DFT+U Calculations and XAS Study: Further Confirmation of the

Article pubs.acs.org/JPCC

DFT+U Calculations and XAS Study: Further Confirmation of the Presence of CoO5 Square-Based Pyramids with IS-Co3+ in LiOverstoichiometric LiCoO2 Dany Carlier,*,† Ju-Hsiang Cheng,‡ Chun-Jern Pan,‡ Michel Ménétrier,† Claude Delmas,† and Bing-Joe Hwang‡ †

CNRS, Université de Bordeaux, ICMCB, IPB-ENSCBP, 87 avenue du Dr. A. Schweitzer, 33608 Pessac Cedex, France Department of Chemical Engineering, National Taiwan University of Science and Technology (NTUST), Taipei 106, Taiwan

‡

S Supporting Information *

ABSTRACT: LiCoO2, one of the major positive electrode materials for Li-ion batteries, can be synthesized with excess Li. Previous experimental work suggested the existence of intermediate spin (IS) Co3+ ions in square-based pyramids to account for the defect in this material. We present here a theoretical study based on density functional theory (DFT) calculations together with an X-ray absorption spectroscopy (XAS) experimental study. In the theoretical study, a hypothetical Li4Co2O5 material, where all the Co ions are in pyramids, was initially considered as a model material. Using DFT+U, the intermediate spin state of the Co3+ ions is found stable for U values around 1.5 eV. The crystal and electronic structures are studied in detail, showing that the defect must actually be considered as a pair of such square-based pyramids, and that Co−Co bonding can explain the position of Co in the basal plane. Using a supercell corresponding to more diluted defects (as in the actual material), the calculations show that the IS state is also stabilized. In order to investigate experimentally the change in the electronic structure in the Li-overstoichiometric LiCoO2, we used X-ray absorption near edge structure (XANES) spectroscopy and propose an interpretation of the O Kedge spectra based on the DFT+U calculations, that fully supports the presence of pairs of intermediate spin state Co3+ defects in Lioverstoichiometric LiCoO2.

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INTRODUCTION LiCoO2 is still the most widely used positive electrode material in commercial Li-ion batteries. One aspect that we believe is key to LiCoO2 is its tendency to accommodate Li-overstoichiometry, which influences the deintercalation mechanism via the electron delocalization mechanism between formally Co3+ and Co4+ via the common edge of adjacent CoO6 octahedra.1 Moreover, lithium overstoichiometry may allow optimizing the cell capacity balance between the positive and negative electrode. 7 Li NMR is very sensitive to traces of electron spins, which are absent in stoichiometric LiCoO2 with a LS Co3+ t2g6 electronic configuration. A single Li NMR signal is thus obtained, with a long T1.2 On the opposite, when excess Li2CO3 is used in the synthesis, and no long annealing is carried out, ICP shows a Li/Co ratio higher than 1 (typically 1.1) and the 7Li NMR spectrum shows additional contributions. Based on a series of observations, we previously hypothesized that the charge due to substitution of some Li+ ions for Co3+ ones is compensated for by oxygen vacancies, leading to the typical formula Li1.04Co0.96O1.96 leaving only Co3+ ions, but some of them (0.08) present in square based pyramids instead of octahedra. This lifts the degeneracy of the eg and the t2g blocs of © 2013 American Chemical Society

orbitals, and we proposed an intermediate spin configuration as illustrated in the Supporting Information, Figure S1.1,3 Besides, Shinova et al. suggested that the layered modification of LiCoO2 is “rigid” with respect to the accommodation of more than one lithium atom compared to the (low temperature) spinel one.4 Based on characterization and calculations, Qian et al. suggested that surface defects in (nominally stoichiometric) nanoparticulate LiCoO2 can also lead to intermediate spin state for Co3+.5 Recently, Koyama et al. reported that Co3+ with IS can be stabilized in presence of an oxygen vacancy.6 However, none of these studies deals with the defect in overstoichiometric LiCoO2, taking into account the charge compensation and an orbital description with the IS configuration. In this paper, we further asses our structural hypothesis for the defect in Li-overstoichiometric LiCoO2 based on DFT+U calculations. In a first part, a hypothetical material with the Li4Co2O5 formula is considered as it corresponds to extreme overstoichiometry in LiCoO2 with all cobalt ions in square based pyramid environment. Then we Received: October 3, 2013 Revised: November 27, 2013 Published: November 27, 2013 26493

