AlF3-coating to improve high voltage cycling performance of Li[Ni1/3Co1/3Mn1/3]O2 cathode materials for lithium secondary batteries

Article abstract of DOI:10.1149/1.2422890

Li [Ni13 Co13 Mn13] O2 powders were modified by coating their surface with amorphous AlF3 as a new coating material. The AlF3 -coated Li [Ni13 Co13 Mn13] O2 electrode showed improved cycle performance and rate capability under a high cutoff voltage range of 4.5 and 4.6 V. AC impedance results showed that the AlF3 -coated Li [Ni13 Co13 Mn13] O2 has stable charge transfer resistance (Rct) regardless of the cycle number. Electron diffraction analysis also showed that no structural transition of the primary particles was observed for the AlF3 -coated electrode. Electrochemical impedance spectroscopy and electron microscopy indicate that AlF3 coating plays an important role of stabilizing the interface between cathode and electrolyte.

Full text of DOI:10.1149/1.2422890

A168
Journal of The Electrochemical Society, 154 ͑3͒ A168-A172 ͑2007͒
0013-4651/2007/154͑3͒/A168/5/$20.00 © The Electrochemical Society
AlF3-Coating to Improve High Voltage Cycling Performance
of Li†Ni1/3Co1/3Mn1/3‡O2 Cathode Materials for Lithium
Secondary Batteries
,z
Y.-K. Sun,a, S.-W. Cho,a S.-W. Lee,a C. S. Yoon,b and K. Aminec,
*
*
aDepartment of Chemical Engineering, and bDivision of Materials Science and Engineering, Center for
Information and Communication Materials, Hanyang University, Seoul 133-791, South Korea
cArgonne National Laboratory, Electrochemical Technology Program, Chemical Engineering Division,
Argonne, Illinois 60439, USA
Li͓Ni1/3Co1/3Mn1/3͔O2 powders were modified by coating their surface with amorphous AlF3 as a new coating material. The
AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 electrode showed improved cycle performance and rate capability under a high cutoff voltage
range of 4.5 and 4.6 V. AC impedance results showed that the AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 has stable charge transfer
resistance ͑Rct͒ regardless of the cycle number. Electron diffraction analysis also showed that no structural transition of the
primary particles was observed for the AlF3-coated electrode. Electrochemical impedance spectroscopy and electron microscopy
indicate that AlF3 coating plays an important role of stabilizing the interface between cathode and electrolyte.
© 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2422890͔ All rights reserved.
Manuscript submitted September 1, 2006; revised manuscript received October 16, 2006. Available electronically January 5, 2007.
In recent years, the lithium transition-metal oxides,
Li͓NixCo1−2xMnx͔O2 have received considerable interest as a candi-
date to replace the commercial cathode material ͑LiCoO2͒ for
lithium secondary batteries. Among them, Li͓Ni1/3Co1/3Mn1/3͔O2
has been studied extensively as a promising cathode material due to
its many advantages such as a high discharge capacity, high rate
capability, and good structural stability. In attempts to increase the
reversible capacity of the cathode material, the upper cutoff voltage
limit has been progressively raised. The increased upper voltage
limit resulted in a moderate increase in the specific discharge capac-
ity as expected. However, the electrode, charged to a high voltage
Li͓Ni1/3Co1/3Mn1/3͔O2 by AlF3 on electrochemical performance at a
high cutoff voltage. We also investigate the reason for the improve-
ment of electrochemical performance of AlF3 coating by comparing
the structural and interfacial properties of pristine and AlF3-coated
Li͓Ni1/3Co1/3Mn1/3͔O2.
