Abstract
The mechanism of thermal stabilization of delithiated 
by cobalt substitution for nickel was closely studied from the structural point of view by thermogravimetry, X-ray diffraction, and X-ray absorption analysis. Delithiated 
with hexagonal 
or monoclinic 
structure was decomposed to a spinel phase (cubic, 
at temperatures around 220°C and then converted to a rock-salt phase (cubic, 
at higher temperatures. Cobalt substitution of nickel in 
stabilized the spinel phase, formed from the thermal decomposition of 
and suppressed the decomposition of this spinel phase to a rock-salt phase. While the highly delithiated 
was eventually converted to a rock salt phase with NiO structure during heating, 
spinel structure was locally formed around the cobalt ions in 
The improvement of the thermal stability of highly delithiated 
by cobalt addition could be explained by local formation of 
spinel structure around the cobalt ions in 
This spinel structure around the cobalt ions was relatively stable at high temperature and therefore, depressed the decomposition of 
to a rock-salt phase. © 2001 The Electrochemical Society. All rights reserved.
Export citation and abstract BibTeX RIS
Due to the increased production and use of portable devices, lithium secondary batteries with extended lives and higher power output are in demand. Consequently, 
has been extensively studied as a positive electrode material for these batteries because of its high specific capacity above 200 mAh/g.1
2
3
4
5 However, a few disadvantages, such as difficulty in preparation of electrochemically active form,4 capacity fading during the cycling,3 and poor thermal stability,6 are still unsolved and prevent 
from being used in the realistic lithium secondary batteries.
In particular, the thermal instability of delithiated 
has been considered as a one of the most crucial drawbacks that must be improved, because the thermal instability of delithiated 
causes safety hazards of the cell.6
7
8
9 It has been reported that delithiated 
is unstable at high temperatures and exothermically decomposed with oxygen liberation.6
9 The released oxygen may lead to an increase in an inner-pressure of the cell and a violent reaction with an electrolyte heated above its flash point. In addition, the heat evolved during the exothermic decomposition reaction of 
may play a role as a trigger for the thermal runaway of the cell. Furthermore, the amount of lithium available in 
i.e., specific capacity of 
is limited by thermal stability of 
because the thermal stability of delithiated 
is significantly degraded by lithium removal.
In order to improve the thermal stability of cathode materials for lithium secondary batteries, substitution of other elements such as Al, Co, Mn, Mg, and/or Ti for Ni and surface coating with 
or MgO have been reported.9
10
11
12
13
14 However, not only the mechanism of the thermal stabilization by the substitution and surface coating but also the detailed thermal decomposition mechanism has not been clearly understood. In previous work, we studied the thermal behavior of 
and reported that delithiated 
(
or 
) was thermally decomposed to a spinel 
phase at around 220°C and then converted to a rock-salt phase 
at higher temperatures.15 In addition, it was also expected that the thermal stability of 
could be improved by stabilization of the spinel structure and suppression of its decomposition to a rock-salt phase.
In this study, we investigated the effect of cobalt addition on the thermal behavior of 
and described the mechanism of the thermal stabilization of 
by cobalt substitution for nickel. For this purpose, a series of 
and 
were prepared by electrochemical delithiation of 
and 
and their thermal and structural characteristics were examined by thermogravimetry (TG), X-ray diffraction (XRD), and X-ray absorption (XAS) measurements.
Experimental
was synthesized by reacting stoichiometric amounts of 
and 
was obtained by solution method using nickel and cobalt nitrate and NaOH solutions. The pellets comprised of a mixture of 
and 
were precalcined at 600°C for 12 h in a stream of oxygen. The precalcined products were powdered, pressed into pellets, and reacted at 750°C for 
and at 800°C for 
for 24 h under oxygen flow. The reaction products were ground and stored in a desiccator. The compositions of the as-prepared 
and 
were determined from the values of Ni/Co ratio and average oxidation state of 
in 
obtained by inductively coupled plasma-Auger electron spectroscopy (ICP-AES) and iodometric titration, respectively, and are listed in Table I.
Table I.