dx.doi.org/10.1021/jp409850q | J. Phys. Chem. C 2013, 117, 26493−26500

The Journal of Physical Chemistry C

Article

then the product was reground and heated at 900 °C under oxygen for typically 15 days as described in ref.2 XAS Acquisition Conditions. X-ray absorption spectroscopy (XAS) was performed at the National Synchrotron Radiation Research Center (NSRRC) at Hsinchu, Taiwan. The storage ring of the electronic accelerator can supply the electronic energy of 1.5 GeV and the operation current at 360 mA. The Co K-edge XAS spectra were collected at the BL17C1 station with a Si double crystal monochromator used to perform energy scan, of which the parallelism can be adjusted to eliminate the high order harmonics. All the Co K-edge spectra were recorded using the transmission mode. Ionization chambers were applied as detectors to monitor the intensity of the incident and transmitted beams through the specimen. The absorption coefficient can be calculated from the logarithm of the intensity ratio of the incident and transmitted beams. The reference Co metal foils were positioned in front of the window of the third ionization chamber and measured simultaneously as a standard energy for calibration in each energy scan. The beam size was limited by horizontal and vertical slits with the area of 2 × 2 mm2 during the XAS measurements. The Co L-edge X-ray absorption measurements were performed at the BL20A1 station and its measurements were acquired in total electron yield (TEY) mode for Co LII,III-edge spectra and fluorescence yield (FY) modes for O K-edge spectra using an ultra high-vacuum (UHV) chamber with a base pressure of 1 × 10−10 Torr. The detection depth for FY and TEY mode are more than 50 Å and less than 10 Å, respectively, which could provide the information of bulk and surface of the particle. However, some distortion of the Co LII,III-edge spectra may arise due to the self-absorption effect.19,20 Therefore, the TEY mode was chosen to measure the Co LII,III-edge. XAS Data Analysis. The extended X-ray absorption fine structure (EXAFS) data reduction was conducted by utilizing the standard procedures. The EXAFS function, χ, was obtained by subtracting the postedge background from the overall absorption and then normalized with respect to the edge jump step. The normalized χ(E) was transformed from energy space to k space, where k is the photoelectron wave vector. The χ(k) data were multiplied by k2 to compensate the damping of EXAFS oscillations in the high k region. Subsequently, k2weighted χ(k) data in k space ranging from 3.2 to 12.1 Å−1 for the Co K-edge was Fourier transformed to r space to separate the EXAFS contributions from the different coordination shells. A nonlinear least-squares algorithm was applied to the curve fitting of an EXAFS in r space between 1.0 and 3.0 Å for Co depending on the bond to be fitted. All the computer programs were implemented in the UWXAFS 3.0 package,21 with the backscattering amplitude and the phase shift for the specific atom pairs being theoretically calculated by using the FEFF7 code. From these analyses, structural parameters such as the coordination numbers (N), bond distance (R), Debye−Waller factor (Δσj2), and the inner potential shift (ΔE0) have been calculated. The amplitude reduction factor (S02) value, which accounts for energy loss due to multiple excitations, for Co is obtained by analyzing the LiCoO2 powder reference samples and by fixing the coordination number in the FEFFIT input file.

considered a supercell with the Li25Co23O47 formula as representative of a diluted defect. In order to investigate the difference in electronic structure between stoichiometric and Li-overstoichiometric LiCoO2, we also use X-ray absorption spectroscopy (XAS) and discuss especially the O−K edge with the support of density functional theory (DFT) calculations.

