Synthesis and electrochemical characterizations of nano-La2O3-coated nanostructure LiMn2O4 cathode materials for rechargeable lithium batteries

Synthesis and electrochemical characterizations of nano-La2O3-coated nanostructure LiMn2O4 cathode materials for rechargeable lithium batteries

Materials Research Bulletin 45 (2010) 1825–1831 Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier...

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Materials Research Bulletin 45 (2010) 1825–1831

Contents lists available at ScienceDirect

Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

Synthesis and electrochemical characterizations of nano-La2O3-coated nanostructure LiMn2O4 cathode materials for rechargeable lithium batteries D. Arumugam, G. Paruthimal Kalaignan * Advanced Lithium Battery Research Lab, Department of Industrial Chemistry, Alagappa University, Alagappa Puram, Karaikudi 630 003, Tamil Nadu, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 3 May 2010 Received in revised form 27 August 2010 Accepted 15 September 2010 Available online 24 September 2010

LiMn2O4 spinel cathode materials were coated with 1.0, 2.0 and 3.0 wt.% of La2O3 by polymeric process, followed by calcinations at 850 8C for 6 h in air. The surface coated LiMn2O4 cathode materials were physically characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and XPS. XRD patterns of La2O3-coated LiMn2O4 revealed that the coating did not affect the crystal structure and 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 100 nm. XPS data illustrated that the La2O3 was completely coated over the surface of the LiMn2O4 core cathode materials. The galvanostatic charge and discharge of the uncoated and La2O3coated LiMn2O4 cathode materials were carried out in the potential range of 3.0 and 4.5 V at 30 8C and 60 8C. Among them, 2.0 wt.% of La2O3-coated spinel LiMn2O4 cathode has improved the structural stability, high reversible capacity and excellent electrochemical performances of the rechargeable lithium batteries. ß 2010 Elsevier Ltd. All rights reserved.

Keywords: A. Oxides B. Sol–gel chemistry C. Photoelectron spectroscopy D. Electrochemical properties

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 tops, computers, calculators 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 4 V (versus Li/ Li+) region suffer from capacity fading, especially at elevated temperature (50–60 8C). 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 can be employed. One way is substitution of heterogeneous atom into the host LiMn2O4

* Corresponding author. Tel.: +91 9486179872; fax: +91 4565 225202. E-mail address: [email protected] (G. Paruthimal Kalaignan). 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.09.021

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 an 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 8C). 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 [16,17], MgO [18], ZnO [19–21], TiO2 [22,23], AlPO4 [24] coated core LiMn2O4 cathode materials has been found to suppress the manganese ions dissolution from the spinel lattice in contact with electrolyte and improve the capacity retention. However, the coated species could strip off during long term cycling and a challenge is to achieve robust coatings that will be stable under aggressive charge/discharge conditions. The main goal of this study was to improve the electrochemical performances of La2O3 nano-layer coated Li–Mn–O spinels cathode materials for the both room temperature and elevated temperature.

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LiMn2O4 powder was prepared by the sol–gel method, Li(OCOCH3)2H2O (AR 99.99% pure) and Mn(OCOCH3)24H2O (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 by adding aqueous ammonia. The resulted solution was evaporated at 80 8C until a transparent gel was obtained. Finally the gel precursor was decomposed at 300 8C for 8 h, followed by calcination at 850 8C for 15 h to get pure LiMn2O4 powder.

with a nickel filtered Cu Ka radiation sources (l = 1.5405 A˚). The diffraction patterns were recorded between scattering angles of 158 and 808 in step of 0.18 min 1. The surface morphology of the coated materials was studied using scanning electron microscopy (Hitachi model S-3000H). The coated layer particles morphology was examined by the transmission electron microscopy (JEOL TEM-2100). TEM samples were prepared by dispersing the cathode materials in ethanol and placing the drop of the clear solution on a carbon coated copper grid with subsequent drying. The X-ray photon spectra of La 3d, O 1s and Mn 2p were recorded by XPS with monochromatic Al Ka radiation at 1450 eV. The spectra were scanned in the range from 0.01 to 1400 eV binding energy in 1 eV steps.

