Synthesis and electrochemical characterizations of Nano-SiO2-coated LiMn2O4 cathode materials for rechargeable lithium batteries

Synthesis and electrochemical characterizations of Nano-SiO2-coated LiMn2O4 cathode materials for rechargeable lithium batteries

Journal of Electroanalytical Chemistry 624 (2008) 197–204 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal h...

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Journal of Electroanalytical Chemistry 624 (2008) 197–204

Contents lists available at ScienceDirect

Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem

Synthesis and electrochemical characterizations of Nano-SiO2-coated LiMn2O4 cathode materials for rechargeable lithium batteries D. Arumugam, G. Paruthimal Kalaignan * Advanced Lithium Battery Research Lab, Department of Industrial Chemistry, Alagappa University, Karaikudi 630 003, Tamil Nadu, India

a r t i c l e

i n f o

Article history: Received 25 May 2008 Received in revised form 10 September 2008 Accepted 15 September 2008 Available online 21 September 2008 Keywords: Cathode materials SiO2-coated LiMn2O4 XRD TEM Electrochemical performances Mn-ions dissolution

a b s t r a c t LiMn2O4 spinel cathode materials were coated with 1.0, 2.0 and 3.0 wt.% of SiO2 by polymeric process, followed by 850 °C for 6h in air. The surface coated LiMn2O4 cathode materials were physically characterized using X-ray diffraction, Scanning electron microscopy, Transmission electron microscopy and X-ray photon spectroscopy. XRD patterns of SiO2-coated LiMn2O4 revealed that the coating did not affect the crystal structure, space group Fd3m of the cathode materials, compared to the uncoated LiMn2O4. The surface morphology and particle agglomeration were investigated using Scanning electron microscopy, and the TEM image showed a compact coating layer on the surface of the core materials that had average thickness of about 50 nm. XPS data illustrated that the SiO2 was coated over surface on the LiMn2O4 core materials. The galvanostatic charge and discharge of the uncoated and SiO2-coated LiMn2O4 cathode materials were carried out 0.1 mA/g in the range of 3.0 and 4.5 V at 30 °C and 60 °C. The discharge capacity of 2.0 wt.% of SiO2-coated LiMn2O4 (120 mAh/g) showed only 4.8% loss of the initial capacity (126 mAh/g) over the 100 cycles at 30 °C and (112 mAh/g) showed only 8.9% of loss of the initial capacity (123 mAh/g) over the 100 cycles at 60 °C. The cycleability improvement of the spinel LiMn2O4 coated with 2.0 wt.% of SiO2 is demonstrated at room temperature and elevated temperature. From the analysis of electrochemical impedance spectroscopy (EIS), the improvement of cycleability may be attributed to the suppression on the formation of the passive film and reduction of Mn dissolution, which results from modifying the surface of the spinel LiMn2O4 with SiO2. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Rechargeable lithium ion batteries have been enjoying a significant commercial success as the most promising portable energy source in electronic products such as lap top computers, calculator and cellular phones mainly by their high working voltages, high energy density, long life etc. Currently, LiCoO2, LiNiO2 and LiMn2O4 are the main cathode materials for rechargeable lithium ion batteries. Among these, spinel LiMn2O4 and its derivatives as the most potential materials were ascribed to its merits of easy preparation, inexpensiveness, more abundance of manganese resources, non toxicity and environmental friendly nature [1–4]. However, LiMn2O4 electrodes in the 4V (versus Li/Li+) region suffer from capacity fading, especially at elevated temperature (50–60 °C). The capacity loss has been ascribed to several factors such as (i) Jahn–Teller distortion due to Mn3+ ions, (ii) the dissolution of manganese ions into the electrolyte, (iii) loss of crystallinity during cycling and (iv) electrolyte decomposition at the high potential regions [5–8]. In order to overcome this capacity fading problem, two kinds of methods