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CALCULATION METHOD We calculated the total energies with the GGA (PBE)7 + U method, using the Projector Augmented Wave (PAW) method8 as implemented in the Vienna Ab Initio Simulation Package (VASP).9 The DFT+U method (LDA+U or GGA+U) allows one to treat more accurately strongly correlated systems, such as transition metals or rare earth-based materials.10,11 In our study, Dudarev’s approach was used to perform the GGA + U. The effective on site Coulomb (U) and exchange (J) parameters are not input separately; only the difference (U − J) is meaningful. It, therefore, allows one to vary a single parameter called Ueff = U − J. Ueff has been varied between 0 and 6 eV to investigate the resulting spin states induced for the cobalt ions. A plane wave cutoff energy of 500 meV and a 6 × 6 × 6 and a 2 × 2 × 2 k-point grid were respectively used for the Li4Co2O5 and Li25Co23O47 cells (see further) to let the total energy converge by less than 5 meV/unit cell. The atomic positions and the lattice parameters of stoichiometric LiCoO212 were used to build the monoclinic initial cells and then cell parameters and atomic positions were fully relaxed with VASP for every Ueff value used. Ferromagnetic (F) and antiferromagnetic-type (AF) configurations between the two adjacent cobalt ions in the square based pyramid environment were considered. For the plot of the partial density of state (DOS) around Co, a sphere radius of 0.55 Å was used for the integration. The method developed by Bader to divide molecules into atoms13 was used in order to determine partial charges on Co ions. The implementation of this method to the VASP output is described in ref.14−16 The Electron Localization Function (ELF)17 is directly available from the VASP code.9 The VESTA code18 was used to plot the 3D spin or ELF maps.

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EXPERIMENTAL SECTION Materials Preparation. Li-overstoichiometric LiCoO2 was prepared from a mixture of Co3O4 (prepared by thermal decomposition of Co(NO3)2·6H2O (Sigma-Aldrich ACS reagent, containing less than 10 mg/kg Fe, Ni, or Cu and less than 20 mg/kg Mn)) and Li2CO3 (Alpha Aesar) with a Li/ Co molar ratio 1.2. The mixture was heated under oxygen flow to 600 °C for 15 h and 800 °C for 1 h. The obtained sample contains LiCoO2 and unreacted Li2O/Li2CO3 that were washed out with water. The obtained LiCoO2 was remixed with an excess of Li2CO3 (1/1 in mass) and heated under oxygen to 900 °C for 48 h. The product was again washed with water to yield the pure Li-overstoichiometric LiCoO2 sample, as identified by X-ray diffraction (XRD) showing no traces of impurity and the following parameters: R3m ̅ space group; a = 2.817(1) Å; c = 14.051(1) Å. Typically, the final composition obtained for the overstoichiometric samples is close to Li1.04Co0.96O1.96 based on the inductively coupled plasma analysis.1 Stoichiometric LiCoO2 was prepared from Co3O4 (prepared as above) and Li2CO3 (Alpha Aesar) mixed in stoichiometric amounts and first reacted for 12 h at 600 °C under oxygen, and

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RESULTS AND DISCUSSION DFT Calculations. In order to check if Co3+ with squarebased pyramid oxygen coordination can be stabilized in overstoichiometric LiCoO2 and what the associated electronic configuration would be, we first considered a hypothetical 26494



