LiCoO2 Cathode Material That Does Not Show a Phase Transition from Hexagonal to Monoclinic Phase

Article abstract of DOI:10.1149/1.1397772

Structural instability of LiCoO₂ can be improved by sol-gel coating of Al₂O₃ and subsequent heat-treatments. While Al₂O₃ phase does not exist after heat-treatments, solid solution LiCo_1-xAl_xO₂ that has discretely higher Al concentration was formed at the surface up to ～500 A inside the particle. However, heat-treatment to 700°C results in the presence of the solid solution beyond ～500 A. The different Al concentration at the surface significantly affects the structural stability of the materials during cycling, and those prepared at 400°C do not show a phase transition from hexagonal to monoclinic phase. Disappearance of such a phase transition improves capacity retention of the cathode. Moreover, cathodes prepared at 400 and 500°C show improved layered characteristics with cation order.

Full text of DOI:10.1149/1.1397772

A1110
Journal of The Electrochemical Society, 148 ͑10͒ A1110-A1115 ͑2001͒
0013-4651/2001/148͑10͒/A1110/6/$7.00 © The Electrochemical Society, Inc.
LiCoO2 Cathode Material That Does Not Show a Phase
Transition from Hexagonal to Monoclinic Phase
,c
, z
Jaephil Cho,a, Yong Jeong Kim,b and Byungwoo Parkb,
*
*
aSchool of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
bSchool of Materials Science and Engineering, Seoul National University, Seoul, Korea
Structural instability of LiCoO2 can be improved by sol-gel coating of Al2O3 and subsequent heat-treatments. While Al2O3 phase
does not exist after heat-treatments, solid solution LiCo1ϪxAlxO2 that has discretely higher Al concentration was formed at the
surface up to ϳ500 Å inside the particle. However, heat-treatment to 700°C results in the presence of the solid solution beyond
ϳ500 Å. The different Al concentration at the surface significantly affects the structural stability of the materials during cycling,
and those prepared at 400°C do not show a phase transition from hexagonal to monoclinic phase. Disappearance of such a phase
transition improves capacity retention of the cathode. Moreover, cathodes prepared at 400 and 500°C show improved layered
characteristics with cation order.
© 2001 The Electrochemical Society. ͓DOI: 10.1149/1.1397772͔ All rights reserved.
Manuscript received February 21, 2001. Available electronically September 4, 2001.
Experimental
Presently, LiCoO2 is the most widely used cathode material in
commercially available Li-ion batteries, due to its high energy den-
sity and good cycle life performance. However, phase transforma-
tion from hexagonal to monoclinic phase, occurring between 4.1 and
4.2 V, induces a nonuniform volume change along the c direction
͑ϳ2% expansion͒.1,2 This change eventually induces strains and ex-
tended defects ͑microcracks͒ between and within the particles, and
leads to the disconnection of electrical contact of particles, resulting
in increased cell capacity fading.3,4 Oxides including LiCoO2, can
only tolerate elastic strain of ϳ0.1% before fracture.5 This phenom-
enon has been observed with transmission electron microscopy
TEM of cycled LiCoO2 cathode particles.3 In addition, cation disor-
der is reported to be an important factor in decreasing the discharge
capacity.6,7 Cation disordering occurs into the structure via exchange
of Li and Co ions between 3a and 3b sites, and increased cation
disordering can be identified by the decreasing intensity ratio of
͑003͒ to ͑104͒ peaks. However, an X-ray diffraction ͑XRD͒ pattern
of the cycled LiCoO2 cathode was reported to show little apparent
change in the structure of LiCoO2 phase because XRD patterns of
the cycled electrode showed very strong ͑003͒ peak intensity, there-
fore other peaks appeared to be negligible.3 This is because com-
pressed LiCoO2 cathode powders are strongly textured along the
͑003͒ plane. A similar fracture event was observed in the LiNiO2
material above 4.2 V by using an optical microscope. Fracture of
particles rose from an abrupt shrinkage along the c direction, corre-
sponding to ϳ9% volume change, observed during the phase tran-
sition between hexagonal phases.8
LiCoO2 powder was prepared from the reaction of mixture of
LiOH•H2O and Co3O4 in the mole ratio of 1.05:1 at 900°C for 24 h.
