ELECTRONIC STRUCTURE AND MAGNETIC PROPERTIES
OF DEFECTIVE AND NANOSTRUCTURED LiCoO2
V.R. Galakhov*, B.A. Gizhevskii*,
A.S. Semenova**, D.G. Kellerman**, L.I. Leont'ev***,
M. Raekers****, M. Neumann****
*Institute of Metal Physics, Russian Academy of Sciences Ural Division, 620219 Yekaterinburg, Russia
**Institute of Solid State Chemistry, Russian Academy of Sciences Ural Division, 620219 Yekaterinburg, Russia
***Institute of Metalurgy, Russian Academy of Sciences Ural Division, 620016 Yekaterinburg, Russia
****Universität Osnabrück - Fachbereich Physik,
D-49069 Osnabrück, Germany
Abstract
X-ray absorption, resonant X-ray emission, and X-ray
photoelectron spectra of the valence band and core levels have been
measured for LixCoO2 (0.6 ≤ x ≤ 1.0). Resonant O Kα X-ray emission
spectra of LiCoO2 showed localized excitonic states due to a d-d
transition between unoccupied and occupied Co 3d states. On the base of
measurements of Co 2p and O 1s X-ray absorption spectra it was
established that in defective cobaltites the electronic holes are localized
mainly in O 2p states. An evidence of phase separation in LixCoO2 has
been found.The magnetic susceptibility and electrical conductivity
measurements of LixCoO2 have been presented. The transition metal-tosemiconductor in defective lithium cobaltites have been found. The
nanostructured LiCoO2 obtained by high pressure torsion method has
been studied. For nanoceramics LixCoO2 subjected to strong plastic
deformation we have determined the decrease of relative Co/Li
concenreation on the surface of the samples.
1. Introduction
Oxides LixCoO2 have served as cathode materials for Li batteries.
Most of the electrochemical and physical properties would be affected
by the electronic structure of these compounds. The electronic structure
67
of LiCoO2 has been investigated with various experimental and
theoretical approaches. Nonetheless, many questions concerning the
electronic structure structure of LiCoO2, especially, of defective
cobaltites LixCoO2 remain unsolved. LiCoO2 is a semiconductor with
the band gap of 2.7 eV [1] while LixCoO2 at Li concentrations below
x = 0.75 is metallic [2], implying that a metal-insulator transition occurs
at intermediate Li concentrations. Electrical conduction in LiCoO2 is
expected to occur not in the Co eg conduction band but in the Co t2g
valence band by the hopping of Co 3d electrons between localized states
near the Fermi level [3]. In LiCoO2, the Li and Co ions reside
alternatively layered between close-packed planes of oxygen ions.
Extraction of Li from LiCoO2 occurs via several phase transitions. In
LixCoO2, two rhombohedral phases co-exist for 0.75 ≤ x ≤ 0.93 which
has been attributed to the insulator metal transition upon Li extraction
[4]. Defective cobaltites LixCoO2 can be expected to contain both Co3+
and Co4+ ions. On the base of the study of Co 2p and O 1s X-ray
absorption spectra of LixCoO2 defective oxides Montoro et al. [5] have
concluded that the trivalent Co ions remain unaffected by Li deintercalation, i.e. in LixCoO2 oxides, doped holes are localized in O 2p
states. The Co ions in defective cobaltites can be regarded as being in the
d6 configuration, as it was shown by the Co 2p X-ray measurements of
NaCo2O4 [6]. On the other hand, Yoon et al.[7] have proposed that the
charge compensation for the electron exchange in the Li-ion deintercalated process could be achieved in both the oxygen sites and
cobalt sites simultaneously. By this means the question concerning of the
nature of doping holes in defective cobaltites LixCoO2 remains to be
solved. The goal of the present work is, firstly, to investigate the
electronic structure of defective cobaltites LixCoO2 by means of X-ray
absorption (XAS), and X-ray photoelectron (XPS) spectroscopy and to
established changes in it after the semiconductor-metal transition and,
secondly, to determine the nature of doping holes induced by Li
deintercalation. Earlier, we have studied the electronic structure of
LiCoO2 by combination of X-ray photoelectron and emission spectra [8]
of defective cobaltites LiCoO2-δ by means of X-ray photoelectron
spectroscopy [9]. We have found that the defiency of oxygen in
LiCoO2-δ give rise of the divalent cobalt ions [9]. In this paper we want
68
to present the results of a combined X-ray spectroscopic study
performed by means of X-ray absorption (XAS), X-ray emission(XES),
and X-ray photoelectron (XPS) spectroscopy of LixCoO2 (0.6 ≤ x ≤ 1.0).
