Improvement of structural and electrochemical properties of commercial LiCoO2 by coating with LaF3

Article abstract of DOI:10.1016/j.electacta.2011.03.017

Commercial LiCoO₂ has been modified with LaF₃ as a new coating material. The surface modified materials were characterized by X-ray diffraction (XRD), transmission electronic microscopy (TEM), field emission scanning electron microscopy (FE-SEM), auger electron spectroscopy (AES) and galvanostatic charge-discharge cycling. The LaF₃-coated LiCoO ₂ had an initial discharge specific capacity of 177.4 mAh g ^-1 within the potential ranges 2.75-4.5 V (vs. Li/Li⁺), and showed a good capacity retention of 90.9% after 50 cycles. It was found that the overcharge tolerance of the coated cathode was significantly better than that of the pristine LiCoO₂ under the same conditions - the capacity retention of the pristine LiCoO₂ was 62.3% after 50 cycles. The improvement could be attributed to the LaF₃ coating layer that hinders interaction between LiCoO₂ and electrolyte and stabilizes the structure of LiCoO₂. Moreover, DSC showed that the coated LiCoO ₂ had a higher thermal stability than the pristine LiCoO₂.

Full text of DOI:10.1016/j.electacta.2011.03.017

Electrochimica Acta 56 (2011) 4791–4796
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Improvement of structural and electrochemical properties of commercial LiCoO2
by coating with LaF3
Zhanxu Yang , Qingdong Qiao , Wensheng Yangb
a
a,∗
a
School of Petrochemical Engineering, Liaoning Shihua University, No. 1, West Dandong Road, Wanghua District, Fushun, Liaoning 113001, PR China
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Commercial LiCoO2 has been modified with LaF3 as a new coating material. The surface modified mate-
rials were characterized by X-ray diffraction (XRD), transmission electronic microscopy (TEM), field
emission scanning electron microscopy (FE-SEM), auger electron spectroscopy (AES) and galvanos-
tatic charge–discharge cycling. The LaF3-coated LiCoO2 had an initial discharge specific capacity of
Received 10 December 2010
Received in revised form 4 March 2011
Accepted 4 March 2011
Available online 12 March 2011
−1
+
1
9
77.4 mAh g within the potential ranges 2.75–4.5 V (vs. Li/Li ), and showed a good capacity retention of
0.9% after 50 cycles. It was found that the overcharge tolerance of the coated cathode was significantly
Keywords:
better than that of the pristine LiCoO2 under the same conditions – the capacity retention of the pristine
LiCoO2 was 62.3% after 50 cycles. The improvement could be attributed to the LaF3 coating layer that
hinders interaction between LiCoO2 and electrolyte and stabilizes the structure of LiCoO2. Moreover, DSC
showed that the coated LiCoO2 had a higher thermal stability than the pristine LiCoO2.
© 2011 Elsevier Ltd. All rights reserved.
Lithium-ion battery
Cathode material
Surface modification
LaF3 coating
Overcharge tolerance
1
. Introduction
Layered LiCoO2 has been the dominant cathode material for
In this work, LaF3 has been introduced as a new coating mate-
rial for the LiCoO2 cathode for the first time. On the one hand,
lanthanum is an effective coating element in the form of lan-
thanide oxide which possesses good thermal stability, increases the
maximum current carrier capacity and supports high potential, to
improve the electrochemical performance of the cathode material
[16,17]. On the other hand, fluorine in the form of fluorides such as
commercial Li-ion batteries since 1991 because of its ease of pro-
duction, high operating potential, low self-discharge rate and high
reversibility below 4.2 V (vs. Li/Li ) [1–3]. However, its available
specific capacity in practical batteries is only about 140 mAh g
roughly half of its theoretical capacity (274 mAh g ). In order to
+
−
1
,
−
1
AlF , as a surface coating material, has successfully improved the
3
obtain a higher capacity from LiCoO , it must be charged above
electrochemical performance of the cathode material, which could
be attributed to the reduction of the structural change of active
material from HF attack by a thin AlF3 coating layer preventing
the highly delithiated cathode from being directly contacted with
liquid electrolyte [18–20]. LaF3 has been used as chemical sensors
because of its high chemical stability and ionic conductivity, but
it has not been introduced as surface coating material for cathode
material in lithium-ion batteries. In this study, we report a study of
the structure, morphology, overcharge behavior and thermal sta-
2
+
4
.2 V (vs. Li /Li) but this would lead to a rapid capacity loss thought
to be caused by side reactions with the electrolyte at high potentials
as well as structural instability [3–8]. To enhance the electrochemi-
cal stability of LiCoO at upper cutoff potential, partial substitution
2
of Co by other metals in LiCoO2 has been shown to be an effective
method of improving the structural stability of the cathode materi-
als. However, such improvements in structural stability have been
realized at the expense of specific capacity [9,10].