* Corresponding author. Tel.: +91 4565 228836; fax: +91 4565 225202; mob.: +91 94431 35307. E-mail address: [email protected] (G. Paruthimal Kalaignan). 0022-0728/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2008.09.007

can be employed. One way is substitution of heterogeneous atom into the host LiMn2O4 structure of cathode materials and other one is surface modification. Several research groups have attempted to stabilize the structure of LiMn2O4 powders during cycling by substituting a small fraction of the manganese ions with several divalent or trivalent metal ions. There was improvement of cycle performance at room temperature by partial substitution of transition metal instead of Mn in LiMn2O4 [8–14], where as methods resulted in LiMn2O4 was still suffered from significant capacity decline at elevated temperature (50–60 °C). Therefore, a different approach has been reported, which involves modifying the surface of the cathode materials by coating it with electrochemically inactive metal oxides or ceramic oxide materials. Amatacci et al. [15] first reported, the surface treatment of LiMn2O4 with lithium boron oxide (LBO) was an attractive way to improve the electrochemical properties of LiMn2O4. Al2O3 [16– 18], ZnO [19–21], TiO2 [22,23], AlPO4 [24–28], CeO2 [29,30], MgO [31], and LiAlO2 [32] coated to core materials enhanced the cycling behaviour of the spinel LiMn2O4 cathode materials. In this work, we have studied the electrochemical performances of the cells, when charged at 4.5 V for the surface coated LiMn2O4 cathode materials at elevated temperature with 1 wt.% of SiO2 by a polymeric process.

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2. Experimental 2.1. Preparation of spinel LiMn2O4 LiMn2O4 powder was prepared by the sol–gel method, Li(OCOCH3)  2H2O (AR 99.99% pure) and Mn(OCOCH3)2  4H2O (AR 99.98% pure) were dissolved using distilled water in the mole ratio of Li:Mn (1:2) and added to an aqueous solution of 1 M citric acid under vigorous stirring and the pH value of the solution was adjusted to 6–7 using aqueous ammonia. The resulted solution was evaporated at 80 °C until a transparent gel was obtained. Finally, the gel precursor was decomposed at 300 °C for 8 h, followed by calcination at 850 °C for 15 h to get LiMn2O4 powder. 2.2. Synthesis of SiO2-coated LiMn2O4 SiO2-coated LiMn2O4 cathode materials were synthesized by polymeric process at the calculation of 1.0, 2.0, and 3.0 wt.% by using silicic acid as the coating of raw materials. The 2 g of the LiMn2O4 powder was dispersed in distilled water by 3 h stirring. At the time, when SiO2 was calculated at 1.0, 2.0 and 3.0 wt.% by using silicic acid, 5 ml of polyvinyl alcohol were mixed in warm triple distilled water and added drop wise to the dispersed LiMn2O4 solution. The mixture was stirred for 5 h at room temperature and heated at 60 °C for 10 h of continuous stirring. Then removed, the excess water, a thick polymer gel was obtained. The obtained gel precursor was dried in an air oven at 110 °C for 12 h to form a fine powder, which was calcined at 850 °C for 6 h to form LiMn2O4 coated with a thin layer of SiO2 in the weight ratios of 99:1, 98:2 and 97:3, respectively. Fig. 1 shows the Flow chart for the synthesis of SiO2-coated LiMn2O4 cathode materials. 2.3. Physical characterizations Structural analysis was carried out by using a powder X-ray diffraction (XRD), Siemens D-5000, Mac Science MXP 18 was equipped with a nickel filtered Cu Ka radiation sources (k = 1.5405 Å). The diffraction patterns were recorded between scattering angles of 10° and 80° in step of 0.1°/min. The surface

