Preparation of submicrocrystal LiMn2O4 used Mn3O4 as precursor and its electrochemical performance for lithium ion battery

Accepted Manuscript Preparation of submicrocrystal LiMn2O4 used Mn3O4 as precursor and its electrochemical performance for lithium ion battery Bao-Sheng Liu, Zhen-Bo Wang, Yin Zhang, Fu-Da Yu, Yuan Xue, Ke Ke, FangFei Li PII: DOI: Reference:

S0925-8388(14)02647-4 http://dx.doi.org/10.1016/j.jallcom.2014.11.004 JALCOM 32553

To appear in:

Journal of Alloys and Compounds

Received Date: Revised Date: Accepted Date:

12 August 2014 30 October 2014 1 November 2014

Please cite this article as: B-S. Liu, Z-B. Wang, Y. Zhang, F-D. Yu, Y. Xue, K. Ke, F-F. Li, Preparation of submicrocrystal LiMn2O4 used Mn3O4 as precursor and its electrochemical performance for lithium ion battery, Journal of Alloys and Compounds (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.11.004

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Preparation of submicrocrystal LiMn2O4 used Mn3O4 as precursor and its electrochemical performance for lithium ion battery Bao-Sheng Liu 1, Zhen-Bo Wang 1*, Yin Zhang 1, Fu-Da Yu 1, Yuan Xue 1, Ke Ke1, 2* Fang-Fei Li 3 1

School of Chemical Engineering and Technology, Harbin Institute of Technology, No. 92 West-Da Zhi Street, Harbin, 150001 China

2

Chilwee Power Co. Ltd., No. 12 Zhizhou Road, Xinxing Industrial Park, Zhicheng, Changxing, Zhejiang Province 313100, China

3

School of Electrical Engineering and Automation, Harbin Institute of Technology, Harbin 150001, China * Corresponding author. Tel.: +86-451-86417853; Fax: +86-451-86418616 E-mail: [email protected] (Z.B. Wang)

Abstract: Spinel LiMn2O4 has been synthesized by solid state reaction with industrial grade Mn3O4 and Li2CO3 as precursors without purification, and its electrochemical performance for lithium ion battery has been investigated by CR2025 coin cell. The results of X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) images show that the size of LiMn2O4 particles grow up with increasing temperature of calcination, and the sample synthesized at 800 oC for 12 hrs has the best crystallinity with a submicron size. It can deliver initial capacity of 112.9 mAh/g with capacity retention ratio of 89.1 % after 200 cycles at charge/discharge rate of 1 C. The results of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) also show that it has the highest electrochemical activity and lowest charge transfer impedance.

1

Keywords: LiMn2O4; Submicrocrystal; Mn3O4; Industrial grade; Cation mixing

1. Introduction The use of LiMn2O4 as an electrode for rechargeable lithium-ion battery was proposed more than 20 years ago [1]. Compared to layered oxides, spinel LiMn2O4 with a three dimensional framework structures has been becoming a superb candidate as cathode material of LIB, which can be applied in high power applications, such as hybrid electric vehicles (HEVs) and power tools. This kind of lithium-ion battery using LiMn2O4 as cathode material has the advantages of low cost, environmently friendly and good safety properties [2–6]. In recent years, layered materials, such as lithium nickel manganese cobalt oxide (NCM) and lithium nickel and cobalt aluminate (NCA) have been developing rapidly, and the blended cathode material composed of these layer materials and spinel LiMn2O4 or core-shell structured materials reveal fascinating performance [7-10]. The electrochemical performance of LiMn2O4 is highly sensitive to the physical properties [11–18] and the purity [19] of the raw materials, as well as the preparation process [20]. Electrolytic manganese dioxide (EMD) has been widely used as manganese compound precursors for synthesis of LiMn2O4 powders. However, EMD usually has disordered appearance and some amount of impurity such as Na+ and SO42-, which cause the irreversible capacity of LiMn2O4 during the storage [21, 22]. Comparing to MnO2, Mn3O4 has a higher utilization rate of Mn atom and a similar price. To sum up, Mn3O4 is more suitable for industry. Tang’s group [23] had studied the effect of different Mn sources on the performance of spinel LiMn2O4. According to his work, LiMn2O4 synthesized from Mn3O4 has better cycle efficiency and cycle performance. But the same synthetic process was used for those different Mn sources as raw materials. Zhang [24] et al had optimized the synthetic process of LiMn2O4 from Mn3O4 and LiOH· H2O as precursors, but LiOH is much more expensive and corrosive than Li2CO3. Park [25] et al had prepared Mn3O4 nanoparticles by a simple sonochemical method with a cubic morphology, and