2.2. Synthesis of La2O3-coated LiMn2O4

2.4. Electrochemical characterizations

La2O3-coated LiMn2O4 cathode materials were synthesized by polymeric process at the calculation of 1.0, 2.0, and 3.0 wt.% by lanthanum nitrate hexahydrate using as the coating of raw materials. The 2 g of the LiMn2O4 powder was dispersed in distilled water by 3 h stirring. La2O3 was calculated at 1.0, 2.0 and 3.0 wt.% by using lanthanum nitrate hexahydrate, 5 ml of polyvinyl alcohol were mixed in warm 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 8C for 10 h of continuous stirring. Then removed the excess water, a thick polymer gel was obtained. Thus obtained gel precursor was dried in an air oven at 120 8C for 12 h to form a fine powder, which was calcined at 850 8C for 6 h to form LiMn2O4 coated with a thin layer of La2O3 in the weight ratios of 99:1, 98:2 and 97:3, respectively. Fig. 1 shows the flow chart for the synthesis of La2O3-coated LiMn2O4 cathode materials.

The cathodes 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 Nmethyl-2-pyrrolidone (NMP) solvent. This mixture was then applied onto an etched aluminium foil current collector and dried at 120 8C for 12 h in a vacuum oven. The coated cathode foil was 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 diethyl carbonate (EC:DEC) 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 in the range between 4.5 and 3.0 V at room temperature and elevated temperature (30 8C and 60 8C). The cyclic voltammogram (CV) experiments were carried out at a scan rate of 100 mV/s between 3.0 and 4.5 V using Auto Lab Modular Electrochemical Instruments (BST 7249).

2. Experimental 2.1. Preparation of spinel LiMn2O4

2.3. Physical characterizations Structural analysis was carried out by using a powder X-ray [()TD$FIG] diffraction (Siemens D-5000, Mac Science MXP 18) was equipped

LiMn2 O4 + Triple distilled water 3h stirred

Dispersed LiMn2 O4

3h stirred after that add drop wise

La (NO3)3.6H2O with Polyvinyl alcohol in Triple distilled water

Stirred for 10h at 60°C

Viscous gel

Dried in air oven at 110°C for 12h

Fine powder Heated at 850°C for 6h Nano La 2 O3-coated LiMn 2 O4 powder Fig. 1. Flow chart for the La2O3-coated spinel LiMn2O4 cathode materials prepared by a polymeric process.

[()TD$FIG]

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3 Wt.% La 2O3 coated LiMn 2O4

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cathode materials, respectively. It indicates that, uniform La2O3 coating was formed over the pristine LiMn2O4 particles with a coating thickness around 100 nm. 3.3. X-ray photon spectroscopy analysis

Intensity (a.u)

2 Wt.% La 2O3 coated LiMn 2O4

1 Wt.% La 2O3 coated LiMn 2O4

Pristine LiMn2O4

20

30

40

50

60

70

80

Two Theta (Degree) Fig. 2. X-ray diffraction patterns of uncoated and various wt.% of La2O3-coated spinel LiMn2O4 cathode materials synthesized at 850 8C.

3. Results and discussion 3.1. X-ray diffraction analysis Fig. 2 shows the XRD patterns for both the uncoated and La2O3coated (1.0, 2.0 and 3.0 wt.%) LiMn2O4 powders. All the powders were well-defined spinel structure with a space group of Fd3m [25]. The presence of crystalline La2O3 was not detected by XRD. In addition, the La2O3 coating has not changed the 2u value of the peaks. Earlier reports have showed that, substitution of transition metal ions form Mn3+ in LiMn2O4 with significant changes in lattice parameters [8,9]. In our case, significant changes in lattice parameters were not obtained. This phenomenon indicates that the mechanism of coating is different from doping. These results revealed that La2O3 is just coated on the surface of the LiMn2O4 powders.