An excess amount of LiOH•H2O was used to compensate for the
loss of Li during firing. The surface area of the precursor bare-
LiCoO2 powder was 0.4 m2/g. To coat Al2O3 on the surface of
LiCoO2 ͑average particle size of 10 ␮m͒, aluminum ethylhexano-
diisopropoxide Al͑OOC8H15͒͑OC3H7͒2 was first dissolved in isopro-
panol, then was continuously stirred for 20 h at 21°C. LiCoO2 par-
ticles were then mixed with the coating solution such that total
amount of Al in the coating solution corresponded to 5 wt % of
LiCoO2 used. Sol-gel coating of Al2O3 on LiCoO2 was accom-
plished as follows
Hydrolysis Al͑OR͒ ϩ H2O → Al͑OR͒ ͑OH͒ ϩ ROH
͓1͔
4
3
Polycondensation Al͑OR͒ ͑OH͒ ϩ Al͑OR͒ ͑OH͒
3
3
→ ͑OR͒3Al-O-Al͑OR͒ ϩ H2O
͓2͔
͓3͔
3
Particle surface-OH ϩ ͑OR͒Al͑OR͒
3
→ particle surface-O-Al͑OR͒ ϩ ROH
3
In these expressions OR is an alkoxy group and ROH is an
alcohol. Metal alkoxides of Al͑OR͒ are first hydrolyzed into
4
Al͑OR͒ ͑OH͒ by the reaction with water vapor in the atmosphere, as
3
shown in Eq. 1. Then, the Al-OH groups in hydrolyzed alkoxides
Al͑OR͒ ͑OH͒ are polycondensed into Al-OR groups to form
3
In order to improve the structural instability of LiCoO2, partial
substitution of Al for Co in LiCo1ϪxAlxO2 was reported by Jang
et al., but the cathode showed both decreased initial capacity ͑127
mAh/g between 4.5 and 2 V͒ relative to LiCoO2, and deteriorated
capacity retention during cycling.9 To overcome this problem,
SnO2-coated LiCoO2 materials have been investigated by Cho
et al.10 Coated LiCoO2 materials prepared at lower temperatures led
to the formation of solid solution LiCo1ϪxSnxO2 near the particle
surface, dominantly distributed near the particle surface. Even
though these cathodes showed excellent structural stability to bare
LiCoO2 during cycling, those exhibited some problems: decreased
initial capacities and voltage profile at the end of discharge. Thus,
we optimize the coated material to prevent such problems, and in
this study, Al2O3-coated LiCoO2 cathode material is reported.
͑OR͒3Al-O-Al͑OR͒3 within the coating layer, and water is produced
following Eq. 2. Finally, Al-OR groups react with the LiCoO2
surface-OH groups, resulting in good adhesion of aluminum oxide
gel to the surface of LiCoO2 particle, according to Eq. 3.11 During
firing, the aluminum alkoxide gel of LiCoO2 surface O-Al͑OR͒ is
3
first decomposed into amorphous Al2O3 and then crystallized. After
drying LiCoO2 coated with aluminum alkoxide gel at 80°C, the
batch was fired at 400, 500, and 700°C, respectively, for 10 h, and
sieved each batch after soft grinding so that an average particle size
became 10 ␮m for all the electrochemical studies. Detailed assem-
bling process of coin-type half cell was described elsewhere.12 The
cathode electrode consisted of 92 wt % LiCoO2, 4 wt % super P
carbon black, and 4 wt % polyvinylidene fluoride ͑PVDF͒. Weight
of active material was 27 Ϯ 1 mg. The cells were first cycled at the
0.1 and 0.2 C rates for 1 and 2 cycles, respectively, and finally at the
0.5 C rate for total 70 cycles at the charge cutoff voltage of 4.4 V.
For cycling tests with charge cutoff voltages of 4.1 and 4.2 V, the
cells were cycled at the rate of 0.5 C for total 50 cycles. Discharge
cutoff voltage was fixed to 2.75 V. The anode electrode for a coin-
* Electrochemical Society Active Member.
c On leave from: Samsung SDI Co., Ltd., Chonan, Chungchongnam-Do, Korea.
z E-mail: byungwoo@snu.ac.kr
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Journal of The Electrochemical Society, 148 ͑10͒ A1110-A1115 ͑2001͒
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Figure 2. Concentration/depth profile of Al in LiCo1ϪxAlxO2 as a function
of heat-treatment temperatures 400, 500, and 700°C.
