Doping Effect on the Grain Growth of Spinel LiMn2O4 ...

26 卷 9 期 2007. 9

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构化学（JIEGOU HUAXUE） Chinese J. Struct. Chem.

Vol. 26, No. 9 1017~1022

Doping Effect on the Grain Growth of Spinel LiMn2O4 Prepared by Sol-Gel Methods① ZUO Xiang-Qing LI Huai-Xiang② CHEN Lu-Sheng ZHOU Hong-Wei XIA Rong-Hua (Department of Chemistry, Shandong Normal University, Jinan 250014, China) ABSTRACT The cathode-active materials, Li1+yMxMn2-xO4 (M = Al, Co, Ni, Zn, y = 0.02, x = 0.02) powder, were synthesized by sol-gel method using LiOH, Mn(NO3)2 as the starting materials, citric acid as a carrier and Al(NO3)3·9H2O or Co(NO3)2·6H2O or Ni(NO3)2·6H2O or Zn(NO3)2·6H2O as dopants. The influence of different doping elements on the structural properties of the as-prepared samples was investigated by X-ray diffraction (XRD), infrared (IR) spectroscopy and scanning electron microscopy (SEM). X-ray diffraction patterns of the prepared samples were identified as the spinel structure with space group Fd3m. The grain size increases gradually as the sintering temperature rises and corresponding activation energies for the grain growth have been estimated using Arrhenius’ empirical relation. Keywords: grain growth, sol-gel, doping, activation energy, LiMn2O4

1 INTRODUCTION The lithium secondary batteries have been rapidly developed since their commercialization in the early 1990’s. Nowadays, improving the preparation technology and electrochemical performance of their electrode materials is one of the major focuses on the research and development of such materials, power sources and chemistry[1]. Spinel lithium manganese oxide, LiMn2O4, is an ideal material as a high-capacity Li-ion battery cathode by virtue of its low toxicity, low cost, and the high natural abundance of manganese. However, its capacity fades slowly, and this prevents its wide commercial applications. This fading is mainly due to the dissolution of Mn3+, Jahn-Teller effect[2] and decomposition of the organic solvents. On the other hand, spinel LiMn2O4 from solid-state reactions usually contains

some impure phases, which is detrimental to electrochemical performance. In order to overcome or partly alleviate these factors, sol-gel methods and doped or coated LiMn2O4 have been exploited. The materials prepared by sol-gel methods have few impurities, a small particle size with a homogeneous size distribution and a controlled morphology in contrast to those prepared by traditional solid-state reaction[3]. The traditional sol-gel method and coprecipitation process, which are easy to cause exclusion and segregation when heating, are not always effective on maintaining a homogenous distribution of precursor. As a result, undesirable phases form during the process of calcination. Among the modified sol-gel methods[4], the process of chelation of citric acid is effective. It is considered that the chelation compound is formed via the che lating of citric acid with lating of citric acid with metal ions in ions

Received 19 December 2006; accepted 23 January 2007 ① This work was supported by the National Natural Science Foundation of China (60671010) and Natural Science Foundation of Shandong Province (Y2006B29) ② Corresponding author. born in 1956, doctor, professor, doing researches on semiconductor materials. Tel: 0531-86271517, E-mail: [email protected]

ZUO X. Q. et al.: Doping Effect on the Grain Growth of Spinel LiMn2O4 Prepared by Sol-Gel Methods

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No. 9

ions in aqueous solution and the transparent viscous

temperature range of 400～850 ℃ for 5 h in air.

gel is afforded through polyesterification reaction

2. 2

Structural characterization of the products

between chelating compound of citric acid and po-

X-ray diffraction (XRD) analysis of the samples

lyatomic alcohol. The synthesis of spinel LiMn2O4

was carried out by powder X-ray diffraction using

by this method has many advantages, such as lower

Cu Kα radiation with a step size of 0.02° in the 2 θ

calcination temperature, homogenous phase-pure

range of 15 ～ 70° (BRUKER ADVANCED D8

LiMn2O4 particles in regular morphology and easily-

X-ray λ = 0.15406 nm, 40 kV, 40 mA). The

controlled dopant. In our present work, the intro-

morphologies of the samples were observed using a

duction of hetero-atoms M like Co, Al, Zn, and Ni in

scanning electron microscope (SEM) (HITACHI

the synthesis of LiMn2O4 by sol-gel method and the

H-800). The infrared (IR) spectra of the samples

effects on the grain growth of spinel LiMn2O4 have

were measured using a BRUKER Tensor 27 spec-

already been investigated.

trophotometer.