LiMn2O4 + Distilled water

3h stirred

Dispersed LiMn2O4

3h stirred after that add drop wise

Silicic acid with 5 ml of Polyvinyl alcohol in Triple distilled water

Stirred for 10h at 60°C

Viscous gel

morphology of the coated materials was studied using Scanning electron microscopy (SEM), Hitachi model S-35000V. The coated layer particles morphology was examined by a JEOL JEM– 200FFX11 transmission electron microscopy (TEM). TEM samples were prepared by dispersing the cathode materials in ethanol, placing the drop of the clear solution on a carbon coated copper grid and subsequent drying. The depth profiles of Si and Mn were recorded from the X-ray photon spectroscopy (XPS) using an electron spectroscopy for chemical analysis (ESCA) instruments (VG Scientific ESCALAB 253) with monochromatic Al Ka (1486.8 eV) at room temperature. Spectra for SiO2 coated LiMn2O4 samples was prepared as pellets by mixing the coated LiMn2O4 with 10% of acetylene black to improve the conductivity and mitigate charging effects. XPS spectra were recorded in the range from 0 to 1400 eV of binding energy (BE) with constant pass energy mode at 5.85 eV and the calibration of XPS depth profile in the range of surface to 50 nm distance within particle. 2.4. Electrochemical characterizations The cathode electrodes studies were prepared by a doctor blade coating method with a slurry of 85 wt.% coated active materials (0.85 g), 10 wt.% of conductive acetylene black and 5 wt.% of PVDF binder, in N-methyl-2-pyrrolidone (NMP) solvent. This mixture was then applied onto an etched aluminium foil current collector and dried at 110 °C for 12 h in a vacuum oven. The coated cathode foil was then pressed and then cut into circular discs of 20 mm in diameter. The button cells were assembled using 2023 stainless steel coin type containers in an argon-filled glove box, in which oxygen and H2O contents were maintained below 2 ppm. Lithium foils were used as the anode and reference electrode and 1 M LiPF6 with 1:1 ratio of ethylene carbonate and dimethyl carbonate (EC:DMC) was used as the electrolyte and a thin polypropylene film acted as the separator. The charge–discharge cycles for assembled cells were performed using WPG100 Potentiostate/Galvanostate cycle life tester with current density 0.1 mA/g in the range of 4.5 and 3.0 V at room temperature and elevated temperature (30 °C and 60 °C). The cyclic voltammogram (CV) experiments were carried out at a scan rate of 100 lV/s between 3.0 and 4.5 V using Auto Lab Modular Electrochemical Instruments (BST 7249). Impedance measurements were done for fully charged coin cells (4.5 V). The impedance spectra were recorded using a schlumber 610 electrochemical interface and frequency response analyzer. The frequency range was 0.001 Hz to 60 kHz and the amplitude of the perturbation signal was 30 mV. The dissolution behaviour of transition metal ions for uncoated and 2.0 wt.% of SiO2 coated LiMn2O4 were investigated by being immersed into electrolyte consisting of 1 M LiPF6 with 1:1 ratio of ethylene carbonate and dimethyl carbonate (EC:DMC) at room temperature and elevated temperature. The 0.2 g weight of the cathode powder was immersed in 20 ml of the electrolyte. The concentration of the dissolved transition metal ions were determined using atomic absorption spectroscopy (AAS).

Dried in air oven at 110°C for 12h

3. Results and discussion Fine powder

3.1. X-ray diffraction analysis Heated at 850°C for 6h Nano SiO2-coated LiMn2O4 powder Fig. 1. Flow chart for the SiO2-coated spinel LiMn2O4 cathode materials prepared by a polymeric process.

Fig. 2 shows the XRD patterns for both the uncoated and SiO2coated (1.0, 2.0 and 3.0 wt.%) LiMn2O4 powders. All the powders were well defined spinel structure with space group of Fd3m, in which the lithium ions occupy the tetrahedral 8a site, Mn3+ and Mn4+ ions reside at the octahedral 16d site and O2 ions are located at the 32e site [33]. The presence of crystalline SiO2 was not

D. Arumugam, G. Paruthimal Kalaignan / Journal of Electroanalytical Chemistry 624 (2008) 197–204

detected by XRD. In addition, the SiO2 coating has not changed the 2h value of the peaks. Earlier reports have showed that, substitution of transition metal ions form Mn3+ in LiMn2O4 significant changes in lattice parameter [29–31]. In this case, significant changes were not obtained. This phenomenon indicates that the mechanism of coating is different from doping. These results revealed that SiO2 is just coated on the surface of the LiMn2O4 powders.

