Synthesis and electrochemical characterization of nano-CeO2-coated nanostructure LiMn2O4 cathode materials for rechargeable lithium batteries

Synthesis and electrochemical characterization of nano-CeO2-coated nanostructure LiMn2O4 cathode materials for rechargeable lithium batteries

Electrochimica Acta 55 (2010) 8709–8716 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elec...

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Electrochimica Acta 55 (2010) 8709–8716

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Synthesis and electrochemical characterization of nano-CeO2 -coated nanostructure LiMn2 O4 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

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Article history: Received 3 May 2010 Received in revised form 2 August 2010 Accepted 2 August 2010 Available online 10 August 2010 Keywords: Cathode materials CeO2 -coated LiMn2 O4 XRD TEM Electrochemical performances

a b s t r a c t LiMn2 O4 spinel cathode materials were coated with 0.5, 1.0, and 1.5 wt.% CeO2 by a polymeric process, followed by calcination at 850 ◦ C for 6 h in air. The surface-coated LiMn2 O4 cathode materials were physically characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron microscopy (XPS). XRD patterns of CeO2 -coated LiMn2 O4 revealed that the coating did not affect the crystal structure or the Fd3m space group of the cathode materials compared to uncoated LiMn2 O4 . The surface morphology and particle agglomeration were investigated using SEM, TEM image showed a compact coating layer on the surface of the core materials that had average thickness of about 20 nm. The XPS data illustrated that the CeO2 completely coated the surface of the LiMn2 O4 core cathode materials. The galvanostatic charge and discharge of the uncoated and CeO2 -coated LiMn2 O4 cathode materials were measured in the potential range of 3.0–4.5 V (0.5 C rate) at 30 ◦ C and 60 ◦ C. Among them, the 1.0 wt.% of CeO2 -coated spinel LiMn2 O4 cathode satisfies the structural stability, high reversible capacity and excellent electrochemical performances of rechargeable lithium batteries. Crown Copyright © 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction Rechargeable lithium-ion batteries have been enjoying great commercial success as the most promising portable energy source in electronic products, such as lap tops, computers, calculators and cellular phones, mainly due to their high working voltages, high energy density, and long life. Currently, LiCoO2 , LiNiO2 and LiMn2 O4 are the main cathode materials for rechargeable lithiumion batteries. Among these, spinel LiMn2 O4 and its derivatives, materials with perhaps the highest potential for these applications, are easily prepared, inexpensive, abundant, non-toxic and environmentally friendly [1–4]. However, LiMn2 O4 electrodes in the 4 V (versus Li/Li+ ) region suffer from capacity fading, especially at elevated temperature (50–60 ◦ C). The capacity loss has been ascribed to several factors, including Jahn–Teller distortion due to Mn3+ ions, the dissolution of manganese ions into the electrolyte, loss of crystallinity during cycling and electrolyte decomposition at the high potential regions [5–8]. To overcome this capacity fading problem, two types of methods can be employed. One is the substitution of a heterogeneous atom into the host LiMn2 O4 structure of cathode materials; the other

∗ Corresponding author. Tel.: +91 9486179872; fax: +91 4565 225202. E-mail address: [email protected] (G.P. Kalaignan).

is surface modification. Several research groups have attempted to stabilize the structure of LiMn2 O4 powders during cycling by substituting a small fraction of the manganese ions with several divalent or trivalent metal ions. There was an improvement in cycle performance at room temperature by partial substitution of transition metals instead of Mn in LiMn2 O4 [8–14], whereas existing dopent methods resulted in LiMn2 O4 still suffered from significant capacity decline at elevated temperature (50–60 ◦ C). A different approach has been reported that involves modifying the surface of the cathode material by coating it with electrochemically inactive metal oxides or ceramic oxide materials. Amatucci et al. [15] first reported that the surface treatment of LiMn2 O4 with lithium boron oxide (LBO) was an attractive way to improve the electrochemical properties of LiMn2 O4 . Al2 O3 - [16,17], MgO- [18], ZnO- [19–21], TiO2 - [22,23], and AlPO4 [24]-coated core LiMn2 O4 cathode materials have been found to suppress the manganese ion dissolution from the spinel lattice in contact with the electrolyte and to improve the capacity retention. However, the coated species could strip off during long-term cycling. An important challenge is to achieve robust coatings that will be stable under aggressive charge/discharge conditions. In this work, we have studied the electrochemical performances of the lithium cells of 4.5 V assembled using the prepared various wt.% of CeO2 -coated LiMn2 O4 cathode materials.

