Variation of the Chemical Bonding Nature of LiMn2-xNixO4 Spinel

The effect of nickel substitution on the chemical bonding nature of spinel lithium manganate has been investigated by performing comparative X-ray abs...
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J. Phys. Chem. B 2001, 105, 335-342

335

Variation of the Chemical Bonding Nature of LiMn2-xNixO4 Spinel Oxides upon Delithiation and Lithiation Reactions Seong-Ju Hwang, Hyo-Suk Park, and Jin-Ho Choy* National Nanohybrid Materials Laboratory, School of Chemistry and Molecular Engineering, Seoul National UniVersity, Seoul 151-747, Korea

Guy Campet Institut de Chimie de la Matie` re Condense´ e de Bordeaux (ICMCB) du CNRS, Chaˆ teau BriVazac, AVenue du Dr. A. Schweitzer, 33608 Pessac, France ReceiVed: July 26, 2000; In Final Form: September 28, 2000

The effect of nickel substitution on the chemical bonding nature of spinel lithium manganate has been investigated by performing comparative X-ray absorption spectroscopic analyses for Ni- and Cr-substituted LiMn2-xMx O4 (M ) Ni and Cr) compounds. The Mn K-edge X-ray absorption spectroscopic results presented here clarify that the replacement of Mn with Ni or Cr gives rise not only to the increase of the Mn oxidation state but also to the suppression of the tetragonal distortion in cubic spinel lattice. From the extended X-ray absorption fine structure analyses at the Ni and Cr K-edges, it has been confirmed that the substitution of nickel ion induces lower structural stability of the spinel lattice compared to that of chromium ion. Especially for the lithiated spinel phase, a remarkable tetragonal distortion is observed for the Ni-substituted case in contrast to the Cr-substituted one. On the basis of the present experimental findings, it is concluded that the substitution of Mn with Ni is less effective in stabilizing the cubic spinel lattice than that with Cr, which is responsible for the inferior electrochemical performance of LiMn2-xNixO4.

Introduction In the past few years, the lithium derivatives of transition metal oxides such as LiCoO2, LiMn2O4, and LiNiO2 have been extensively studied as cathode materials for rechargeable lithium batteries, since these can be reversibly deintercalated and reintercalated by lithium ions at high potential.1-3 Recently, special attention has been paid to the spinel lithium manganate due to its economic and environmental advantages.4,5 Moreover, in contrast to layered metal oxides, this spinel compound LixMn2O4 can be used as a positive electrode for both 3 and 4 V regions corresponding to the compositional ranges 1 e x e 2 and 0.1 e x e 1, respectively.6 Despite such advantages, the remarkable capacity loss during the electrochemical chargedischarge process frustrates the commercial use of this cathode material. Although the origin of this capacity fading has not been fully understood, it is rather well-established that the structural phase transition from cubic symmetry to a tetragonal one during the discharging process is one of the main causes of capacity loss in this spinel oxide.7 Since the formation of the tetragonally distorted Li2Mn2O4 phase is induced by the Jahn-Teller effect of the trivalent Mn+III ion (high spin d4), numerous of attempts have been made to stabilize the cubic spinel lattice by increasing the average oxidation state of manganese.7-12 Through such investigations, it has been established that the partial substitution of manganese ions with other transition metal ions such as Ni+II, Cr+III, and Co+III improves the electrochemical performance of LiMn2O4 spinel.8,9 Taking into account the preferential oxidation state of the * To whom all correspondences should be addressed. Telephone: (82) 2-880-6658. Fax: (82) 2-872-9864. E-mail: [email protected].

substituent cation, the substitution of Mn with Ni+II is expected to be more effective in increasing the Mn oxidation state compared to substitution with Cr+III or Co+III. In fact, it was reported that nickel substitution via the sol-gel route enhances the stability of the electrochemical charge-discharge process in 3 and 4 V regions.9,11 On the contrary, according to the recent study on the cation-substituted LiMn1.5M0.5O4 electrode (M ) Ni, Co, etc.), severe capacity fading for electrochemical cycling in the 3 V region was observed for the nickel-substituted spinel compound prepared by the solid-state method.12 To understand such contrasting results for the Ni-substituted spinel oxides, it is quite important to examine the effects of Ni substitution on the electronic and crystal structures of the spinel compound, which are closely related to its electrochemical property. For this purpose, X-ray absorption spectroscopy (XAS) is thought to be very suitable, since this method allows us to probe in detail the local atomic arrangement and electronic configuration of the absorbing metal ion. Actually this tool has been successfully applied to the study of the chemical bonding nature of lithium manganate spinel compounds as well as other cathode materials such as LiMnO2, LiNi1-xCoxO2, etc.13-17 In this work, we have performed XAS analyses for the nickelsubstituted LiMn2-xNixO4 spinel oxide to investigate how nickel substitution influences the chemical bonding character of lithium manganese spinel oxide. We have also examined the evolution of local symmetry and the electronic configuration of nickel and manganese ions upon lithium deintercalation and intercalation reactions, which would provide some clues regarding the stabilization of the lithium manganate cathode during the electrochemical charge-discharge process. In addition, the XAS spectra of Cr-substituted spinel compounds were also measured

10.1021/jp002673+ CCC: $20.00 © 2001 American Chemical Society Published on Web 12/09/2000

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TABLE 1: Lattice Parameters, Unit Cell Volumes (Vc), Crystal Symmetries, Chemical Formulas, and the Mn+III/Mn+IV Mole Ratios of the Spinel LiMn2--xMxO4 (M ) Ni and Cr) Compounds and Their Delithiated and Lithiated Derivatives nominal compn

a (Å)