Experimental
Li͓Ni1/3Co1/3Mn1/3͔O2 powders were synthesized by the co-
precipitation method because this method gives rises to highly ho-
͑Ͼ4.5 V͒ lead to
a significant deterioration of the cycle
mogeneous ͓Ni1/3Co1/3Mn1/3͔͑OH͒ hydroxide powders; therefore,
2
performance.1,2 The origin of this capacity fading was related to the
increase in the surface reactivity between the highly delithiated and
instable cathode and the electrolyte, resulting in significant interfa-
cial impedance rise. In addition, operating at high voltage can lead
to either an increase in cobalt dissolution into the electrolyte in case
of cobalt based cathodes,3,4 or structural changes of the host
material.5
simple calcination of the hydroxide and lithium salt can result in a
phase-pure product with high homogeneity. The detailed procedure
for preparing the ͓Ni1/3Co1/3Mn1/3͔͑OH͒ powders was reported
2
previously.16 A mixture of the dehydrated ͓Ni1/3Co1/3Mn1/3͔͑OH͒
2
and LiOH·H2O was heated at 950°C for 15 h, then annealed at
700°C for 5 h in air. To coat the surface of Li͓Ni1/3Co1/3Mn1/3͔O2
with AlF3, ammonium fluoride ͑Aldrich͒, and aluminum nitrate non-
ahydrate ͑Aldrich͒ were separately dissolved in distilled water. After
the prepared Li͓Ni1/3Co1/3Mn1/3͔O2 powders were immersed into
the aluminum nitrate nonahydrate solution, the ammonium fluoride
solution was slowly added to the solution. The molar ratio of Al to F
was fixed at 3 and the amount of AlF3 in the solution corresponded
to 2 mol % of the Li͓Ni1/3Co1/3Mn1/3͔O2 powders. Afterward, the
solution containing the Li͓Ni1/3Co1/3Mn1/3͔O2 powder was continu-
ously stirred at 80°C for 5 h and then filtered by distilled water. The
resulting Li͓Ni1/3Co1/3Mn1/3͔O2 powders were heated at 400°C for
5 h in flowing nitrogen to avoid the formation of Al2O3.
For fabrication of the cathode, the prepared powders were mixed
with carbon black and polyvinylidene fluoride ͑94:3:3͒ in
N-methylpyrrolidinon. The slurry thus obtained was coated onto Al
foil and roll-pressed at 120°C in air. The electrodes were dried at
120°C overnight in vacuum state prior to use. Preliminary cell tests
were done using a 2032 coin-type cell adopting Li metal as the
anode. The electrolyte solution was 1 M LiPF6 in ethylene
carbonate–diethyl carbonate ͑1:1 in volume, Cheil Industries, Inc.,
Korea͒.
To solve this problem, various metal oxides were coated on
LiCoO2.6-11 It is well known that a metal-oxide-coated LiCoO2 has
a high reversible capacity and improved cycle characteristics. Al-
though the reason for the enhanced electrochemical performance is
not well understood, the coating technology is being used for com-
mercialized batteries due to the enhanced electrochemical perfor-
mance. For example, Cho et al.7 reported that an Al2O3 coating on
LiCoO2 effectively suppressed the lattice-constant changes and
thereby resulted in zero-strain cathode material. Meanwhile, Chen
and Dahn12 proposed that oxide coating might inhibit side reactions
involving oxygen loss from LixCoO2 to the electrolyte and hence
improve the cycling stability. Recently, Sun et al.13 first reported that
LiCoO2 with AlF3 coating had improved electrochemical properties
at a high cutoff voltage, which originated from the lower charge
transfer resistance and reduced cobalt dissolution. In addition, Fumi-
hiro et al.14 found that anion substitution for O in LiCoO2 substan-
tially suppressed the Co dissolution, even at the moderately high
upper voltage limit of 4.3 V. In a previous study,15 Kim et al. re-
ported that a fluorine-substituted Li͓Ni1/3Co1/3Mn1/3͔O2 exhibited
stable cycling performance and improvement of high rate capability
compared to the bare Li͓Ni1/3Co1/3Mn1/3͔O2, which resulted from
the smaller c-axis variation and fluorine coating effect.
Powder X-ray diffraction ͑XRD͒ ͑Rint-2000, Rigaku, Japan͒
measurement using Cu K␣ radiation was employed to identify the
crystalline phase of the synthesized powders and AlF3-coated pow-
ders. From the XRD patterns, lattice parameters were calculated by
a least-squares method. High-resolution transmission electron mi-
croscope ͑HR-TEM, JEM2010, JEOL͒ was employed to character-
ize the prepared powders and cycled electrodes. AC impedance mea-
In this work, we report the effect of surface modification of
*
Electrochemical Society Active Member.
z E-mail: yksun@hanyang.ac.kr
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Journal of The Electrochemical Society, 154 ͑3͒ A168-A172 ͑2007͒
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Figure 1. Powder XRD patterns of ͑a͒ pristine and ͑b͒ AlF3-coated
Li͓Ni1/3Co1/3Mn1/3͔O2.
surements were performed using a Zahner Elektrik IM6 impedance
analyzer over the frequency range from 1 MHz to 5 mHz with the
amplitude of 10 mVrms
.
