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Journal of Sol-Gel Science andTechnology ISSN 0928-0707Volume 68Number 2 J Sol-Gel Sci Technol (2013) 68:169-174DOI 10.1007/s10971-013-3148-9
Synthesis of LiNi1/3Co1/3Mn1/3O2 cathodematerial by a modified sol–gel method forlithium-ion battery
Yaoyao Zhang, Xiaoyan Wu, Ye Lin, DanWang, Chunming Zhang & Dannong He
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ORIGINAL PAPER
Synthesis of LiNi1/3Co1/3Mn1/3O2 cathode material by a modifiedsol–gel method for lithium-ion battery
Yaoyao Zhang • Xiaoyan Wu • Ye Lin •
Dan Wang • Chunming Zhang • Dannong He
Received: 5 July 2013 / Accepted: 26 August 2013 / Published online: 4 September 2013
� Springer Science+Business Media New York 2013
Abstract LiNi1/3Co1/3Mn1/3O2 was prepared by a modi-
fied sol–gel method, selecting ethylene diamine tetraacetic
acid and citric acid as the co-chelating agent. The mole
ratios of ethylene diamine tetraacetic acid (EDTA) to metal
ion (Mn?) were 0:1, 1:1 and 2:1. The obtained samples
were characterized by XRD, BET and SEM. The XRD
showed that LiNi1/3Co1/3Mn1/3O2 had good crystallinity
and well-ordered layered structure. After calcined at
850 �C, the LiNi1/3Co1/3Mn1/3O2 particles exhibited a
three-dimensional space network structure, which was
greatly correlated with the ratio of EDTA to metal ion. The
LiNi1/3Co1/3Mn1/3O2 obtained from a mole ratio of 1:1
(EDTA:Mn?) had the best electrochemical performance.
The reversible capacities were reached 168 and 100 mAh/g
at 1C and 10C discharge rate, respectively. The result of
the cycling performance showed a high capacity mainte-
nance ratio of 89.3 % at 1C and 25 �C after 50 cycles. The
further electrochemical performance was evaluated by
electrochemical impedance spectroscopy and cyclic
voltammetry.
Keywords LiNi1/3Co1/3Mn1/3O2 � Sol–gel method �Co-chelating agent � Lithium-ion battery
1 Introduction
Rechargeable lithium-ion batteries have revolutionized the
power portable electronics market and have been consid-
ered a prime candidate for power storage devices of electric
vehicles (EVs), hybrid electric vehicles (HEVs) and plug-
in hybrid electric vehicles (PHEVs) applications [1, 2].
Among the commercialized lithium-ion batteries, LiCoO2
is the most popular cathode material because of its con-
venience for synthesis, good cycling performance and
remarkable thermal stability. However, the relative high
cost, toxic nature of cobalt and only 50 % attainable
capacity of the theoretical capacity limit its large-scale
application in lithium-ion batteries [3–5].
Recently, layered transition-metal oxide LiNi1/3Co1/3
Mn1/3O2 with rhombohedral structure has been viewed as a
possible substitution for LiCoO2 due to its lower toxicity
and cost, higher reversible capacity and improved chemical
stability [6–9]. These advantages of LiNi1/3Co1/3Mn1/3O2
are attributed to its a-NaFeO2 type structure with the space
group of R-3m. The oxidation states of transition-metal
ions are identified as Ni2?, Co3? and Mn4?, which have
been revealed by XPS [10–12]. Moreover, Ni2?/3?, Ni3?/4?
and Co3?/4? redox couples play important roles in the
electrochemical stability during the cycling process and
Mn4? does not participate in the redox reaction and does
not act as an electrochemical active species [10].
