Disordering and electronic state of cobalt ions in mechanochemically synthesized LiCoO2

Article abstract of DOI:10.1006/jssc.2002.9493

Mechanical activation (MA) combined with heat treatment at moderate temperatures was used to prepare disordered and highly dispersed LiCoO₂ starting from the mixtures of various cobalt precursors (CoOOH, Co(OH)₂, and Co) and LiOH. X-ray powder diffraction and IR spectroscopy were used to investigate the phase composition and the crystal structure of as-prepared samples, while the electronic state of cobalt ions was characterized by diffuse reflectance electron spectroscopy. MA of the LiOH+CoOOH mixture led to the formation of LT-LiCoO₂ with a cubic spinel-related structure. Heat treatment at 600°C of the latter resulted in the formation of HT-LiCoO₂ with a hexagonal layered structure similar to ceramic LiCoO₂. However, as-prepared HT-LiCoO₂ is characterized by Co³⁺ O₆ octahedra less perfect than those of ceramic LiCoO₂. All MA-LiCoO₂ samples are exclusively described by localized d electrons.

Full text of DOI:10.1006/jssc.2002.9493

Journal of Solid State Chemistry 165, 56–64 (2002)
doi:10.1006/jssc.2002.9493, available online at http://www/idealibrary.com on
Disordering and Electronic State of Cobalt Ions in Mechanochemically
Synthesized LiCoO2
N. V. Kosova,*,1 V. F. Anufrienko,w T. V. Larina,w A. Rougier,z L. Aymard,z and J. M. Tarasconz
*Institute of Solid State Chemistry, wBoreskov’ Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, 18 Kutateladze, Novosibirsk
´
´
´
630128, Russia; and zLaboratoire de Reactivite et Chimie des Solides, Universite de Picardie Jules Verne, France
E-mail: kosava@solid.nsc.ru
Received August 7, 2001; in revised form November 19, 2001; accepted December 21, 2001
(4). For example, while cycling an LiCoO2/Li electroche-
Mechanical activation (MA) combined with heat treatment at
moderate temperatures was used to prepare disordered and
highly dispersed LiCoO2 starting from the mixtures of various
mical cell, LiCoO2 exhibits a monoclinic distortion for
x=0.5, which was associated with an interslab Li/vacancy
ordering (5). Thus, the need for cathode materials with
suitable crystal and electronic structure that could provide
intercalation/deintercalation without sharp structural dis-
tortion and, therefore, band destabilization is highlighted.
Structural disordering in LiCoO2 can be achieved by
cation or anion nonstoichiometry, cation mixing, and a
disturbance of oxygen layers stacking. Generally, it
corresponds to the weakening of d-ions correlation,
resulting in the full localization of d electrons on the
transition metals ions, as a limit (6). Low-temperature
methods are usually used to prepare disordered LiCoO2.
As reported in the literature, LiCoO2 exhibits low-and
high-temperature forms, depending on the synthesis
temperature. HT-LiCoO2 has a hexagonal structure, while
LT-LiCoO2 has a cubic structure. The hexagonal and cubic
structures are based on the same oxygen sublattice, and are
distinguished by the spatial arrangement of cations. HT-
LiCoO2 exhibits an ideal a-NaFeO2 layered structure
(S.G.: R3m) with an ABCABC oxygen stacking (7). LT-
LiCoO2 is believed to adopt a spinel-related structure
ꢀ
(S.G.:Fd3m) based on a cubic closed packed oxygen
network with alternating cation layers of 0.75 Co, 0.25 Li
and 0.75 Li, 0.25 Co composition perpendicular to each of
the four cubic (111) directions (1).
Rossen et al. obtained LT-LiCoO2 by mixing
LiOH ꢀ H2O and CoCO3 at 4001C in alumina boats under
extra dry air for 90 h, with a final grinding (1). Sometimes,
low-temperature methods were found to produce HT-
LiCoO2. In this case, cobalt hydroxides (Co(OH)2 or
CoOOH) were usually found to be more reactive pre-
cursors than Co3O4. The higher reactivity of these
compounds in the synthesis of layered LiCoO2 is most
likely nested in the similarity of their structures (R3m for
CoOOH and P-3-m1 for Co(OH)2) and in their larger
specific surface area. When interacting with LiOH,
cobalt precursors (CoOOH,Co(OH) ,and Co) and LiOH. X-
2
ray powder diffraction and IR spectroscopy were used to
investigate the phase composition and the crystal structure of
as-prepared samples,while the electronic state of cobalt ions was
characterized by diffuse reflectance electron spectroscopy. MA
of the LiOH+CoOOH mixture led to the formation of LT-
LiCoO2 with a cubic spinel-related structure. Heat treatment at
6001C of the latter resulted in the formation of HT-LiCoO2 with
a hexagonal layered structure similar to ceramic LiCoO2.