As an alternative approach, there has been extensive research
bility of LaF -coated LiCoO2 material.
3
into coating of LiCoO with various metal oxides in order to enhance
2
their electrochemical properties and structural stability [11–15],
however, most of the metal oxides as coating materials such as
Al O , ZnO, ZrO , cannot remain stable under the corrosion of HF
2
. Experimental
2
3
2
2.1. Synthesis of materials
from the electrolyte.
Commercial LiCoO2 with an average particle size of 10 ␮m was
obtained from Beijing Easpring Material Technology Co. Ltd. The
required amount of LiCoO2 (10 g) was first suspended in distilled
water with vigorous agitation, and acetic acid was added drop by
drop into the mixture to adjust the pH value at 5.5. The amount of
∗ Corresponding author. Tel.: +86 413 6860 686; fax: +86 413 6860 958.
E-mail addresses: qiaoqd@lnpu.edu.cn, qiaoqingdong@163.com (Q. Qiao).
0
013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.03.017

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Z. Yang et al. / Electrochimica Acta 56 (2011) 4791–4796
LaF in the solution was 0.5 mol% of the LiCoO powders. A solution
3
2
containing La(NO3)3 (0.1674 g) was then slowly added drop-wise
to the suspension. At the same time, a solution containing NH4F
(
0.0611 g) was added drop-wise. The suspension was subsequently
aged at room temperature with vigorous agitation for 1 h. The final
◦
powder was filtered and dried at 100 C for 12 h, and subsequently
◦
heat-treated in a furnace at 400 C for 5 h in air.
2.2. Characterization of materials
The particle morphologies of the products were observed by
means of a field emission scanning electron microscope (FE-
SEM) (JSM-7500F). X-ray powder diffraction (XRD) measurements
were obtained on a Rigaku D/MAX-1A diffractometer operated at
◦
4
0 kV and 30 mA in the 2ꢀ range 10–70 with Cu-K␣ radiation
(
ꢁ = 0.15406 nm). Transmission electronic microscopy (TEM) was
carried out on a Hitachi H-800 instrument for investigating the
microstructure of the samples. Auger electron spectroscopy (AES)
(
ULVAC-PHI, AES-PHI 700) was used to examine the spatial distri-
bution of constituent ions in the coated particles. Co dissolution
◦
Fig. 1. XRD patterns of the LaF3 calcined at (a) 100 and (b) 400 C for 5 h in air.
content of the sample after storage in electrolyte for 7 days at
◦
5
5 C was examined by inductively coupled plasma-atomic emis-
sion spectroscopy (ICP-AES) (Shimadzu ICPS-7500).
Electrochemical behavior during charge–discharge cycles was
examined using a two-electrode test cell with lithium foil as the
negative electrode. A positive electrode was made by coating a
paste of the active material, acetylene black and a polyvinylidene
fluoride (PVDF) binder (90:5:5 wt%) on an aluminum-foil collector.