3% SiO2 coated LiMn2O4

3.2. Surface morphology and particle size analysis

Intensity (a.u)

2% SiO2 coated LiMn2O4

Fig. 3a and b show the SEM images of uncoated and 2.0 wt.% SiO2-coated LiMn2O4, respectively. It was observed that the surface of the cathode particles was distinctly changed upon coating. The brightness of the surface increased in the coated samples compared to the pristine sample. It is associated with the accumulation of charge on the non-conducting coating materials (SiO2) as the electron beam impinges on it. Fig. 4a and b present the Transmission electron micrographs of the uncoated and 2.0 wt.% of SiO2 coated LiMn2O4 cathode materials, respectively. It indicates that uniform SiO2 coating was formed over the pristine LiMn2O4 particles with a coating thickness around 50 nm.

1% SiO2 coated LiMn2O4

Uncoated LiMn2O4

20

30

40

50

60

70

199

80

2 theta (Degree) Fig. 2. X-ray diffraction patterns of uncoated and various wt.% of SiO2-coated spinel LiMn2O4 cathode materials synthesized at 850 °C.

3.3. X-ray photon spectroscopy analysis XPS has been used extensively to study the surface composition and to determine whether SiO2 coating remained on the surface of the core materials. Fig. 5a–c show the XPS spectra

Fig. 3. SEM images of (a) uncoated and (b) 2.0 wt.% of SiO2-coated spinel LiMn2O4 cathode materials synthesized at 850 °C.

Fig. 4. TEM images of (a) uncoated and (b) 2.0 wt.% of SiO2-coated spinel LiMn2O4 cathode materials synthesized at 850 °C.

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of O 1s, Si 2p and Mn 2p at the surface depth of 0 and 50 nm, respectively, for SiO2-coated LiMn2O4. Fig. 5a represents the XPS profile at the surface and 50 nm depth for O 1s peak centered at 533 eV, which corresponds to O 1s bonded with Si [34]. At 50 nm depth profile of O 1s has very low peak intensity compared to the surface, which reveals that the coated materials mainly remained on the surface. Fig. 5b shows the characteristic binding energy of Si 2p around 103 eV at the surface level, but at the depth of 50 nm, the same peak was found to be very shallow. The differences in peak intensity indicate that the SiO2 remained on the surface of the core materials. As the depth increased, it is clearly noticeable that there was a decrease in SiO2 concentration. Therefore, it could be concluded that there was no influence on the chemical state or binding energy of the different ions in the SiO2-coated LiMn2O4 samples. Also, both

O 1s and Si 2p binding energy demonstrated that the coated LiMn2O4 remained on the surface of the core materials and does not react to form any solid solution with the pristine materials which is also evident from XRD. Fig. 5c shows the XPS spectra of Mn 2p, the peak observed around 642.2 eV at the surface and 50 nm depth may be assigned to characterize the binding energy value of Mn 2p spectra in LiMn2O4 [35–37]. 50 nm depth profile has higher intensity peak compared to surface level. It reveals that LiMn2O4 core materials have covered by nano layer of SiO2. Fig. 5d shows the distribution of Mn and Si atomic concentrations in SiO2-coated LiMn2O4 with a depth profile of the particle up to 50 nm. The concentration of Mn increased to a depth of about 30 nm and then leveled off. The high atomic concentration of Si at the surface of the core material is reasonable due to the presence of

a

b

O 1s 50 nm depth Si 2p

Intensity (a.u)

Intensity (a.u)

533 eV

O 1s 533 eV

103 eV

50 nm depth Si 2p

103 eV

At surface

500

520

540

At surface

560

80

100

Binding energy (eV)