As one result, on the base of O 1s and Co 2p absorption spectra it is
shown that in defective cobaltites, electronic holes have mainly O 2p
nature.
2. Experiment
A single-phase, homogeneous ceramic sample of LiCoO2 was
prepared by sintering a mixture of Co3O4 and Li2CO3. The synthesis was
carried out in air at a temperature of 700°C for 20 h and after that at a
temperature of 850°C for 25 h, followed by slow cooling. Samples of the
LixCoO2 (x = 0.60-0.96) oxides were obtained by chemical deintercalation in 0.7 N H2SO4. A de-intercalation time of
=20 min
corresponds to the oxide with x = 0.96, and
=520 min corresponds to
x = 0.6. Li concentrations were estimated by means of atom-absorption
analysis using a spectrophotometer Perkin-Elmer 503. The phase
composition was checked by X-ray diffraction analysis using an X-ray
diffractometer STADI-P (STOE) and by magnetic susceptibility
measurements. Characterization of the samples is more detail described
elsewhere [10]. Nanostructured LiCoO2 were obtained by high pressure
torsion method. The anvil rotation angles φ indicate levels of
deformations. The X-ray photoelectron spectra of the LixCoO2 oxides
were obtained with a PHI 5600 ci Multitechnique System XPS
spectrometer using monochromatized Al Kα radiation. The energy
resolution was about 0.4 eV. The Co 2p and O 1s X-ray absorption
spectra (XAS) of the LixCoO2 oxides were carried out at BESSY at the
Russian-German Beam Line using a MUSTANG experimental station
equipped with a PHOIBOS analyzer (SPECS). O Kα X-ray emission
spectra were measured at the bulk branch line of beamline I511 at MAX
II (MAX-lab National Laboratory, Sweden). The spectra were obtained
using a high-resolution Rowland-mount grazing-incidence grating
spectrometer with a two-dimensional detector.
3. Results and discussion
In the LiCoO2 oxides, trivalent ions are in the low-spin state t2g↑3
69
t2g↓3eg0. The absence of unpaired d electrons in Co3+ ions is responsible
for their zero magnetic moment. Upon deintercalation, lithium cobaltites
contain not only lithium vacancies but also defects in the sublattice of
the transition metal. The formation of the defects is associated with the
charge compensation during the low-temperature oxidation of LiCoO2.
In this case, Co4+ ions have unpaired electrons and the defects formed
upon deintercalation are paramagnetic centers. The effective magnetic
moments obtained from measurements of temperature dependences of
the magnetic susceptibilities of LixCoO2 are approximately equal to ~0.9
μB [10]. This value is less than the one expected for Co4+ ions in the low
spin state t2g↑3 t2g↓2eg1 (1.73 μB ). This circumstance can by explained by
partial oxidation of oxygen ions (O2– → O1–).
In order to determine the valence states of cobalt and oxygen ions,
we examine the O 1s X-ray absorption spectra presented in Fig. 1.
Fig. 1. O 1s X-ray absorption spectra of LixCoO2.