2

LiMn2O4 nanoparticles were prepared on this basis. But the electrochemical performance of the prepared LiMn2O4 nanoparticles had not been studied in his work. Jiang et al [26] had synthesized helical spherical Mn3O4 by the controlled crystallization method. Then, micro-spherical particle of LiMn2O4 was synthesized at different temperatures using the as-prepared helical spherical Mn3O4 and Li2CO3. The purity of as-prepared Mn3O4 reaches 99.5% using analytical grade raw material and controlled crystallization method which is less effective and more expensive for industry production. In order to investigate an optimal synthetic process, LiMn2O4 was prepared via solid reaction with Mn3O4 and Li2CO3 as precursors without purification. After that the structure, morphology and composition of LiMn2O4 were obtained by X-ray diffraction (XRD) patterns, scanning electron microscope (SEM) and inductively coupled plasma mass spectrometry (ICP-MS), respectively. Furthermore, the initial discharge curves, cyclic tests, rate capability and cyclic voltammetry measurements were taken to investigate the electrochemical performance of different process.

2. Experimental 2.1. Synthesis For preparing LiMn2O4, the mixture of as-prepared Mn3O4 and Li2CO3 (the mole ratio of Li/Mn = 1.05:2) was mixed with alcohol in the ball mill at a speed of 400 r/min for 6 hrs, then dried at 55oC for 8 hrs. Then the mixtures were annealed at 550oC for 6 hrs and sintering at 750, 800, 850, 900 oC, respectively, for 12 hrs and then natural cooling to room temperature under oxygen atmosphere through the whole process [27-29]. Finally a black powder (LiMn2O4) was obtained. Pristine Mn3O4 and Li2CO3 come from industrial manufactures, which mean lower price and less purity than analytical reagent. 2.2. Physical characterization In order to identify the crystal structure and the purity of the samples, X-ray diffraction

3

(XRD) analysis was performed by using a XRD 6100 (Shimadzu) instrument with Cu Kα radiation, and the scan speed is 2 degree/min from 10° to 90°. Scanning electronic microscopy (SEM) was performed by using a SU8000 (Hitachi) instrument to detect the surface morphology and analyze the size of the particles. The element composition of raw materials and synthesized LiMn2O4 were analyzed by inductively coupled plasma mass spectrometry (ICP-MS, PerkinElmer, Optima 5300DV) tests. 2.3. Electrochemical characterization The electrochemical properties of LiMn2O4 samples were tested in CR2025 coin type cells. A composite positive electrode was built by mixing the LiMn2O4 powders, Super P and PVDF binder in a weight ratio of 80:15:5. These slurries were then cast onto an Al foil current collector and dried at 120oC for 10 hrs in a vacuum oven. Circular discs with a diameter of 14 mm were punched out as the positive electrodes. A typical disk electrode contains 8 mg of active material with a thickness of 80 µm. A disk of lithium foil was used as the negative electrode. A piece of Celgard 2400 membrane was used as a separator. 1 mol/L LiPF6 in EC-DMC-EMC (1:1:1, volume ratio) was applied as the electrolyte. The test cells were assembled in a glove box with an excellent environment control, the concentrations of both H2O and O2 were below 1 ppm. And then the cells were standing for 8 hrs for the sake of activation. Charge–discharge performance, CV and EIS of the cells were conducted on a battery test system (Neware CT 3008, Neware Co. China) between 3.20~4.25V.

3. Results and discussion 3.1 Optimization of sintering temperature 3.1.1 Morphology and structure of the materials Fig.1 presents the XRD patterns of the final products obtained at four different temperatures of 750 oC, 800 oC, 850 oC and 900oC for 12 hrs, which were marked as LMO750, LMO800, LMO850 and LMO900, respectively. And the crystal cell parameters calculated

4

from the XRD data are given in Table 1.