Figure 1. Plots of ͑a͒ lattice constant a, ͑b͒ lattice constant c, and ͑c͒ c/a
ratio, for bare LiCoO2 , LiCoO2 coated with gel solution of Al2O3, followed
by drying at 80°C, and the materials heat-treated at 400, 500, and 700°C for
10 h.
type Li-ion cell test was prepared by mixing mesophase pitch carbon
fiber ͑MCF͒ and PVDF binder in a weight ratio of 90:10 in n-methyl
pyrrolidone. In order to balance coin-type Li-ion cells C/LiCoO2,
weight ratio of MCF to LiCoO2 used was 1.1. The positive electrode
contained 35 Ϯ 2 mg of active material and had 2 cm2 of area. A
rate capability test for the Li-ion cells was carried out at 0.5, 1, and
2 C rates.
Results and Discussion
XRD patterns of Al2O3-coated LiCoO2 powders heat-treated at
400, 500, and 700°C were identical to that of bare LiCoO2, and
indexed to space group R-3m.13 Those patterns did not show any
peaks of Al2O3 phase, indicating that Al2O3 exists as a thin film or
has reacted with LiCoO2, thus forming solid solution LiCo1ϪxAlxO2
at the particle surface. To examine these possibilities, changes of
lattice constants a and c along with c/a ratio are plotted as a func-
tion of heat-treatment temperature of Al2O3-coated LiCoO2 ͑Fig. 1͒.
In the course of drying at 80°C, Al-OR groups reacted with the
LiCoO2 surface-OH groups, thus forming strong bonds with oxygen
in LiCoO2. Enclosure of LiCoO2 particles by thin-film oxides hav-
ing high fracture toughness is believed to change the measured lat-
tice constants of the powders prepared at 80°C. The effect of firing
temperature on the lattice constants, as shown in Fig. 1, was found
Figure 3. ͑a͒ SEM micrograph of coated LiCoO2 prepared at 700°C. The
number on the surface indicates spots for Auger profile. ͑b͒ Distribution of
Al surface concentration at the numbered positions.
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Journal of The Electrochemical Society, 148 ͑10͒ A1110-A1115 ͑2001͒
Figure 5. Cycle life performance of bare- and coated-LiCoO2 materials pre-
pared at 400, 500, and 700°C with a charge cutoff voltage of ͑a͒ 4.1 V and
͑b͒ 4.2 V at the rate of 0.5 C. Discharge cutoff voltage for both cases is
2.75 V.
Figure 4. ͑a͒ Voltage profiles of bare- and Al2O3-coated LiCoO2 prepared at
400, 500, and 700°C in Li/LiCoO2 cells between 4.4 and 2.75 V at the rate of
0.1 C. ͑b͒ Discharge capacity vs. cycle number of the same at the 0.1 C, 0.2
C, and 0.5 C rates up to 70 cycles.
the samples at 0.1 C rate ͑14 mA/g͒ show negligible differences,
depending on the firing temperature of the sample. Those prepared
at 400 and 500°C show 174 mAh/g, while that prepared at 700°C
shows 172 mAh/g. This difference is believed to be associated with
the solid solution formation beyond ϳ1000 Å range. Even though
cycle life data of the coated LiCoO2 prepared at 400 and 500°C
show a little difference at the rate of 0.5 C after 70 cycles ͑4 and 6%
capacity losses, respectively͒, that prepared at 700°C shows much
deteriorated capacity retention ͑15% loss͒. This result is much better
than SnO2-coated LiCoO2 in terms of initial capacity and capacity
retention. For instance, initial capacities and capacity retention of
SnO2-coated LiCoO2 prepared at 400 and 500°C were 164-166
mAh/g and 84%, respectively.10 In addition, samples prepared at
400 and 500°C do not show decreased voltage profiles at the end of
discharge ͑Fig. 4͒. In the case of bare LiCoO2, the capacity fading is
much faster than that of the coated material, showing 40% capacity
loss only after 30 cycles. Five coin cells were assembled to test the
reproducibility of the capacity loss, showing deviations of only Ϯ2
mAh/g after 30 cycles.