LiMn2O4 LiMn1.8Ni0.2O4 LiMn1.67Ni0.33O4 LiMn1.8Cr0.2O4 λ-MnO2 λ-LiyMn1.8Ni0.2O4 λ-LiyMn1.67Ni0.33O4 λ-LiyMn1.8Cr0.2O4 Li2Mn2O4 Li2Mn1.8Ni0.2O4 Li2Mn1.67Ni0.33O4 Li2Mn1.8Cr0.2O4

8.249 8.222 8.203 8.229 8.038 8.092 8.129 8.092 5.659 5.669 5.685 5.676

c (Å)

Vc

cryst sym

chem formula

Mn+III mole ratio (%)

Mn+IV mole ratio (%)

9.291 9.272 9.248 9.186

561.29 555.78 551.95 557.24 519.33 529.91 537.23 529.93 297.59 298.02 298.93 296.01

cubic cubic cubic cubic cubic cubic cubic cubic tetragonal tetragonal tetragonal tetragonal

Li1.07Mn2O4 Li1.02Mn1.78Ni0.22O4 Li1.01Mn1.69Ni0.31O4 Li0.99Mn1.76Cr0.24O4 MnO2 Li0.43Mn1.78Ni0.22O4 Li0.52Mn1.74Ni0.26O4 Li0.22Mn1.81Cr0.19O4 Li2.34Mn2O4 Li2.69Mn1.82Ni0.18O4 Li2.68Mn1.69Ni0.31O4 Li2.47Mn1.82Cr0.18O4

53 33 23 43 0 0 0 2 83a 72a 78a 74a

47 67 77 57λ 100 100 100 98 0 0 0 0

a There is a small fraction of divalent Mn ion in the tetragonal spinel phases (17% for Li2Mn2O4, 28% for Li2Mn1.8Ni0.2O4, 22% for Li2Mn1.67Ni0.33O4, 26% for Li2Mn1.8Cr0.2O4).

and carefully compared with those of Ni-substituted compounds in order to differentiate the effects of substitution of Mn with divalent and trivalent cations. Experimental Section Sample Preparation and Characterization. The polycrystalline LiMn2-xMxO4 (M ) Ni and Cr) samples were prepared by heating the stoichiometric mixture of Li2CO3, Mn2O3, NiO, and Cr2O3 at 730-750 °C for 40 h with intermittent grindings. The chemical delithiation reaction was carried out by stirring these samples with 2.5 M H2SO4 solution, as reported previously.18 On the other hand, the lithiated Li2Mn2-xMxO4 (M ) Ni and Cr) samples with tetragonal spinel structure were synthesized by reacting the pristine cubic spinel compounds with 1.6 M n-BuLi in hexane.19 The crystal structure of these spinelstructured materials was studied by powder X-ray diffraction (XRD) using Ni-filtered Cu KR radiation with a graphite diffracted beam monochromator, and their chemical compositions were determined by atomic absorption (AA) spectrometry. As listed in Table 1, the present experimental results indicate that the nickel and chromium ions are successfully incorporated into the crystal lattice of lithium manganate with spinel structure. X-ray Absorption Measurement. XAS experiments were carried out for LiMn2-xMxO4 (M ) Ni and Cr) and their delithiated and lithiated derivatives by using the extended X-ray absorption fine structure (EXAFS) facility installed at the beam line 7C at the Photon Factory in Tsukuba.20 The XAS data were collected at room temperature in a transmission mode using gasionization detectors. All the present spectra were calibrated by measuring the spectra of Mn, Ni, and Cr metal foils. The data analyses for the experimental spectra were performed by using the standard procedure reported previously.13 All the present X-ray absorption near-edge structure (XANES) spectra were normalized by fitting the smooth EXAFS high-energy region with a linear function after subtracting the background extrapolated from the preedge region. The EXAFS oscillations were separated from the absorption background by using a cubic spline background removal technique. The resulting χ(k) oscillations were weighted by k3 in order to compensate for the diminishing amplitude of the EXAFS at the high-k region. For the analysis of the EXAFS data, we carried out nonlinear leastsquares curve fitting to the Fourier-filtered coordination shells by minimizing the value of F (F ) [∑k6(χcal - χexp)2]1/2/n, where the summation was performed over the data points (n) in the analyzed k range) with the use of well-known single scattering EXAFS theory.21

Figure 1. Powder XRD patterns for (a) the cubic spinel LiMn1.8Ni0.2O4, (b) λ-LiyMn1.8Ni0.2O4, and (c) the tetragonal spinel Li2Mn1.8Ni0.2O4.

Results and Discussion Powder XRD Analysis. The powder XRD pattern of the pristine LiMn1.8Ni0.2O4 is shown in Figure 1, together with those of its delithiated and lithiated derivatives. While all the intense XRD peaks of LiMn1.8Ni0.2O4 and λ-LiyMn1.8Ni0.2O4 can be well-indexed on the basis of the cubic spinel structure with the space group of Fd3m, the lithiated Li2Mn1.8Ni0.2O4 compound shows the characteristic peaks of the tetragonal spinel phase.22 The lattice parameters and unit cell volumes of these spinelstructured compounds were calculated from least-squares fitting analysis, as summarized in Table 1. The lattice parameters of pristine spinel LiMn2-xNixO4 compounds decrease as the Ni content increases, even though the manganese ion is replaced by the larger Ni+II ion (Mn+III (6) ) 0.65 Å, Mn+IV (6) ) 0.53 Å, and Ni+II (6) ) 0.70 Å, where the number in parentheses represents the coordination number23). Such an unexpected lattice contraction caused by the substitution of Mn with Ni indicates that the lattice parameters of lithium manganate with spinel structure are mainly dependent on the average oxidation state of manganese, which is enhanced by Ni substitution. In contrast to the pristine spinel phase, the unit cell volumes of λ-MnO2 and tetragonal spinel Li2Mn2O4 increase as the manganese ions are replaced by nickel ions. Taking into account the fact that the manganese valence in these compounds remains unchanged before and after nickel substitution, i.e., Mn+IV for λ-LiyMn2-xNixO4 and Mn+III for Li2Mn2-xNixO4, such a lattice