Fig.1 Table 1

The XRD patterns as shown in Fig.1 match well with standard data of spinel LiMn2O4 (JPDS NO.88-1030). Those indicate that all samples are identified as a single-phase spinel with a space group Fd-3m, in which the lithium ions occupy the tetrahedral (8a) sites while the manganese ions reside on the octahedral (16d) sites [30]. No peaks from other phase are observed, indicating the high purity of the LiMn2O4. The strong and sharp reflection peaks suggest that the as-prepared LiMn2O4 was well crystallized. The I(311)/I(400) peak intensity ratio (R) is also marked in Fig.1, which reflects the degree of tetragonal distortion from the cubic spinel structure [31,32]. The (311)/(400) peak intensity ratio (R) of LMO800 is 0.881. Its value is lower than that of other samples, indicating that the material LMO800 has the smallest distortion degree. The cation mixing of Li+/Mn3+ is usually characterized by the (220) peak intensity. The smaller peak intensity means smaller cation mixing [33,34]. The (220) peaks could not be found in the XRD patterns of four samples in Fig.1, indicating that there are slightly cation mixing in all of the four samples. Thus, it can be expected that the LiMn2O4 material synthesized at 800 oC should exhibit a good electrochemical performance. The cell parameter is calculated by a least squares method using eight diffraction lines (as shown in Fig. 1) and is listed in Table 1. It can be seen that the volume of LiMn2O4 unit cell increases while the temperature rising below 800 oC and then decreases, all of which are smaller than that of standard PDF data. This may be caused by tetragonal distortion and cation mixing. The morphology of Mn3O4 and LiMn2O4 powders synthesized at different temperatures is observed by scanning electron microscopy. The results are shown in Fig.2. It can be seen 5

that Mn3O4 has a spherical-like shape and LiMn2O4 powders have typical cubic spinel structure and the estimated particle size is about 0.5~1.0 µm in diameter in the range of submicrocrystal. Obviously, the particles of Mn3 O4 broke up through the synthetic process and the spherical-like shape didn’t remained. There are little differences among 750oC, 800 oC and 850oC on particle size. But the size of LiMn2O4 particles grows up and the boundary of crystalline becomes indistinct while the temperature rose to 900oC, which may be caused by oversintering. Fig. 2

Generally, the smaller tetragonal distortion degree, cation mixing and most similar cell volume to the standard data it has, the better electrochemical performance it exhibits. Although the sample prepared at 900oC has a smaller distortion degree, the biggest particle size, which hinders the diffusion of Li+, may be the main reason of the worst electrochemical performance. According to the results of inductively coupled plasma mass spectrometry (ICP-MS) test (Table 2), the main impurities of the raw materials and LiMn2O4 are Na+, Ca2+, Mg2+ and Fe3+. It can be seen that most of the impurities in LiMn2O4 come from Mn3O4.These ions would spread through electrolyte into anode electrode during charge-discharge process, and then may be reduced to metal atoms and deposited on the surface of anode electrode due to their lower reduction potential than Li+[35], which would cause the decrease of capacity and cyclic performance of lithium-ion battery.

Table 2

3.1.2. Electrochemical characters In order to examine the electrochemical reactivity and cyclic stability of LiMn2O4 6

prepared here, charge-discharge tests between 3.20 V and 4.25 V are carried out at a specific discharge rate of 0.2C at room temperature. The following cyclic tests and rate capability tests are demonstrated by the same coin cell. As is shown in Fig.3, the initial discharge curves of different sintering temperatures have two plateaus around 4.10V and 3.95V. With the increasing of sintering temperature, the initial discharge capacity of sample increases first and then decreases. The LiMn2O4 prepared at 800 oC presents the highest capacity of 112.9 mAh/g, which results from its higher crystallized microstructure and less cation defect. And the particle sizes of samples rise while the temperature increasing, which hinder the diffusion of lithium-ions during the charge-discharge process and lead to their initial discharge capacities decrease. This is a little lower than 122.8 mAh/g reported by Jiang [26] et al in whose work the analytical grade raw material were used.