The electric dipole-allowed 1s → 2p transition of O 1s X-ray
absorption spectra provides direct information of the oxygen charge state
and Co-O bonding interaction, since the 2p orbitals of oxygen ligand are
involved in bonding configuration with Co metal ions. The sharp peak at
70
530.5 eV corresponds to the transition of O 1s electrons to the
hybridized state of Co 3d (eg*) and O 2pσ orbitals whereas the broad
bumps at energies of 538 eV and 542-544 eV reveal O 2p states mixed
in the Co 4sp bands. The small feature at 534 eV in the spectrum of
LiCoO2 is formed by Li2CO3 contaminations. The O 1s spectra of
deintercalated cobaltites exhibit additional features (indicated by
arrows). This suggests an increase in the density of O 2p vacant states.
On the other hand, Co 2p X-ray absorption spectra of defective
cobaltites are nearly identical to the spectrum of the initial oxide LiCoO2
(see Fig. 2).
Fig. 2. Co 2p X-ray absorption spectra of LixCoO2. The spectra of
Li0.60CoO2 are measured at temperatures of 300 K and 90 K. The inset displays
a temperature dependence of the magnetic sysceptibilty χ (T) of Li0.60CoO2.
Therefore, we can conclude that the charge compensation occurs
through the formation of holes in the O 2p states, whereas the electron
confuguration of the cobalt ions remains unchanged. Our results confirm
those obtained by by Montoro et al. [5] and Yoon et al. [7], but in
71
distinction of these works, our O 1s X-ray absorption spectra also show
the effect for small concentration of holes. An appearance of the three
peak features in the O 1s spectrum of the sample with low Li
deintercalation can be probably explained by a lower Li concentration in
the surface region of the Li0.96CoO2 sample than obtained in bulk probe.
The temperature depences of the magnetic susceptibility of
LixCoO2 compounds (x ≤ 0.75) exhibit an anomaly in the form of a
small shoulder at T > 150 K. The dependence χ (T) of Li0.60CoO2 for
Li0.60CoO2 is shown in inset from Fig. 2.
Fig. 3. Valence band X-ray photoelectron spectrum and O Kα X-ray emission
(XES) spectra of LixCoO2. The XES spectrum (measured at the excitation
energy of 530.5 eV) is given on a common energy scale based on the binding
energy of the O 1s levels
72
One of the reasons of the jump in the magnetic susceptibility is a
change of the Co 3d electronic configuration. In order to verify that the
above effect is not correlate to the change of the Co 3d electronic
configuration, we have measured the Co 2p X-ray absorption spectrum
of Li0.6CoO2 at temperature of 90 K which is less then the temperature of
this jump in the magnetic susceptibility. Fig. 2) shows that the Co 2p Xray absorption spectrum of Li0.6CoO2 remains uтchanged.
We have shown that the anomaly in the temperature depences of
the magnetic susceptibility of LixCoO2 (x < 0.75) at T ~ 150 K is caused
by the change in the conductivity type [10]. Therefore, one can conclude
that the metal-to-semiconductor transition in LixCoO2 does not connect
with the change of the Co 3d electronic confuguration.
An information about occupied Co 3d and O 2p states can be
obtain from valence band photoelectron and X-ray emission spectra
plotted in a common energy scale. Figure 3 presents valence band X-ray
photoelectron (XPS) spectra of LiCoO2, Li0.96CoO2, and Li0.6CoO2. For
LiCoO2, O K
X-ray emission spectrum excited at 530.5 eV is shown.
The XES O Kα spectrum has been brought to a common energy scale
based on the O 1s binding energies obtained from our X-ray
photoelectron experiment. The XPS valence band spectra for the Al Kα
excitation result mainly Co 3d states. The strongest peak A of the XPS
spectra near the Fermi level derives from the t2g orbitals. According to
calculations made by Czyzyk et al. [11] feature B mainly formed by t1u
orbitals, which derive primarily from the O 2p states with a slight
admixture of the Co 4p states. Band C reveals the hybridized O 2p-Co
3d(eg) states. Band D reflects hybridization of the Co 4s and 4p states
(a1g and t1u orbitals) with O 2p states. Features B and D manifest
themselves in the O Kαspectrum as main peak b and feature c,
respectively. Feature a3 in the O K
spectrum coincides with peak A of
the XPS spectrum and reveals, probably, hybridization between the Co
t2g and O 2pπ states.