Bonding Nature of LiMn2-xNixO4 Spinel Oxides

Figure 2. Mn K-edge XANES spectra for (a) the cubic spinel LiMn2-xMxO4, (b) λ-LiyMn2-xMxO4, and (c) the tetragonal spinel Li2Mn2-xMxO4, in comparison with those for the references (d) Mn2O3 and (e) MnO2. The solid lines, dotted lines, dashed lines, and dotteddashed lines represent the data for the Ni content (x) of 0, 0.20, 0.33 and those for the Cr content (x) of 0.20, respectively.

expansion for Ni-substituted λ-MnO2 and tetragonal spinel phases is ascribed to the larger ionic size of divalent nickel ion with respect to trivalent or tetravalent manganese ion. In the case of Cr-substituted compounds, the lattice volumes of the pristine spinel and its lithiated derivative are smaller than those of unsubstituted homologues, while the crystal lattice of λ-MnO2 is expanded by Cr substitution. As discussed above, the lattice contraction of the pristine compound with spinel structure upon Cr substitution can be understood as a result of the increase of the Mn oxidation state. On the other hand, the changes in the lattice volume of λ-MnO2 and that of the tetragonal spinel upon Cr substitution reflect the relative ionic size of manganese and chromium ions, i.e., r(Mn+III) > r(Cr+III) > r(Mn+IV).23 Mn K-Edge XANES Analysis. The variation of the chemical bonding character of lithium manganates with spinel structure upon nickel substitution has been investigated by using XANES spectroscopy. The Mn K-edge XANES spectra for LiMn2-xNixO4 (x ) 0, 0.2, and 0.33) and their delithiated and lithiated derivatives are shown in Figure 2, in comparison with those for the references LiMn1.8Cr0.2O4, λ-LiyMn1.8Cr0.2O4, Li2Mn1.8Cr0.2O4, Mn2O3, and MnO2. In the preedge region, a small peak P corresponding to the quadrupole-allowed 1s f 3d transition appeared in all the spectra presented here. Since this peak intensity is proportional to the distortion of local structure around the absorbing metal ion from inversion symmetry,24 the negligible intensity of this peak implies that all the manganese ions in the samples are stabilized in the octahedral site with an inversion center. The position of this preedge peak for LiMn2-xMxO4 is found to be higher than that for Mn+III2O3 but lower than that for Mn+IVO2, whereas the delithiated λ-LiyMn2-xMxO4 derivatives exhibit nearly the same peak energy as Mn+IVO2, which indicates that the mixed oxidation state (Mn+III/Mn+IV) converges to the tetravalent one upon acid treatment. In the case of Li2Mn2-xMxO4 with tetragonal spinel structure, the energy of the preedge peak is nearly identical to that for reference Mn+III2O3, manifesting the trivalent oxidation state of manganese ion. In the main-edge region, there are some strong peaks corresponding to the dipole-allowed transitions from the core 1s to unoccupied 4p levels. The pristine LiMn2-xMxO4 compounds and their delithiated derivatives show

J. Phys. Chem. B, Vol. 105, No. 1, 2001 337

Figure 3. Ni K-edge XANES spectra for (a) the cubic spinel LiMn1.8Ni0.2O4, (b) λ-LiyMn1.8Ni0.2O4, and (c) the tetragonal spinel Li2Mn1.8Ni0.2O4, in comparison with those for the references (d) NiO and (e) LiNiO2.

nearly identical spectral features to those characteristic of the cubic spinel phase.13,14 As shown in Figure 2, the position of the edge jump for LiMn2-xNixO4 is shifted toward the highenergy side with increasing nickel substitution rate, indicating an increase of the Mn oxidation state. In addition to the edge shift, the main-edge peaks become sharper upon nickel substitution, suggesting that the structural disorder around manganese is suppressed by decreasing the concentration of the Jahn-Teller Mn+III (3d4) ion. For the delithiated λ-LiyMn2-xNixO4 compounds, there is no remarkable variation in edge position with respect to the nickel-substitution rate, confirming that all the manganese ions are oxidized into the tetravalent state after acidic treatment. However, the intensity of the peak C becomes weaker as the Ni content increases, which suggests an enhancement of the structural disorder around manganese upon Ni substitution. Such spectral variations upon Ni substitution are also detected for the Cr-substituted LiMn1.8Cr0.2O4 and λ-LiyMn1.8Cr0.2O4 compounds. However, the degree of spectral modification is less prominent for Cr-substituted materials than for Nisubstituted ones, indicating that the replacement of Mn with Cr+III induces relatively small changes in the oxidation state of manganese and its local structure compared to replacement with Ni+II. On the other hand, the lithiated Li2Mn2-xMxO4 compounds exhibit quite different spectral shapes from the pristine and delithiated spinel ones, which would originate from the presence of tetragonal distortion caused by the collective Jahn-Teller effect. In contrast to the cubic spinel and λ-MnO2 phases, the tetragonal spinel compounds show a strong peak A which reflects the presence of trivalent manganese ion stabilized in the tetragonally distorted octahedra.15 It is also observed that this peak A is weaker for the Cr-substituted compound than for the Ni-substituted one, indicating that the tetragonal distortion upon lithiation is less severe for the former. Ni K-Edge XANES Analysis. The Ni K-edge XANES spectra of LiMn1.8Ni0.2O4, and its delithiated and lithiated derivatives are plotted in Figure 3, together with those of references NiO and LiNiO2. The edge energies of lithium nickel manganese oxides are almost identical to that of NiO, revealing the divalent oxidation state of nickel in these compounds. This is in good agreement with the previous X-ray photoelectron spectroscopy (XPS) results.11 In the preedge region, a small peak

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Figure 4. Cr K-edge XANES spectra for (a) the cubic spinel LiMn1.8Cr0.2O4, (b) λ-LiyMn1.8Cr0.2O4, and (c) the tetragonal spinel Li2Mn1.8Cr0.2O4, in comparison with those for the references (d) Cr2O3 and (e) CrO3.