Fig.3

There are two lower plateaus while LiMn2O4 discharge at 1C current comparing to that at 0.2C, which could be seen in Fig.4(a), it releases less charge at the first plateaus. After 200 cycles at 1C current, the capacity retention ratios of LiMn2O4 preapred at 750oC, 800 oC, 850 oC and 900 oC are 91.5%, 89.1%, 80.8%, and 83.1%, respectively, which is revealed in Fig.4 (b).

Fig.4

In the present study, rate capability is investigated at different C-rates (current densities). For each 10 cycle average, the charge and discharge process are performed at the same C-rate between 4.25 -3.2 V at room temperature (Fig. 5). The variation rule goes the same with the initial discharge capacity above. That is, with the increasing of temperature, the rate capability 7

of LiMn2O4 increases firstly and then decreases. Between 4.25 and 3.2 V, LMO800 has the discharge capacity of 108.6, 105.3, 100.6 and 92.5 mAh/g at 0.1, 0.2, 0.5 and 1C, respectively. The results indicate that LiMn2O4 sintered at 800oC has the best rate capability, while LMO900 has the worst one.

Fig. 5

Cyclic voltammetry tests on LiMn2O4 electrode prepared at different temperatures taken at a scanning rate of 0.1 mV/s between 3.20-4.25V are displayed in Fig.6.

Fig. 6

The cathodic peaks at 3.92V and 4.04V correspond to Li+ intercalation into the oxide (as shown equation (1)), while the anodic peaks at 4.09V and 4.21V correspond to Li+ deintercalation from the oxide (as shown equation (2))[36,37]. The area cathodic peaks covered roughly equal with that of anode dose, which indicate the reaction carried out reversibly. LMO800 electrode presents the highest peaks, which means the highest reactivity. LiMn2O4 ↔ Li1-xMn2O4 + x Li+ + x e-

(x<0.5)

(1)

Li1-xMn2O4 ↔ Mn2O4 + (1-x) Li+ + (1-x) e-

(x≥0.5)

(2)

In summary, LMO800 sample shows the best electrochemical performance so that 800 oC is chosen to be the optimal sintering temperature. 3.2 Optimization of sintering time The influence of sintering time is studied in this section by the tests of LiMn2O4 electrode sintered for 8, 10, 12 and 14 hrs. XRD and electrochemical measurement are carried out to investigate the optimal sintering time. 3.2.1 Structure of the homemade materials 8

The XRD patterns of LiMn2O4 samples prepared for different sintering times are also in good agreement with the reported data of cubic spinel LiMn2O4 (JPDS NO.88-1030). No peaks from other phases are observed, indicating the high purity of the LiMn2O4. For XRD patterns, the sharper peak is, the higher degree of crystallinity is. As shown in the Fig.7, the peaks of LMO12h’s pattern are much sharper than those of other three samples, and reveal a lower (311)/(400) peak intensity ratio of 0.881, which signify the smallest distortion degree of the cubic spinel structure.

Fig.7

The cell parameter is calculated by a Least Squares Method using eight diffraction lines (as shown in Fig. 7) and is listed in Table 3. It can be seen that the volume of LiMn2O4 crystal cell increases with the sintering time when reaction time is less than 12 hrs, and then decreases in 14 hrs, all of which are smaller than that of standard PDF data.

Table 3

3.2.2. Electrochemical characters In this section, the same electrochemical measurements with 3.1.2 are presented under the same testing conditions. There is a same law among following tests, which is, with the increasing of sintering time, the electrochemical performance of sample increasing firstly and then decreasing. For instance, the initial discharge capacity and cycling performance of sample increase first and then decrease. This may be attributed to the degree of crystallinity and cation mixing. Their initial discharge capacities are 98.7, 90.7, 112.9 and 98.7 mAh/g at 0.2C for LMO8h, LMO10h, LMO12h and LMO14h as shown in Fig.8(a). It can be seen from cyclic 9

life curves (Fig.8 (b)) that the capacity retention ratios of LMO8h, LMO10h, LMO12h, LMO14h are 91.5%, 91.2%, 89.1% and 98.0%. This is better than Kim’s[38] work. Kim has prepared the single-crystalline MnO2 nanorods by hydrothermal synthesis method and their chemical conversion into free-standing single-crystalline LiMn2O4 nanorods using a simple solid-state reaction. About 85% of the initial charge storage capacity at high power rates is maintained for over 100 cycles. Rate capability tests in Fig.9 shows LMO12h owns higher capacity than those of others at all discharge rates. Their capacities decrease, while the discharge current increase.