0013-4686/$ – see front matter. Crown Copyright © 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.08.016

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diffraction patterns were recorded between scattering angles of 15◦ and 80◦ in 0.1◦ /min steps. The surface morphologies of the coated materials were studied using a scanning electron microscope (Hitachi model S-3000H). The coated particles’ morphology was examined by a transmission electron microscope (JEOL TEM2100). TEM samples were prepared by dispersing the cathode materials in ethanol and placing a drop of the clear solution on a carbon-coated copper grid with subsequent drying. The X-ray photon spectra of Ce 3d, O 1s and Mn 2p were recorded by XPS with monochromatic Al K␣ radiation at 1450 eV. The spectra were scanned in the range of 0.01–1400 eV binding energy in 1 eV steps. 2.4. Electrochemical characterization

Fig. 1. Flow chart for the CeO2 -coated spinel LiMn2 O4 cathode materials prepared by a polymeric process.

2. Experimental 2.1. Preparation of spinel LiMn2 O4 LiMn2 O4 powder was prepared by the sol–gel method. Li(OCOCH3 )·2H2 O (AR 99.99% pure) and Mn(OCOCH3 )2 ·4H2 O (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. The pH value of the solution was adjusted to 6–7 by adding 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 pure LiMn2 O4 powder.

The cathodes were prepared using a doctor blade coating method with a slurry of 85 wt.% of 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 aluminum foil current collector and dried at 110 ◦ C for 12 h in a vacuum oven. The coated cathode foil was pressed and then cut into circular discs of 20-mm diameter. The button cells were assembled using stainless steel coin-type containers in an argon-filled glove box, in which oxygen and H2 O concentrations were maintained below 2 ppm. Pieces of lithium foil were used as the counter and reference electrodes, 1 M LiPF6 with a 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 measured using a WPG100 Potentiostate/Galvanostate cycle life tester in the potential range between 4.5 and 3.0 V at room temperature and at elevated temperatures (30 ◦ C and 60 ◦ C). The cyclic voltammogram (CV) experiments were carried out at a scan rate of 100 ␮V/s between 3.0 and 4.5 V using Auto Lab Modular Electrochemical Instruments (BST 7249). The electrochemical impedance spectra of uncoated and CeO2 -coated LiMn2 O4 were carried out at room temperature using a 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. 3. Results and discussion

2.2. Synthesis of CeO2 -coated LiMn2 O4

3.1. XRD analysis

CeO2 -coated LiMn2 O4 cathode materials were synthesized using a polymeric process at 0.5, 1.0 and 1.5 wt.% with cerium (IV) ammonium nitrate used as the coating of raw materials. LiMn2 O4 powder (2 g) was dispersed in distilled water with 3 h of stirring. CeO2 was prepared at 0.5, 1.0 and 1.5 wt.% by mixing cerium nitrate hexahydrate and 5 ml of polyvinyl alcohol in warm distilled water and adding the mixture dropwise into the dispersed LiMn2 O4 solution. The mixture was stirred for 5 h at room temperature and heated at 60 ◦ C for 10 h with continuous stirring. The excess water was removed, and a thick polymer gel was obtained. The obtained gel precursor was dried in an air oven at 120 ◦ C for 12 h to form a fine powder, which was calcined at 850 ◦ C for 6 h to form LiMn2 O4 coated with a thin layer of CeO2 in 99.5:0.5, 99.0:1.0 and 98.5:1.5 weight ratios. Fig. 1 shows the flow chart for the synthesis of CeO2 coated LiMn2 O4 cathode materials.

Fig. 2 shows the XRD patterns for both the uncoated and CeO2 coated (0.5, 1.0 and 1.5 wt.%) LiMn2 O4 powders. All the powders had a well-defined spinel structure with a Fd3m space group in which the lithium ions occupy the tetrahedral 8a site, Mn3+ and Mn4+ ions the octahedral 16d site, and O2− ions the 32e site [25]. The presence of crystalline CeO2 was not detected by XRD. In addition, the CeO2 coating did not change the 2 value of the peaks. The peak intensities of LiMn2 O4 decreased with increasing the wt.% of CeO2 coating content on the surface of the LiMn2 O4 particles. It reveals that, CeO2 has completely covered the pristine LiMn2 O4 particles. Earlier reports have shown that substitution of transition metal ions form Mn3+ in LiMn2 O4 with significant changes in lattice parameters [29–31]. In our case, significant changes in lattice parameters were not obtained. This phenomenon indicates that the coating mechanism is different from doping. These results reveal that CeO2 is coated only on the surface of the LiMn2 O4 powders.