Ni/Co ratio, average oxidation states of  in   and compositions of  | |||
|---|---|---|---|
| Samplea | Ni/Co ratiob | Average oxidation state of 
c | Composition |
 | - | 2.907 |
 |
 | 0.865/0.135 | 2.936 |
 |
| a Nominal compositions of the samples are calculated from the relative content of nickel nitrate, cobalt nitrate, and lithium nitrate. | |||
| b Obtained by ICP-AES. | |||
| c Obtained by iodometric titration. | |||
The crystal structures of the as-prepared 
and 
were characterized by neutron diffraction. The neutron powder diffraction data were analyzed by Rietveld refinement analysis using the General Structure Analysis System (GSAS) program.16 The classic reliability factors, defined in the caption of Table II, were used.
Table II.
Rietveld refinement results for as-prepared  and  | |||||
|---|---|---|---|---|---|
| Sample |
a | a (Å) | c (Å) | z (O) | Cationic distribution |
 | 8.34 | 2.8813 | 14.201 | 0.2415 |
 |
 | 7.98 | 2.8718 | 14.184 | 0.2407 |
 |
a  where the quality minimized is  with  and  being the observed and calculated intensities, respectively, and w a weight related to the error. | |||||
A composite electrode was prepared by mixing 88 wt % 
6 wt % acetylene black as a conductor, and 6 wt % polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) copolymers (Kynar FLEX 2801, Elf-Atochem America) as a binder dissolved in n-methyl-2-pyrrolidinone. The mixed slurry was coated onto Al foil and dried for >24 h. A three-electrode cell system was employed for the electrochemical measurements. Reference electrode and counter electrode were constructed from lithium foil, and 1 M 
in propylene carbonate (PC) was used as the electrolyte. Electrochemical delithiation of 
and 
was carried out at room temperature in a glove box filled with purified argon.
In order to obtain a delithiated 
the cell was charged to a desired composition at a constant current corresponding to a 20 h rate. After relaxation of several days, the electrode was taken out of the cell. It was then washed with tetrahydrofuran and dried thoroughly in vacuum. Thermal behavior of the charged compounds was examined up to 400°C using a TG (TGA 2050, TA Instruments) in air at a heating rate of 5°C/min. The structural change of 
as a function of temperature was investigated by means of XRD measurement with Cu Kα radiation. XAS spectra, especially X-ray absorption near edge structure (XANES), were measured before and after heating the 
composite electrodes at 300°C for 5 h in order to investigate the changes in the oxidation state of nickel and cobalt ions and local structure around the nickel and cobalt ions in 
Results and Discussion
Characterization of the as-prepared 
.—
The neutron powder diffraction patterns of as-prepared 
could be indexed assuming a space group of 
and the Rietveld refinement results of the patterns using the GSAS programs are listed in Table II. The structural parameters of 
were refined assuming the following atomic positions: lithium-rich 3a site at 000, nickel- and cobalt-rich 3b site at 001/2, and oxygen anion site (6c) at 
where 
Compositions of as-prepared 
and 
as listed in Table I, were 
and 
respectively, and held constant during the refinement. Detailed structural parameters, the assumption, and constraints employed during the refinements were described previously.17 The cationic distributions for 
and 
were determined as 
and 
respectively. However, for convenience, the phases are still referred by their nominal chemical formula of 
and 
in the following discussion.
Electrochemical delithiation from the .—
Figure 1 shows charge curves of 
and 
electrodes in 1 M 
charged at a constant current corresponding to a 20 h charge rate. The charge curve of 
shows a few voltage plateaus at potentials of 3.66, 4.03, and 4.20 V vs. 
and these are reported to be caused by phase transitions of 
on lithium deintercalation.3 However, 
shows a monotonous charge curve until 0.8Li are removed, and it means a single phase is maintained during the lithium deintercalation in this composition range.
Figure 1. Continuous charge curves of (a) 
and (b) 
obtained during the constant-current charging at a 20 h rate in 1 M 
in PC. Bar shown in the inset indicates the compositions at which the thermal and structural analysis was carried out.
In order to investigate the thermal behavior of delithiated 
and 
thermal analysis using TG and structural analysis using XRD and XAS were carried out. The bar shown in the inset of Fig. 1 indicates the compositions at which the thermal and structural analysis was conducted.