2 EXPERIMENTAL

3 RESULTS AND DISCUSSION

2. 1

3. 1

Synthesis of doped LiMn2O4

Li1+yMxMn2-xO4 powders were synthesized by

Grain growth of Li1+yMxMn2-xO4

with sintering temperature

sol-gel method using citric acid as a chelating agent.

Fig. 1 shows the XRD patterns of Li1+yMn2O4 sin-

A stoichiometric lithium hydroxide, manganese

tered for 5 h between 673 K and 1123 K. The pat-

nitrate and doped element nitrate was dissolved into

terns were identified as the cubic spinel phase hav-

de-ionized water, respectively and mixed with an

ing space group Fd3m in which the lithium ions

aqueous solution of citric acid. All chemicals were

occupy the tetrahedral (8a) sites and the manganese

of analytical grade (AR) and used without further

ions locate at the octahedral (16d) site[5]. It is found

purification. Considering the loss of lithium in the

that the XRD peaks become gradually sharper with

heating process, excessive lithium salt was added in

increasing temperature, indicating the particle grows

the initial components. The resulting solution was

larger in size. Fig. 2 shows the XRD patterns of

stirred for 0.5 h at room temperature and its pH

Li1+yMxMn2-xO4 sintered for 5 h at 750 ℃. They are

value was adjusted to 6.0 by ammonia solution to

all identified as the cubic spinel phase with space

form a sol. A viscous gel was obtained by

group Fd3m. Generally speaking, hetero-atoms M

evaporating the water at 60～80 ℃ for about 4 h.

are located at the manganese octahedral (16d) site.

The gel was dried in a drying oven at 110～120 ℃

The lithium ions also occupy the tetrahedral (8a)

for 18 h and then at 300 ℃ for 3 h to form black

sites, which may stabilize the structure by raising the

powders. The samples of Li1+yMxMn2-xO4 were

average valence number. 400

311

111

111

obtained by sintering the dried powders in

Intensity/ a.u.

Intensity / a.u.

Li1+yMn2O4 Al Co

Ni Zn 20

30

40

50

60

20

70

40

50

60

70

2θ / degree

2 θ / degree

Fig. 1. XRD patterns of Li1+yMn2O4 prepared from 673 to 1123 K for 5 h

30

Fig. 2.

XRD patterns of Li1+yMxMn2-xO4 sintered at 1023 K for 5 h

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The average size, D, of primary crystals was

sintering temperature are given in Fig. 3. The grain

calculated from the (111) diffraction peaks using the

size of every sample increases gradually as the

[5]

Scherrer formula : D = 0.9λ/βcos(θ), where λ is the

increase of sintering temperature and the grain size

X-ray wavelength, β the full width at intensity equal

of doped samples is larger than that of Li1+yMn2O4

to half maximum (FWHM) and θ the Bragg angle of

samples. In addition, the structure of the cubic spinel

the diffraction peak considered. The variations of the

phase of Li1+yMxMn2-xO4 maintained none the worse

grain size of Li1+yMxMn2-xO4 samples with the

for the 1% doped elements.

60

60

A : A l- d o p e d 50

B : C o -d o p e d

50 40

40

Grain size / nm

30

30 20

20 10

60 60

C : N i- d o p e d

50

50

40

40 30

30

20

20

600

D : Z n -d o p e d

700

800

900

1000

1100

1200

600

700

800

900

1000 1100 1200

T e m p e r a tu r e / K Fig. 3.