XPS has been used extensively to study the surface composition and to determine whether La2O3 coating remained on the surface of the core materials. Fig. 5(a–c) shows the XPS spectra of O 1s, La 3d and Mn 2p at the surface depth between 0 and 100 nm respectively for La2O3-coated LiMn2O4. Fig. 5a represents the XPS spectra of O 1s peak centered at 531.35 eV at the surface level, which corresponds to O 1s bonded with La [26,27]. Fig. 5b shows the characteristic binding energy of La 3d around 835.13 eV at the surface level, but at the depth of 100 nm, the same peak was found to be very shallow. The differences in peak intensity indicates that the La2O3 remained on the surface of the core materials. As the depth increased, it is clearly noticeable that there was a decrease in La2O3 concentration. Therefore, it could be concluded that there was no influence on the chemical state or binding energy of the different ions in the La2O3-coated LiMn2O4 samples. Also, both O 1s and La 3d 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 642.15 eV at the surface and 100 nm depth may be assigned to characterize the binding energy value of Mn 2p spectra in LiMn2O4 [28,29]. 100 nm depth profile has higher intensity peak compared to surface level. It reveals that, LiMn2O4 core materials have covered by nano-layer of La2O3. Fig. 5d shows the distribution of Mn and La atomic concentrations in La2O3-coated LiMn2O4 with a depth profile of the particle up to 100 nm. The concentration of Mn increased to a depth of about 20 nm and then leveled off. The high atomic concentration of La at the surface of the core material is reasonable due to the presence of La2O3 content. The concentration of La at 100 nm depth was low typically less than 10 at% compared to surface level. Beyond that, there was rapid decrease in the La concentration with the depth of particle value, which approximately corresponds to the thickness of the compact layer observed with a TEM image. 3.4. Galvanostatic charge/discharge studies

3.2. Surface morphology and particle size analysis Fig. 3(a and b) shows the SEM images of uncoated and 2.0 wt.% of La2O3-coated LiMn2O4, respectively. It indicates the surface modified LiMn2O4 particles has uniform surface and homogeneous particle size. Fig. 4(a and b) represents the transmission electron micrographs of uncoated and 2.0 wt.% of La2O3-coated LiMn2O4

[()TD$FIG]

Fig. 6(a and b) shows the typical charge–discharge curves of bare and 1.0, 2.0 and 3.0 wt.% of La2O3-coated spinel LiMn2O4 samples at a discharge rate of 0.5 C between 3.0 and 4.5 V at 30 8C and 60 8C, respectively. It can be seen that the LiMn2O4 samples with and without the La2O3 coating have similar charge–discharge profiles, exhibiting two charge–discharge plateaus in the potential region of

Fig. 3. SEM images of (a) uncoated and (b) 2.0 wt.% of La2O3-coated spinel LiMn2O4 cathode materials synthesized at 850 8C.

[()TD$FIG]

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Fig. 4. TEM images of (a) uncoated and (b) 2.0 wt.% of La2O3-coated spinel LiMn2O4 cathode materials synthesized at 850 8C.

4.0–4.3 V, which are ascribed to the remarkable characteristics of a well defined LiMn2O4 spinel and the voltage plateaus indicated that the insertion and extraction of lithium-ions occur in two states [9]. The first voltage plateau at about 4.02 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 4.26 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 La2O3 coatings at both 30 8C and 60 8C. This may be due to the little higher electrode impedance resulting from the La2O3 coatings. 3.5. Cycling performances Fig. 7(a and b) shows the results of discharge cycling at 0.5 C rate between 3.0 and 4.5 V for uncoated LiMn2O4, 1.0, 2.0 and 3.0 wt.% of La2O3 coated LiMn2O4 performed at 30 8C and 60 8C,