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to 2 with a step for S = 1 indicating a clear change in electronic configuration. When the AF and F configurations lead to a similar resulting spin value (S) per Co and are performed for a same Ueff value, one can compare the relative stability of these two magnetic configurations (F and AF configurations between the two adjacent cobalt ions in the square based pyramid environment in the unit cell). In all such cases, the AF arrangement between adjacent Co ions in the layer is predicted to be more stable than the F one by 95−150 meV/Li4Co2O5 formula unit depending on the spin configuration of the Co ions. For the pure GGA calculation (Ueff = 0), an unexpected spin value/Co is computed, namely, S = 0.5 (Figure 1b), that should correspond to a single unpaired electron per Co ion. A Badertype charge analysis was performed in order to check that the Co ions are still in the trivalent state: a +1.10 charge was calculated for Co ions in Li4Co2O5, which is closer to the value obtained for LS-Co3+ ions in LiCoO2 (Bader charge = +1.23) than that obtained for LS-Co4+ ions in CoO2 (Bader charge = +1.40). An electron localization function (ELF) 3D map was also plotted to better understand the electronic structure (Figure 2a). A strong electronic density clearly appears at the position of the O vacancy. This could be due to the pairing of one electron/Co3+ ion resulting from the overlap of the dz2 orbitals pointing toward the O vacancy as shown in Figure 2b,

phase corresponding to extreme overstoichiometry in LiCoO2 with all cobalt ions in such square based pyramid environment. According to the Li1+tCo1−tO2−t formulation, this corresponds to t = 1/3, that is, to the Li4/3Co2/3O5/3 (i.e., Li4Co2O5) formula. The resulting cell is monoclinic (S.G. = Cm) and is shown in Figure 1a. The local axis system chosen for the

Figure 1. (a) In-plane view of the Li4Co2O5 hypothetical cell considered to model the defect in overstoichiometric LiCoO2 showing the excess Li at the Co site and the two CoO5 ions in square-based pyramids. The structure is then built by the successive stacking (O3type) of the MO2 layer as shown and the pure Li layer. (b) Calculated spin/Co in Li4Co2O5 with GGA (Ueff = 0 eV) and GGA+U calculations varying the Ueff value used. Two magnetic coupling configurations were used: ferromagnetic (F) or antiferromagnetic (AF) between adjacent Co ions in the cell.

designation of the d orbitals is also shown in the Supporting Information (Figure S2), with the z-axis being the direction from the Co nucleus to the oxygen vacancy. In the following, we considered the dxy, dxz, dyz, and the dz2, dx2‑y2 orbitals designated respectively by t2g and eg, although (i) the degeneracy of the t2g and eg levels is lifted because of the change in symmetry due to the CoO5 environment vs CoO6 octahedra and (ii) the actual relevant levels might result from a hybridization between the 3d orbitals. Figure 1b shows the resulting spin value (S) per Co ions in the cell vs the Ueff = U − J parameter (for LS-Co3+, IS-Co3+, and HS-Co3+, S = 0 (no unpaired electrons), S = 1 (two unpaired electrons), and S = 2 (four unpaired electrons) spin values are, respectively, expected). As Ueff increases from 0 (pure GGA) to 4 eV, the spin value per Co increases from 0.5

Figure 2. (a) 3D ELF map calculated for Li4Co2O5 with the pure GGA method (Ueff = 0 eV) and plotted for a level equal to 0.1. The map was plotted for x between (0,1), y between (0,1), and z between (0.5,1.5). (b) Possible overlap of the dz2 orbitals between adjacent Co ions resulting in a pairing of the spin. 26495



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therefore resulting in a Co−Co bond and a single remaining unpaired electron per Co ion. This electronic picture, even if not in agreement with the experimental studies of the overstoichiometric LiCoO2,1 emphasizes that one should really consider the defect in this material as a pair of Co ions in 5-fold coordination. Detailed analysis of the band structure would indeed require further investigation. For 1 ≤ Ueff ≤ 1.5 (for the AF configuration) and for 1.5 ≤ Ueff ≤ 2 eV (for the F configuration), the GGA+U calculations lead to a resulting spin value/Co, S = 1, in agreement with the expected value for IS-Co3+ ions (Figure 1b). This case will be analyzed and discussed below. Calculations performed with larger Ueff values (Ueff ≥ 2 eV for the AF configuration and Ueff ≥ 2.5 eV for the F configuration) lead to a resulting spin S = 2, corresponding to HS-Co3+ ions (Figure 1b). As this electronic configuration does not seem in agreement with the experimental data for a diluted defect case,1 this case was not further considered. We now focus on the calculations performed with Ueff = 1.5 eV, leading to S = 1 per Co (as expected for IS-Co3+ ions) in the two magnetic configurations. The relaxed cell parameters and some Co−O and Li−O distances are given in the Supporting Information (Figure S3) (the Co−O ones for AF configuration are also reported in the Supporting Information, Figure S2). Somewhat surprisingly, during the geometry optimization, the Co ions remain very close to the basal plane of the pyramid. In order to characterize the electronic configuration of this Co3+ ion, we plotted in Figure 3 the partial