Although the defective cobaltites LixCoO2 are expected to be hole
doped, the Co t2g peak remains sharp. Therefore, one can suggest that
the Co3+ state remains in the low-spin t2g configuration and that the
spectral weight near the Fermi level is dominated by the t2g states.
73
Complex impedance measurements [2] show that LixCoO2 becomes an
electronic conductor for x ≤ 0.9. However, according to the results
presented in Fig. 1, the Li0.96CoO2 compound should be a metal. It
should be noted that XAS measurements in TEY are surface sensitive to
a few hundred Angstroms, while the Li concentration of the samples has
been estimated by applying bulk sensitive methods. From this
discrepancy one can conclude that the surface region has a significant
lower Li concentration then the bulk. To our opinion, this discrepancy
may be explained suggesting a so-called phase separation effect. A
lithium de-intercalation leads to the appearance of small metallic islands
with small Li concentrations surrounded by the insulating host material.
The decrease in Li-content leads to the increase of sizes and the quantity
of these islands, their overlapping and, as a result, to the appearance of
metallic properties of the Li-deintercalated cobaltites. An existent of
such metallic islands in a semiconducting matrix in LixCoO2 has been
mentioned in experiments of nuclear magnetic resonance [12].
Fig. 4. Co 2p X-ray photoelectron spectra of nanostructured LiCoO 2 obtained
by high pressure torsion method. The anvil rotation angles φ indicate leveles of
deformations. For comparison, the spectrum of initial LiCoO 2 is presented.
74
The shape of core-level X-ray photoelectron spectra (XPS) in
oxides are known to be sensitive to the electronic structure of the
compounds, including the valence state of the transition metal ion and
the degree of covalency of metal-oxygen bond. We have shown that Co
2p XPS depend on the oxygen stoichiometry in LiCoO2-δ [9].
Fig. 4 shows Co 2p X-ray photoelectron spectra of nanostructured
LiCoO2 obtained by high pressure torsion method. The anvil rotation
angles φ indicate leveles of deformations. Deformation of LiCoO2 may
lead either to defects in oxygen and therefore to forming Co2+ ions or to
defects in lithium those accompanied nominally Co4+ ions. The spectrim
of initial LiCoO2 is also given. In addition to the spin-orbit components,
Co 2p3/2 and Co 2p1/2, the spectra exhibit satellites due to charge-transfer
effect. While the main line is characterized primarily by the 2p53d7L
configuration of the final state in photoemission, the satellite should
correspond to the sum of the 2p53d8L2 and 2p53d6 configurations. Here,
L and L2 reflect the presence of one and two holes in the oxygen 2p
band, respectively. We have found no dufference between the Co 2p
XPS spectra of nanostructured and initial (coarse grain) samples of
LiCoO2. This means that the valence state of Co ions are not changed.
Fig. 5. Co 3p and Li 1s X-ray photoelectron spectra of nanostructured LiCoO 2
obtained by high pressure torsion method. The anvil rotation angles φ indicate
leveles of deformations. For comparison, the spectrum of initial LiCoO 2 is
presented.
75
Fig. 5 shows Co 3p and Li 1s X-ray photoelectron spectra of
nanostructured and coarse grain samples of LiCoO2. One can see that the
increase of the anvil rotation angles leads to an increase of relative
intensity of Li 1s / Co 3p spectra. Note that photoelectron spectra give
an information about the surface of materials. This means that relative
concentration of Li atoms on the surface of the samples increase. One
can suggest that the inside of the sample under deformation, a
dissolution of LiCoO2 occurs into Co3O4 and Li2O and Li2O molecules
transfer to the surface.