The positive film was subjected to roll pressing and electrodes of
1
.2 mm diameter were punched out. The positive electrodes were
◦
dried at 120 C for 12 h in a vacuum oven. Coin-type cells (CR 2032)
were assembled in an argon filled glove box with an electrolyte
of 1 mol dm LiPF6 in EC-EMC-DMC (1:1:1 volume ratio) solution
and a Celgard 2400 separator. The electrochemical data were col-
−
3
lected using a LAND CT2001A test system within the potential range
+
2
.75–4.3, 2.75–4.5 or 2.75–4.7 V (vs. Li/Li ) at a constant current
−2
density of 0.2 mA cm
.
The thermal stabilities of the pristine and LaF -coated LiCoO2
3
were studied using differential scanning calorimetry (DSC)
(
PerkinElmer Pyris 1). The coin cell was first galvanostatically
charged to 4.5 V at a constant current and then opened in a dry
room. The measurement was performed in a nitrogen atmosphere
Fig. 2. XRD patterns of (a) the pristine LiCoO and (b) LaF -coated LiCoO2 calcined
2
3
◦
−1
◦
at 400 C for 5 h in air.
at a heating rate of 10 C min
.
3
. Results and discussion
After precipitation of a solution containing La(NO ) and a solu-
Fig. 3 shows TEM images of the pristine and coated LiCoO . The
2
pristine material shows a very smooth edge lines, and there is no
other layer on the surface in Fig. 3a. The coating material formed
a coating layer with a thickness of about 10 nm around the LiCoO2
particle shown in Fig. 3b. FE-SEM images of the pristine and coated
3
3
◦
tion of NH F, the precipitate was dried at 100 C, and subsequently
heat-treated in a furnace at 400 C for 5 h in air. The XRD pattern of
4
◦
the product was shown in Fig. 1, and the pure phase of the syn-
thesized LaF3 material calcined at 400 C for 5 h in air could be
LiCoO particles are shown in Fig. 4. A smooth and clean surface was
2
◦
observed on pristine LiCoO particle at high magnification in Fig. 4a.
2
indexed to the JCPDS No.74-1324 pattern, which it does not show
any additional peaks about the lanthanum oxides.
After coated with LaF , the change of coated particle morphology is
obvious even at low magnification in Fig. 4b, which is more obvious
3
The XRD patterns of the pristine and LaF -coated LiCoO2 are
at high magnification. The differences of pristine LiCoO and coated
3
2
shown in Fig. 2. It was confirmed that both materials can be indexed
in the hexagonal-type space group R3m. The XRD pattern of the
LiCoO2 in the images, we could conclude that the surface of the
LiCoO2 is covered with a coating thin film.
¯
LaF -coated LiCoO2 material does not show any additional peaks
AES was carried out to examine the spatial distribution of La
atoms near the surface of the coated particles, and the results are
shown in Fig. 5. It can be clearly seen that Co is hardly detected
3
other than those of LiCoO . This is presumably because the content
2
of is low and LaF forms only a thin film on the surface of LiCoO . The
3
2
lattice parameters were calculated by a least square method from
on the top surface of the coated LiCoO particles. The La concentra-
2
Fig. 2. The calculated lattice parameters of the LaF -coated LiCoO2
tion on the top surface of the coated LiCoO particles is initially very
3
2
were a = 0.2816 nm and c = 1.4079 nm, respectively. The values are
close to those of pristine, a = 0.2816 nm, c = 1.4072 nm, indicating
that the LaF3 was not incorporated into the host structure since no
changes were seen in the structure.
high, decreases sharply before a depth of about 9 nm, and Co shows
the opposite trend to the La concentration, as shown in Fig. 5. There-
fore, the thickness of LaF3 coating layer is about 9 nm. The result
is consistent with that of the TEM. The La levels off at a depth of

Z. Yang et al. / Electrochimica Acta 56 (2011) 4791–4796
4793
Fig. 4. FE-SEM images of the (a) pristine and (b) coated LiCoO2 powders. The inset
is the corresponding FE-SEM image of the cathode surface at high magnification.