P related to the quadrupole-allowed transition from the core 1s to unoccupied 3d levels is observed for all the present compounds. The weak intensity of the preedge peak P for LiMn2-xNixO4 indicates that the nickel ions are incorporated into the octahedral manganese site. In the main-edge region, the overall spectral features of the pristine LiMn1.8Ni0.2O4 are nearly the same as those of λ-LiyMn1.8Ni0.2O4, whereas the lithiated Li2Mn1.8Ni0.2O4 differs in spectral shape from the above two compounds, as observed in the Mn K-edge XANES region. Such spectral differences indicate that the local structure around nickel ion remains unchanged before and after delithiation reaction, whereas the lithiation leads to a significant perturbation around nickel ion, as in the case of manganese ion. Cr K-Edge XANES Analysis. The Cr K-edge XANES spectra of LiMn1.8Cr0.2O4, and its delithiated and lithiated derivatives are presented in Figure 4, in comparison with those of references Cr2O3 and CrO3. The edge positions of all the spinel-related compounds are determined to be almost the same as one another and nearly identical to that of Cr2O3, suggesting that the oxidation state of chromium (Cr+III) is not changed as a result of the delithiation and lithiation process. While the preedge peaks P related to the quadrupole-allowed 1s f 3d transitions are fairly weak in intensity for the spinel lithium chromium manganates and Cr2O3 with trivalent chromium ion stabilized in the octahedral site, a very intense feature P is observed for the reference CrO3 with the tetrahedral Cr+VI ion. Such an inconsistency is attributed to the differences not only in the hole concentrations of Cr 3d orbitals but also in the local symmetry around chromium, i.e., Oh for the former and Td for the latter.16 In this regard, the weak intensity of the preedge peaks P for spinel-related compounds allows us to exclude the possibility of the existence of hexavalent chromium ions in the present compounds, as would be expected. In the main-edge region, the overall spectral features of the delithiated and lithiated spinel compounds are rather similar to those of the pristine material, in contrast to the Mn K- and Ni K-edge XANES results. On the basis of such spectral variations, it is concluded that both the delithiation and lithiation processes have little influence on the local structure around chromium, which is in marked contrast to the cases of manganese and nickel ions. This experimental finding provides clear evidence of the higher structural stability of the regular CrO6 octahedron with respect

Figure 5. (a) Fourier transformed Mn K-edge EXAFS spectra and (b) their inverse Fourier transforms for the cubic spinel LiMn2-xMxO4 compounds with Ni content (x) of (i) 0 (solid lines), (ii) 0.20 (dotted lines), and (iii) 0.33 (dashed lines) and those for Cr content (x) of (iv) 0.20 (dotted-dashed lines), respectively. The information in parentheses is for the data in a. The Fourier filtering range is shown by the arrows. In b, the solid lines and empty circles represent the fitted and experimental data, respectively.

to the Li deintercalation/intercalation, which can surely be attributed to the great octahedral site stabilization energy of trivalent chromium ions with an electronic configuration of [Ar]3d3.25 Mn K-Edge EXAFS Analysis. The evolution of Mn local structure in the spinel lattice upon cation substitution has been quantitatively determined by EXAFS spectroscopy. The k3weighted Mn K-edge EXAFS spectra for LiMn2-xMxO4 (M ) Ni and Cr; x ) 0, 0.2, and 0.33) are Fourier transformed in Figure 5a. All the spinel lithium manganates exhibit two intense Fourier transform (FT) peaks at ∼1.5 and ∼2.5 Å, which are attributed to the (Mn-O) and (Mn-M) shells (M ) Mn, Ni, or Cr), respectively. As the manganese ion is replaced by nickel or chromium ion, the intensities of both FT peaks become enhanced with a more prominent peak for the Ni-substituted compound. Since the trivalent manganese ion in the oxide lattice is generally stable in the tetragonally distorted octahedron,26 there are three different manganese-oxygen bond distances in the unsubstituted LiMn2O4 compound, i.e., R(Mn+III-Oeq), R(Mn+III-Oax), and R(Mn+IV-O). Among them, the longest (Mn+III-Oax) bond has been known to contribute to a damping of the EXAFS signal.14 In this regard, the enhancement of FT peaks upon cation substitution can be understood as the result of an increase of the average Mn oxidation state, leading to a decrease of the Mn+III ion concentration with Jahn-Teller character. To perform the nonlinear curve fitting analysis, the FT spectra were inversely Fourier transformed to k space (Figure 5b). All the present compounds show nearly the same EXAFS oscillation, which can be well-reproduced on the basis of cubic

Bonding Nature of LiMn2-xNixO4 Spinel Oxides

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TABLE 2: Results of Nonlinear Least Squares Curve Fittings for the Mn K-edge EXAFS Spectra of Cubic Spinel LiMn2-xMxO4 (M ) Ni and Cr) Compounds sample LiMn2O4 LiMn1.8Ni0.2O4 LiMn1.67Ni0.33O4 LiMn1.8Cr0.2O4

bond (Mn-O) (Mn-Mn) (Mn-O) (Mn-Mn,Ni) (Mn-O) (Mn-Mn,Ni) (Mn-O) (Mn-Mn,Cr)