3. Conclusions
In conclusion, we have studied the change in the electronic
structure of defective oxides LixCoO2 (0.6 ≤ x ≤ 1) by of means X-ray
photoelectron, resonant X-ray emission and X-ray absorption
spectroscopy, leading to a number of results. It was established that in
defective cobaltites, electronic holes are localized mainly in O 2p states.
An evidence of phase separation in LixCoO2 was found. The
nanostructured LiCoO2 obtained by high pressure torsion method has
been studied. For nanoceramics LixCoO2 subjected to strong plastic
deformation we have determined the decrease of relative Co/Li
concenreation on the surface of the samples.
This work was supported by the Russian Foundation for Basic
Research (Grants Nos 07-02-00540 and 06-03-72003), by the grant
"Cooperation between the Ural and Sibirian Divisions of the Russian
Academy of Sciences", by the Research Council of the President of the
Russian Federation (Project NSH-1929.2008.2), and by the bilateral
Program "Russian-German Laboratory at BESSY".
References
1. Van ElpJ, Wieland J L, Eskes H, Kuiper P, Sawatzky G A, de Groot F
M F, Turner T S, Electronic structure of CoO, Li-doped CoO and
LiCoO2.' Phys. Rev. B 1991 44 6090-6103.
2. Molenda J, Stoklosa A, Bak T: Modification in the electronic
structure of cobalt bronze LixCoO2 and the resulting electrochemical
properties.' Solid State Ionics 1989 36, 53-58.
3. Kushida K and Kuriyama K: Mott-type hopping conduction in the
76
ordered and disordered phases of LiCoO2.' Solid State Commun.
2004 129 525-528.
4. Rossen Reimers J N and Dahn J R: Synthesis and Electrochemistry of
Spinel LT-LiCoO2 .' Solid State Ionics 1993 62 53-60.
5. Montoro L. A. , Abbate M., and Rosolen J. M. : Changes in the
electronic structure of chemical deintercalated LiCoO2.'
Electrochem. Solid-State Letter 2000 3 410-412.
6. Matsushita T, Mizumaki M, Ikeda N, Nakazawa M, Agui A, Saitoh Y,
Nakatani T, Yoshigoe A, and Nakamura S: .' X-ray absorption
spectroscopy in Na2O4, LaCoO3, SrCoO3.' Surf. Rev. Letters 2002 9
1327
7. Yoon W-S, Kim K-B, Kim M-G, Lee M-K, Shin H-J, Lee J-S, and Yo
V-H: Oxygen contribution on Li ion intercalation-deintercalation ob
LiCoO2 investigated bu O K-edge and Co L-edge X-ray absorption
spectroscopy.' J. Phys. Chem. B 2002 106 2526-2532.
8 Galakhov V R, Kurmaev W Z, Uhlenbrock S, Neumann M,
Kellerman D G and Gorshkov V S: Degree of covalency of LiCoO2:
X-ray emission and photoelectron study.' Solid State Commun 1996
99 221-224.
9. Galakhov V R, Karelina V V, Kellerman D G, Gorshkov V S,
Ovechkina N A, and Neumann M: Electronic structure, X-ray
spectra, and magnetic properties of the LiCoO2-δ and NaxCoO2
nonstoichiometric oxides.' Phys. Solid State 2002 44 266-272.
10. Kellerman D G, Galakhov V R, Semenova A S, Blinovskov Ya N, and
Leonidova O N: Semiconductor–metal transition in defect lithium
cobaltite.' Phys. Solid State. 2006 48 548-556.
11. Czyzyk M T, Potze R and Sawatzky G A: Band-theory description of
high-energy spectroscopy and the electronic structureb of LiCoO2.'
Phys. Rev. B 1992 46 3729-3735.
12. Levasseur S, Menetrier M, Suard E, and Delmas C, Evidence for
structural
defects
in
non-stoichiometric
HT-LiCoO2:
electrochemical, electronic properties and 7Li NMR studies.' Solid
State Ionics 2000 128 11-24.
77