2.3. Physical characterization 3.2. Surface morphology and particle size analysis Structural analysis was carried out using a powder X-ray diffraction instrument (Siemens D-5000, Mac Science MXP 18) equipped ˚ The with a nickel filtered Cu-K␣ radiation source ( = 1.5405 A).

Fig. 3a and b shows the SEM images of uncoated and 1.0 wt.% CeO2 -coated LiMn2 O4 , respectively. The surface of the cathode

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Fig. 2. XRD patterns of uncoated and various wt.% CeO2 -coated spinel LiMn2 O4 cathode materials synthesized at 850 ◦ C.

particles changed distinctly upon coating due to the CeO2 layer covering the core particle. The surface brightness increased in the coated sample compared to the pristine sample. This increase in brightness is associated with the accumulation of charge on the CeO2 coating materials as the electron beam impinges it. The spherical shaped pristine LiMn2 O4 particle size was increased from around 0.15 ␮m to 0.19 ␮m after the CeO2 coating. It reveals that, small layer of CeO2 coated on the surface of the LiMn2 O4 enhance the particle size of the pristine. Fig. 4a and b presents the TEM images of the uncoated and 1.0 wt.% CeO2 -coated LiMn2 O4 cathode materials, respectively. These images indicate that a uniform CeO2 coating formed over the pristine LiMn2 O4 particles with a coating thickness around 20 nm. 3.3. XPS analysis XPS has been used extensively to study the surface composition and to determine whether the CeO2 coating remained on the surface of the core materials. Fig. 5a,b and c shows the XPS spectra of O

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1s, Ce 3d and Mn 2p, respectively, at surface depths between 0 and 100 nm for CeO2 -coated LiMn2 O4 . Fig. 5a represents the XPS profile at the surface and at 100 nm depth for the O 1s peak centered at 529.03 eV, which corresponds to O 1s bonded with Ce at the surface [26,27]. At 100-nm depth, the O 1s peak has shifted slightly from 529.03 eV to 530.89 eV, revealing that the peak centered at 530.89 eV corresponds to O 1s bonded to Mn and that the coated materials remained mainly on the surface. Fig. 5b shows the characteristic binding energy of Ce 3d as around 915.26 eV at the surface level, but at a depth of 100 nm, the same peak is very shallow. The differences in peak intensity indicate that the CeO2 remained on the surface of the core materials. As the depth increased, there was a clear decrease in CeO2 concentration. Therefore, there was no influence on the chemical state or binding energy of the different ions in the CeO2 -coated LiMn2 O4 samples. Also, both the O 1s and Ce 3d binding energies demonstrated that the coated LiMn2 O4 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 642.19 eV at the surface and 100 nm depth may be assigned to the binding energy value of Mn 2p spectra in LiMn2 O4 [28,29]. The 100-nm depth profile has higher peak intensities than the surface level, revealing that the LiMn2 O4 core materials are covered by a nano-layer of CeO2 . XPS spectra of O 1s, Mn 2p and Ce 3d showed small peaks that may be attributed to either an amorphous or a semi-crystalline CeO2 nano-layer coated on the surface of the LiMn2 O4 core particles. Fig. 5d shows the distribution of Mn and Ce atomic concentrations in CeO2 -coated LiMn2 O4 with a depth profile of up to 100 nm. The Mn concentration increased to a depth of about 20 nm and then leveled off. The high atomic concentration of Ce at the surface of the core material is reasonable because of the presence of CeO2 . The concentration of Ce at 100 nm was low, typically less than 10 at.%, compared to the surface level. Beyond that, there was a rapid decrease in the Ce concentration with the depth of particle value, which corresponds approximately to the thickness of the compact layer observed with a TEM image.

3.4. Galvanostatic charge/discharge studies Fig. 6a and b shows the typical charge/discharge curves of bare and 0.5, 1.0 and 1.5 wt.% CeO2 -coated spinel LiMn2 O4 samples at a discharge rate of 0.5 C between 3.0 and 4.5 V at 30 ◦ C and 60 ◦ C, respectively. The LiMn2 O4 samples with and without the CeO2 coating have similar charge/discharge profiles, exhibiting two charge/discharge plateaus in the potential regions of 4.09

Fig. 3. SEM images of (a) uncoated and (b) 1.0 wt.% CeO2 -coated spinel LiMn2 O4 cathode materials synthesized at 850 ◦ C.