Fig.8 Fig. 9

Cyclic voltammetry on LiMn2O4 prepared for different sintering times are shown in Fig. 10. The cathodic/anodic peaks of LMO12h are higher and sharper than others, which mean LMO12h has a higher electrochemical activity.

Fig.10

Electrochemical impedance spectroscopy is conducted by ac perturbation signal of ±5 mV, and the frequency range is from 105 to 10 -1 Hz(Fig. 11). Their results have been normalized. The electrochemical impedance spectroscopy (EIS) may be considered as one of the most sensitive tools for the study of differences in the electrode behavior due to surface modification. The EIS curves had been fitting by software and the equivalent circuit was given in Fig.11. Fig. 11

10

The radius of the semicircle in EIS represents charge transfer impedance while the slope of the following oblique line represents diffusion impedance. Smaller radius means smaller charge transfer impedance. The parameters of the equivalent circuit are presented in Table 4. The standard deviation(Chi-Squared value in Table 4) of four samples are all under 10-3, which means the fitting results are credible. It can be seen that LMO12h has a smallest Rct value (charge-transfer resistance) of 49.1 Ω, which means LMO12h has the best electrochemistry activity. Also, the diffusion coefficient values of the lithium ions in the bulk electrode materials can be calculated by using some equations [39].

Table 4

4. Conclusions Spinel LiMn2O4 was synthesized by a typical solid reaction with industrial grade Mn3O4 and Li2CO3 as precursors without purification in the box furnace under air atmosphere, and the synthesis conditions including sintering temperature and time were studied. The as-prepared Mn3O4 and Li2CO3 sintered at 800 oC for 12 hrs has the most regular morphology, the highest degree of crystallinity and the best electrochemical performance. Its initial discharge capacity is 112.9 mAh/g and retains 95.2 mAh/g after 200 cycles at 1C current. Acknowledgment We acknowledge the National Natural Science Foundation of China (Grant No. 21273058),

China

postdoctoral

science

foundation

(Grant

No.2012M520731

and

2014T70350), Heilongjiang postdoctoral financial assistance (LBH-Z12089) for their financial support.

References 11

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List of figure and table captions Fig.1 XRD patterns of LiMn2O4 prepared at different temperatures Fig. 2 SEM micrographs of LiMn2O4 samples: (a) 750 oC (b) 800 oC (c) 850oC (d) 900oC and Mn3O4 Fig.3 Initial discharge curves of LiMn2O4 synthesized from different temperatures at 0.2C Fig. 4 Initial discharge curves at 1C (a) and cycling test (b) of LiMn2O4 synthesized at different temperatures Fig.5 Rate capability of LiMn2O4 synthesized at different temperatures Fig.6 Cyclic voltammetry test of LiMn2O4 synthesized at different temperatures Fig.7 XRD patterns of LiMn2O4 sintered for different times Fig. 8 Initial discharge curves (a) and cycling test (b) of LiMn2O4 synthesized for different times Fig.9 Rate capability of LiMn2O4 synthesized at different times Fig. 10 Cyclic voltammetry of LiMn2O4 synthesized for different times Fig. 11 Electrochemical impedance spectroscopy of LiMn2O4 synthesized for different times

Table 1 Crystal cell parameters of LiMn2O4 samples prepared at different temperatures Table 2 Element analysis of the raw materials and LiMn2O4 Table 3 Crystal cell parameters of LiMn2O4 samples for different sintering times Table 4 Parameters of the equivalent circuit

15

440 531

331

Intensity (a.u.)