However,as-prepared HT-LiCoO 2 is characterized by Co3+O6
octahedra less perfect than those of ceramic LiCoO2. All MA-
LiCoO2 samples are exclusively described by localized d
electrons. # 2002 Elsevier Science (USA)
INTRODUCTION
LiCoO2 is widely studied by numerous researchers as a
promising positive electrode material for secondary lithium
batteries (1–3). Over the past few years a great deal of
attention has been devoted to the preparation of meta-
stable lithium–transition metal oxides of unusual ordering
(structural, magnetic, charge) and valence states of ions
that cannot be obtained by conventional high-temperature
techniques. The structure of such products, as expected,
should be more stable to intercalation/deintercalation of
lithium ions in the course of discharge/charge processes,
thus, providing better electrochemical behavior and pre-
venting structural distortion. Phase transition occurs when
an electron density, arising from the inserted atoms,
destabilizes vacant levels of the matrix, and a more
effective electron stabilization of the system can be
provided as a result of irreversible structural transition
ꢀ
1To whom correspondence should be addressed. Fax: 7-3832-322847.
56
0022-4596/02 $35.00
r 2002 Elsevier Science (USA)
All rights reserved.

DISORDERING AND ELECTRONIC STATE OF COBALT IONS
EXPERIMENTAL
57
CoOOH and Co(OH)2 exhibit their amphotheric proper-
ties. Chiang et al. prepared HT-LiCoO2 by freeze-drying
finely divided cobalt hydroxide–lithium hydroxide mix-
tures, obtained by precipitation (2). Amatucci et al.
prepared HT-LiCoO2 powders at temperature as low as
1001C through the use of a two-day ion exchange reaction
between CoOOH and an excess of LiOH ꢀ H2O in the
presence of a small amount of added water as well as in
autoclave (3). The product had some amounts of carbonate
and hydroxyl species. By heating at moderate tempera-
tures, around 2501C, the authors succeeded in removing
these species and improving the electrochemical cycling
properties.
Among other promising low-temperature techniques is
mechanical activation (MA). This technique makes it
possible to obtain highly dispersed and disordered com-
pounds at nearly room temperature. Fernandez-Rodriguez
et al. found that LiCoO2 was formed after extensive
grinding (40 h) of a mixture of LiOH ꢀ H2O and Co(OH)2 in
agate milling jars in air (8). However, they experienced
some difficulties due to the partial decomposition of
LiCoO2 with the formation of Co3O4. Obrovac et al. tried
to obtain disordered LiCoO2 with improved electrochemi-
cal performance by MA, but failed due to the noticeable
instability of LiCoO2 under mechanical loading (9). In
both studies the researchers obtained disordered and
nonhomogeneous LiCoO2. The authors claimed that the
sample phase nonhomogeneity was the main reason for the
degradation of their electrochemical performance. How-
ever, the change in the electronic state of cobalt ions in
MA-LiCoO2 was not investigated.
LiOH (Aldrich), CoOOH, Co(OH)2 (UM Belgium), and
Co (polyol) were used as initial reagents. CoOOH was
prepared by oxidation of Co(OH)2 using 8 M NaClO as
oxidizing agent in the present of 5 M KOH (10). Ceramic
LiCoO2 was synthesized from a stoichiometric mixture of
Li2O and Co3O4 heat treated at 9001C.
Mechanical activation was carried out in a Spex 8000
mixer mill that generates normal mechanical strain, using
stainless jars and balls. The mass ratio of material to balls
was 1/20. Grinding time varied from 1 to 10 h. MA was
performed in air or oxygen. The stoichiometric Li/Co
molar ratio was used in the starting mixtures. The activated
samples were then heated at 4001C for 4 h in air. Ceramic
HT-LiCoO2 was ground under similar milling conditions.