120

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160

180

Binding energy (eV) 100

d

c Mn 2p

Si Mn

80

Atomic concentration (%)

Intensity (a.u)

642.2 eV 50 nm depth

60

40

20

Mn 2p At surface

642.2 eV

0 600

620

640

660

Binding energy (eV)

680

700

0

10

20

30

40

Distance with in particle (nm)

Fig. 5. XPS spectra of (a) O 1s, (b) Si 2p, (c) Mn 2p and (d) depth profile of a 2.0 wt.% SiO2-coated LiMn2O4 particle.

50

60

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SiO2 content. The concentration of Si at 50 nm depth was low typically less than ten atomic percent compared to surface level. The atomic concentration of Si and Mn was observed at the surface around 39% and 0.14%, at the 50 nm depth around 3% and 52%, respectively. The atomic concentration (%) of Si was much higher at surface level compared to 50 nm depth, which depicts SiO2 layer was coated over the LiMn2O4 core materials. Beyond that, there was rapid decrease in the Si concentration with the depth of particle value, which approximately corresponds to the thickness of the compact layer observed with a TEM image.

plateaus indicated that the insertion and extraction of lithium ions occur in two states [38]. The first voltage plateau at about 4.01 V is attributed to the removal of lithium ions from half of the tetrahedral sites in which Li–Li interaction occurs. The second voltage plateau observed at around 4.21 V is ascribed to the removal of lithium ions from the remaining tetrahedral sites. In the initial charge–discharge curves, the pristine LiMn2O4 samples display slightly larger capacities than sample with SiO2 coatings at both 30 °C and 60 °C. This may be due to the little higher electrode impedance resulting from the SiO2 coatings.

3.4. Galvanostatic charge/discharge studies

3.5. Cycling performances

Fig. 6a and b show the typical charge-discharge curves of bare and 1.0, 2.0 and 3.0 wt.% of SiO2-coated spinel LiMn2O4 samples at a discharge rate of 0.5C between 3.0 and 4.5 V at 30 °C and 60 °C, respectively. It can be seen that the LiMn2O4 samples with and without the SiO2 coating have similar charge–discharge profiles, exhibiting two charge–discharge plateaus in the potential region of 4.0–4.3 V, which are ascribed to the remarkable characteristics of a well defined LiMn2O4 spinel and the voltage

Fig. 7a and b show the results of discharge cycling at 0.5C rate between 3.0 and 4.5 V for uncoated LiMn2O4, 1.0, 2.0 and 3.0 wt.% of SiO2 coated LiMn2O4 performed at 30 °C and 60 °C, respectively, up to 100 cycles. The first cycle discharge capacities,

a

Potential (V) Vs Li/Li

+

3.0 Wt.% 2.0 Wt.% 1.0 Wt.% uncoated LiMn2o4

4.5

4.0

3.5

Specific discharge capacity (mAh/g)

5.0

180

a

LiMn2O4 1% SiO2 2% SiO2 3% SiO2

160

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120

100

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60 uncoated LiMn2o4

3.0 3.0 Wt.%

0

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1.0 Wt.%

140

Specific discharge capacity (mAh/g)

+

Potential (V) Vs Li/Li

2.0 Wt.% 1.0 Wt.%

4.0

3.5 uncoated LiMn2o4 3.0 Wt.% 2.0 Wt.%

0

20

40

60

80

100

120

80

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180

uncoated LiMn2o4

3.0

60

160

b 3.0 Wt.%

40

Cycle number (n)

Specific capacity (mAh/g)

4.5

20

160

140

120

100

80

60

1.0 Wt.%

140

b

LiMn2O4 1% SiO2 2% SiO2 3% SiO2

160

Specific capacity (mAh/g) Fig. 6. Initial charge/discharge curves of uncoated LiMn2O4, 1.0 wt.%, 2.0 wt.% and 3.0 wt.% of SiO2-coated LiMn2O4 cathode materials cycled between the range of 3.0– 4.5 V; (a) room temperature (30 °C) and (b) elevated temperature (60 °C).