ions differs slightly from the one previously suggested, since the t2g level carrying the spin is the dyz orbital and not the dxy one as would be expected from an elongation of the Co−O distance due to an O vacancy along the z direction for an isolated CoO5 pyramid. In our case, the hypothetical defect in overstoichiometric LiCoO2 leads locally to a pair of CoO5 pyramids. The dyz orbital points toward the former common edge between the (former) two CoO6 octahedra of LiCoO2. Indeed, the 3D spin density map plotted in Figure 4a for the AF

Figure 4. 3D spin density map (a) and 3D ELF map (b) calculated for Li4Co2O5 with the GGA+U method (Ueff = 1.5 eV) and the AF magnetic coupling configuration. For the spin density map, an isosurface value equal to 0.002 spin/Å2 was used and yellow and blue surfaces indicate respectively positive and negative spin densities. For the ELF, a level equal to 0.1 was used similar to the one used in Figure 2a.

Figure 3. Partial spin DOS calculated around Co in Li4Co2O5 in the AF coupling configuration with GGA+U (Ueff = 1.5 eV). The Fermi level is indicated by the dashed line. The orientation of the local axis system is given in the Supporting Information, Figure S2.

configuration shows that the spin density globally points toward the O vacancy and the missing edge. Strong hybridization between the dz2 and dyz orbitals of adjacent Co can lead to a local Co−Co bond as drawn schematically in Supporting Information Figure S4, and is most probably responsible for the remaining of Co nearby the base of the pyramid during the geometrical optimization. The Co−Co bonding for U = 1.5 eV is however not associated with the pairing of one electron per Co as in the case of the pure GGA calculation, since the localization of electrons on Co is made stronger by the U parameter. The ELF map (Figure 4b) plotted for the same level as used in Figure 2a indeed shows that the electronic density is not anymore localized at the position of the O vacancy as in the case of the pure GGA calculation but more around the Co ions.

DOS around Co projected onto the 5 d orbitals according to the local axis system chosen (see the Supporting Information, Figure S2). Based on the position of the Fermi level, the electronic structure of the Co ions in such environment is (dxy2, dxz2, dyz1 dz21 dx2‑y20) that, indeed, corresponds to an S = 1 intermediate spin configuration. Thus, as was recently shown for other occurrences of CoO5 square-based pyramids at a surface of LiCoO2 or resulting from an O-vacancy created in LiCoO2,5,6 it is theoretically possible to stabilize IS-Co3+ in overstoichiometric LiCoO2 using the defect model proposed by Levasseur et al.1 However, the electronic structure of the Co3+ 26496



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the presence of the spin in the adjacent CoO5 and are thus slightly polarized. The calculated spin DOS of Li25Co23O47 will be discussed in the following. XAS. In order to better understand the change in electronic structure locally induced by the defect in Li-overstoichiometric LiCoO2, we studied the two phases by X-ray absorption spectroscopy. The Co K-edge X-ray absorption near edge structure (XANES) spectrum is shown in Figure 6a. Both the edge

All these calculations were carried out for the hypothetical Li4Co2O5 compound, which was, to our knowledge, never experimentally observed. In order to verify if IS-Co3+ can also be stabilized in a more realistic case, we considered a larger cell with a diluted defect. According to the Li 1+t Co 1‑t O 2‑t formulation, this phase would correspond to t = 1/24, that is, Li25/24Co23/24O47/24 (Li25Co23O47) cell with one defect every two Co-layer and two CoO5 among the 12 positions in the (a,b) plane (monoclinic cell with the Cm space group) as represented in Figure 5a. This cell has the advantage to exhibit