Thermal decomposition of .—
Figure 2 shows the thermogravimetric analysis (TGA) results for washed composite electrodes containing a series of 
acetylene black, and PVDF-HFP copolymer at a heating rate of 5°C/min in an air atmosphere. In general, weight losses measured with the TGA were found to be almost entirely due to oxygen liberated from the samples.6
7
8 In previous work, we studied the thermal behavior of 
and reported that delithiated 
was thermally decomposed to a spinel phase at around 220°C and then converted to a rock-salt phase at higher temperature.15 The phase transitions of 
with temperature were observed by high-temperature XRD analysis, and the representative XRD patterns of 
at various temperatures are shown in Fig. 3. As the temperature is increased, the trigonal phases with space group of 
transformed into a spinel phase 
at 200°C and then converted to a rock-salt phase 
at 300°C. More details about the phase transitions of 
with temperature have been described earlier.15
Figure 2. TGA results for (a) 
and (b) 
composites in air atmosphere at a heating rate of 5°C/min. The data have been offset vertically by 5% sequentially for clarity.
Figure 3. XRD patterns of 
during heating at a rate of 5°C/min. (*) Al foil and sample holder, Pt.
Therefore, it could be noted that the weight loss at around 220°C in TG curves of 
shown in Fig. 2a resulted from the phase transition of 
to a spinel. For the samples with 
it is suggested that there is no release of oxygen to form a spinel; no weight loss is observed. However, for 
the conversion of 
to a spinel is accompanied by oxygen release because there are more than four oxygen atoms for three cations in such samples. In addition, the weight losses at temperatures higher than the transformation temperature to spinel are due to the formation of a rock-salt phase. The temperatures at which the transformation to a rock salt occurred significantly decreased with x in 
as reported previously.15
was thermally decomposed at temperatures higher than those of 
as shown in Fig. 2b, and it indicated that the thermal stability of 
was improved by cobalt substitution for nickel. The increase in the decomposition temperature by cobalt addition is more remarkable for the transformation to a rock-salt phase than that to a spinel phase. Therefore, it could be noted that cobalt ions stabilized the spinel phase, formed from thermal decomposition of trigonal 
and suppressed decomposition of this phase to a rock-salt phase.
In order to investigate the effect of cobalt substitution for nickel on the phase transition from spinel to rock-salt phase, XRD patterns of 
and 
were measured after heating at 300°C for 5 h. 
heated at 300°C for 5 h showed an XRD pattern of spinel with space group of 
as shown in Fig. 4a, because little decomposition to a rock-salt phase occurred at this temperature. The temperature of decomposition of 
to a rock-salt phase tends to decrease with x; therefore, further decomposition of 
to a rock-salt phase was expected to proceed during heating at 300°C for 5 h with increase in x. For 
the decomposition to rock-salt phase was almost complete during heating at 300°C for 5 h, therefore, the XRD pattern of heat-treated 
could be indexed to the rock-salt phase with a space group of 
as shown in Fig. 4a.
Figure 4. XRD patterns of (a) 
and (b) 
after heating at 300°C for 5 h. (*) Al foil.
As shown in Fig. 4b, XRD patterns of 
after heating at 300°C for 5 h are similar to those of 
The XRD pattern of heat-treated 
could be indexed assuming a spinel structure, and further decomposition to a rock-salt phase was observed for the samples with higher x. However, contrary to 
in addition to a rock-salt phase, there is also a small amount of spinel phase still observed in the heat-treated 
This can be seen by the remnants of 
and 
peaks, marked by downward pointing arrows, in XRD pattern of 
after heating at 300°C for 5 h.
From the XRD patterns of 
and 
heated at 300°C for 5 h, it could be noted that cobalt ions stabilized the spinel phase, formed from the thermal decomposition of 
and shifted the onset of the decomposition of this spinel phase to a rock-salt phase to higher temperatures.
X-ray absorption analysis of .—
In order to investigate the thermal behavior of 
and the mechanism of thermal stabilization of this phase by cobalt addition, changes in oxidation states of nickel and cobalt ions and local structures around these ions in 
with temperature were studied by XAS. Figure 5 shows the Ni K-edge XANES spectra of 
both before and after heating at 300°C at 5 h. Two peaks were observed in the Ni K-edge XANES spectra of 
in Fig. 5a. A weak pre-edge (peak A) represents the transition of the 1s electron to unoccupied 3d orbitals of 
ions. The 
transition, which is formally forbidden, becomes dipole-allowed because of a combination of strong 3d-4p mixing and an overlap of the metal 3d orbitals with the oxygen 2p orbitals due to the noncentrosymmetric environment of slightly distorted 
octahedral site.17
18
19
20 A strong main-edge (peak B) appears due to the electric dipole-allowed transition of a 1s core electron to a unoccupied 4p orbitals.17
18
19
20 As x is increased, both the pre-edge and the main edge shifts to a higher energy region, suggesting that 
ions were oxidized to 
ions upon lithium deintercalation.