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Fig. 4. TEM images of (a) uncoated and (b) 1.0 wt.% CeO2 -coated spinel LiMn2 O4 cathode materials synthesized at 850 ◦ C.

and 4.15 V that are ascribed to the remarkable characteristics of a well-defined LiMn2 O4 spinel. The voltage plateaus indicate that the insertion and extraction of lithium ions occur in two states [12]. The first voltage plateau at 4.09 V is attributed to the removal of lithium ions from half of the tetrahedral sites in which Li–Li interactions occur. The second voltage plateau observed at 4.15 V is

ascribed to the removal of lithium ions from the remaining tetrahedral sites. In the initial charge/discharge curves, the pristine LiMn2 O4 samples display slightly larger capacities than the samples with CeO2 coatings at both 30 ◦ C and 60 ◦ C. This may be due to the slightly higher electrode impedance resulting from the CeO2 coatings.

Fig. 5. XPS spectra of (a) O 1s, (b) Ce 3d, (c) Mn 2p and (d) depth profile of a 1.0 wt.% CeO2 -coated LiMn2 O4 particle.

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Fig. 6. Initial charge/discharge curves of uncoated LiMn2 O4 , 0.5 wt.%, 1.0 wt.%, and 1.5 wt.% CeO2 -coated LiMn2 O4 cathode materials cycled over the range of 3.0–4.5 V: (a) room temperature (30 ◦ C) and (b) elevated temperature (60 ◦ C).

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Fig. 7. Discharge capacities of uncoated LiMn2 O4 , 0.5 wt.%, 1.0 wt.%, and 1.5 wt.% CeO2 -coated LiMn2 O4 cathode materials over the range of 3.0–4.5 V at a rate of 0.5 C: (a) room temperature (30 ◦ C) and (b) elevated temperature (60 ◦ C).

3.5. Cycling performance Fig. 7a and b shows the results of discharge cycling at a 0.5 C rate between 3.0 and 4.5 V for uncoated LiMn2 O4 and 0.5, 1.0 and 1.5 wt.% CeO2 -coated LiMn2 O4 performed at 30 ◦ C and 60 ◦ C, respectively, up to 100 cycles. The first cycle discharge capacities, 100th cycle discharge capacities and capacity retention ratios of CeO2 -coated and uncoated LiMn2 O4 are summarized in Table 1. The discharge capacity of Li/electrolyte/LiMn2 O4 is initially 135 mAh/g and declines to 81 mAh/g after 100 cycles with a capacity loss of 40% (Fig. 7a). In contrast, the 1.0 wt.% CeO2 -coated LiMn2 O4 exhibits an initial discharge capacity of 126 mAh/g, and the sample exhibited only 5% capacity loss and 120 mAh/g discharge capacity after 100 cycles. This cycling behavior of the CeO2 -coated LiMn2 O4 electrodes indicates that the CeO2 coating significantly improved the electrochemical performances at room temperature (30 ◦ C). The major issue with spinel LiMn2 O4 materials was their poor electrochemical performances at elevated temperature caused by the dissolution of manganese ions into the electrolyte. Fig. 7b shows the results of electrochemical cycling at a 0.5 C rate between 3.0 and 4.5 V at 60 ◦ C. Surface modification significantly improved the cyclability of LiMn2 O4 even at elevated temperature (60 ◦ C). For

pristine LiMn2 O4 , the discharge capacity declined from 125 to 62 mAh/g, with a capacity loss of 50.4% after 100 cycles. The 1.0 wt.% CeO2 -coated LiMn2 O4 exhibited an initial discharge capacity of 125 mAh/g that reached 117 mAh/g after 100 cycles, with capacity loss of 7%. This coated sample shows similar characteristics of two potential plateaus that were obtained at 4.09 and 4.15 V as the uncoated electrode; this indicates that the CeO2 coating does not change the intrinsic properties of LiMn2 O4 during the insertion and extraction of lithium ions. A high percentage of CeO2 -coated LiMn2 O4 has a lower capacity compared to the uncoated samples and samples with lower concentrations of CeO2 because the addition of cerium oxide replaces some of the pristine LiMn2 O4 . Even in coated LiMn2 O4 , Mn3+ only contributes to the capacity during charge/discharge cycling. The 1.0 wt.% CeO2 -coated LiMn2 O4 sample is optimal for enhancing the stability and cycling performances of the electrode. The CeO2 coating provides 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 CeO2 coating suppresses the resistance increase caused by repeated insertion and extraction of lithium ions during cycling in the potential range of 3.0–4.5 V