Results and Discussion
Figure 1 shows XRD patterns of the pristine and AlF3-coated
Li͓Ni1/3Co1/3Mn1/3͔O2 powders. Both powders were confirmed to
be well-defined hexagonal ␣-NaFeO2 structure with a space group
Figure 2. TEM images of ͑a͒ AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 with mag-
nified images around the edge and ͑b͒ pristine Li͓Ni1/3Co1/3Mn1/3͔O2.
¯
of R3m. No impurity peaks corresponding to Al-related compounds
were observed from the XRD patterns shown in Fig. 1b. The lattice
constants of the AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 powder were
a = 2.864͑6͒ and c = 14.277͑8͒ Å, respectively, as calculated by a
least square method. The values are close to those of the pristine
material, a = 2.867͑5͒, c = 14.250͑4͒ Å, indicating that the AlF3
was not incorporated into the host structure as no changes were seen
in the structure.
Figure 2 presents TEM images of the pristine and AlF3-coated
Li͓Ni1/3Co1/3Mn1/3͔O2. The images reveal that each AlF3-coated
Li͓Ni1/3Co1/3Mn1/3͔O2 individual particle consists of primary
particles with sizes of 100–1000 nm. Figure 2a shows a TEM
172–173 mAh/g. The pristine Li͓Ni1/3Co1/3Mn1/3͔O2 showed a
gradual decrease in capacity, leading to a capacity retention of 75%
after 50 cycles. Meanwhile, the AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2
showed remarkably enhanced capacity retention of 93% after same
cycling period. Similarly to the 3.0–4.5 V range, the capacity reten-
tion of the AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 in the voltage range
of 3.0–4.6 V was significantly improved ͑92% capacity retention͒
compared to pristine cathode ͑72% capacity retention͒.
The rate capability properties also display advantages of the AlF3
coating. Figure 5 shows the discharge capacities for the Li/pristine
and AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 cells as a function of cur-
rent density ͑32–640 mA/g͒ between 3.0 and 4.5 V vs Li/Li+. The
cells were charged galvanostatically with a current density of
16 mA/g ͑0.1 C rate͒ before each discharge test and then discharged
of
a
typical primary particle from the AlF3-coated
Li͓Ni1/3Co1/3Mn1/3͔O2 powders. Magnified images around the edge
of the particle clearly reveal that it is uniformly encapsulated by a 5
nm thick AlF3 film. The grainy uniform contrast of the AlF3 film
confirms that the AlF3 coating layer is amorphous. The TEM images
provide conclusive evidence that the primary particle is coated by
the AlF3 film. In comparison, the bright field TEM of an uncoated
primary Li͓Ni1/3Co1/3Mn1/3͔O2 particle shown in Fig. 2b did not
have an extra film on the particle surface, as one would expect.
Figure 3 shows the initial charge/discharge curves of the Li/
pristine and AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 cells by applying a
constant current of 16 mA/g between 3.0 and 4.5 V vs Li/Li+. Both
cells show very stable and smooth charge/discharge curves, and the
curves for both cells nearly coincide even up to the cutoff voltage of
4.5 V. This result indicates that the coating medium of AlF3 was not
introduced into the bulk of Li͓Ni1/3Co1/3Mn1/3͔O2 structure. The
charge curve for the AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 electrode is
slightly higher than that of the pristine Li͓Ni1/3Co1/3Mn1/3͔O2. This
result also indicates that a resistive coating layer exists on the sur-
face of AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 particle.
Figure 4 shows the discharge capacity of Li/pristine and
AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 cells over 50 cycles in the range
of 3.0–4.5 and 3.0–4.6 V at a current rate of 80 mA/g ͑0.5 C-rate͒,
respectively. For the 3.0–4.5 V range, the pristine and AlF3-coated
Li͓Ni1/3Co1/3Mn1/3͔O2 delivered the same discharge capacity of
Figure 3. Initial charge and discharge curves of Li/pristine
Li͓Ni1/3Co1/3Mn1/3͔O2 and Li/AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 cells.