Figure 5. (a) In-plane view of the Li25Co23O47 hypothetical cell considered to model a diluted defect for the overstoichiometric LiCoO2 showing the excess Li at the Co site and the two associated CoO5 ions in square-based pyramids. (b) The structure then consists of one Co-layer over two as shown separated by Li layers.

some IS-Co3+ ions only in one layer over two and therefore also exhibit CoO2 layers identical to the one in stoichiometric LiCoO2 (Figure 5b). Moreover, even in the Co layer presenting some defects, these ones are diluted and several Co3+ ions are still located in octahedral sites. For this cell, IS-Co3+ ions were also found to be stable if a Ueff value equal to 1.3 eV is applied for all Co ions in the cell (for Ueff = 1.5 eV, HS state for the all Co3+ ions is already obtained). Even starting with a ferromagnetic configuration between adjacent CoO5, the calculation in this cell converges to antiferromagnetic arrangement, showing that the ferromagnetic case is really unstable. The optimized cell parameters and distances are given in the Supporting Information, Figure S3. As in the case of the model Li4Co2O5 compound, the two IS-Co3+ ions in CoO5 environment are also located nearby the base of the pyramid, probably because of the formation of the Co−Co bonds. The other Co ions in the cell, located in an octahedral environment, remain in first approximation LS-Co3+. However, some of these ions feel

Figure 6. Normalized (a) Co K-edge XANES spectra (pre-edge and edge shown) and (b) Co LII,III-edge XAS spectra of LiCoO2 and overstoichiometric Li1+tCo1‑tO2‑t.

and the pre-edge absorption peak energy positions are located at the same energy positions for LiCoO2 and Li-overstoichiometric LiCoO2, indicating no significant change in the Co oxidation state. In principle, the change in the local environment expected for some Co ions should affect the shape of the pre-edge, but no significant modification is observed here (see inset of Figure 6a) The oxidation state of cobalt can be further confirmed by Co L-edge XAS measurement, which is more sensitive to the electronic structure of metal ions (Figure 6b). The two spectra also exhibit really close peak positions, indicating that the valence state of Co cations in the two compounds is approximately the same, that is, +III, in agreement with our Bader charge analysis as discussed above. 26497



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According to our DFT+U calculations on the electronic structure of the defect in Li-overstoichiometric LiCoO2, we expect a strong modification of the 3d orbitals of Co. Since no clear modification was observed in the Co K pre-edge, that is a forbidden (1s→3d) and therefore weak transition, we investigated the 3d empty levels through the study of the XAS at the oxygen K-edge, since it mainly results from the transition of O 1s electron to O 2p orbitals, that are strongly hybridized with cobalt 3d orbitals. Figure 8 shows the O K-edge

The Fourier transform (FT) of the Co K-edge EXAFS spectra for the stoichiometric and Li-overstoichiometric LiCoO2 is shown in Figure 7. The first peak is assigned to

Figure 8. O K-edge XAS spectra in fluorescence yield (FY) mode for LiCoO2 and overstoichiometric Li1+tCo1‑tO2‑t.