The synthesis method of LiNi1/3Co1/3Mn1/3O2 has a great
influence on the particle morphology, grain size, specific
surface area, cation distribution and the mechanical properties
[13–18]. Compared with the traditional solid state reaction
Y. Zhang � D. He
School of Material Science and Engineering, Shanghai Jiao Tong
University, No. 800 Dongchuan Road, Shanghai 200240,
People’s Republic of China
X. Wu � D. Wang � C. Zhang (&) � D. He (&)
National Engineering Research Center for Nanotechnology,
No. 28 East Jiangchuan Road, Shanghai 200241,
People’s Republic of China
e-mail: [email protected]
D. He
e-mail: [email protected]
Y. Lin
Solid Oxide Fuel Cell SmartState Center, University of South
Carolina, 541 Main Street, Columbia, SC 29208, USA
123
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DOI 10.1007/s10971-013-3148-9
Author's personal copy
method, sol–gel and co-precipitation methods are considered
as the alternate approaches to improve powder properties [19–
22]. According to the previous research, the hydroxide co-
precipitation method may prevent the stratified precipitation
of precipitated Ni/Co/Mn transition-metal hydroxide to
deviate from the original formula at the molecular level which
is difficult to be obtained. On the other hand, the sol–gel
method could effectively eliminate the difference between
various metal ions and then help the molecule level mixing of
the metal ions in the precursor.
In this work, LiNi1/3Co1/3Mn1/3O2 was prepared by a
modified sol–gel method using ethylene diamine tetraacetic
acid and citric acid (EDTA–CA) as the bi-component
chelating agent. Influence of different mole ratios of
EDTA–CA to metal ion (Mn?) on the particle size and the
microstructure of LiNi1/3Co1/3Mn1/3O2 powder were stud-
ied. The effect of EDTA on the electrochemical perfor-
mance of LiNi1/3Co1/3Mn1/3O2 was also investigated.
2 Experimental
2.1 Powder synthesis
LiNi1/3Co1/3Mn1/3O2 cathode material was synthesized by
a modified sol–gel method using EDTA–CA as co-chelat-
ing agents. Stoichiometric amounts of Li(CH3COO)�2H2O,
Mn(CH3COO)2�4H2O, Ni(CH3COO)2�4H2O and Co(CH3-
COO)2�4H2O were used as raw materials and dissolved in
distilled water. The necessary amount of EDTA and CA,
which were pre-dissolved in ammonia, was then gradually
dropped into the mixed metal ion solution (the mole ratios
of EDTA to Mn? were 0:1, 1:1 or 2:1, and the mole ratio of
CA to Mn? was fixed at 2:1). NH3�H2O was used to adjust
the pH of the solution to the aimed value (pH = 7.0), and
then the mixed solution was heated up to 80 �C with
continuous stirring until the transparent gel was formed.
Then the gel was heated and dried at 240 �C in an electric
oven for 6 h and calcined in open air at 850 �C for 5 h to
obtain final powder.
2.2 Physical characterization
The phase characterization was verified by a powder X-ray
diffraction (XRD) measurement using a Bruker D8 advance
diffractometer with nickel filtered Cu ka radiation
(k = 1.5418 A) in the 2h range from 10� to 80�. The
powder morphology was observed using JEOL-6930 scan-
ning electron microscopy (SEM). The specific surface area
of the sample was measured by a 3H-2000 specific surface
area instrument (Beishide Instrument-ST Co., Ltd., Beijing,
China) using N2 adsorption. The samples were treated at
200 �C for 3–5 h in a vacuum to remove the surface
adsorbed species.
2.3 Electrode preparation and electrochemical
characterization
The charge and discharge characteristics were performed in
CR 2025 coin-type cells. The cell was composed of the
cathode and a lithium metal anode separated by the porous
polypropylene separator (Celgard 2400), and 1M LiPF6 in
ethylene carbonate (EC)-dimethyl carbonate (DMC)-
methyl ethyl carbonate (EMC) was used as the electrolyte
(EC:DMC:EMC = 1:1:1, w/w). The cathode was obtained
by mixing 80:10:10 (w/w) ratio of active material (LiNi1/3
Co1/3Mn1/3O2), conductive material (acetylene black) and
polyvinylidene fluoride (PVDF) binder in N-methyl-2-
pyrrolidinone (NMP). The slurry was then coated on the
aluminium foil (*10 lm) current collector and dried
under vacuum at 120 �C for 12 h. The electrode disks were
punched and weighed, and then the cells were assembled in
a glove box filled with pure argon.
The charge/discharge characteristics of the cells were
performed over the potential range between 2.5 and 4.3 V
using a NEWARE BTS 5 V–10 mA computer-controlled
Galvanostat (Shenzhen, China) at different rates of
0.5–10C at room temperature. Electrochemical impedance
spectroscopy (EIS) was carried out using an electrochem-
ical workstation (CHI660D) in the frequency range from
0.1 Hz to 1 MHz. Cyclic voltammetry (CV) tests were
performed on this apparatus in the potential window of
2.5–4.8 V versus Li?/Li at the scanning rate of
0.1 mV s-1.