Figure 5. Ni K-edge XANES spectra of 
(a) before and (b) after heating at 300°C for 5 h. The insets show expanded views of the region containing the 
transitions.
Delithiated 
was thermally decomposed to a rock-salt phase at high temperature, because highly oxidized nickel ions are unstable and reduced to divalent state. The decomposition reaction was reported to proceed according to the following reaction8
15
Therefore, the oxidation state of nickel ions in 
must be reduced to values lower than +3 during the decomposition to a rock-salt phase, and the oxidation state of nickel ions in heated 
decreases with x after the decomposition.
Figure 5b shows the Ni K-edge XANES spectra of 
after heating at 300°C for 5 h. For all compositions, both the pre-edge and the main edge were located in a lower energy region than those of 
and it shows that the reduction of nickel ions in 
to 
occurred during heating. Reduction of nickel ions led to oxygen evolution from the 
in order to satisfy the charge neutrality in the oxides. Decreases in both the pre-edge and the main-edge energies with x were also observed in the XANES spectra of 
after heating, indicating that the oxidation state of nickel ions in the heated 
decreased with x. Figure 6 clearly shows the changes in the pre-edge and the main-edge energies in the Ni K-edge XANES spectra of 
which correspond to the changes in the oxidation state of nickel ions, before and after heating 300°C for 5 h, as a function of x.
Figure 6. (a) Pre-edge and (b) main-edge energies in the Ni K-edge XANES spectra of 
before and after heating at 300°C for 5 h.
Figures 7 and 8 show the Ni K-edge XANES spectra of 
and the energies of the pre-edge and the main edge in the spectra before and after heating at 300°C for 5 h as a function of x, respectively. The Ni K-edge XANES spectra of 
as a function of lithium content both before and after heating were identical to that of 
presented in Fig. 5. The spectra showed that the nickel ions in 
have the same local surroundings and show the same behavior on lithium deintercalation and temperature as in 
In addition, both the Ni K-edge XANES spectra of highly delithiated 
and 
after heating at 300°C for 5 h were in good agreement with that of NiO, as shown in Fig. 9. Therefore, it should be noted that the oxidation states of the nickel ions in the delithiated 
and 
were reduced to the values lower than +3, and the local structures around the nickel ions in these oxides were converted to that in NiO with rock-salt structure during heating.
Figure 7. Ni K-edge XANES spectra of 
(a) before and (b) after heating at 300°C for 5 h. The inset shows expanded views of the region containing the 
transitions.
Figure 8. (a) Pre-edge and (b) main-edge energies in the Ni K-edge XANES spectra of 
before and after heating at 300°C for 5 h.
Figure 9. Ni K-edge XANES spectra of (a) 
and (b) 
after heating at 300°C for 5 h, and (c) NiO.
Changes in oxidation state of cobalt ions and local structure around the cobalt ions in 
both before and after heating at 300°C for 5 h were also investigated by Co K-edge XANES analysis. The Co K-edge XANES spectra of 
as a function of lithium content are shown in Fig. 10a and are identical to the previously reported Co K-edge XANES spectra of 
17
18 This indicates that the cobalt ions in 
have the same local surroundings and show the same behavior on lithium deintercalation as in 
As x is increased, both the pre-edge and the main edge in the Co K-edge XANES spectra of 
shift to a higher energy region. 
ions in 
were oxidized to 
ions on lithium deintercalation.
Figure 10. Co K-edge XANES spectra of 
(a) before and (b) after heating at 300°C for 5 h. The inset shows expanded views of the region containing 
transitions.