The final products were characterized by X-ray diffrac-
tion using a Scintag diffractometer with CuKa radiation
(1.5418 A). IR spectra were recorded on a Bruker FTIR
spectrometer using CsI pellets. Thermal analysis was
performed in air with a Setaram TG-DTA 92 apparatus
using alumina crucibles. The rate of heating was 101C/min.
Diffuse reflectance electron spectra (DRS) were recorded
using a Shimadzu spectrophotometer in the range 11,000–
50,000 cmꢁ1. A Scanning Philips field effect gun (FEG) XL-
30 electron microscope (SEM) with a resolution of about 1
nm was used to study morphology changes. Electrochemi-
cal tests were carried out in a galvanostatic mode with a C/
10 charge/discharge rate, using a MacPile system (Biologic
S.A., Claix, France).
In the present study we tried to determine whether the
MA method is suitable for providing a disordered but
homogeneous LiCoO2 with therefore a more effective
electron correlation of cobalt ions. LiCoO2 samples were
prepared both by MA of the mixtures of various initial
reagents (‘‘reactive grinding’’) and by MA of ceramically
prepared LiCoO2 (‘‘nonreactive grinding’’). The main goal
of this study was to determine whether a relation between
the structural parameters and the electronic state of cobalt
ions in mechanochemically prepared LiCoO2, as well as its
electrochemical performance as cathode material, does
exists.
RESULTS AND DISCUSSION
Table 1 displays the results of the thermodynamic
estimation of DG2098
for the reactions of LiCoO2
K
formation from LiOH and cobalt compounds exhibiting
various oxidation states of cobalt ions. It shows that at
room temperature the reaction with metallic cobalt is
thermodynamically more feasible. As the cobalt oxidation
state of the initial precursors increases, the DG0298 K value
becomes more positive. Note that only reaction (1) occurs
in the absence of oxygen. When using metallic cobalt or
Co(OH)2, LiCoO2 synthesis proceeds through an oxidation
TABLE 1
DG0298 for Reactions of the LiCoO2 Formationa
Reaction
Co oxidation state
DG kJ/mol LiCoO2
1
2
3
4
LiOH+CoOOH-LiCoO2+H2O
+3
+2, +3
+2
ꢁ31.3
ꢁ34.1
LiOH+1/3Co3O4+1/12O2-LiCoO2+1/2H2O
LiOH+Co(OH)2+1/4O2-LiCoO2+3/2H2O
LiOH+Co+3/4O2-LiCoO2+1/2H2O
ꢁ105.1
0
ꢁ281.9
aDG0f, 298 of compounds being used: ꢁ619.ꢀkJ/mol for LiCoO2 (11); ꢁ455.6 kJ/mol for LiOH (12); ꢁ413.8 kJ/mol for Co(OH)2 (13); ꢁ744.ꢀ kJ/mol for
Co3O4 (13); ꢁ369.1 kJ/mol for CoOOH (14)

58
KOSOVA ET AL.
of Co or Co2+ to Co3+, while using CoOOH such Antolini when heating a mixture of Li2CO3 with Co3O4,
oxidation is not necessary, except that the dehydration of and interpreted as a LiCoO2 formation (15).
CoOOH with the formation of Co3O4 does take place.
Similar differences were observed for the initial and the
activated LiOH+Co(OH)2 mixtures. The DTA heating
curve of the initial mixture is characterized by two
endothermic peaks in the low-temperature region (below
3001C) and one endothermic at 400–5001C. Both peaks
showing at low and high temperature have the same origin
as those mentioned above while the middle one is
associated with the dehydration of initial cobalt hydroxide
into Co3O4 as deduced by a separate X-ray study (8). Such
differences disappear after MA; the DTA line for the latter
sample is close to that observed for the activated
LiOH+CoOOH mixture.
As shown in Fig. 1b, the heating of the nonactivated
LiOH+Co mixture is accompanied by an endothermic
peak at 1001C and an intense exothermic one at
380–4801C, associated with the elimination of adsorbed
water and the metallic cobalt oxidation concomitant with
the LiCoO2 formation, respectively. For the activated
mixture, both peaks are still present with, however, an
increase in intensity for the first one and a decrease for the
second, with the latter becoming wider (350–5501C).