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Journal of The Electrochemical Society, 154 ͑3͒ A168-A172 ͑2007͒
Figure 4. Cycling performance of Li/pristine Li͓Ni1/3Co1/3Mn1/3͔O2 and
Li/AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 cells cycled at ͑a͒ 3.0–4.5 and ͑b͒
3.0–4.6 V range.
at current densities from 32 ͑0.2 C rate͒ to 640 mA/g ͑4 C rate͒. At
low current density ͑32 mA/g͒, the two electrodes showed similar
electrochemical performance. With increased current density, the
AlF3-coated Li͓Ni1/3Mn1/3Co1/3͔O2 electrode clearly exhibited a
higher discharge capacity than the pristine electrode. For example,
the AlF3-coated Li͓Ni1/3Mn1/3Co1/3͔O2 delivered a discharge capac-
ity of 127 mAh/g with good cyclability at 640 mAh/g, whereas
the pristine electrode reached 102 mAh/g at the same current
density. Another interesting feature is different capacity retention
behavior of the two electrodes. The capacity of the pristine
Li͓Ni1/3Co1/3Mn1/3͔O2 electrode exhibited an accelerated deteriora-
tion with cycling while the AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 elec-
trode showed excellent capacity retention. This could be due to both
intrinsic poor rate capability and decreased cycling behavior of pris-
tine Li͓Ni1/3Co1/3Mn1/3͔O2 electrode at higher cutoff voltage above
4.5 V as observed in Fig. 4.
Figure 6. Nyquist plots of ͑a͒ the Li/pristine Li͓Ni1/3Co1/3Mn1/3͔O2 and ͑b͒
the Li/AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 cell at 1st, 10th, 30th and 50th cycle.
To explore the reason for the enhanced electrochemical proper-
ties of the AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2, electrochemical im-
pedance spectroscopy ͑EIS͒ was carried out for the pristine and
AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 at different cycle numbers in the
charged state to 4.5 V vs Li/Li+. Nyquist plots derived from the
analyses are shown in Fig. 6. The insets in Fig. 6a and b present
expanded views for the high-to-medium frequency region. The
equivalent circuit used in this study was reported in our previous
study.17 The open symbols and continuous lines represent experi-
mental data and fitted results with the equivalent circuit, respec-
tively. These simulated lines correspond well to the observed plots.
The Nyquist plots for both electrodes present two semicircles, one in
the high-to-medium frequency region and the other in the low fre-
quency region. The semicircle at high-to-medium frequency is at-
tributed to resistance of surface film ͑Rsf͒ that covers cathode par-
ticle. The semicircle at low frequency is associated with the charge-
transfer resistance ͑Rct͒ coupled with a double-layer capacitance.
Tables I and II give Rsf and Rct as a function of cycle number for the
pristine and AlF3-coated LiCoO2 electrodes, respectively. The Rsf
for the both electrode drastically decreases after the first cycle. This
decline could be due to the improvement of ionic conductivity
for the lithium ion, although the passivation film or the reduction
of electrode/electrolyte reactivity. The Rsf of AlF3-coated
Li͓Ni1/3Co1/3Mn1/3͔O2 was about one and one-half times larger than
that of pristine Li͓Ni1/3Co1/3Mn1/3͔O2 at the first cycle as well as
subsequent cycles. For example, Rsf value of AlF3-coated
Li͓Ni1/3Co1/3Mn1/3͔O2 changed from 16.9 ⍀ at the first cycle to
3.7 ⍀ after 30 cycles, whereas that of pristine decreased from
Figure 5. Rate capability of Li/pristine Li͓Ni1/3Co1/3Mn1/3͔O2 and
Li/AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 cells at various current densities.
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Journal of The Electrochemical Society, 154 ͑3͒ A168-A172 ͑2007͒
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Table I. Surface film resistance „Rsf… for pristine and AlF3-coated
Li†Ni1/3Co1/3Mn1/3‡O2 cells with cycle number.
Rsf ͑⍀͒
Cycle
Pristine
AlF3-coated
number
Li͓Ni1/3Co1/3Mn1/3͔O2
Li͓Ni1/3Co1/3Mn1/3͔O2
1
10
20
30
40
50
10.5
2.99
2.38
2.00
1.76
1.81
16.9
3.85
3.45
3.69
3.63
4.26
10.5 ⍀ at the first cycle to 2 ⍀ after 30 cycles. These results indi-
cate that the AlF3 coating influences the formation of passivation
film on the cathode particle surface. The large Rsf of AlF3-coated
Li͓Ni1/3Co1/3Mn1/3͔O2 at the first cycle is attributed to more resis-
tive and thicker surface film compared to the other cycle numbers.