Figure 10b shows the Co K-edge XANES spectra of 
after heating at 300°C for 5 h as a function of x. While both the pre-edge and the main edge in the Ni K-edge XANES spectra of 
and 
shifted to the energy regions lower than those of 
after heating, the Co K-edge XANES spectra of heated 
had the same pre-edge and main-edge energies as those of pristine 
as shown in Fig. 10b. This indicates that while the oxidation state of nickel ions in 
and 
was reduced to the values lower than +3 after heating requiring the reduction of 
to 
the reduction of cobalt ions took place only to the oxidation state of +3 at this temperature. The pre-edge and the main-edge energies in the Co K-edge XANES spectra of 
both before and after heating were plotted in Fig. 11 for clarity.
Figure 11. (a) Pre-edge and (b) main-edge energies in the Co K-edge XANES spectra of 
before and after heating at 300°C for 5 h.
ion is much more readily reducible than 
ion, and this is consistent with the fact that 
is easily synthesized while stoichiometric 
is difficult to synthesize. Therefore, contrary to nickel ions in delithiated 
and 
which are easily reduced to 
ions, the reduction of cobalt ions to the oxidation state below +3 is very difficult even at high temperature. It indicates that cobalt ions in 
can hold more oxygen in oxides at high temperature compared with nickel ions and therefore, suppress the thermal decomposition of 
to a rock-salt phase accompanied by oxygen release.
Figure 12 shows the Co K-edge XANES spectra of 
after heating at 300°C for 5 h. The Co K-edge XANES spectra of the heated 
and heated 
were very similar to those of 
and 
spinel, respectively, and the Co K-edge XANES spectrum of heated 
showed the intermediate feature between the 
and the 
spinel. The electronic structure of cobalt ions and the local structure around the cobalt ions in the heated 
and 
are very similar to those in 
and 
respectively. For the heated 
some cobalt ions are in the same surroundings as in the 
and the others are in the 
From these results, it can be said that the local structure around the cobalt ions in 
changed to that in the mixed phase of 
and 
during heating at 300°C for 5 h, and the fraction of the 
spinel increased with x.
Figure 12. Co K-edge XANES spectra of (a) heated 
(b) 
(c) heated 
(d) heated 
and (e) 
It has been reported that delithiated 
is decomposed to 
and 
spinel on heating according to the following reaction6
Because the cobalt ions in 
have the same local surroundings as in the 
as mentioned previously, the local structural changes around the cobalt ions in 
with temperature may be similar to those in 
Therefore, it can be noted that 
and 
structures are locally formed around the cobalt ions in 
during heating, and the relative amount of 
structure increases as x increases.
In highly oxidized 
NiO and 
structure may be dominantly formed around the nickel and cobalt ions at high temperatures, respectively. Therefore, the heated 
shows the Ni K-edge XANES spectrum similar to that of NiO, as shown in Fig. 9, and the Co K-edge XANES spectrum similar to that of 
as shown in Fig. 12. The XRD patterns of heated 
as shown in Fig. 4b, in which the diffraction peaks of the spinel still remained, could be explained by coexistence of rock-salt phase of NiO structure and spinel phase of 
structure locally formed around the cobalt ions.
From the Ni and Co K-edge XANES analysis of 
and 
it can be noted that the decomposition of 
to a rock-salt phase was suppressed by cobalt substitution for nickel. 
structure was locally formed around the cobalt ions in 
and this spinel structure is relatively stable at high temperatures.
While 
spinel structure was locally formed around the cobalt ions in the heated 
the cobalt ions in the heated 
showed an oxidation state of +3, which is slightly higher than the oxidation state (+2.67) of cobalt ions in stoichiometric 
spinel. It might be due to the incorporation of a small amount of 
and/or 
within the spinel structure locally formed around the cobalt ions in the heated 
However, further studies should be pursued to obtain accurate structural information about this spinel structure formed around the cobalt ions in the heated 
Conclusions
The thermal stability of highly delithiated 
was improved by cobalt substitution for nickel. The thermal stabilization mechanism was explained by local structural changes of 
by cobalt addition.
Delithiated 
was thermally decomposed to a spinel around 220°C and then turned into a rock-salt phase at higher temperature. Because 
spinel structure was locally formed around the cobalt ions added in the highly delithiated 
and this spinel structure was relatively stable at high temperature, the decomposition of spinel phase, formed from a thermal decomposition of 
was suppressed by cobalt addition.
Acknowledgments
The authors thank Korea Basic Science Institute for TG measurements and LG Chemical for financial support. This study has been supported by the Brain Korea 21 Projects.
Yonsei University assisted in meeting the publication costs of this article.