1. Reactive Grinding
TG-DTA data highlight an appreciable difference in the
thermal behavior of the activated and nonactivated
mixtures. The heating of the nonactivated LiOH+CoOOH
mixture is accompanied by three endothermic effects at
50–150, 320–330, and 4501C (Fig. 1a). They were assigned
(i) to the adsorbed water removal from LiOH in the course
of the sample preparation, and (ii) to the processes of
CoOOH decomposition and (iii) of LiOH melting,
respectively. All effects are coupled with the corresponding
mass loss. In contrast, the DTA curve for activated mixture
is quite less busy with the presence of a single endothermic
peak at 50–2001C, associated with the elimination of
gaseous products. Surprisingly, the interaction with the
LiCoO2 formation was not found to occur with a notice-
able thermal effect in both cases. This phenomenon
contrasts with the exothermic effect observed at 3801C by
FIG. 1. TG and DTA curves of initial (1) and ground (2) mixtures: LiOH+CoOOH (a) and LiOH+Co (b).

DISORDERING AND ELECTRONIC STATE OF COBALT IONS
59
FIG. 2. X-ray patterns of ground samples (1 h): CoOOH (1);
LiOH+CoOOH (2); LiOH+Co (3); LiOH+Co(OH)2 (4); LiOH+
CoOOH ground and heated at 6001C (5).
FIG. 3. FTIR spectra of ground samples (1 h); LiOH+CoOOH (1);
LiOH+Co(OH)2 (3); LiOH+Co (5); LiCoO2 (6) and initial compounds
CoOOH (2); Co(OH)2 (4); LiCoO2 (7).
X-ray diffraction patterns of the mixtures of LiOH with
CoOOH, Co(OH)2, and Co, ground for 1 h in air, are
shown in Fig. 2. The reflections of LiOH are absent in all
the patterns due to the hydroxide amorphization and a comparison with the initial compounds. The spectra of
weak scattering of lithium atoms. For the LiOH+Co CoOOH (curve 2) and Co(OH)2 (curve 4) are described by
(curve 3) and LiOH+Co(OH)2 (curve 4) mixtures, the X- intense bands in the 500–650 and 300–600 cmꢁ1 ranges,
ray patterns of the ground samples exhibit characteristics respectively. The IR characterization of ceramic layered
similar to those of the individual Co and Co(OH)2, LiCoO2 is well described in the literature (17). Briefly,
respectively, with however wider and less intensive reflec- based on the factor group analysis of D53d, four IR active
tions due to the amorphization of the starting compounds modes (2A2u+2Eu) distributed over two different fre-
upon grinding. In addition, color changes are observed as quency regions are present. The bands of the CoO6
the pink initial Co(OH)2-based mixture turns brown due to vibrations are within the range 400–650 cmꢁ1, while the
a partial oxidation of the cobalt hydroxide.
LiO6 vibrations are within the range 200–400 cmꢁ1. The
The ground LiOH+CoOOH mixture exhibits a poor FTIR spectrum of ceramic LiCoO2 (curve 7) matches well
crystallinity (Fig. 2, curve 2). However, four main broad X- the existing literature data through the presence of one
ray peaks can be distinguished. A new broad peak in the band at 270 cmꢁ1, corresponding to the Li–O vibration,
44–481 2ꢁ range appears, which does not exist in the and a larger one centered at 600 cmꢁ1, corresponding to the
ground CoOOH (curve 1). Based on the crystallographic Co–O vibration.
data of LT-LiCoO2 reported in the literature (see above),
Noticeable change in the spectra of all ground samples
we can conclude that LT-LiCoO2 is formed upon MA of are observed. Whatever the starting compounds, the bands
the LiOH+CoOOH mixture. Note that You et al. have become broader and less intense after 1 h of MA, most
recently reported the formation of LT-LiCoO2 by grinding likely resulting from sample amorphization and a decrease
the stoichiometric Li2O2+Co mixture for 10 h (16).
in particle size. A broad band in the range 450–600 cmꢁ1
Since crystallinity of the ground samples is rather poor, appears in the spectrum of the activated LiOH+CoOOH
the X-ray diffraction technique provides limited informa- mixture, instead of a single band at 595 cmꢁ1 in the
tion and needs to be completed by other methods. The spectrum of the initial CoOOH, indicating the early stage
local environment of the cations against neighboring of the LiCoO2 formation (Fig. 3 curve 1). In the spectrum
oxygen atoms has been determined by infrared spectro- of the LiOH+Co(OH)2 mixture (curve 3), a sharp decrease
scopy.
in the band intensity, corresponding to initial Co(OH)2, is
Figure 3 shows FTIR spectra of as-prepared LiCoO2 as observed. In addition, one can see a new broadband
well as of the ceramic one before and after grinding in appearing at 500–700 cmꢁ1. A sharp decrease in band

60
KOSOVA ET AL.
intensity and the appearance of continuous absorption of
the LiOH+Co mixture (curve 5) suggest the presence of a
large amount of conductive electrons in the system.