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Figure 7. The k -weighted Fourier transform magnitudes of the Co Kedge EXAFS spectra of LiCoO2 and overstoichiometric Li1+tCo1‑tO2‑t.

spectra acquired for the two materials in the fluorescence mode (that allows providing bulk information). The first energy range (between 528 and 536 eV) corresponds to transitions which occur from O 1s to hybridized states between Co 3d and O 2p sub-bands leading to pre-edge peaks. The second energy range (between 536 and 555 eV) corresponds to transitions from O 1s toward hybridized levels between mixed Co 4sp and O 2p sub-bands. Contrarily to the Co K and L-edges, strong differences between the two materials are observed here in the first energy domain, as emphasized in Figure 8 showing only the pre-edge region. In stoichiometric LiCoO2, a single prepeak is observed around 532 eV, as expected for transition from O 1s to eg empty levels resulting from the hybridization between O 2p and Co 3d orbitals, with Co3+ being in LS configuration in octahedral symmetry (t2g6eg0 configuration) and in agreement with literature.22 In Li-overstoichiometric LiCoO2, however, we observe a broadening of the main peak located around 532 eV in comparison to stoichiometric LiCoO2 and the presence of other peaks located at lower energy values: namely, three additional peaks are clearly observed at 529, 530, and 530.8 eV. The presence of additional peaks located at lower energy values is indicative of the presence of empty levels involving O 2p and Co 3d orbitals closer to the Fermi level. In order to interpret this O K pre-edge spectrum, we computed the spin DOS obtained for our model structure of a diluted defect in overstoichiometric LiCoO2, that is, Li25Co23O47. This allowed us to figure out the position of 3d orbitals of the ISCo3+ in square based pyramids versus those of LS-Co3+ in octahedral sites. Indeed, two types of layers were used to build our supercell (Figure 5b). (i) In layer 1, several kinds of Co ions are found: Co3+ in square based pyramid and some Co3+ ions in octahedral environment. (ii) In layer 2, only Co3+ ions in octahedral sites are present, similarly to a Co layer in stoichiometric LiCoO2. The resulting partial DOS are given in Figure 9a for the various Co ions (partial 3d) and in Figure 9b for the various O ions (partial 2p). For the Co ions of layer 2, as expected, the partial DOS (in black in Figure 9a) indicated that they exhibit full t2g up and down levels and empty eg up

the scattering process of the ejected electron at the oxygencontaining coordination shell, and the second peak represents the scattering process of the ejected electron at cobaltcontaining coordination shells. From Figure 7, it is clearly observed that the intensity of both the Co−O and Co−Co peaks for the Li-overstoichiometric LiCoO2 sample are lower than that of LiCoO2. We undertook a fitting of the data with a structural model, applying the same Debye−Waller factors for the two compounds. The fits of the curves are given in Supporting Information Figure S5 together with the table of resulting parameters. They lead to mean Co−O and Co−Co coordination numbers lower than 6 for Li-overstoichiometric LiCoO2 (4.7 for Co−O and 4.8 for Co−Co, against 6.0 for stoichiometric LiCoO2), in agreement with our hypothesis for the defect: five oxygen coordination for some Co and Li, with low scattering ability, replacing some Co. However, the mean coordination numbers obtained are not realistic, since they are much lower than expected from the amount of defects in the structure (indeed, the expected Li1.04Co0.96O1.96 formula discussed in the Introduction leads to 0.08 Co with 5-fold O coordination and 0.88 Co with 6-fold O coordination, leading to an average coordination number of 5.68). We therefore studied the influence of the Debye−Waller factors on the resulting mean-coordination numbers obtained by the fit as given in Supporting Information Figure S6. As more disordering is expected in Li-overstoichiometric LiCoO2, larger Debye− Waller factors than the one used for LiCoO2 are expected; they were therefore varied from 1.398 × 10−3 to 3.144 × 10−3 Å2 for Co−Co and from 1.040 × 10−3 to 4.295 × 10−3 Å2 for Co−O. From this analysis, we can conclude, whatever the Debye− Waller factors used for Li-overstoichiomtric LiCoO2 (within what we consider as a standard value domain), the mean Co−O and Co−Co coordination numbers are still lower than 6, in agreement with our hypothesis for the defect. However, one cannot conclude on the best Debye−Waller factor to be used since the amount of defect is really low. 26498



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respectively, IS-Co3+ dyz, dx2‑y2, and dz2 orbitals, that also exhibit an energy separation between 0.5 and 1 eV in the DFT +U calculations. Detailed analysis of the calculation outputs in Figure 9 also suggest that the broadening of the main peak at 532 eV can be due to the contribution of transitions to the eg levels of LS-Co3+ ions siting close to IS ones and to transitions from O 1s to O 2p hybridized with dx2‑y2 (down).