[()TD$FIG]

a

respectively, up to 100 cycles. The first cycle discharge capacities, 100th cycle discharge capacities and capacity retention ratios of La2O3-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 La2O3-coated, the 2.0 wt% of La2O3 coated LiMn2O4 exhibits initial discharge capacity of 117 mAh/g, but after 100 cycles only 13.7% capacity loss was obtained and the discharge capacity still maintains at 101 mAh/g. This cycling behaviour of the La2O3-coated LiMn2O4 electrodes indicates the impact of La2O3 coating significantly which improved the electrochemical performances at room temperature (30 8C). 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 0.5 C rate between 3.0 and 4.5 V at 60 8C. It is clear that, surface modification has significantly

b

O 1s

Intensity (a.u)

Intensity (a.u.)

La 3d 100 nm depth

La 3d

At the surface

460

480

500

520

540

560

580

800

600

810

Binding Energy (eV)

820

830

840

850

Binding Energy (eV)

c Intensity (a.u)

100 nm depth

Mn 2p At the surface

Atomic Concentration (%)

100 Mn 2p

d 80

60

Mn

40

20 La

0 600

650

700

Binding Energy (eV)

750

0

20

40

60

80

100

Distance with in particle (nm)

Fig. 5. XPS spectra of (a) O 1s, (b) La 3d, (c) Mn 2p and (d) depth profile of a 2.0 wt.% La2O3-coated LiMn2O4 particle.

[()TD$FIG]

[()TD$FIG]

D. Arumugam, G. Paruthimal Kalaignan / Materials Research Bulletin 45 (2010) 1825–1831

150

5.0

a

140

Potential (V) Vs Li/Li

+

Specific Capacity (mAh/g)

4.5

4.0

3.5

3.0

1829

130 120 110 100 90 80 70 60 50

2.5 0

20

40

60

80

100

120

0

140

20

60

80

100

120

100

120

Cycle number (n)

Specific Capacity (mAh/g) 150

5.0

b

Specific Capacity (mAh/g)

140

+

4.5

Potential (V) Vs Li/Li

40

4.0

3.5

3.0

130 120 110 100 90 80 70 60

2.5 0

20

40

60

80

100

120

50

140

0

20

Specific Capacity (mAh/g)

40

60

80

Cycle number (n)

Fig. 6. Initial charge/discharge curves of uncoated LiMn2O4, 1.0 wt.%, 2.0 wt.% and 3.0 wt.% of La2O3-coated LiMn2O4 cathode materials cycled between the range of 3.0 and 4.5 V; (a) room temperature (30 8C), (b) elevated temperature (60 8C).

Fig. 7. Discharge capacities of uncoated LiMn2O4, 1.0 wt.%, 2.0 wt.% and 3.0 wt.% of La2O3-coated LiMn2O4 cathode materials between the range of 3.0 and 4.5 V at a rate of 0.5C; (a) room temperature (30 8C), (b) elevated temperature (60 8C).

improved the cycleability of LiMn2O4 even at elevated temperature (60 8C). 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 La2O3-coated LiMn2O4 provides initial discharge capacity of 113 mAh/g and remains at 92 mAh/g after 100 cycles, with capacity loss of 18.5%. On the other hand, 2.0 wt% of La2O3-coated LiMn2O4 shows the similar characteristics of two potential plateaus which were obtained at 3.92 and 4.26 V compared with uncoated electrode, this indicates that the La2O3 coating does not change the intrinsic property of LiMn2O4 during insertion and extraction of lithium ions. High percentage of La2O3 coated LiMn2O4 has lower capacity compared to uncoated and lower La2O3 coated LiMn2O4 because the addition of lanthanum oxide content replace the quantity of pristine LiMn2O4. Even