Fig. 3. TEM images of the (a) pristine and (b) coated LiCoO2 powders.
about 40 nm, as shown in Fig. 5. The result implies that the coating
elements diffuse into the bulk LiCoO during calcination process to
2
form an doped LiCoO2 phase with a thickness of about 31 nm.
An electrochemical test is conducted with
a constant
−
2
charge/discharge current density of 0.2 mA cm in the different
upper cutoff potential. Typical charge/discharge curves of the pris-
tine and LaF -coated LiCoO cathode are shown in Fig. 6. The initial
3
2
−
1
1
discharge specific capacities of the pristine LiCoO (152.8 mAh g
)
)
2
−
is slight higher than that of the LaF -coated LiCoO (150.7 mAh g
3
2
cathode within the potential range 2.75–4.3 V, as shown in Fig. 6.
The capacity retention of LaF -coated LiCoO2 within the potential
3
range 2.75–4.3 V is 96.6% after 50 cycles which is much higher than
the corresponding value for the pristine LiCoO . Rate capability is
2
an important and desirable feature in secondary battery applica-
tions. Fig. 7 shows the rate capability and cyclability of the pristine
and LaF3 coated LiCoO2 between 2.75 and 4.3 V at various C-rates.
Fig. 5. Depth profiles of coated LiCoO2 particle determined by AES.
It was clearly observed that the LaF -coated electrode has better
rate capability than the pristine LiCoO2.
In order to further study the overcharge behavior of the cathode,
the upper charge cutoff potential was increased to 4.5 and 4.7 V,
particle. Sun et al. have also reported the similar result [18–20]. The
initial discharge specific capacity of LaF -coated LiCoO within the
3
3
2
−
1
potential range 2.75–4.7 V is 232.6 mAh g , slightly higher than
−
1
the value (221.3 mAh g ) for the pristine LiCoO . Zhao et al. have
2
respectively. Cycle-life plots of the pristine and LaF -coated LiCoO2
also reported that LiCoO2 charged at higher upper charge cutoff
potential has a lower discharge specific capacity than that of coated
cathodes [13]. This suggests that the LaF3 coating layer, like other
coating materials, stabilizes the structure of LiCoO2 and allows
more Li ions to be intercalated into the cathode during the discharge
process. At the upper charge cutoff potential of 4.7 V, the discharge
specific capacity of the pristine LiCoO2 shows a dramatic fading in
3
cathode in the different upper cutoff potential are presented in
Fig. 8. The initial discharge specific capacity of LaF -coated LiCoO
3
2
−
1
within the potential range 2.75–4.5 V is 177.4 mAh g , and the
capacity retention is 90.9% after 50 cycles, much higher than that of
the pristine LiCoO2 (62.3%). In addition, the discharge curve for the
LaF -coated LiCoO electrodes at the later stage are slightly higher
3
2
than that of the pristine LiCoO , which may be contributed that a
capacity with cycling, while LaF -coated LiCoO2 shows much less
2
3
resistive coating layer exists on the surface of LaF -coated LiCoO2
fading in capacity. After 30 cycles, the discharge specific capacity
3

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Z. Yang et al. / Electrochimica Acta 56 (2011) 4791–4796
Fig. 6. The charge/discharge curves of the pristine (a–c) and LaF3-coated LiCoO2 (d–f) within different potential ranges: (a and d) 2.75–4.3 V, (b and e) 2.75–4.5 V and (c and
f) 2.75–4.7 V. The inset is the discharge curves of the pristine (a–c) and LaF3-coated LiCoO2 (d–f) at the later stage for the discharge process.
Fig. 8. Comparison of the cyclabilities of the pristine (a–c) and LaF3-coated LiCoO2
(
d–f) cycled in different potential ranges: (a and d) 2.75–4.3 V, (b and e) 2.75–4.5 V
and (c and f) 2.75–4.7 V.
Fig. 7. Rate capability of the pristine and LaF3-coated LiCoO2 between 2.75 and 4.3 V.