Variations of the grain size with the sintering temperature (A: Li1+yAl0.02Mn1..98O4;

B: Li1+yCo0.02Mn1..98O4; C: Li1+yNi0.02Mn1..98O4; D: Li1+yZn0.02Mn1..98O4; - - - ○: Li1+yMn2O4)

linearity relationship between ln(D) and 1/T

Assuming the grain growth obeys the first order [6]

kinetics, Arrhenius empirical relation , D =

obviously. Plot of natural logarithm of D for

Doexp(–Ea / RT), could be used to estimate the

Li1+yNi0.02- Mn1.98O4 versus reciprocal absolute

activation energy for the grain growth, where D is

temperature is shown in Fig. 4. From the slope of the

the grain size (nm), Do the pre-exponential constant,

plot, the activation energy of 29.63 kJ/mol could be

Ea the activation energy for grain growth, R universe

observed. For other doped samples, their activation

gas constant and T absolute temperature. There is a

energies are given in Table 1.

Table 1. Activation Energy of Different Hetero-atoms M Doped Samples Samples

Activation energy (kJ/mol)

Li1+yMn2O4

31.04

Li1+yAl0.02Mn1.98O4

36.93

Li1+yCo0.02Mn1.98O4

21.39

Li1+yNi0.02Mn1.98O4

29.63

Li1+yZn0.02Mn1.98O4

26.35

The activation energies for the growth of Co-, Niand Zn-doped samples are smaller than those of Li1+yMn2O4, while the activation energy of Al-doped sample is larger. Particular mechanism of the grain growth needs further exploration.

3. 2

Lattice-parameter variation

of Li1+yMxMn2-xO4 with sintering temperature The lattice parameters of cubic structure of LiMn2O4 could be determined from plane (111) diffraction peaks according to[7]


1020

-16.6 -16.8

ln(D)

-17.0 -17.2 -17.4 -17.6 -17.8

8

10

12 4

14

16

-1

10 / T (K )

Fig. 4. Arrhenius plot of the grain growth in size of Li1+yNi0.02Mn1.98O4 in the temperature range of 673～1123 K

1 = d

h 2 + k 2 + l 2 , where d is the spacing of the ac

crystal planes and h, k and l are the Miller indices of the measured reflection. The lattice constant of stoichiometric spinel LiMn2O4 is 0.82476 nm. Obvious lower lattice constants of the prepared Li1+y- Mn2O4 were found when compared with those of the stoichiometric spinel LiMn2O4 or other doped stoichimetric spinels studied by previous ～ investigators[8 11]. However, the present lattice constants are comparable to those of the oxidized samples re- ported by Kock et al.[2]. This is in agreement with the trend observed for lithium-doped spinel[12], which indicates that the samples were all defect spinels. Main reason might be attributed to the substitutions. Different heteroatom Co, Al, Zn, or Ni substitution of Mn in Li1+yMn2O4 could lower the lattice constants at different extent. For example,

the shrinkage of the cubic lattice as Co3+ replaces Mn3+ is explained by the smaller ionic size of Co3+ (0.0545 nm) compared to Mn3+ (0.0645 nm)[9]. The Co–O bond (bond energy, 1067 kJ/mol), stronger than Mn–O (bond energy, 949 kJ/mol)[9], contributes to the overall stabilization of the spinel octahedral sites. But the lattice constant variations might be similar reason for different dopants. In addition, most lattice constants of the samples become longer as the sintering temperature increases. Table 2 gives the computing results of the lattice constants. On the one hand, in lower temperature range, the gel will not turn into normal spinel phase with high purity. With increasing the sintering temperature, the spinel phase of Li1+y- MxMn2-xO4 becomes gradually purer so that most lattice parameters of the samples become longer, close to those of the normal spinel phase with high purity. On the other hand, lower sintering temperature results in the formation of a more oxidized manganese cation because Mn4+ is a more stable manganese ion at lower temperature. The atomic radius of Mn4+ (0.067 nm) is smaller than that of Mn3+ (0.072 nm)[13], so the lattice constant slightly increases with the increase of sintering temperature. However, due to the lower initial lattice constants observed for doped samples, the lattice constant variations at different sintering temperature were irregular. In general, the contraction of spinel phase Li1+yMxMn2-xO4 keeps the three-dimensional intrinsic structure more stable, which seems favorable for the superior resistance to microscope distortion, erosion and dissolution.

A

B

1μm

1μm

Fig. 5.

No. 9

SEM images of (A) Li1+yMn2O4 and

(B) Li1+yCo0.02Mn1.98O4 sintered at 1023 K for 5 h

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Table 2.

学（JIEGOU HUAXUE）Chinese

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Struct.