In the range 1000 to 4000 cmꢁ1 (not reported here) all
spectra of activated mixtures indicate the presence of CO32ꢁ
and proton-containing groups with strong hydrogen
bonds. It is well known that MA increases the sensitivity
of oxides to the ambient atmosphere.
Overall, the IR study leads to the conclusion that for
both precursors, Co(OH)2 and CoOOH, the environment
of cobalt ions in the activated mixtures is altered due to the
formation of poor crystallized LiCoO2.
SEM micrographs (Fig. 4a) show the appearance of
mechanically driven layered dense composites in the
activated mixture of lithium and cobalt hydroxides instead
of plate-like hexagonal particles characteristic of cobalt
hydroxides. Surprisingly, the metallic cobalt spherical
particles were not significantly destroyed under MA in
the mixtures with LiOH (Fig. 4b), in contrast to their full
distortion observed when activated alone. In our opinion,
this is due to the layered structure of LiOH and its good
plasticity, preventing the process of strong distortion of
metallic particles.
Thus, MA of the mixtures under study brings about
different results, depending on the nature of the reagents
used. When using metallic cobalt, an intimate mixing of
reagents occurs, accompanied by the hardly noticeable
oxidation process of Co0. Partial oxidation of Co(OH)2 is
observed for the Co(OH)2-based mixture, whereas the LT-
LiCoO2 formation occurs in the CoOOH-based mixture.
Considering thermodynamic data (Table 1), this result
indicates that the formation of LiCoO2 from Co(OH)2 and
Co are hampered, probably due either to a lack of sufficient
amount of oxygen in the jars or to the reaction of the Co2+
ions oxidation being a diffusion-controlled one. Special
experiments, performed with the LiOH+Co mixture in an
oxygen atmosphere (20 atm), did not show a noticeable
influence of gas atmosphere on the mechanochemical
synthesis of LiCoO2, suggesting a more complex reacting
mechanism to be identified.
2. Thermal Treatment of Activated Samples
All activated mixtures were heat treated at 6001C for 4 h
in air. Whatever the precursors, the obtained heat-treated
compounds exhibited X-ray patterns similar to that of
layered HT-LiCoO2 (Fig. 2, curve 5). Therefore, the spinel-
frame LT-LiCoO2, formed from the activated LiOH+
CoOOH mixture, was changed into a hexagonal HT-
LiCoO2 phase upon heating. The formation of HT-LiCoO2
after heat treatment of the activated mixture was confirmed
by IR spectroscopy (spectra not shown here).
FIG. 4. SEM micrographs of ground samples (1 h): LiOH+CoOOH
(a); LiOH+Co (b); LiOH+CoOOH ground and heated at 6001C (c).

DISORDERING AND ELECTRONIC STATE OF COBALT IONS
61
According to electron microscopy data, the heated
(a) Co2+ ions in octahedral oxygen crystal fieldF
4
samples consists of small spherical particles having an [Co2+
]
.
The F term is the ground atom term for a
Oh
average size of 100–200 A (Fig. 4c).
free Co2+ ion (electron configuration d7) and 4P is the first
excited term. According to Tanabe–Sugano diagrams, for
the oxygen octahedral complexes of high-spin Co2+ ions,
3. Nonreactive Grinding
4
the following three transitions from the ground state T1g
are possible: 4T1g–4T2g (n1) 4T1g –4A2g (n2), 4T1g(F) –4T1g(P)
(n3). The energy of the n1 transition is usually small (less
than 10,000 cmꢁ1). The n2 transition is a two-electron
transition and therefore characterized by a small extinction
(intensity). Thus, for this case, the n3 transition is more
likely to occur. It can be displayed by a multiple structure
or a broadening of the spectrum due to an admixing of the
excited states. The analysis of literature data shows that for
Co2+ ions, stabilized in perfect oxygen octahedral co-
ordination, for example, in solid Co3Si4(OH)2 ꢀ nH2O (20),
the n3 transition is observed in the region 19,000–
20,000 cmꢁ1, thus, being a good signature of these ions.