We have also examined the influence of cutoff voltages at 4.1
and 4.2 V on the electrochemical cycling stability ͑Fig. 5͒. At the
cutoff voltage of 4.1 V, the capacity losses of all samples are similar
to one another regardless of coating. However, increasing the cutoff
voltage to 4.2 V leads to the obvious capacity fading of bare and
coated LiCoO2 prepared at 700°C, whereas those prepared at 400
and 500°C show enhanced cycle life performance. For instance, ca-
pacity losses in bare LiCoO2 at 4.1 and 4.2 V were 5 and 17%,
respectively, from the initial capacity, while those in coated LiCoO2
prepared at 400°C were 5 and 4%, respectively, after 50 cycles at the
rate of 0.5 C.
to be reproducible with other samples and diffractometers. Hence,
authors believe that the effect is real, and not an experimental arti-
fact. Even though lattice constant c shows similar behavior to lattice
constant a, an abrupt increase of lattice constant c at 700°C ͑becom-
ing similar to the bare case͒ is noticeable. This change indicates
possible formation of LiCo1ϪxAlxO2 solid solution at the particle
surface through interdiffusion.
In order to obtain accurate distribution of Al atoms near the
particle surface, Auger electron spectroscopy ͑AES͒ was used, and
its concentration was measured in depth from the particle surface
͑Fig. 2͒. Sputtering depth was obtained by assuming a sputtering
rate of 260 Å/min, which is calibrated from SiO2. For samples pre-
pared at 400 and 500°C, Al concentrations decay to nearly zero at
ϳ500 Å. However, 700°C sample shows appreciable amount of Al
inside the particle, approximately 2 atom % Al at ϳ1000 Å. Surface
concentration from randomly selected points on the particle shows
noticeable variations, as shown in Fig. 3. Inductively coupled
plasma ͑ICP͒ mass spectroscopy showed that bare LiCoO2 powders
prepared at 900°C showed a little Li deficiency, i.e., x ϭ 0.03 in
Li1ϪxCoO2. While Al2O3-coated LiCoO2 showed similar Li defi-
ciency from ICP measurement, the exact Li concentration profile as
a function of depth needs to be further studied, e.g., in thin film
geometry.
Electrochemical cycling tests of the Al2O3-coated samples were
carried using coin-type half cell (Li/LiCoO2) between 4.4 and 2.75
V at 0.1 and 0.2 C rates for 1 and 2 cycles, respectively, and at the
0.5 C rate for the remaining 67 cycles ͑Fig. 4͒. Initial capacities of
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Journal of The Electrochemical Society, 148 ͑10͒ A1110-A1115 ͑2001͒
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Figure 7. Changes of lattice constants a and c as a function of x in
Li1ϪxCoO2 for bare and coated samples.
in Li/LiCoO2 were charged at a 0.1 C rate to the predetermined
voltages, then potentiostated for 10 h until the current was equiva-
lent to 0.01 C. An empirical relationship between the equilibrium
potential and the degree of Liϩ extraction from the Li1ϪxCoO2 was
Figure 6. CVs of bare and coated LiCoO2 heat-treated at 400, 500, and
700°C. The scan rate was 0.02 mV/s. The symbols H and M indicate hex-
agonal to monoclinic phases, respectively.
To investigate the origin of the capacity loss, cyclic voltammo-
grams ͑CVs͒ of the coated samples were carried out, using
Li/Al2O3-coated LiCoO2 cells ͑Fig. 6͒. The symbols H and M rep-
resent hexagonal and monoclinic phases, respectively. Samples heat-
treated at 400°C did not show any phase transition from hexagonal
to monoclinic phase at ϳ4.1 V, and no peak representing from
monoclinic to hexagonal at ϳ4.2 V ͑during charging͒. However,
very little tiny peaks at ϳ4.1 and ϳ4.2 V during charging ͑ϳ4.15
and ϳ4.05 V during discharging͒ appeared for the sample prepared
at 500°C. Such tiny peaks may result from deeper diffusion of Al
beyond 500 Å as firing temperature increases. As firing temperature
increases to 700°C, these peaks become remarkable. Still, the peak
heights are smaller than that of bare LiCoO2, implying that it may
undergo a less severe degree of phase transition than does the bare
sample. This phase transition significantly influences the capacity
retention of the sample, and should be suppressed in order to en-
hance the capacity retention. This is well supported by the result of
SnO2-coated LiCoO2 also.10 The origin of the disappearance of this
phase transition needs to be identified, and more detailed investiga-
tions regarding this phenomenon are underway. Another interesting
point is sharper peak around 3.95-4.05 V ͑during charging͒ in the
coated sample prepared at 400 and 500°C, than that in the sample
prepared at 700°C. This indicates that the diffusivity of Li is faster
in the Al2O3-coated powders prepared at lower temperatures. The
thin film LiCo1ϪxAlxO2 layer formed at the surface contracts the
bulk lattice constant c ͑Fig. 1b͒, and may provide an easier pathway
for Li-ion migration.