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Lattice Constant ac (nm) of the Li1+yMxMn2-xO4 Samples

Samples

673 K

Li1+yMn2O4 Li1+yAl0.02Mn1.98O4

773 K

873 K

923 K

973 K

1023 K

1073 K

1123 K

0.80625 0.80142 0.81473 0.80748 0.80535

0.80415

0.80910

0.80806

0.80344 0.80554 0.80050 0.80927 0.80819

0.80931

0.81165

0.81094

Li1+yCo0.02Mn1.98O4

0.81201 0.80806 0.81036 0.80911 0.81432

0.81336

0.81407

0.81295

Li1+yNi0.02Mn1.98O4

0.80876 0.80811 0.81036 0.80515 0.81340

0.81328

0.81310

0.81690

Lli1+yZn0.02Mn1.98O4 0.80584 0.81153 0.81024 0.81079 0.81461

0.81146

0.81436

0.81593

Table 3.

IR Vibration Absorption of Mn (IV)–O and Mn (III)–O Bonds －

Wave number (cm 1) of Mn (IV)–O

Sample

3. 3

化

－

Wave number (cm 1) of Mn (III)–O

Li1+yMn2O4

628.44

514.93

Li1+yNi0.02Mn1.98O4

627.78

513.85

Li1+y Co0.02Mn1.98O4

627.24

515.76

Li1+y Zn0.02Mn1.98O4

628.18

515.65

Li1+yAl0.02Mn1.98O

628.04

519.19

are ascribed to asymmetric stretching vibrations of

Morphology and infrared

Mn (Ⅳ)–O and Mn (Ⅲ)–O bonds, respectively, in

analysis of Li1+yMxMn2-xO4 Particle morphology as examined by scanning

the crystals. There are slight shifts compared to 507 －1

electron microscopy (SEM) was found to be

and 612 cm

influenced by the doping. Fig. 5 shows the SEM

bands in LiMn2O4[14]. The shift results are

images of Li1+yMn2O4 (A) and Li1+yCo0.02Mn1.98O4

summarized in Table 3. These differences are

(B) sintered at 1023 K for 5 h. The co-doped

probably due to the substitution of hetero-atoms Ni,

samples showed larger particle in size than

Co, Zn and Al for the manganese atoms at the

Li1+yMn2O4, which was partially consistent with the

octahedral (16d) site, causing the lattice constriction

results obtained by the analysis of diffraction lines.

and strengthened Mn–O bonds.

from the vibrations of corresponding

1.0

4 CONCLUSION Intensity / a.u

0.8

Li1+yM0.02Mn1.98O4 was 0.6

A

structure with space group Fd3m. The grain size of the samples increases gradually as the sintering

628

0.2 515

E

400

500

sol-gel

the prepared samples were identified as the spinel

C D

0.0 300

by

method with citric acid as a carrier. XRD patterns of

B 0.4

prepared

temperature rises. The lattice constants also increase 600

700

800

900

slightly with the sintering temperature but they are

1000

-1

lower than those of stoichiometric spinel LiMn2O4.

Wave number / cm

Fig. 6. Infrared spectra for the samples A: Li1+yNi0.02Mn1.98O4; B: Li1+yAl0.02Mn1.98O4; C: Li1+yCo0.02Mn1.98O; D: Li1+yMn2O4;

The activation energies for the grain growth are estimated to be 31.04, 36.93, 21.39, 29.63 and 26.35 kJ/mol for Li1+yMn2O4, Li1+yAl0.02Mn1.98O4, Li1+y-

E: Li1+yZn0.02Mn1.98O4

Co0.02Mn1.98O4, Li1+yNi0.02Mn1.98O4 and Li1+yZn0.02Fig. 6 shows the IR spectroscopy of Li1+yMx-

Mn1.98O4, respectively, using Arrhenius’ empirical

－1

relation. The substitution of hetero-atoms Ni, Co, Zn

Mn2-xO4 samples. The bands about 515 and 628 cm


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No. 9

and Al for the manganese atoms in LiMn2O4 also

asymmetric stretching vibrations of Mn (Ⅳ)–O and

causes slight infrared absorption shift from the

Mn (Ⅲ)–O in the crystals.

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