3 Results and discussion
3.1 Powder characterization
The XRD patterns of the spherical LiNi1/3Co1/3Mn1/3O2
materials obtained by a modified sol–gel method are pre-
sented in Fig. 1. All the diffraction peaks are well-defined
and sharp. It suggests that all the samples have good
crystallinity and a layered oxide structure based on a
hexagonal a-NaFeO2 structure, which can be indexed with
the R-3m space group. The splits of (006)/(102) and (108)/
(110) peak pairs in the XRD patterns reveal that these
oxides own a highly ordered layered structure [23]. Fur-
thermore, as shown in Table 1, the integrated intensity
ratios of (003) and (104) peaks I003/I104 are both higher
than 1.2 when the mole ratios of EDTA to Mn? are 1:1 and
2:1, which indicate the LiNi1/3Co1/3Mn1/3O2 powders have
good cation ordering and no ‘‘cation-mixing’’ [24]. On the
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contrary, the sample without EDTA has undesirable cation
mixing, in which the I003/I104 value is about 1.2 [25].
Ethylene diamine tetraacetic acid is a versatile chelating
agent, the chelating reaction is fast and fully, moreover, it
is highly stable even under heat and light conditions for
forming the strong five-membered ring structure. Com-
pared to CA, EDTA forms more stable complex with most
metal ions than CA and should thus be more effective in
immobilizing metal ions. However, –OH group in citric
acid makes the polymerization reaction possible between
citric acid and EDTA or between citric acids molecules
[22, 26]. In our previous work [22, 27], the EDTA–CA sol–
gel synthesis route with the only 1:1 mole ratio of EDTA/
Mn? was used to prepare the Ba0.5Sr0.5Co0.8Fe0.2O3-d
cathode material in solid oxide fuel cells and Li4Ti5O12
anode material in Li-ion batteries. Obviously we didn’t
consider the impact on the particle size, the microstructure
and the electrochemical performance of LiNi1/3Co1/3Mn1/3O2
powder while changing the EDTA to Mn?. Thus, in this
study, the morphologies of the LiNi1/3Co1/3Mn1/3O2 pow-
ders synthesized with different amounts of EDTA are
firstly shown in Fig. 2. The LiNi1/3Co1/3Mn1/3O2 particles
all exhibit a three-dimensional space network structure, and
moreover, powders derived with higher EDTA to metal ion
ratio show more porous network structures. The average
crystallite sizes and specific surface areas of the LiNi1/3
Co1/3Mn1/3O2 oxides with different EDTA amount are
listed in Table 1. The average crystallite sizes were cal-
culated according to XRD line broadening with Scherer’s
equation. According to Fig. 2 and Table 1, it is obvious
that the crystallite sizes reduce and the BET surface areas
Fig. 1 XRD patterns of LiNi1/3Co1/3Mn1/3O2 material, the mole
ratios of EDTA to Mn? are a 0:1, b 1:1 and c 2:1
Table 1 Structural parameters and BET surface area of spherical
LiNi1/3Co1/3Mn1/3O2 powders
LiNi1/3Co1/3
Mn1/3O2
I003/I104 Crystallite
sizes (nm)
BET surface
area (m2/g)
EDTA:Mn?
0:1 1.2 107.8 4.4
1:1 1.5 78.5 5.8
2:1 1.7 67.4 6.7
Fig. 2 SEM images of LiNi1/3Co1/3Mn1/3O2 material, the mole ratios
of EDTA to Mn? are a 0:1, b 1:1 and c 2:1
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increase with the increase of EDTA/Mn? ratio. It illustrates
that the chelating agent of EDTA can create pore space
during the calcination process and contributing to the for-
mation of particle with small size.