coated LiMn2O4, Mn3+ only contribute the capacity during charge/ discharge cycling. 2.0 wt.% of La2O3 coated LiMn2O4 is the optimum composition to enhance the stability and cycling performances of the electrode. Fig. 8 shows the capacity retention of 2.0 wt.% of La2O3-coated LiMn2O4 cell at different discharge rates (charge at 0.5 C) characterized at elevated temperature (60 8C) in the potential range between 3.0 and 4.5 V. The initial and 100th discharge capacities and capacity retention with different discharge rates are summarized in Table 2. The initial discharge capacities for 2.0 wt.% of La2O3-coated LiMn2O4 are 113 mAh/g at 0.5 C, 112 mAh/g at 5 C, 101 mAh/g at 10 C, 97 mAh/g at 15 C and 82 mAh/g at 20 C rates. The reversible capacity of the cell gradually reduced in the first 20 cycles, and then keeps well stable. The 2.0 wt.% of La2O3-coated

Table 1 Electrochemical capacities and retention ratios of the lithium ion cells with La2O3-coated LiMn2O4 cathode materials. Electrode compositions

Initial discharge capacity at 30 8C (mAh/g)

100th discharge capacity at 30 8C (mAh/g)

Initial discharge capacity at 60 8C (mAh/g)

100th discharge capacity at 60 8C (mAh/g)

Capacity retention at 30 8C (%)

Capacity retention at 60 8C (%)

Uncoated LiMn2O4 1% La2O3 coated LiMn2O4 2% La2O3 coated LiMn2O4 3% La2O3 coated LiMn2O4

135 123 117 107

81 93 101 86

125 115 113 102

62 86 92 75

60.0 75.6 86.3 80.3

49.6 74.7 81.5 73.5

[()TD$FIG]

[()TD$FIG]

D. Arumugam, G. Paruthimal Kalaignan / Materials Research Bulletin 45 (2010) 1825–1831

160

180

140

160

Specific Capacity (mAh/g)

Specific Capacity (mAh/g)

1830

120 100 80 60 0.5C 5 C 10 C 15 C 20 C

40 20

140 120 100 80 60

Charge (5C) / Discharge (C/5) Charge (2C) / Discharge (C/5)

40 20 0

0 0

20

40

60

80

100

0

20

40

60

80

100

Cycle mumber (n)

Cycle number (n)

LiMn2O4 cathode delivers the discharge capacities of 97, 84, 89, 88, and 69 mAh/g after 100 cycles at a rate of 0.5 C, 5 C, 10 C, 15 C and 20 C, respectively. Few similar results have also been included at such current rates. The excellent capacity retention may be obtained at moderate rates. Based on the above results, the 2.0 wt.% of La2O3-coated LiMn2O4 sample is attractive material for practical applications. La2O3 coating provide the interface with a chemically stable but highly Li+ conducting barrier layer that effectively reduces the chemical reaction between the charged active materials and the electrolyte. The La2O3 coating suppress the increase of resistance caused by repeated insertion and extraction of lithium ions while be cycled in the potential range of 3.0–4.5 V at 0.5 C to 20 C rates versus Li/Li+. In addition to possessing good rate capabilities, an appropriate electrode material must meet the requirements for real life battery operation; thus, it must be able to withstand fast charges and slow discharges. This property was tested under the following conditions in the 2.0 wt.% of La2O3-coated LiMn2O4; cells were charged under two fast regimes (2 C and 5 C) and further discharged under one slow regime (C/5) at room temperature. Fig. 9 shows the variation of the capacity as a function of cycle number and Table 3 lists the calculated coulombic efficiencies. As can be seen, capacity retention by the cells was quite good (the capacity faded less than 20% after 100 cycles). Table 2 Rate of discharge capacity with function of cycle number for 2.0 wt.% of La2O3coated spinel LiMn2O4. Rate of discharge (C)

Initial discharge capacity (mAh/g)

100th discharge capacity (mAh/g)

Capacity retention (%)

0.5 5 10 15 20

113 112 101 97 82

93 84 89 88 69

82 75 88 90 84

Fig. 9. Variation of the capacity as a function of cycle number for 2.0 wt.% of La2O3coated LiMn2O4 cycled the potential range of 2.5 and 4.5 V at room temperature for real life battery operation.