Let’s just remember the low intensity of the transition for
perfect structures. However, a structural disordering (i.e.,
the presence of structural defects) or a cation–cation
magnetic exchange interaction enhances the transition
intensity.
In parallel to the reactive grinding experiments described
above, a study dealing with the nonreactive grinding of
ceramic HT-LiCoO2 was also carried out. The evolution of
the X-ray patterns with grinding time is shown in Fig. 5.
The X-ray line broadening suggests a sample amorphiza-
tion coupled with a reduction in particle size upon
grinding. We did not observe an inversion of the relative
intensities of the 003 and 104 reflections as reported by
Fernandez-Rodriguez et al. (8). The full destruction of the
layered structure (i.e., disappearance of the 003 peak) was
observed only after 10 h of grinding. The X-ray pattern of
the sample ground for 10 h exhibits three main peaks
(marked with *) corresponding to a phase that can be
indexed in a cubic symmetry and whose exact stoichio-
metry is unknown. Upon grinding ceramic LiCoO2, the IR
study, showing the appearance of bands characteristic of
the compound Li2CO3 (spectra not given here), suggests a
delithiation of LiCoO2 while the broadening of the Co–O
bands in the region 400–700 cmꢁ1 (Fig. 3, curve 6) indicates
a decreases in sample homogeneity.
(b) Co2+ ions in tetrahedral oxygen crystal fieldF
[Co2+
]
.
For high-spin Co2+ ions in a tetrahedral
Td
coordination, three types of transitions from the ground
4
4
4
state A2g are also possible: A2g–4T2g (n1), A2g–4T1g (n2),
4A2g(F) –4T1g(P) (n3). For oxygen ligands, the n1 and n2
transitions appear in the low-frequency region (less than
10,000 cmꢁ1). A good example of Co2+ ions in a
tetrahedral crystal field is CoCr2O4. For solid CoCr2O4,
the n3 transition is observed at 15,000–17,000 cmꢁ1 (20). It
is well known (18) that the extinction of all transitions for
4. Electronic State of Cobalt Ions in as Prepared
LiCoO2 Samples
The electronic state of Co ions in the as-prepared
samples was investigated by diffuse reflectance electron
spectroscopy (DRS). Spectra interpretation based on the
analysis of the peculiarities of Co2+ or Co3+ ion
absorption bands versus the oxygen crystal field (18,19)
was performed.
[Co2+
]
is significantly higher than for [Co2+
]Oh, making
]
Oh
Td
it easy to distinguish this cobalt state along with [Co2+
even if it is present in small amounts.
(c) Co3+ ions in octahedral oxygen crystal fieldF
[Co3+
]
.
For free Co3+ ion (electron configuration
Oh
d6) D appears to be the atom ground term. For oxygen
5
1
ligands, a low-spin state with the lowest A1g state is
usually reported (21). In this case the following transitions
1
1
1
are possible: A1g–3T1g (n1), A1g–1T2g (n2), A1g–1T1g (n3),
and A1g–1T2g (n4). Two transitions with the energy of
1
about 16,000–17,000 cmꢁ1 (n3) and 22,000–24,000 cmꢁ1 (n4)
are characteristic of the low-spin Co3+ ions in octahedral
coordination. Besides, d–d transitions from the ground to
the excited state in the ultraviolet region (25,000–
35,000 cmꢁ1) are also possible.
(d) Experimental spectra of CoOOH, Co(OH)2, and
LiCoO2. In the initial Co(OH)2 spectrum (space group
FIG. 5. X-ray pattern of ceramic LiCoO2 as-prepared (1) and ground
for 1 (2), 3 (3), 5 (4), and 10 h (5). * corresponds to a new phase indexed in
a cubic symmetry.
2+
ꢀ
P3m1, high-spin Co
ions in Oh positions), the n3
transition is observed at 19,000–20,000 cmꢁ1 (Fig. 6).

62
KOSOVA ET AL.
FIG. 6. DR spectra of CoOOH (1); Co(OH)2 (2); and LiCoO2 (3).