CNa ∆E eV) R (Å) σ2 (10-3 × Å) 6 6 6 6 6 6 6 6

1.20 1.30 0.34 3.10 0.31 4.51 0.83 2.40

1.91 2.91 1.91 2.90 1.91 2.89 1.91 2.90

4.57 5.00 3.75 3.89 2.88 3.19 4.59 4.29

a To determine the effect of cation substitution on the local crystal order around manganese ion, all the fitting analyses were carried out with the calculated amplitude reduction factor (S02) of unsubstituted LiMn2O4.

spinel structure. The best fitting results are compared to the experimental spectra in Figure 5b, and the fitted structural parameters are summarized in Table 2. The substitution of Mn with Ni+II or Cr+III has little influence on the Mn-O bond distance, while it gives rise to a decrease in the Mn-M bond distance (M ) Mn, Ni, or Cr), as previously reported for Crsubstituted spinel oxide.14 Such a remarkable shortening of the metal-metal bond distance is in good agreement with the cell contraction of spinel lattice upon cation substitution. It is also found that the Debye-Waller factor is decreased with an increase of the Ni+II (or Cr+III) substitution rate, confirming a remarkable decrease in structural disorder upon cation substitution due to the further oxidation of trivalent manganese ion. This is well-correlated with the sharpening of XANES features for Ni- and Cr-substituted phases. Comparing the Ni- and Crsubstituted compounds, the Debye-Waller factors of both coordination shells are estimated to be smaller for LiMn1.8Ni0.2O4 than for LiMn1.8Cr0.2O4, confirming that the substitution with divalent nickel ion is more effective in increasing the Mn oxidation state than replacement with trivalent chromium ion. The influence of Li deintercalation on the local atomic arrangement of manganese has also been probed by performing Mn K-edge EXAFS analyses for the delithiated λ-LiyMn2-xMxO4 (M ) Ni and Cr; x ) 0, 0.2, and 0.33), as shown in Figure 6a,b. The best fitting results to these FT peaks are compared to the experimental spectra in Figure 6b, and the fitted structural parameters are listed in Table 3. While the manganese-oxygen bond distance is independent of the Ni (or Cr) substitution rate, the Mn-M bond distance becomes longer with an increase of the cation substitution rate. Such a variation of bond length can be explained by the fact that the shorter Mn-Mn bond is replaced by the longer Mn-Ni,Cr one, whereas the Mn oxidation state is fixed to be tetravalent for all the present compounds. As expected from a decrease of FT peak intensity (Figure 6a), the Debye-Waller factor becomes slightly greater as the manganese ion is substituted with a nickel (or chromium) ion. Such a finding can be regarded as clear evidence of the increase of structural disorder caused by the replacement of Mn with Ni+II (or Cr+III). On the other hand, Mn K-edge EXAFS analyses have also been carried out for the lithiated Li2Mn1.8M0.2O4 (M ) Ni and Cr) spinel compounds (Figure 7a,b) in order to probe the effect of cation substitution on the structural variation during the lithiation process. As can be seen clearly from Figure 7a, there are remarkable differences between the FT data of the two compounds as follows: the Ni-substituted compound shows three FT peaks corresponding to the Mn-O, Mn-M, and Mn-M shells (M ) Mn or Ni) at ∼1.6, ∼2.2, and ∼2.7 Å, while only two intense FT peaks appear for the chromium-substituted Li2Mn1.8Cr0.2O4 compound. Considering

Figure 6. (a) Fourier transformed Mn K-edge EXAFS spectra and (b) their inverse Fourier transforms for the λ-LiyMn2-xMxO4 compounds with Ni content (x) of (i) 0 (solid lines), (ii) 0.20 (dotted lines), and (iii) 0.33 (dashed lines) and those for Cr content (x) of (iv) 0.20 (dotteddashed lines), respectively. The information in parentheses is for the data in a. The Fourier filtering range is shown by the arrows. In b, the solid lines and empty circles represent the fitted and experimental data, respectively.

the fact that the splitting of Mn-M shells originates from JahnTeller distortion of the trivalent manganese ion,14-16 the appearance of two FT peaks for the Cr-substituted phase indicates that the cubic symmetry of the pristine LiMn1.8Cr0.2O4 is not completely changed into tetragonal symmetry upon lithiation reaction, in contrast to the Ni-substituted case. Such a suppression of tetragonal distortion in Li2Mn1.8Cr0.2O4 is further supported by the weak intensity of the peak A in the XANES region. To obtain structural parameters, we have tried to fit the present spectra of both compounds on the basis of tetragonal spinel structure (Figure 7b).27 As listed in Table 4, while the Mn-O bond lengths are almost the same for both samples, the difference between the Mn-M bond distances is smaller for the Cr-substituted compound than for the Nisubstituted one, which suggests that the tetragonal distortion caused by the lithiation process is significantly suppressed by replacing Mn with Cr+III. Moreover, the Debye-Waller factors of Mn-M shells in Li2Mn1.8Cr0.2O4 are found to be significantly greater than those in Li2Mn1.8Ni0.2O4, indicating the perturbation of tetragonal symmetry of the lithiated spinel phase upon Cr substitution. This is well-correlated with the very broad features A and B in the XANES spectrum of the Cr-substituted compound (Figure 2). Ni K- and Cr K-Edge EXAFS Analyses. In addition, the change of local symmetry around nickel and chromium has been quantitatively examined by performing EXAFS analyses at the Ni K- and Cr K-edges. The FTs of k3-weighted Ni K-edge EXAFS spectra for LiMn1.8Ni0.2O4 and its delithiated and lithiated derivatives are plotted in Figure 8a and the corresponding Fourier-filtered oscillations in Figure 8b, respectively. For

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TABLE 3: Results of Nonlinear Least Squares Curve Fittings for the Mn K-Edge EXAFS Spectra of λ-MnO2-Type LiyMn2-xMxO4 (M ) Ni and Cr) Compounds sample λ-MnO2 λ-LiyMn1.8Ni0.2O4 λ-LiyMn1.67Ni0.33O4 λ-LiyMn1.8Cr0.2O4

bond

CNa

∆E (eV)