In contrast to the Rsf trends, the Rct value of pristine
Li͓Ni1/3Co1/3Mn1/3͔O2 greatly increased with increasing cycles,
even though the Rct of the first cycle was nearly same for the both
pristine and AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2. The Rct value of
the pristine was 11.1 ⍀ after the first cycle, drastically increased
with cycling, and reached to 230.9 ⍀ after 50 cycles. On the other
hand, the Rct value of AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 remained
within 5.3–6.03 ⍀, except for the first cycle ͑10.35 ⍀͒, as shown in
Fig. 6 and Table II. Different from the Rsf, Rct of the AlF3-coated
Li͓Ni1/3Co1/3Mn1/3͔O2 remained constant in spite of successive cy-
cling except for the first cycle, while the Rct of pristine
Li͓Ni1/3Co1/3Mn1/3͔O2 rapidly increased with cycling.
It is known that resistive surface films hinder the charge transfer
kinetics. However, in our study, although the surface film resistance
of the AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 is larger than that of the
pristine electrode, a stable surface film on the AlF3-coated
Li͓Ni1/3Co1/3Mn1/3͔O2 particle might have prevented interfacial
degradation between the electrolyte and cathode material at a high
voltage ͑4.5 V͒. Recently, Miyashiro et al.9 reported that the en-
hanced cycling properties of ZrO2-coated LiCoO2 for the high cutoff
voltage of 4.6 V were not due to the structural change of the host
but to the improved interfacial properties at the LiCoO2 surface.
More recently, Sun et al.13 also reported that the excellent capacity
retention of AlF3-coated LiCoO2 at the 4.5 V cutoff voltage origi-
nated from the lower charge transfer resistance and reduced cobalt
dissolution. It is clear that AlF3 coating greatly reduces the increase
of charge transfer resistance during cycling, which, in turn, limits
the increase the total cell resistance.
Figure 7. TEM ͑a͒ image of primary particle from the cycled AlF3-coated
Li͓Ni1/3Co1/3Mn1/3͔O2 electrode and ͑b͒ magnified image of the circled re-
gion in ͑a͒.
contrast of the AlF3 film indicates that the film remained amorphous
during cycling. The coating thickness also appears to have remained
uniform; however, the magnified image in Fig. 7b shows that the
AlF3 film thickness is strikingly different from one face of the par-
ticle to another. While 2 nm thick AlF3 film is well maintained along
the left face of the particle, the AlF3 film thickness on the right face
is limited to ϳ3 nm. This observation suggests that, although fur-
ther investigation is needed, the AlF3 coating may preferentially
either be coated or deteriorate along a certain crystallographic direc-
tion of the primary particle.
Electron diffraction analysis of the cycled electrodes demon-
strated that the integrity of the layered structure during cycling was
also improved by the AlF3 coating. For the pristine sample, it ap-
pears that some of primary particle underwent a phase transition to a
cubic spinel structure during cycling, as evidenced by a 100 zone
diffraction pattern of the cubic spinel structure in Fig. 8a obtained
from the cycled electrode that was not coated with AlF3. In com-
parison, no such phase transition was noticed for the observed pri-
mary particles of the AlF3-coated electrode. All of the
observed particles from the AlF3-coated electrode exhibited
symmetries consistent with the hexagonal structure of layered
Figure 7a shows a TEM of a primary particle from the
AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 electrode after electrochemical
cycling. The image indicates a 20 nm thick AlF3 film preserved on
the particle surface even after 50 cycles. Again, the uniform grainy
Table II. Charge transfer resistance „Rct… for pristine and
AlF3-coated Li†Ni1/3Co1/3Mn1/3‡O2 cells with cycle number.
Rct͑⍀͒
Cycle
Pristine
AlF3-coated
number
Li͓Ni1/3Co1/3Mn1/3͔O2
Li͓Ni1/3Co1/3Mn1/3͔O2
1
10
20
30
40
50
11.1
26.5
43.6
102.8
155.7
230.9
10.35
6.03
4.18
5.66
5.04
5.30
Figure 8. Electron diffraction analysis of the ͑a͒ pristine
Li͓Ni1/3Co1/3Mn1/3͔O2 and ͑b͒ AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 cells after
50 cycles.
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Journal of The Electrochemical Society, 154 ͑3͒ A168-A172 ͑2007͒
Hanyang University assisted in meeting the publication costs of this
article.
Li͓Ni1/3Co1/3Mn1/3͔O2. As an example, the 100 zone diffraction pat-
tern of the hexagonal Li͓Ni1/3Co1/3Mn1/3͔O2 structure obtained
from the AlF3-coated electrode after cycling is shown in Fig. 8b.
