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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: zhangchm2003@163.com

D. He

e-mail: hdnbill@sh163.net

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

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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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