R (Å)

σ2 (10-3 × Å)

(Mn-O) (Mn-Mn) (Mn-O) (Mn-Mn,Ni) (Mn-O) (Mn-Mn,Ni) (Mn-O) (Mn-Mn,Cr)

6 6 6 6 6 6 6 6

2.25 0.63 2.76 2.16 3.57 3.57 2.32 1.58

1.90 2.84 1.90 2.85 1.90 2.86 1.90 2.85

3.19 3.74 3.97 4.35 3.88 4.49 3.44 4.42

a

To determine the effect of cation substitution on the local crystal order around manganese ion, all the fitting analyses were carried out with the calculated amplitude reduction factor (S02) of unsubstituted λ-MnO2.

Figure 7. (a) Fourier transformed Mn K-edge EXAFS spectra and (b) their inverse Fourier transforms for (i) Li2Mn1.8Ni0.2O4 and (ii) Li2Mn1.8Cr0.2O4, respectively. The Fourier filtering range is shown by the arrows. In b, the solid lines and empty circles represent the fitted and experimental data, respectively.

TABLE 4: Results of Nonlinear Least Squares Curve Fittings for the Mn K-Edge EXAFS Spectra of the Tetragonal Spinel Li2Mn2-xMxO4 (M ) Ni and Cr) Compounds sample

bond

Li2Mn1.8Ni0.2O4 (Mn-Oeq) (Mn-Oax) (Mn-Mn,Ni) (Mn-Mn,Ni) Li2Mn1.8Cr0.2O4 (Mn-Oeq) (Mn-Oax) (Mn-Mn,Cr) (Mn-Mn,Cr)

CN ∆E (eV) R (Å) σ2 (10-3 × Å) 4 2 2 4 4 2 2 4

1.42 1.42 11.78 11.78 2.95 2.95 10.53 10.53

1.92 2.26 2.79 2.99 1.92 2.26 2.84 2.98

2.67 8.94 4.82 10.49 4.21 12.60 6.14 18.31

all the present lithium nickel manganates, two FT peaks corresponding to the Ni-O and Ni-M shells (M ) Mn or Ni) are observed around 1.6 and 2.6 Å, which underlines that the substituted nickel ions are placed in regular octahedra. Such a local symmetry of nickel accounts for the absence of JahnTeller distortion in the divalent nickel ion with an electronic

Figure 8. (a) Fourier transformed Ni K-edge EXAFS spectra and (b) their inverse Fourier transforms for (i) LiMn1.8Ni0.2O4, (ii) λ-LiyMn1.8Ni0.2O4, and (iii) Li2Mn1.8Ni0.2O4, respectively. The Fourier filtering range is shown by the arrows. In b, the solid lines and empty circles represent the fitted and experimental data, respectively.

configuration of [Ar]3d.8 While the FT data for the pristine LiMn1.8Ni0.2O4 are similar to those for the delithiated derivative, the lithiated Li2Mn1.8Ni0.2O4 shows a marked depression in the intensity of FT peaks and in the amplitude of EXAFS oscillation at the high-k region compared to the other compounds. Such spectral changes indicate that the local atomic arrangement around nickel ion becomes remarkably frustrated after the lithiation process, as expected from the Ni K-edge XANES analyses. As shown in Figure 8b, all the present EXAFS data can be well-reproduced on the basis of a regular NiO6 octahedron with six neighboring metal ions at the same distance. From the fitting results in Table 5, the nickel ion in the pristine LiMn1.8Ni0.2O4 is found to be stabilized in the regular octahedron with the Ni-O bond distance of 2.07 Å, which is fully consistent with the bond length of Ni+II-O in the reference NiO (2.08 Å).28,29 The Ni-O and Ni-M bond distances are slightly shortened upon delithiation and elongated upon lithiation, which appears to be related to cell contraction and expansion.

Bonding Nature of LiMn2-xNixO4 Spinel Oxides

J. Phys. Chem. B, Vol. 105, No. 1, 2001 341

TABLE 5: Results of Nonlinear Least Squares Curve Fittings for the Ni K-Edge EXAFS Spectra of Cubic Spinel LiMn1.8Ni0.2O4 and Its Delithiated and Lithiated Derivatives sample LiMn1.8Ni0.2O4

bond

(Ni-O) (Ni-Mn,Ni) λ-LiyMn1.8Ni0.2O4 (Ni-O) (Ni-Mn,Ni) Li2Mn1.8Ni0.2O4 (Ni-O) (Ni-Mn,Ni)

CN ∆E (eV) R (Å) σ2 (10-3 × Å) 6 6 6 6 6 6

7.41 2.01 12.00 7.89 7.64 2.19

2.07 2.91 2.05 2.88 2.07 2.93

3.88 2.78 4.03 1.64 10.76 8.02

Figure 9. (a) Fourier transformed Cr K-edge EXAFS spectra and (b) their inverse Fourier transforms for (i) LiMn1.8Cr0.2O4, (ii) λ-LiyMn1.8Cr0.2O4, and (iii) Li2Mn1.8Cr0.2O4, respectively. The Fourier filtering range is shown by the arrows. In b, the solid lines and empty circles represent the fitted and experimental data, respectively.