We have further investigated the evolution of lattice constants a
and c as a function of x in Li1ϪxCoO2 because the structural change
is sensitive to the lattice parameters ͑Fig. 7͒. The coin-type half-cells
Figure 8. XRD patterns of ͑a͒ cycled LiCoO2 electrode after dissembling
from Al foil, and ͑b͒ as-prepared LiCoO2 electrode.
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Journal of The Electrochemical Society, 148 ͑10͒ A1110-A1115 ͑2001͒
Figure 11. ͑a͒ Plots of rate capability of bare- and Al2O3-coated LiCoO2 at
0.5, 1, and 2 C discharge rates between 4.4 and 2.75 V in coin-type Li-ion
cells ͑charge rate, 1 C͒. ͑b͒ Cycle life performance of the same materials at
higher discharge rates between 4.4 and 2.75 V at 1, 20, 40, and 80 cycles in
coin-type Li-ion cells ͑charge rate, 1 C͒.
Figure 9. Changes of lattice constants a, c, and c/a ratio for Al2 -coated
3
LiCoO2 cathodes before and after cycling.
used to determine the x value in Li1ϪxCoO2. The extracted Liϩ
composition ͑x͒ in Li1ϪxCoO2 was calculated from the charge ca-
pacity, assuming 100% columbic efficiency for the reaction
LiCoO2 → Li1ϪxCoO2 ϩ xLiϩ ϩ xeϪ
͓4͔
The lattice constant, a, of all the samples shows only minor changes
during charging, quite opposite to the lattice constant c. Note that
bare LiCoO2 undergoes the most pronounced lattice change among
the samples from x ϳ 0.4 to 0.5, where a hexagonal to monoclinic
phase transition occurs, showing 1.3 Ϯ 0.2% abrupt expansion
along the c axis. However, the lattice constants of coated samples
prepared at 400, 500, and 700°C show 0.5 Ϯ 0.2, 0.5 Ϯ 0.2, and
0.9 Ϯ 0.3% expansion, respectively ͑from x ϳ 0.4 to 0.5͒. The re-
sult shows that the degree of phase transition decreases with de-
creasing firing temperatures of the Al2O3-coated samples. This is
well consistent with the results of cycle life performance and CVs
͑Fig. 4, 5, and 6͒.
However, beyond x ϳ 0.5, the lattice constant c contracts, ac-
companying another phase transition from monoclinic to hexagonal
phase. Upon increasing the charge cutoff voltage from 4.2 to 4.4 V
͑corresponding to x ϳ 0.5 and 0.7, respectively͒, the capacity loss
of bare LiCoO2 increased from 17 to 60% after 30 cycles ͑Fig. 4 and
5͒. Hence, the structural stability of bare LiCoO2 depends largely on
the extent of lattice contraction. On the other hand, the capacity loss
Figure 10. Comparison of peak intensity ratios of ͑003͒/͑104͒ before and
after cycling as a function of heat-treatment temperatures.
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Journal of The Electrochemical Society, 148 ͑10͒ A1110-A1115 ͑2001͒
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of coated materials are less sensitive to the charge cutoff voltage
than that of bare one. This is because of significant decrease in the
lattice contraction between x ϳ 0.5 and x ϳ 0.7.
life test. Capacity fading of the coated sample is six times less than
that of bare one ͑31%͒, showing 5% capacity loss of the initial
capacity after 80 cycles at the 1 C rate between 4.4 and 2.75 V.