511

400 222

)deg(raht

311

ytnieIs

2t0 4

45031

u).a1(23

111

Fig 1

o

900 C R=0.893 o

850 C R=0.965 o

800 C R=0.881 o

750 C R=1.089 20

40

60

2 theta (degree) 16

80

Fig. 2

Mn3O4

17

Fig. 3

4.2

4.10V 3.95V

Voltage (V)

4.0 3.8 o

3.6

900 C

3.4

750 C

o

800 C

o

o

850 C

3.2 0

20

40

60

80

100

Specific Capacity (mAh/g) 18

120

140

Fig. 4

120 4.2

Capacity (mAh/g)

4.0 Voltage (V)

100

o

800 C @ 0.2C

3.8 o

o

800 C

750 C

3.6

o

850 C

o

3.4

900 C

o

800oC 750 C

80

o

850 C o

900 C

60 40

b

a

3.2 0

20

40

60

80

100

120

140

Specific Capacity (mAh/g)

19

20

0

50

100 150 Cycle Number

200

Fig. 5

0.1C

Specific Capacity ( mAh/g)

120

0.1C

0.2C

0.5C 1C

100 80 60 750 800 850 900

40 20 0

0

10

20

30

Cycle Number 20

40

50

Fig. 6

0.20 0.15 Current ( mAh/g)

4.21V

750 800 850 900

0.10

4.09V

0.05 0.00 -0.05 -0.10 4.04V

-0.15 -0.20

3.92V 3.2

3.4

3.6

3.8

Voltage( V ) 21

4.0

4.2

531

440

511

400 331

222

311

111

Fig. 7

14 h R=0.984

Intensity ( a.u.)

12 h R=0.881 10 h R=0.932 8h R=0.896 20

40

60

2 theta (degree) 22

80

Fig. 8

120

Specific Capacity ( mAh/g )

4.2

Voltage (V)

4.0 3.8

8h 14h

3.6 3.4

a

3.2 0

10h 20

40

60

80

12 h 100

100 80 60 40 20

120

Specific Capacity ( mAh/g )

12h 10h 14h 8h

b 0

50

100

150

Cycle Number

23

200

Fig. 9

Capacity ( mAh/g )

120

100 0.1C

0.2C

80

1C

14h 12h 10h 8h

60

40

0.1C

0.5C

0

10

20

30

Cycle Number 24

40

50

Fig. 10

0.20

14h 12h 10h 8h

0.15 0.10 Current (A/g)

4.19V 4.08V

0.05 0.00 -0.05 -0.10 3.94V

-0.15 -0.20

3.2

3.4

3.6

3.8 Voltage (V) 25

4.07V 4.0

4.2

4.4

Fig. 11

100

-Z'' / ohm

80

14h 12h

60 40

8h 20 10h 0

0

20

40

60

Z' / ohm 26

80

100

Table 1

Temperature / oC

a×10 / nm

V ×103/ nm3

750

8.2162

554.6

800

8.2278

557.0

850

8.2229

556.0

900

8.2205

555.5

standard

8.2363

558.7

27

Table 2

Na+ / %

Mg2+ / %

Ca2+ / %

Fe3+ / %

Li2CO3

0.0160

<0.0080

0.0020

<0.0010

Mn3O4

0.0175

0.0055

0.0115

0.0024

LiMn2O4

0.0079

0.0080

0.0207

0.0170

28

Table 3

Time / h 8 10 12 14 standard

a ×10 / nm

V ×103/ nm3

8.1970

550.8

8.1933

550.0

8.2278

557.0

8.1940

550.2

8.2363

558.7

29

Table 4

Re / Ω

Rct / Ω

Chi-Squared / 10-4

LMO8h

4.5

75.2

7.29

LMO10h

6.3

75.9

8.26

LMO12h

7.0

49.1

8.78

LMO14h

6.3

63.3

8.41

30

Contents entry

31

Spinal LiMn2O4 particles synthesized at 800 o C for 12 h has the best crystallinity with a submicron size and smallest cation disorder, resulting in a superior capacity retention ratio of 90.4% after 200 cycles at 1C at room temperature, which possesses an initial capacity of 106.8 mAh/g.

32

Highlights
High purity spinel LiMn2O4 was synthesized from industrial grade raw materials.
LiMn2O4 prepared by optimal conditions has the smallest cation mixing.
Optimized LiMn2O4 has the highest initial capacity with 112.9 mAh/g
Capacity retention of optimized LiMn2O4 is 90.4% after 200 cycles at 1 C.

33