FIG. 7. DR spectra of ground samples: LiOH+CoOOH (1);
LiOH+Co(OH)2 (2); LiOH+Co (3); and LiCoO2 (4).
Moreover, a strong band in the ultraviolet region quantitative data on the concentration of varied cobalt ions
(432,000 cmꢁ1) is present, which, to our opinion, is due to different extinctions of their absorption bands.
associated with a ligand–metal charge transfer (CT). The
shape of the observed spectrum together with its low
intensity are indicative of a high degree of homogeneity of heat treatment. Let’s analyze the spectra of ground
(e) Co3+ Co2+ ions in ground samples before and after
an octahedral environment of Co2+ ions in the sample.
mixtures on the basis of the above-mentioned data. The
ions in Oh main electronic state of cobalt ions in the LiOH+CoOOH
3+
ꢀ
In CoOOH (space group R3m, Co
positions), the majority of cobalt ions are low-spin Co3+ mixture (Fig. 7, curve 1) is found to be low-spin [Co3+
]
Oh
ions with two different types of an oxygen octahedron (28,000 cmꢁ1). A small amount of high-spin [Co2+
]
ions
Td
environment as the appearance of two bands at 27,000 and (15,000 cmꢁ1) is also present. For the LiOH+Co(OH)2
22,000 cmꢁ1 suggests (Fig. 6, curve 1). The former is mixture, a large overall absorption is observed as
associated with a more perfect octahedron while the latter compared with the initial Co(OH)2, suggesting a pro-
with a less perfect one, composed of O2ꢁ ions and OH nounced structural disorder (Fig. 7, curve 2). Three bands
groups. The band at 22,000 cmꢁ1 unlikely corresponds to located at 28,000, 18,000–19,000, and 15,000 cmꢁ1 corre-
[Co2+
observed, probably due to the presence of Co3O4 as an also present. The spectrum of the LiOH+Co mixture is
admixture. characterized by a large continuous absorption, probably
The spectrum of ceramic LiCoO2 (space group R3m, resulting from the presence of delocalized electrons in the
Co3+ ions in Oh positions) is characterized by a system (metal state) with traces of [Co2+
Td, which are
]
Oh. Small traces of [Co2+
]
(15,000 cmꢁ1) are also sponding to [Co3+ Oh, [Co2+ Oh, and [Co2+
] ] ]Td (traces) are
Td
ꢀ
]
comparatively low intensity, the same as for Co(OH)2 badly noticeable in the background (Fig. 7, curve 3). In the
(Fig. 6, curve 3). The absorption takes place in two regions: spectrum of the activated LiCoO2, a more intense band
in the low-frequency region (13,000–20,000 cmꢁ1) and in an belonging to [Co3+
]
is significantly shifted to a low-
Oh
ultraviolet region (28,000–35,000 cmꢁ1). The bands at frequency region as compared with nonactivated LiCoO2.
18,000–19,000 cmꢁ1 and 15,000 cmꢁ1 are associated with This phenomenon may be explained by a weakening of the
the n3 transition of [Co2+
]
[Co2+
]
ions, respectively. crystal field effect as a result of the distortion of the anionic
Td
Oh
The latter (at 28,000–35,000 cmꢁ1) is a characteristic of framework by residual OHꢁ Co23ꢁ groups. Moreover, the
[Co3+ in a highly perfect octahedral environment. bands corresponding to [Co2+ Td and [Co2+
Oh increase in
Hereby, the ceramic LiCoO2 is characterized by small intensity.
]
]
]
Oh
amounts of Co2+ ions in its structure and Co3O4 as an
admixture.
Thus, DRS can provide qualitative information on the [Co3+
]Oh, with a localization in a more perfect oxygen
The DR spectra of all ground mixtures heated at 6001C
become similar (Fig. 8). The main state of the cobalt is
electronic state of cobalt ions (valence and spin state, crystal field (32,000–33,000 cmꢁ1) as compared with those
oxygen environment) in LiCoO2 prepared by different without heating. However, the observed shift of the right
methods. Unfortunately, it is impossible to get accurate edge of this band to a high-frequency UV region indicates

DISORDERING AND ELECTRONIC STATE OF COBALT IONS
63
FIG. 9. Charge–discharge curve of ground sample LiOH+CoOOH.