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Table 1 Electrochemical capacities and retention ratios of lithium-ion cells with various CeO2 -coated LiMn2 O4 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 LiMn2 O4 0.5% CeO2 -coated LiMn2 O4 1% CeO2 -coated LiMn2 O4 1.5% CeO2 -coated LiMn2 O4

135 131 126 125

81 112 120 100

125 126 125 121

62 88 117 83

60.0 85.0 95.0 80.0

49.6 69.0 93.0 68.0

Fig. 8. Rate of discharge performance of a 1.0 wt.% CeO2 -coated LiMn2 O4 cathode cycled over the range of 3.0–4.5 V at 60 ◦ C (charge 0.5 C).

at 0.5 C to 20 C rates versus Li/Li+ . The initial capacities of both uncoated and all the CeO2 -coated LiMn2 O4 were cycled at elevated temperature significantly lower than room temperature. It reveals that, the higher amount of Mn3+ dissolved from the electrode to electrolyte at 60 ◦ C than 30 ◦ C due to faster electrode electrolyte reaction occurs at 60 ◦ C than 30 ◦ C. Fig. 8 shows the capacity retention of a 1.0 wt.% CeO2 -coated LiMn2 O4 cell at different discharge rates (charged at 0.5 C) at elevated temperature (60 ◦ C) in the potential range between 3 and 4.5 V. The initial and 100th discharge capacities and the capacity retention with different discharge rates are summarized in Table 2. The initial discharge capacities for 1.0 wt.% CeO2 -coated LiMn2 O4 are 125 mAh/g at 0.5 C, 125 mAh/g at 5 C, 121 mAh/g at 10 C, 117 mAh/g at 15 C and 102 mAh/g at 20 C rates. The reversible capacity of the cell gradually decreased in the first 10 cycles and then stabilized. The 1.0 wt.% CeO2 -coated LiMn2 O4 cathode delivered discharge capacities of 117, 108, 101, 99 and 90 mAh/g after 100 cycles at rates 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. Excellent capacity retention may be obtained at moderate rates. Based on the above results, the 1.0 wt.% CeO2 -coated LiMn2 O4 sample is an attractive material for practical applications. The CeO2 nanoTable 2 Electrochemical performance of 1 wt.% of CeO2 -coated spinel electrode. Rate of discharge

Initial discharge capacity (mAh/g)

100th discharge capacity (mAh/g)

Capacity retention (%)

0.5 C 5C 10 C 15 C 20 C

125 125 121 117 102

117 108 101 99 90

93 86 83 84 88

Fig. 9. Typical CV curves of undoped and 1.0 wt.% CeO2 -coated LiMn2 O4 cycled over the potential range of 3.0–4.5 V at 30 ◦ C and 60 ◦ C. (a) Uncoated LiMn2 O4 and (b) 1.0 wt.% CeO2 -coated LiMn2 O4 .

layer coated LiMn2 O4 has superior electrochemical performance compared to bare LiMn2 O4 or other metal oxide-coated cathodes. This result may be attributed to the strong Ce–O bond, which is more resistant to chemical attack and is thermally stable. Cerium oxide also has good electrical conductivity; it enhances the electrical conductivity of the core LiMn2 O4 particle after coating. These beneficial properties of cerium oxide led to the compound’s use as a coating material. The unwanted parasitic reaction and Mn dissolution between the interface of the cathode electrode were reduced significantly by surface modification of CeO2 in the LiMn2 O4 . 3.6. CV studies Fig. 9a and b represents the typical cyclic voltammograms of uncoated and 1.0 wt.% CeO2 -coated LiMn2 O4 electrodes at 30 ◦ C and 60 ◦ C and a scan rate of 100 ␮V/s. CV curves of uncoated LiMn2 O4 have similar anodic and cathodic redox potentials at room temperature and elevated temperature. However, the intensities of the peak currents for uncoated LiMn2 O4 have higher and sharper peaks at 30 ◦ C than at elevated temperature. The reduction of peak intensities for the uncoated LiMn2 O4 at elevated temperature reveals that the uncoated LiMn2 O4 has fast capacity fading at 60 ◦ C due to Mn ion dissolution from the electrode–electrolyte interface. In contrast, the CV curves for 1.0 wt.% CeO2 -coated LiMn2 O4 have similar redox potentials and intensities of peak currents for both anodic and cathodic curves at 30 ◦ C and 60 ◦ C. This finding reveals that

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Fig. 10. The electrochemical impedance spectra of CeO2 -coated and uncoated LiMn2 O4 electrodes as a function of cycle number. (a) LiMn2 O4 and (b) 1 wt.% CeO2 -coated LiMn2 O4 .