In addition, we have investigated the structural modification of
Al2O3-coated LiCoO2 after cycling. Since compressed electrode is
strongly oriented in the ͑003͒ direction, it is impossible to detect any
change in the XRD patterns. Thus, composite cathodes were dis-
solved in acetone so that it was dissembled from the aluminum
electrode. As shown in Fig. 8, such texture was significantly de-
creased in the XRD patterns of the dissembled cathode, thus detect-
ing any change in the microstructure after cycling. Changes of lat-
tice constants are investigated, and the lattice constant, c, in the
samples heat-treated at 400 and 500°C increases after cycling, but
not that heat-treated at 700°C ͑Fig. 9͒. This behavior is in opposite
to the trend of lattice constant a. This indicates that an increased
interlayer distance, c, increases during cycling in the coated LiCoO2
heat-treated at 400 and 500°C. The ratio of ͑003͒/͑104͒ peaks in the
sample heat-treated at 700°C, which is indicative of cation order,
decreases more than in those heat-treated at 400 and 500°C, as
shown in Fig. 10. This result clearly indicates that Al2O3 coating in
LiCoO2 at lower temperature reduces the amount of cation disorder,
due to the suppression of hexagonal to monoclinic phase transition,
as shown in Fig. 6. Similar behavior has been reported in
LiNi1ϪxCoxO2 material, and the capacity loss after extended cycling
increased with increasing cation disorder.6
Finally, we have studied the rate capability and cycle-life perfor-
mance of bare and coated LiCoO2 prepared at 400°C in coin-type
Li-ion cells ͑C/LiCoO2͒, as shown in Fig. 11. Capacity loss of the
Al2O3-coated LiCoO2 is much less than that of bare one, showing
only 0.5% when increasing C rate from 0.5 to 2 C. In addition, the
voltage profile of the coated LiCoO2 is higher than that of bare one,
and it is related to the faster Li diffusion rates in the coated sam-
ple,as evidenced by the cyclic voltammogram data. Superior cycle
life performance of the coated LiCoO2 is also observed in the cycle
Conclusions
Structural stability of LiCoO2 cathode material was improved by
sol-gel coating of Al2O3. Even though an Al2O3 phase did not exist
after heat-treatment, thin-film LiCo1ϪxAlxO2 formation within a
ϳ500 Å region from the particle surface greatly affected the cycling
stability of LiCoO2. This effect also led to the disappearance of
hexagonal to monoclinic phase transition. Al2O3-coated LiCoO2 pre-
pared at 400 and 500°C showed improved capacity retention with
suppressed cation disorder and improved layered characteristics.
Seoul National University assisted in meeting the publication costs of this
article.
References
1. T. Ohzuku and A. Ueda, J. Electrochem. Soc., 141, 2972 ͑1994͒.
2. J. N. Reimers and J. R. Dahn, J. Electrochem. Soc., 139, 2091 ͑1992͒.
3. H. Wang, Y.-I. Jang, B. Huang, D. R. Sadoway, and Y.-M. Chiang, J. Electrochem.
Soc., 146, 473 ͑1999͒.
4. L. H. Van Vlack, Physical Ceramics for Engineers, Addison-Wesley Publishing
Inc., MA ͑1964͒.
5. G. G. Amatucci, J. M. Tarascon, and L. C. Klein, Solid State Ionics, 83, 167 ͑1996͒.
6. J. Cho, G. Kim, and H. Lim, J. Electrochem. Soc., 146, 3571 ͑1999͒.
7. R. J. Gummow, M. M. Thackeray, W. I. F. David, and S. Hull, Mater. Res. Bull., 27,
327 ͑1992͒.
8. K. Dokko, M. Nishizawa, S. Horikoshi, T. Itoh, M. Mohamedi, and I. Uchida,
Electrochem. Solid-State Lett., 3, 125 ͑2000͒.
9. Y.-I. Jang, B. Huang, H. Wang, D. R. Sadoway, G. Ceder, Y.-M. Chiang, H. Liu,
and H. Tamura, J. Electrochem. Soc., 146, 862 ͑1999͒.
10. J. Cho, C.-S. Kim, and S.-I. Yoo, Electrochem. Solid-State Lett., 3, 362 ͑2000͒.
11. D. Segal, Chemical Synthesis of Advanced Ceramic Materials, Cambridge Univer-
sity Press, Cambridge, England ͑1989͒.
12. J. Cho, Y. J. Kim, and B. Park, Chem. Mater., 12, 3788 ͑2000͒.
13. H. J. Orman and P. J. Wiseman, Acta Crystallogr., Sect. C: Cryst. Struct. Commun.,
40, 12 ͑1984͒.
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