3.2 Electrochemical characterization
In order to clarify the improvement of electrochemical
properties by adding EDTA, some electrochemical tests
were carried out. Figure 3 shows the initial cyclic perfor-
mance of the LiNi1/3Co1/3Mn1/3O2 cathode with mole
ratios of EDTA to Mn? (0:1, 1:1 and 2:1) at different
discharge rates (0.5C, 1C, 2C, 5C, 8C and 10C) between
2.5 and 4.3 V at room temperature. The rate was increased
from 0.5C to 10C, the process was taken with 5 cycles for
each stage. The pristine spherical LiNi1/3Co1/3Mn1/3O2
synthesized with EDTA presents higher rate capacities than
the one without EDTA, especially at low discharge/charge
rate. According to the interpretation given in Fig. 2 and
Table 1, it is obvious that the particle size of LiNi1/3Co1/3
Mn1/3O2 synthesized with 2:1 mole ratio of EDTA/Mn? is
the smallest. On the contrary, the particle size of LiNi1/3
Co1/3Mn1/3O2 without EDTA is the largest. Abundant
EDTA can create sufficient pore space and be contributed
to form the particle with small size during the calcination
process. The smaller particle size of the LiNi1/3Co1/3Mn1/3
O2 could reduce the diffusion length of Li? and electrons.
In the other words, the LiNi1/3Co1/3Mn1/3O2 electrode with
higher BET surface area may have more active sites for the
charge transfer processes. Therefore, LiNi1/3Co1/3Mn1/3O2
synthesized with 2:1 mole ratio of EDTA/Mn? has the
highest initial discharge capacity at 0.5C. However, with
the increase of discharge rate, the discharge capacity
decreases rapidly. It is generally believed that this is due to
the increase of electrode polarization at the higher current
rate. Among three LiNi1/3Co1/3Mn1/3O2 samples, the LiNi1/3
Co1/3Mn1/3O2 synthesized with 1:1 mole ratio of EDTA/
Mn? has the highest discharge capacity of about 100 mAh/
g at 10C, which is nearly 57.2 % of that at 0.5C. Mean-
while, the pristine one with 2:1 mole ratio of EDTA/Mn?
shows only 77 mAh/g at 10C, about 43.1 % of that at 0.5C.
Thus, the appropriate amount of EDTA has a profound
influence on achieving the optimized structure of LiNi1/3
Co1/3Mn1/3O2 to improve electrochemical performances.
To further understand the effect of EDTA on the cycling
behavior of LiNi1/3Co1/3Mn1/3O2, the long cyclic perfor-
mance was examined between 2.5 and 4.3 V at 1C rate
(278 mA/g) for 50 cycles. As shown in Fig. 4, the dis-
charge capacities of LiNi1/3Co1/3Mn1/3O2 with 0:1, 1:1 and
2:1 mole ratios of EDTA/Mn? are 145, 168 and 170 mAh/g
for first cycle, respectively. The powders synthesized with
EDTA show better performance than the one without
EDTA, indicating that adding EDTA is a very effective way
to enhance the electrochemical performance of LiNi1/3Co1/3
Mn1/3O2. After 50 cycles, the discharge capacities are
reduced to 123, 150 and 137 mAh/g, and 84.8, 89.3 and
76.5 % of its initial discharge capacity remains, respec-
tively. LiNi1/3Co1/3Mn1/3O2 synthesized with 2:1 mole
ratio of EDTA/Mn? has more pore space, higher surface
area and more active sites for the charge transfer processes
than that with 1:1 mole ratio. This would cause more side
reactions between the active particles or the active particles
and the electrolyte than that with 0:1 and 1:1 mole ratios.
Thus, the decrease of discharge capacities may be attrib-
uted to the progressive agglomeration of nano-sized parti-
cles during continuous tests [28], on the other side, may be
due to the possible phase structure change during the
charge/discharge processes [6, 29].