1.0 wt.% CeO2 -coated LiMn2 O4 has significantly reduced the Mn ion dissolution into the electrolyte at high temperature. The peak potential difference (Ep ) of the two redox peaks was around 0.09 and 0.15 V for 30 ◦ C and 60 ◦ C, respectively. The resultant lower Ep value, which is attributed to 1.0 wt.% CeO2 -coated LiMn2 O4 , has good reversibility at both room temperature and elevated temperature.

3.7. Electrochemical impedance spectroscopy (EIS) studies EIS has been performed to understand the cycle life and rate capability characteristics of the bare and CeO2 -coated LiMn2 O4 cathode materials. Similar EIS studies were also reported (fit to this model) in many cathode materials, such as LiMn2 O4 , LBO-coated LiMn2 O4 [30] and ZnO-coated LiMn2 O4 [31]. In general, the EIS spectra of LiMn2 O4 have two semicircles (high and low frequency), and low frequency tails are observed. The high frequency semicircle is related to the passive surface film that forms on the surface of the cathode materials, the so-called solid electrolyte interface (SEI). The intermediate frequency semicircle reveals the charge transfer resistance in the electrode–electrolyte interfaces. The low frequency tails is associated with the lithium ion diffusion process in the cathode [32–36]. In this EIS spectra, we observe only one semicircle, which is indicative of the combination of two semicircles (high and low frequency). Fig. 10a and b compares the EIS spectra of uncoated and 1.0 wt.% CeO2 -coated LiMn2 O4 (room temperature) samples at a charge potential of 4.5 V as a function of cycle number. Coated and uncoated LiMn2 O4 cathode materials were used as working electrodes, lithium foil was used as both counter and reference electrodes, and 1 M LiPF6 with 1:1 ratio of ethylene carbonate and dimethyl carbonate was used as the electrolyte. 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 4.5 V as a function of cycle number shows that the resistance of the surface film on the cathode particles was 21  after 10 cycles to 98  after 100 cycles for uncoated LiMn2 O4 . In contrast, the resistance was 26  after the first cycle and 42  after 100 cycles for the 1.0 wt.% CeO2 -coated LiMn2 O4 sample. The uncoated sample showed a resistance increase of 77  for 100 cycles compared to 16  for the coated samples. The impedance results revealed that the passive surface film that formed on the uncoated cathode particles caused a faster

increase in the resistance with cycle number (Fig. 10a). In contrast, the formation of a passive surface film was controlled and the reaction between the electrolyte and oxide particles was suppressed on the CeO2 -coated LiMn2 O4 surface (Fig. 10b). During cycling, the LiMn2 O4 surface was influenced by the electrolyte, and capacity faded. Finally, the formation of a passive surface film, increasing the impedance growth of the uncoated LiMn2 O4 , confirmed the faster capacity decrease and the lower cycle stability during repeated cycling. The 1.0 wt.% CeO2 -coated LiMn2 O4 sample showed less impedance growth and could sustain its cycle stability. The surface-modified cathode exhibits better cyclability and rate capability retention as compared to the pristine sample, which reveals that nano-CeO2 -coated LiMn2 O4 cathode materials decrease in SEI resistance and suppress the reaction between the cathode surface and electrolyte. 4. Conclusions The LiMn2 O4 cathode materials were successfully coated with various wt.% of CeO2 using cerium (IV) ammonium nitrate by a polymeric process. XRD patterns for CeO2 -coated LiMn2 O4 did not show any change in the 2 value of the peaks or lattice parameters, and no impurity was detected. TEM images confirmed that CeO2 was formed as a compact coating over the cathode particles with a thickness of about 20 nm. XPS revealed that nano-CeO2 was coated over the surface of the core LiMn2 O4 cathode materials. The surface-modified 1.0 wt.% CeO2 -coated LiMn2 O4 sample exhibited significantly improved capacity retention and excellent cyclability for 30 ◦ C and 60 ◦ C compared to the uncoated spinel LiMn2 O4 . A careful investigation of the cathode by electrochemical impedance spectroscopy before and after surface modification with nano-CeO2 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 to improve their cyclability and rate capability during long-term cycling. Acknowledgment 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.

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