[()TD$FIG]

0.20 0.15

Current (mA)

Fig. 8. Rate of discharge capacity performance (charge at 0.5 C) of 2.0 wt.% of La2O3coated LiMn2O4 cycled the potential range of 2.5 and 4.5 V at 60 8C.

room temperature

0.10 Elevated temperature

0.05 0.00 -0.05 -0.10 3.0

3.2

3.4

3.6

3.8

4.0

Potential (V) Vs Li/Li

4.2

4.4

+

Fig. 10. Typical cyclic voltammogram curves of 2.0 wt.% of La2O3-coated LiMn2O4 cycled the potential range of 2.8 and 4.8 V at 30 8C and 60 8C.

Therefore, the spinel exhibited excellent electrochemical response under these rather drastic working conditions and its coulombic efficiency approached 100% upon cycling. 3.6. Cyclic voltammetry studies Fig. 10 presents the typical cyclic voltammograms of 2.0 wt% of La2O3 coated LiMn2O4 electrodes characterized at 30 8C and 60 8C at a scan rate of 100 mV/s. CV curves for 2.0 wt.% of La2O3-coated LiMn2O4 has similar intensity of peak currents for both anodic and cathodic curves at 30 8C and 60 8C. It reveals that, 2.0 wt.% of La2O3coated LiMn2O4 significantly reduced the Mn ions dissolution into the electrolyte at high temperature. The peak potential difference

Table 3 Coulombic efficiency of the cells charged at 2 C and 5 C rates and discharged at C/5 rate for 2.0 wt.% of La2O3-coated spinel LiMn2O4. Cycle number, n

Coulombic efficiency (%) charge (2 C)/discharge (C/5) Coulombic efficiency (%) charge (5 C)/discharge (C/5)

1

10

20

30

40

50

60

70

80

90

100

93

93

93

95

98

98

98

99

100

100

100

91

91

91

92

93

95

95

95

95

95

95

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(DEp) of two redox peaks around 0.16 and 0.20 V for 30 8C and 60 8C, respectively. The resulting lower value of DEp, which attributed to 2.0 wt.% of La2O3-coated LiMn2O4 has good reversibility at both room temperature and elevated temperature. 4. Conclusions The LiMn2O4 cathode materials were successfully coated with various wt.% of La2O3 using lanthanum nitrate hexahydrate by a polymeric process. XRD patterns for La2O3-coated LiMn2O4 did not show any change in the 2u value of the peaks, lattice parameters and no impurity such as La2O3 were detected. The TEM images confirmed that, the La2O3 was formed as a compact coating over the cathode particles with a thickness of about 100 nm. The XPS revealed that, the nano-La2O3 was coated over the surface of the core LiMn2O4 cathode materials. The 2.0 wt.% of La2O3-coated LiMn2O4 sample has significantly improved the capacity retention and excellent cycleability for 30 8C and 60 8C compared to the uncoated spinel LiMn2O4. A careful investigation of the cathode by electrochemical impedance spectroscopy before and after surface modification with nano-La2O3 reveals that, the improvement is due to a decrease in both solid electrolyte interfacial (SEI) resistances and electron transfer resistances. These results suggest that surface modification is an effective way to improve the chemical stability of the electrode in contact with the electrolyte and improve their cyclability and rate capability during long term cycling. Acknowledgments One of the authors (D. Arumugam) thanks the University Grants Commission (UGC), New Delhi, India for the award of Research Fellowship in Sciences for Meritorious Students (RFSMS) to carry out this work at the Alagappa University in India and also gratefully acknowledge the University Grants Commission (UGC), New Delhi, India.

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