0

20

40

60

80

100

Cycle number (n) Fig. 7. Discharge capacities of uncoated LiMn2O4, 1.0 wt.%, 2.0 wt.% and 3.0 wt.% of SiO2-coated LiMn2O4 cathode materials between the range of 3.0–4.5 V at a rate of 0.5C; (a) room temperature (30 °C) and (b) elevated temperature (60 °C).

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Table 1 Electrochemical capacities and retention ratios of the lithium ion cells with SiO2-coated LiMn2O4 cathode materials Electrode compositions

Initial discharge capacity at 30 °C (mAh/g)

100th Discharge capacity at 30 °C (mAh/g)

Initial discharge capacity at 60 °C (mAh/g)

100th Discharge capacity at 60 °C (mAh/g)

Capacity retention at 30 °C (%)

Capacity retention at 60 °C (%)

Uncoated LiMn2O4 1% SiO2 coated LiMn2O4 2% SiO2 coated LiMn2O4 3% SiO2 coated LiMn2O4

135

81

125

62

60.0

49.6

132

110

123

95

83.3

77.2

126

120

123

112

95.2

91.1

113

89

109

80

78.8

73.4

100th cycle discharge capacities and capacity retention ratios of SiO2-coated and uncoated LiMn2O4 were summarized in Table 1. The initial discharge capacity of Li/electrolyte/LiMn2O4 is 135 mAh/g; it declines to 81 mAh/g after 100 cycles with capacity loss of 40% Fig. 7a. By contrast, among the SiO2-coated, the 2.0 wt% of SiO2 coated LiMn2O4 exhibits initial discharge capacity of 126 mAh/g, but after 100 cycles only 4.8% capacity loss was obtained and the discharge capacity still maintains at 120 mAh/g. This cycling behaviour of the SiO2-coated LiMn2O4 electrodes indicates the impact of SiO2 coating significantly which improved the electrochemical performances at room temperature (30 °C). The major issue with spinel LiMn2O4 materials was their poor electrochemical performances at elevated temperature owing to manganese ions dissolution into the electrolyte. Fig. 7b shows the results of electrochemical cycling at 0.5C rate between 3.0 and 4.5 V at 60 °C. It is clear that, surface modification has significantly improved the cycleability of LiMn2O4 even at elevated temperature (60 °C). For pristine LiMn2O4, the discharge capacity declined from 125 mAh/g to 62 mAh/g with a capacity loss of 50.4% after 100 cycles. But, the 2.0 wt.% of SiO2-coated LiMn2O4 provides initial discharge capacity of 123 mAh/g and remains at 112 mAh/g after 100 cycles, with capacity loss of 8.9%. On the other hand, 2.0 wt% of SiO2-coated LiMn2O4 shows the similar characteristics of two potential plateaus which were obtained at 3.9 and 4.2 V compared with uncoated electrode, this indicates that the SiO2 coating does not change the intrinsic property of LiMn2O4 during insertion and extraction of lithium ions. High percentage of SiO2 coated LiMn2O4 has lower capacity compared to uncoated and lower SiO2-coated LiMn2O4 because the addition of silica content replace the quantity of pristine LiMn2O4. Even coated LiMn2O4, Mn3+ only contribute the capacity during charge/discharge cycling. 2.0 wt.% SiO2-coated LiMn2O4 is the optimum composition to enhance the stability and cycling performances of the electrode.

