is primarily determined by the local bonding forces (corresponding to the cation–oxygen bonds) and by the vibrational interactions between identical, near-neighbor coordinated group. 5. Conclusion Some aspects of the application of the Raman scattering spectroscopy have been applied to the study of local structure of several lithium transition-metal oxides. We have investigated the crystal structures of ordered spinel phases including ¯ the partially delithiated ␭0 -Li0.5 Mn2 O4 (F 43m SG), the partial charge-ordered LiMn2 O4 orthorhombic form (Fddd SG) and the LiNi0.5 Mn1.5 O4 substituted oxide (P41 32 SG). Analysis of spectroscopic data has been performed using the classical factor group theory and the vibration features are compared with those of the ordered lithium ferrite ␣-LiFe5 O8 and the normal spinels LiMn2 O4 and LiNi0.5 Mn1.5 O4 (Fd3m SG), and the inverse spinel LiNiVO4 . Depending of the preparation procedure two varieties of samples were obtained: the normal-spinel structure (sol–gel method) and the ordered-spinel structure (pyrolysis method). These structures refined with the Fd3m and P41 32 space group, respectively, show a smaller cubic unit cell than the undoped LiMn2 O4 spinel. The shortening of Mn O bonds is observed in Raman spectra by a shift of the stretching mode of MnO6 entities. The superstructure of LiNi0.5 Mn1.5 O4 material grown by pyrolysis method is confirmed by probing the local cationic environment by Raman spectroscopy. Measurements reveal that the oxidation state of Ni was +2 in LiNi0.5 Mn1.5 O4 spinels.

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