Although the role of AlF3 coating for the improved electrochemical
properties of layered Li͓Ni1/3Co1/3Mn1/3͔O2 has not been fully in-
vestigated, the this coating brought about stabilization of the host
structure as well as lowering of the charge transfer resistance.
References
1. K. M. Shaju, G. V. Subba Rao, and B. V. R. Chowdari, Electrochim. Acta, 48, 145
͑2002͒.
2. G.-H. Kim, S.-T. Myung, H. J. Bang, J. Prakash, and Y.-K. Sun, Electrochem.
Solid-State Lett., 7, A477 ͑2004͒.
3. G. G. Amatucci, J. M. Tarascon, and L. C. Klein, Solid State Ionics, 83, 167
͑1996͒.
4. S. T. Myung, K. Izumi, S. Komaba, Y.-K. Sun, H. Yashiro, and N. Kumagai, Chem.
Mater., 17, 3695 ͑2005͒.
Conclusions
Layered Li͓Ni1/3Co1/3Mn1/3͔O2 was coated with AlF3 to investi-
gate its cycling performance at high cutoff voltage ͑Ͼ4.5 V͒. A
uniform and thin AlF3-coating layer of 5–10 nm formed on the sur-
face of Li͓Ni1/3Co1/3Mn1/3͔O2, and the layer had an amorphous
structure. The AlF3-coated Li͓Ni1/3Co1/3Mn1/3͔O2 electrode had
much enhanced cycling stability and rate capability compared to
pristine Li͓Ni1/3Co1/3Mn1/3͔O2. The AlF3-coated electrode showed
only 7–8% capacity loss after 50 cycles at high cutoff voltage of 4.5
and 4.6 V, while the pristine samples suffered 25–28% capacity
loss. Such excellent properties could have originated from the lower
and stable charge transfer resistance between cathode and electrolyte
and from the stabilized host structure of Li͓Ni1/3Co1/3Mn1/3͔O2.
5. H. Wang, Y.-I. Jang, B. Huang, D. R. Sadoway, and Y.-M. Chiang, J. Electrochem.
Soc., 146, 473 ͑1999͒.
6. H.-J. Kweon, G.-B. Kim, and D.-G. Park, Korean Pat. Appl., 1998/0012005 ͑1998͒.
7. J. Cho, T.-J. Kim, Y. J. Kim, and B. Park, Angew. Chem., Int. Ed., 40, 3367 ͑2001͒.
8. Y.-K. Sun, Y.-S. Lee, M. Yoshio, and K. Amine, Electrochem. Solid-State Lett., 5,
A99 ͑2002͒.
9. H. Miyashiro, A. Yamanaka, M. Tabuchi, S. Seki, M. Nakayama, Y. Ohno, Y.
Kobayashi, Y. Mita, A. Usami, and M. Wakihara, J. Electrochem. Soc., 153, A348
͑2006͒.
10. J. Cho, T.-J. Kim, Y. J. Kim, and B. Park, Electrochem. Solid-State Lett., 4, A159
͑2001͒.
11. H. J. Kweon, J. Park, J. Seo, G. Kim, B. Jung, and H. S. Lim, J. Power Sources,
126, 156 ͑2004͒.
12. Z. Chen and J. R. Dahn, Electrochem. Solid-State Lett., 5, A213 ͑2002͒.
13. Y.-K. Sun, J.-M. Han, S.-T. Myung, S.-W. Lee, and K. Amine, Electrochem. Com-
mun., 8, 821 ͑2006͒.
14. Y. Fumihiro, O. Hirokuni, and Y. Nobuyuki, Jpn. Pat. 2003-221235 ͑2003͒.
15. G.-H. Kim, J.-H. Kim, S.-T. Myung, C. S. Yoon, and Y.-K. Sun, J. Electrochem.
Soc., 152, A1707 ͑2005͒.
16. M.-H. Lee, Y.-J. Kang, S.-T. Myung, and Y.-K. Sun, Electrochim. Acta, 50, 939
͑2004͒.
Acknowledgments
This research was supported by a grant ͑06K1501-01210͒ from
the Center for Nanostructured Materials Technology under “21st
Century Frontier R&D Programs” of the Ministry of Science and
Technology, Korea.
17. Y.-J. Kang, J.-H. Kim, S.-W. Lee, and Y.-K. Sun, Electrochim. Acta, 50, 4784
͑2005͒.
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