3.7. Electrochemical impedance spectroscopy studies Fig. 9a and b have compared the Electrochemical impedance spectra of the uncoated and 2.0 wt% of SiO2-coated LiMn2O4 samples at a charge potential of 4.5 V as a function of cycle number. It is clear that, a high frequency semicircle represents the impedance due to a solid-state interface layer formed on the surface of the electrode and the diffusion effects of lithium ions on the interface between the active material particles and electrolyte, which is generally indicated by an inclined straight line at the low frequency end. The high frequency resistance for the cells charged at a potential of 4.5 V as a function of cycle number showed the resistance of the surface film on the cathode particles was 21 X after 10 cycles to 98 X after 100 cycles for uncoated LiMn2O4. In contrast, 25 X after 10 cycles to 51 X after 100 cycles for the 2.0 wt% SiO2-coated LiMn2O4 sample. Therefore, the uncoated sample has showed an increase in resistance of 77 X for 100 cycles, while the coated sample showed an increase of 26 X for 100 cycles. The impedance results revealed that, the passive surface film which formed on the uncoated cathode particles, increased the resistance faster with cycle number (Fig. 9a). In contrast, formation of passive surface film was controlled and the reaction between the electrolyte and oxide particles was suppressed on the SiO2-coated LiMn2O4 surface (Fig. 9b). During cycling, the surface of the LiMn2O4 was influenced by the electrolyte and thereby capacity faded. Finally, we can say that the formation of passive surface film increasing the impedance growth of the uncoated LiMn2O4 confirmed the faster capacity fade and lower cycle stability during re-

3.6. Cyclic voltammetry studies Fig. 8 presents the typical cyclic voltammographs of 2.0 wt% of SiO2 coated LiMn2O4 electrodes characterized at 30 °C and 60 °C at a scan rate of 100 lV/s. In general, CV results of pristine LiMn2O4 has less reversibility and low intensity of peak current at 60 °C compared to 30 °C due to the dissolution of Mn3+ ions into the electrolyte (Jahn–Teller distortion). In contrast, the CV curves for 2.0 wt.% of SiO2-coated LiMn2O4 has similar intensity of peak currents for both anodic and cathodic curves at 30 °C and 60 °C. It reveals that, 2.0 wt.% of SiO2-coated LiMn2O4 significantly reduced the Mn ions dissolution into the electrolyte at high temperature. The peak potential difference (DEp) of two redox peaks around 0.12 and 0.14 V for 30 °C and 60 °C, respectively. The resulting lower value of DEp, which attributed to 2.0 wt.% of SiO2-coated LiMn2O4 has good reversibility at both room temperature and elevated temperature.

Fig. 8. Typical cyclic voltammogram curves of 2.0 wt.% of SiO2-coated LiMn2O4 cycled the potential range of 2.8 and 4.8 V at 30 °C and 60 °C.

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600

200

Manganese ions concentration (ppm)

a

Z'' imaginary (Ohm)

150

100

10

th

25

th

50

th

100

th

50

500

400

d

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100

c b a

0 0

0 0

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150

Z'' imaginary (Ohm)

b

200

300

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500

600

Immersion time (h)

200

Z' real (Ohm)

100

Fig. 10. Relationship between the apparent concentrations of dissolved manganese ions from uncoated and 2.0 wt.% of SiO2-coated LiMn2O4 and the immersion time. (a) 2.0 wt.% SiO2-coated LiMn2O4 30 °C, (b) uncoated LiMn2O4 at 30 °C, (c) 2.0 wt.% SiO2-coated LiMn2O4 60 °C and (d) uncoated LiMn2O4 at 60 °C.

4. Conclusions

100

th

10

th

25

th

50 th 100

50

0 0

50

100

150

Z' real (Ohm) Fig. 9. Electrochemical impedance spectra of (a) uncoated LiMn2O4 and (b) 2.0 wt.% of SiO2-coated LiMn2O4 as a function of cycle number.