Especially for the lithiated derivative, the Debye-Waller factors are markedly enhanced with respect to the pristine compound, confirming that the local structure around nickel ion is severely disordered upon lithiation reaction. Parts a and b of Figure 9 present the FTs of k3-weighted Cr K-edge EXAFS spectra for LiMn1.8Cr0.2O4, and its delithiated and lithiated derivatives, and the corresponding Fourier-filtered oscillations, respectively. They are very similar to one another, which highlights the fact that the local atomic arrangement around chromium ion remains unchanged before and after delithiation and lithiation reactions. As observed for the Ni K-edge data of Ni-substituted spinel oxides, two intense FT peaks corresponding to the Cr-O and Cr-M shells (M ) Mn or Cr) are discernible for all the compounds, indicating that the substituted chromium ions are placed in regular octahedra due to the absence of Jahn-Teller distortion. As in the Ni K-edge case, we could fit these spectra successfully on the basis of a regular CrO6 octahedron with six neighboring metal ions located at the same distance from Cr (Figure 9b). As listed in Table 6, the chromium ions are stabilized in a regular octahedron with a Cr-O bond distance of 1.98 Å, as reported previously.18 For all the present compounds, there are small discrepancies in bond

TABLE 6: Results of Nonlinear Least Squares Curve Fittings for the Cr K-Edge EXAFS Spectra of Cubic Spinel LiMn1.8Cr0.2O4 and Its Delithiated and Lithiated Derivatives sample LiMn1.8Cr0.2O4

bond

(Cr-O) (Cr-Mn,Cr) λ-LiyMn1.8Cr0.2O4 (Cr-O) (Cr-Mn,Cr) Li2Mn1.8Cr0.2O4 (Cr-O) (Cr-Mn,Cr)

CN ∆E (eV) R (Å) σ2 (10-3 × Å) 6 6 6 6 6 6

3.73 0.84 10.44 5.67 4.99 1.10

1.98 2.89 1.97 2.86 1.98 2.89

2.58 3.22 2.96 3.96 3.24 6.01

distances and Debye-Waller factors, implying that the chemical environment of chromium ion is quite stable with respect to the delithiation and lithiation processes. Relationship between Structural Stability and Electrochemical Performance. Summarizing the present experimental findings, it becomes clear that the substitution of Mn with Ni+II or Cr+III suppresses the tetragonal distortion in spinel lattice by increasing the average oxidation state of manganese. The replacement of Mn with Ni+II is less effective than with Cr+III in enhancing the structural stability of cubic spinel lattice, especially for the lithiation reaction. This is due to the fact that Ni+II ions with 3d8 electronic configuration have smaller octahedral site preferential energy than Cr+III ions, resulting in an incomplete fixation of Ni+II in octahedral sites during Li intercalation and deintercalation processes. In addition, the larger cation size of Ni+II would induce a strain in the local atomic site around Mn, which makes it easier to distort the regular MnO6 octahedra. In this regard, we can expect that the Crsubstituted spinel compound would exhibit better cyclability in the 3 V region than the Ni-substituted compound, while both spinel compounds would possess commonly improVed electrochemical properties for the 4 V region. Actually it has been reported that the replacement of Mn with Cr or Ni is effectiVe in improVing the electrochemical property of spinel compounds in the 4 V region.8 According to the systematic study on Various cation-substituted spinel phases,8b the Cr- and Ni-substituted spinel compounds commonly show excellent capacity retention after charge-discharge cycling process with an applied current density of 1 mA/cm2 and Voltage range of 3.5-4.35 V (Crsubstituted spinel, 102 mAh/g for the first cycle, 98 mAh/g for the thirty cycle; Ni-substituted spinel, 102 mAh/g for the first cycle, 101 mAh/g for the thirty cycle). In the case of the 3 V region, there is no aVailable literature proViding the direct comparison of electrochemical performances of Ni- and Crsubstituted spinel phases under the same measurement condition. HoweVer, we can eValuate indirectly the efficiency of cation substitutions by comparing the copublished data of unsubstituted spinel compound. As for the Cr substitution, it was reported that, after the fifth discharge-charge cycling (0.25 mA/cm2; 2.0-3.8 V), the Cr-substituted spinel LiMn1.6Cr0.4O4 exhibits the smaller capacity loss of 12% compared to the 30% loss of the unsubstituted compound.9 On the contrary, according to the recent study on LiMn1.5M0.5O4 compounds (M ) Ni, Co, etc.),12 the Ni-substituted spinel phase demonstrates poorer cyclability than the unsubstituted and the Co-substituted spinel phases at a low cycling rate C/40 (Ni-doped spinel, 132 mAh/g for the first cycle, 78 mAh/g for the fifth cycle; Co-doped spinel, 103 mAh/g for the first cycle, 115 mAh/g for the fifth cycle; unsubstituted spinel, 92 mAh/g for the first cycle, 112 mAh/g for the fifth cycle).30 Summarizing the aboVe literature data, it becomes clear that the replacement of Mn with Cr (or Co) is more effectiVe than substitution with Ni in improVing the electrochemical property of spinel compounds for 3 V region. However, as mentioned in the Introduction, there is a contradictory report in which the Ni-substituted LiMn2-xNixO4 prepared