FIG. 8. DR spectra of ground samples after heating at 6001C:
LiOH+CoOOH (1); LiOH+Co(OH)2 (2); LiOH+Co (3); and LiCoO2
(4).
curve of LiCoO2/Li cells made with the ground LiOH+Co
mixture heat treated at 6001C is shown in Fig. 10. The cell
behavior is similar to that usually observed for layered HT-
probably crystal field inhomogeneity around Co3+ ions LiCoO2. A reversible capacity of 0.35 Li is obtained with,
(oxygen defects) or the formation of Co3+ ions’ clusters. It however, a noticeable irreversibility at the first cycle. This
should be noted that heating the LiOH+Co mixture leads sample was shown by DRS to present a better electron
to the full oxidation of cobalt ions localized in more homogeneity of Co3+ ions.
homogeneous Co3+O6 octahedra. The lower intensity of
the 15,000 cmꢁ1 band most likely corresponds, as men-
CONCLUSION
tioned above, to the d–d transition of high-spin Co2+ ions
in Co3O4 (admixture). In contrast, the heating of the
It has been demonstrated that a short mechanical
activated ceramic LiCoO2 does not lead to significant
activation combined with a subsequent annealing at
improvements of Co3+ octahedra; the band at 28,000–
moderate temperatures enables the formation of disor-
29,000 cmꢁ1 is still observed.
dered, highly dispersed, and rather homogeneous LiCoO2
Thus, according to DRS study, the ground mixtures are
samples. The main electronic state of cobalt ions in as-
characterized by strong nonhomogeneity in the electronic
prepared LiCoO2 is a low-spin [Co3+
]Oh with some oxygen
crystal field nonhomogeneity, probably due to the forma-
state of cobalt ions as well as their various oxygen
environments. The annealing of the samples results in an
enhancement of the Co3+ homogeneity. All spectra of the
annealed samples are characterized by localized states only
(i.e., ions). No delocalized electrons were detected in the
structure of as-prepared LiCoO2 samples.
5. Electrochemical Performance of As-Prepared
LiCoO2 Samples
The voltage–composition curves of the LiCoO2//Li cells
made with the ground LiOH+CoOOH mixture, which is
believed to correspond to LT-LiCoO2, are shown in Fig. 9.
The very large capacity of 0.7 Li observed upon the first
charge is followed by a first discharge of 0.1 Li, indicating a
very poor reversibility of the system. The large capacity of
the first charge may be attributed to the reactivity of
adsorbed impurities versus lithium or electrolyte.
The heating of the ground samples at 6001C resulting in
the formation of HT-LiCoO2 leads to a strong improve-
ment of the cycling properties. The voltage–composition
FIG. 10. Charge–discharge curve of ground sample LiOH+CoOOH
after heating at 6001C.

64
KOSOVA ET AL.
tion of oxygen defects. Using the DRS technique, traces of
[Co2+
Oh ions in the structure of LiCoO2 and Co3O4 as an
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]
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33, 1031 (1997).
admixture were detected, while they were practically
undetectable by X-ray and IR spectroscopy. Furthermore,
this technique has indicated that all observed d electrons in
the as-prepared LiCoO2 samples are localized. The main
reason is probably the formation of definite mechan-
ochemically induced defects (e.g., oxygen vacancies, Oꢁ
ions), which are preserved in the activated samples even
after their heating at 6001C. These defects appeared to act
as traps for delocalized electrons, producing cobalt ions
with localized electrons. In light of these results, one can
conclude that in order to prepare cathode material with
enhanced electrochemical performance, it appears neces-
sary to obtain an electronic structure placed midway
between full delocalization (electron gas) and full localiza-
tion (ions). Probably, this could be achieved by a fine
tuning of the mechanochemical conditions and of the
annealing temperature. Such electron structures were
observed for LixCoO2 (x41) (5) as well as in our study
of mechanochemically prepared LiV3O8 (22).
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Ionics 128, 11 (2000).
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ACKNOWLEDGMENTS
The authors thank S. Langella, N. Penin, B. Roch, and S. Soiron for
their fruitful discussions.
19. N. V. Kosova, V. F. Anufrienko, T. V. Larina, and E. T. Devyatkina,
Chem. Sustainable Develop. 9, 235 (2001). [in Russian]
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Innovation 4, 251 (2001).
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