peated cycling. On the other hand, 2.0 wt.% of SiO2-coated LiMn2O4 sample showed less impedance growth and could sustain its cycle stability. 3.8. Analysis of manganese ions dissolution into electrolyte Fig. 10 presents the relationship between the concentration of dissolved cations and the immersion time. The uncoated and 2.0 wt.% of SiO2-coated LiMn2O4 powders were soaked in the 1 M LiPF6 contains 1:1 ratio of ethylene carbonate and dimethyl carbonate at room temperature and elevated temperature. It was found that, more manganese ions were dissolved from uncoated LiMn2O4 powder than 2.0 wt.% of SiO2-coated LiMn2O4 powders at 60 °C, which is ascribed that, SiO2 coating over the spinel LiMn2O4 particles could suppress the dissolution of manganese ions into the electrolyte at 60 °C. However, at room temperature, the amount of manganese ion dissolution of LiMn2O4 and SiO2 coated LiMn2O4 has very small difference due to no reaction takes place between electrode and electrolyte at 30 °C. These results indicated that the SiO2-coated LiMn2O4 could suppress the reaction of Mn dissolution at elevated temperature and clearly improve the cycleability of the spinel LiMn2O4 cathode materials.

The LiMn2O4 cathode materials were successfully coated with various wt.% of SiO2 by a polymeric process. Silicic acid was used as the precursor for coating LiMn2O4 cathode samples with SiO2. The coated materials, XRD patterns did not show any change in the 2h value of the peaks, lattice parameters and no impurity such as SiO2 were detected. The TEM images confirmed that, the SiO2 was formed as a compact coating over the cathode particles with a thickness of about 50 nm. The XPS revealed that, the nano SiO2 was coated over the surface of the core LiMn2O4 cathode materials. The 2.0 wt.% of SiO2-coated LiMn2O4 sample has significantly improved the capacity retention and excellent cycleability for 30 °C and 60 °C compared to the uncoated spinel LiMn2O4. Electrochemical Impedance spectra of the 2.0 wt.% of SiO2-coated cathode suggested that, the SiO2 surface coating on LiMn2O4 controlled the formation of a passive layer film during electrochemical cycling, resulting in enhanced cycle stability. The amount of dissolved manganese ions in 2.0 wt.% of SiO2-coated LiMn2O4 is significantly lower than the uncoated LiMn2O4 systems even at elevated temperature. These results indicated that, SiO2 coating over LiMn2O4 can suppress the dissolution reaction of manganese ions at elevated temperature and clearly improved the cycleability of the spinel LiMn2O4 cathode materials. References [1] G.G. Amatucci, N. Pereira, T. Zheng, I. Plitz, J.M. Tarascon, J. Power Sources 81– 82 (1999) 39. [2] Y.W. Tsai, R. Santhanam, B.J. Hwang, S.K. Hu, H.S. Sheu, J. Power Sources 119– 121 (2003) 701. [3] S.S. Zhang, K. Xu, T.R. Jow, J. Electrochem. Soc. 149 (2002) A1521. [4] D. Im, A. Manthiram, J. Electrochem. Soc. 150 (2003) A742. [5] G.G. Amatucci, C.N. Schmutz, A. Blyr, C. Sigala, A.S. Gozdz, D. Larcher, J.M. Tarascon, J. Power Sources 69 (1997) 11. [6] R.J. Cummow, A. de Kock, M.M. Thackeray, Solid State Ionics 69 (1994) 59. [7] Y. Xia, M. Yoshio, J. Electrochem. Soc. 143 (1996) 825. [8] Y.K. Sun, D.W. Kim, Y.M. Choi, J. Power Sources 79 (1999) 231. [9] S. Shi, C. Ouyung, D.S. Wang, L. Chen, X. Huang, Solid-State Commun. 126 (2003) 531. [10] K. Dokko, S. Hovikoshi, T. Itoh, M. Nishizawa, M. Mohamedi, I. Uchide, J. Power Sources 90 (2000) 153. [11] M. Hosoya, H. Ikuta, M. Wakihara, Solid State Ionics 111 (1998) 153. [12] H.J. Bang, V.S. Donepudi, J. Prakash, Electrochim. Acta 48 (2002) 443. [13] K. Amine, H. Tukamoto, H. Yasuda, Y. Fujita, J. Power Sources 68 (1997) 604.

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