342 J. Phys. Chem. B, Vol. 105, No. 1, 2001 by the sol-gel route shows an excellent cyclability for the 3 V region.11 Such contrasting results can be explained on the basis of the recent systematic study of the relationship between the particle size and electrochemical performance of LiMn2O4 spinel oxide.13 From this study, it has been established that the lowering of synthetic temperature via the sol-gel route decreases the size of the crystallite and modifies the surface properties, which facilitates the grafting process of the Li+ ion and thereby enhances the electrochemical properties in the 3 V region corresponding to the Li insertion.13 In this regard, it is concluded that the excellent electrochemical performance of Ni-substituted LiMn2-xNixO4 prepared by the sol-gel route is attributed to the decrease of crystallite size rather than to the substitution of Mn with Ni. Conclusion We have performed systematic XAS analyses for the Nisubstituted LiMn2-xMxO4 (x ) 0, 0.2, and 0.33) spinel oxides and their delithiated/lithiated derivatives in comparison with the Cr-substituted homologues. According to the Mn K-edge XANES/EXAFS results presented here, it is certain that the replacement of Mn with Ni+II or Cr+III gives rise to a suppression of tetragonal distortion in cubic spinel lattice as well as to the increase of the Mn oxidation state. Moreover, the Ni K- and Cr K-edge XAS analyses reveal that the Ni+II ion shows lower structural stability in the spinel lattice compared to the Cr+III ion, leading to a remarkable structural frustration around nickel ion upon lithiation reaction. On the basis of the present experimental findings, it is concluded that the inferior electrochemical performance of LiMn2-xNixO4 with respect to LiMn2-xCrxO4 can surely be attributed to the less effective stabilization of cubic spinel lattice upon Ni substitution. Acknowledgment. This work was supported in part by the Korean Ministry of Science and Technology through the 1999 National Research Laboratory (NRL) project. The authors are grateful to Prof. M. Nomura for helping us to get the XAS data in the Photon Factory. H.-S.P. thanks the Ministry of Education for the Brain Korea 21 fellowship. References and Notes (1) Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. Mater. Res. Bull. 1980, 15, 783. (2) Dahn, J. R.; von Sacken, U.; Jukow, M. R.; Al-Janaby, H. J. Electrochem. Soc. 1991, 138, 2207. (3) Nagaura, T.; Tazawa, K. Prog. Batteries Sol. Cells 1990, 9, 20. (4) Thackeray, M. M.; Johnson, P. J.; de Piccioto, L. A.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1984, 19, 179.

Hwang et al. (5) Thackeray, M. M. Prog. Solid State Chem. 1997, 25, 1. (6) Tarascon, J. M.; Wang, E.; Shokoohi, F. K.; McKinnon, W. R.; Colson, S. J. Electrochem. Soc. 1991, 138, 2859. (7) Gummow, R. J.; de Kock, A.; Thackeray, M. M. Solid State Ionics 1994, 69, 59. (8) (a) Guohua, L.; Ikuta, H.; Uchida, T.; Wakihara, M. J. Electrochem. Soc. 1996, 143, 178. (b) Pistoia, G.; Antonini, A.; Rosati, R.; Bellitto, C.; Ingo, G. M. Chem. Mater. 1997, 9, 1443. (9) Baochen, W.; Yongyao, X.; Li, F.; Dongjiang, Z. J. Power Sources 1993, 43-44, 539. (10) Sigala, C.; Verbaere, A.; Mansot, J. L.; Guyomard, D.; Piffard, Y.; Tournoux, M. J. Solid State Chem. 1997, 132, 372. (11) Amine, K.; Tukamoto, H.; Yasuda, H.; Fujita, Y. J. Electrochem. Soc. 1996, 143, 1607. (12) Strobel, P.; Palos, A. I.; Anne, M.; Le Cras, F. J. Mater. Chem. 2000, 10, 429. (13) Treuil, N.; Labruge`re, C.; Menetrier, M.; Portier, J.; Campet, G.; Deshayes, A.; Frison, J. C.; Hwang, S. J.; Song, S. W.; Choy, J. H. J. Phys. Chem. B 1999, 103, 2100. (14) Ammundsen, B.; Jones, D. J.; Rozie`re, J.; Villain, F. J. Phys. Chem. B 1998, 102, 7939. (15) Hwang, S. J.; Park, H. S.; Choy, J. H.; Campet, G. Chem. Mater. 2000, 12, 1818. (16) Hwang, S. J.; Park, H. S.; Choy, J. H.; Campet, G. J. Phys. Chem. B 2000, 104, 7612. (17) Nakai, I.; Takahashi, K.; Shiraishi, Y.; Nakagome, T.; Nishikawa, F. J. Solid State Chem. 1998, 140, 145. (18) Larcher, D.; Courjal, P.; Urbina, R. H.; Gerand, B.; Blyr, A.; Du Pasquier, A.; Tarascon, J. M. J. Electrochem. Soc. 1998, 145, 3392. (19) Thackeray, M. M.; David, W. I. F.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1983, 18, 461. (20) Oyanagi, H.; Matsushida, T.; Ito, M.; Kuroda, H. KEK Rep. 1984, 83, 30. (21) Teo, B. K. EXAFS: Basic Principles and Data Analysis; SpringerVerlag: Berlin, 1986. (22) Although the XRD patterns are presented only for the compounds with Ni content of 0.2, the other samples with Ni content (x ) 0 and 0.33) and Cr content (x ) 0.2) exhibit nearly the same diffraction data except a slight shift in peak positions. (23) Shannon, R. D. Acta Crystallogr. A 1976, 32, 751. (24) Hahn, J. E.; Scott, R. A.; Hodgson, K. O.; Doniach, S.; Desjardins, S. R.; Solomon, E. I. Chem. Phys. Lett. 1982, 88, 595. (25) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry; HarperCollins College Publishers: 1993. (26) Wells, A. F. Structural Inorganic Chemistry; Clarendon Press: Oxford, U.K., 1984. (27) In the case of Li2Mn1.8Cr0.2O4, we have also tried to reproduce the experimental data with the cubic spinel structure. However, only the unreasonable result was obtained for this case, i.e., too low coordination numbers. (28) Li, W.; Reimers, J. N.; Dahn, J. R. Phys. ReV. B 1992, 46, 3236. (29) Pickering, I. J.; George, G. N.; Lewandowski, J. T.; Jacobson, A. J. J. Am. Chem. Soc. 1993, 115, 4137. (30) Considering the fact that the trivalent chromium ion (3d3) possesses large octahedral stabilization energy like the trivalent cobalt ion (3d6), it can be reasonably expected that the substitution of Mn with Cr+III would give rise to the remarkable improvement of electrochemical performance under this measurement condition, like the substitution of Mn with Co+III.