Fig. 3 Rate capacities of LiNi1/3Co1/3Mn1/3O2 electrodes from 0.1C
to 10C in the voltage range of 2.5–4.3 V, the mole ratios of EDTA to
Mn? are a 0:1, b 1:1 and c 2:1
Fig. 4 Cycle performance of LiNi1/3Co1/3Mn1/3O2 electrodes at 1C
rate (278 mA/g) between 2.5 and 4.3 V for 50 cycles, the mole ratios
of EDTA to Mn? are a 0:1, b 1:1 and c 2:1
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The first and second CV curves of LiNi1/3Co1/3Mn1/3O2
cathode with different mole ratios of EDTA to Mn? (0:1,
1:1 and 2:1) are displayed in Fig. 5a–c. The cells were
tested in the voltage range of 2.5–4.8 V at a scan rate of
0.1 mV/s. In the case of the LiNi1/3Co1/3Mn1/3O2 with and
without EDTA, two pairs of oxidation and reduction peaks
at *3.82/3.70 and *4.62/4.58 V are clearly observed. The
first oxidation peak at *3.82 V is mainly attributed to the
oxidation of Ni2?/Ni4? couple and the second oxidation
peak at *4.62 V is mainly due to the oxidation of Co3?/
Co4? couple. The reduction peak at *3.70 V is mainly due
to the reduction of Ni4?/Ni2? couple and a weak peak at
*4.58 V is due to the reduction of Co4?/Co3?. These
results confirm that Ni and Co with 2? and 3? valence
states possess electrochemical activities, and Mn with 4?
valence state is inactive. Furthermore, the peak current
densities of the LiNi1/3Co1/3Mn1/3O2 with 1:1 and 2:1 mole
ratios of EDTA/Mn? are larger than that of the one without
EDTA. It indicates that greater capacity would be obtained
from the LiNi1/3Co1/3Mn1/3O2 with EDTA. Among the
three samples, the Ni4?/Ni2? potential difference between
anodic and cathodic peaks at the second cycle only of
LiNi1/3Co1/3Mn1/3O2 with 1:1 mole ratio of EDTA/Mn? is
reduced as compared to that at the first cycle. Such dif-
ference may be caused by the electrode polarization, and
then it will be reflected in the electrochemical performance.
To provide more information about the differences of
electrochemical performances, the EIS tests were carried
out after above CV tests and the results are presented in
Fig. 6. All Nyquist plots are comprised of a depressed
semicircle in high frequency range and a straight line in the
frequency range below 2 Hz. The depressed semicircle can
simply be assigned to the charge transfer resistance. It can
be seen in Fig. 6 that the amount of EDTA has a significant
effect on the charge transfer resistance. The charge transfer
resistance of LiNi1/3Co1/3Mn1/3O2 with 0:1, 1:1 and
2:1 mole ratios of EDTA/Mn? is about 108, 40 and 69 X.
The result indicates that the EDTA–CA process is a
promising way to synthesize the LiNi1/3Co1/3Mn1/3O2
material. Thus, adding optimized ratio of EDTA to metal
Fig. 5 CV curves of LiNi1/3Co1/3Mn1/3O2 electrodes between 2.5 and
4.8 V, the mole ratios of EDTA to Mn? are a 0:1, b 1:1 and c 2:1
Fig. 6 Impedance plots of LiNi1/3Co1/3Mn1/3O2 electrodes, the mole
ratios of EDTA to Mn? are a 0:1, b 1:1 and c 2:1
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ion can significantly improve the electrochemical perfor-
mance of LiNi1/3Co1/3Mn1/3O2 material.
4 Conclusions
The LiNi1/3Co1/3Mn1/3O2 powders were successfully pre-
pared by the modified sol–gel method with EDTA–CA as the
co-chelating agents. The mole ratios of EDTA to Mn? are
0:1, 1:1 and 2:1. The co-chelating agent could effectively
prevent the aggregation and grain growth of LiNi1/3Co1/3
Mn1/3O2 nanoparticles. The powders synthesized with
EDTA–CA agents showed porous network appearance and
the particle sizes in the range of 50–300 nm. The materials
also had better crystallinity and higher ordered layered
structures than the one without EDTA agent. The electro-
chemical performance of this LiNi1/3Co1/3Mn1/3O2 with
1:1 mole ratio of EDTA to Mn? exhibited excellent dis-
charge capacities of 176 and 100 mAh/g at 0.5C and 10C,
respectively. After 50 cycles at 1C rate, the discharge
capacity decreased from 168 to 150 mAh/g. Further inves-
tigations of CV and EIS also revealed that the LiNi1/3Co1/3
Mn1/3O2 electrode with 1:1 mole ratio of EDTA to Mn? had
the best electrochemical performance.
Acknowledgments This work was supported by the International
Science and Technology Cooperation Program of China under Con-
tract No. 2012DFG11660 and the National Natural Science Founda-
tion of China under Contract No. 21171116.
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