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CONCLUSION Using DFT+U, we demonstrated that the intermediate spin state of the Co3+ ions can be stabilized in square based pyramid for overstoichiometric LiCoO2 (Li1+tCo1‑tO2‑t). A theoretical cell was used in order to analyze in details the electronic structure associated with these Co ions. We showed that the defect must actually be considered as a pair of such squarebased pyramids, and that Co−Co bonding can explain the position of Co in the basal plane. In order to investigate experimentally the change in the electronic structure in the Lioverstoichiometric LiCoO2, we used XANES spectroscopy. The most striking difference between the stoichiometric and overstoichiometric samples was observed at the O K-edge. The overstoichiometric material exhibits additional peaks located at lower energy values indicative of the presence of empty levels involving O 2p and Co 3d orbitals closer to the Fermi level. We could interpret the spectrum based on the DFT+U calculations performed on a supercell corresponding to diluted defects (as in the actual material) and assign the additional lines to states involving the O 2p orbital mixed with the 3d orbitals of the IS-Co3+ ions.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic representation of the hypothetical defect in overstoichiometric LiCoO2; local view of the two CoO5 and the LiO5 environments in the Li4Co2O5 cell and orientation of the local axis system used to plot the partial DOS of Co; calculated optimized cell parameters; possible overlap of the dyz (a) and the dz2 (b) orbitals between adjacent Co ions; curve fitting for the EXAFS spectra; fitted mean coordination Co−Co and Co− O numbers from the Co K-edge EXAFS part, depending on the Debye−Waller factors used. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. Calculated partial spin DOS of the Li25Co23O47 model compound described in Figure 5 on (a) 3d Co orbitals and (b) O 2p orbitals.

and down levels (t2g6eg0 configuration), corresponding to the LS configuration. In layer 1 (Figure 5b), the Co3+ in square based pyramid (DOS in red in Figure 9a) exhibits the same IS electronic structure than the one of our initial model cell (Figure 3), and the Co3+ ions in octahedral environment (DOS in blue in Figure 9a) exhibit globally a LS configuration (t2g6 eg0) but with level positions slightly different from those of layer 1, since some of them are located just next to a CoO5 or LiO5 and undergo some local distortion and slight polarization. In Figure 9b, the partial O 2p DOS globally follows the shape of the Co 3d DOS depending on the type of layer the O belongs to (layer 1 or 2), showing the O 2p−Co 3d hybridization. From these partial DOS, we can clearly conclude that levels involving O 2p and IS-Co3+ 3d orbitals are located closer to the Fermi level than levels involving O 2p and LSCo3+ 3d orbitals, as observed in the experimental O K-edge spectrum. Thus, based on our DFT+U calculations, the O preedge spectrum further confirms the presence of IS-Co3+ in overstoichiometric LiCoO2. Moreover, since really well-defined peaks are observed at 529, 530, and 530.8 eV, one can tentatively assign the three additional peaks, neglecting the influence of the O 1s core hole23 to transitions from O 1s to localized levels due to the defect involving O 2p with,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +33 5 40 00 35 69. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work benefited from grants awarded by the Agence Nationale de la Recherche (Blanc Inter II, SIMI 8) No. 2011IS08-001-01, the Ministry of Economic Affairs of Taiwan (101EC-17-A-08-S1-183), and the Joint project between French National Research Agency and the National Science Council of Taiwan (ANR-NSC-101-2923-E-011-001-MY3, LaNaMOx). Région Aquitaine is also acknowledged for financial support, and The Mésocentre de Calcul Intensif Aquitain (MCIA) for computing facilities. C. Denage and P. Aurel (ISM) are acknowledged for technical assistance. 26499



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