Z. Yang et al. / Electrochimica Acta 56 (2011) 4791–4796
4795
Fig. 10. DSC scans of the pristine and coated LiCoO2 cathodes after charging to 4.5 V.
◦
The scan rate was 10 C/min.
1
.16 V, while that of LaF -coated LiCoO is increased by only 0.04 V.
3 2
Since the charge–discharge plateaus strongly depend on the struc-
ture of the cathode, based on the above results, we could draw a
conclusion that the LaF3 coating layer is very helpful to stabilize
the structure of the LiCoO2 cathode during the charge–discharge
process in a wider potential range.
The exothermic decomposition reaction is the most important
factor affecting the thermal stability of Li-ion batteries. DSC mea-
surements were performed in order to study the thermal stability
of the cathodes, and the results are shown in Fig. 10. The pris-
◦
tine LiCoO2 had a large exothermic peak at about 206 C, and the
−
1
enthalpy of reaction of the pristine LiCoO2 is 27.5 J g . However,
Fig. 9. dQ/dV vs. potential curves of (a) the pristine and (b) LaF3-coated LiCoO2 in
the potential range of 2.75–4.5 V.
thermal stability of the LaF -coated LiCoO2 was improved more
3
obviously; its DSC profile exhibited a relatively small peak about
◦
−1
2
18 C with the enthalpy of 14.3 J g . The enhanced thermal stabil-
ity of the coated LiCoO compared with the pristine LiCoO could be
of the coated cathode is as high as 172.3 mAh g−1, while that of
the pristine cathode is only 101.6 mAh g , as shown in Fig. 8. The
2
2
attributed to the thin LaF3 coating layer preventing the highly oxi-
dized positive electrode particles coming into direct contact with
the electrolyte solution, thereby reducing the exothermic reaction.
−1
enhanced overcharge behavior of the LaF -coated LiCoO2 may be
3
attributed to the improvement of the structural stability of active
core material from HF attack by a thin LaF coating layer preventing
3
4
. Conclusions
the highly delithiated cathode from being directly contacted with
liquid electrolyte and thus preventing dissolution of the cobalt. To
analyze the Co dissolution behavior in the product, the pristine
Commercial LiCoO2 has been successfully coated with a layer
of LaF . The LaF -coated LiCoO exhibited high capacity retention
of 90.9% after 50 cycles within the potential range 2.75–4.5 V com-
pared with the 62.3% capacity retention for the pristine LiCoO2.
3
3
2
LiCoO2 (0.50 g)and LaF -coated LiCoO2 (0.50 g) positive electrodes
3
◦
were immersed in 30 mL electrolyte solution for 7 days at 55 C, and
the Co content was determined by ICP-AES. The ICP analysis results
for the pristine and LaF -coated LiCoO dissolution have shown that
The LaF -coated LiCoO2 showed an excellent overcharge tolerance
3
3
2
and structure stability during the high upper cutoff potential. Fur-
thermore, the LaF3 coating could be significantly reduce the Co
dissolution from LiCoO2 and improve thermal stability of LiCoO2.
Therefore, LaF3 has considerable potential as a new coating mate-
rial to give enhanced overcharge tolerance and thermal stability of
LiCoO2 cathode material.
the amount of Co dissolution from the pristine LiCoO2 (0.25 mg)
was much higher than that of LaF -coated LiCoO2 (0.051 mg). The
3
result illustrates that LaF -coated LiCoO can effectively reduce the
3
2
Co dissolution into the electrolyte.
Fig. 9 shows the profiles of dQ/dV vs. potential for the pris-
tine and LaF -coated LiCoO2 cathode within the potential range of
3
2
.75–4.5 V at the 1st and 50th cycle, respectively. It is observed that
Acknowledgments
the oxidation–reduction peaks of the pristine LiCoO2 are centered
at 3.94 and 3.86 V at the first cycle, respectively, and the peaks are
narrow and sharp, as shown in Fig. 9a. However, after 50 cycles, it
is shown that the peaks become so broader that the plateau almost
disappears, and shift to 4.38 and 3.22 V, respectively. For LaF3-
coated LiCoO2 cathode in Fig. 9b, the oxidation–reduction peaks
corresponding to the phase transition during charge–discharge
process are centered at 3.96 and 3.87 V at the first cycle, respec-
tively, are narrow and sharp, whereas after 50 cycles, the oxidation
peak almost never moves and the reduction peak only shifts to
This work was supported by the opening funds of State Key Lab
of Chemical Resource Engineering, the Nature Science Foundation
of Liaoning Province (Grant No. 20082187). The authors wish to
acknowledge Dr. Guoming Liu for fruitful discussions.
References
[
[
[
1] G.T.-K. Fey, C.-Z. Lu, T.P. Kumar, Y.-C. Chang, Surf. Coat. Technol. 199 (2005) 22.
2] H. Wang, W.-D. Zhang, L.-Y. Zhu, M.-C. Chen, Solid State Ionics 178 (2007) 131.
3] Y.-K. Sun, S.-W. Cho, S.-T. Myung, K. Amine, J. Prakash, Electrochim. Acta 53
(2007) 1013.
3
.83 V, and both peaks are still narrow and sharp. Moreover, the
potential gap (ꢂV) of pristine LiCoO2 is increased from 0.08 V to

4
796
Z. Yang et al. / Electrochimica Acta 56 (2011) 4791–4796
[
4] T. Ohzuku, A. Ueda, J. Electrochem. Soc. 141 (1994) 2972.
[13] H. Zhao, L. Gao, W. Qiu, X. Zhang, J. Power Sources 132 (2004) 195.
[14] N.V. Kosova, E.T. Devyatkina, J. Power Sources 174 (2007) 959.
[15] Z. Chen, J.R. Dahn, Electrochem. Solid-State Lett. 5 (10) (2002) A213.
[16] G.T.-K. Fey, P. Muralidharan, C.-Z. Lu, Y.-D. Cho, Electrochim. Acta 51 (2006)
4850.
[5] W.S. Yang, X.M. Li, L. Yang, D.G. Evans, X. Duan, J. Phys. Chem. Solids 67 (2006)
1
343.
[
[
6] G.G. Amatucci, J.M. Tarascon, L.C. Klein, J. Electrochem. Soc. 143 (1996) 1114.
7] Z. Yang, W. Yang, D.G. Evans, G. Li, Y. Zhao, Electrochem. Commun. 10 (2008)
1
138.
[17] C. Lu, J. Chen, Y. Cho, W. Hsu, P. Muralidharan, G.T. Fey, J. Power Sources 184
(2008) 392.
[18] Y.-K. Sun, J.-M. Han, S.-T. Myung, S.-W. Lee, K. Amine, Electrochem. Commun.
8 (2006) 821.
[
[
8] Z. Chen, J.R. Dahn, Electrochim. Acta 49 (2004) 1079.
9] S. Madhavi, G.V.S. Rao, B.V.R. Chowdari, S.F.Y. Li, J. Electrochem. Soc. 148 (2001)
A1279.
[10] Y. Bai, H. Shi, Z. Wang, L. Chen, J. Power Sources 167 (2007) 504.
[11] T. Fang, J.-G. Duh, Surf. Coat. Technol. 201 (2006) 1886.
[12] K.Y. Chung, W. Yoon, J.Mc. Breen, X. Yang, S.H. Oh, H.C. Shin, W.I. Cho, B.W. Cho,
J. Power Sources 174 (2007) 619.
[19] Y.-K. Sun, S.-W. Cho, S.-W. Lee, C.S. Yoon, K. Amine, J. Electrochem. Soc. 154
(2007) A168.
[20] B.-C. Park, H.-B. Kim, S.-T. Myung, K. Amine, I. Belharouak, S.-M. Lee, Y.-K. Sun,
J. Power Sources 178 (2008) 826.