LiMn2O4 thin films prepared by pulsed laser deposition for rechargeable batteries

Thin Solid Films 506 – 507 (2006) 120 – 122 www.elsevier.com/locate/tsf

LiMn2O4 thin films prepared by pulsed laser deposition for rechargeable batteries H. Otsuji *, K. Kawahara, T. Ikegami, K. Ebihara Department of Electrical and Computer Engineering, Graduate School of Science and Technology, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan Available online 20 December 2005

Abstract LiMn2O4 thin films were prepared on ITO (Indium Tin Oxide) coated glass substrate under various deposition conditions by pulsed laser deposition (PLD). LiMn2O4 thin film prepared at the substrate temperature of 700 -C and the oxygen pressure of 100 mTorr shows the good crystallinity of (111) preferred orientation, and its cyclic voltammogram shows two couples of redox peaks around 3.7 and 4.3 V and the peak separation was about 0.7 V. D 2005 Elsevier B.V. All rights reserved. Keywords: LiMn2O4; Indium tin oxide; Pulsed laser deposition; Lithium ion battery

1. Introduction The all-solid-state thin film lithium ion rechargeable batteries have several attractive features including possible integration of battery fabrication with that of the microelectronic devices such as smart cards [1]. In the lithium ion batteries, lithium ion insertion/extraction with redox reaction of host is made use of as both cathode and anode reactions. The high performance electrode materials become an important factor for the development of lithium ion batteries. LiMn2O4 is considered to be more attractive cathode material for lithium ion batteries than LiCoO2 and LiNiO2 [2]. In this study, we prepared LiMn2O4 thin films on ITO (Indium Tin Oxide) coated glass substrate under various deposition conditions using the pulsed laser deposition (PLD) technique. In many experimental works on electrochemical properties, most of LiMn2O4 thin films have been prepared on Pt or Pt coated substrates [3,4]. ITO coated glass has conducting properties and is inexpensive compared with Pt or Pt coated materials. To our knowledge, there is no report about preparing LiMn2O4 thin films on ITO coated glass substrate. In this paper, we investigated

optimum deposition condition of LiMn2O4 thin films on ITO coated glass substrate.

2. Experimental LiMn2O4 thin films were prepared by pulsed laser deposition using KrF excimer laser of wavelength of 248 KrF Excimer Laser Mirror

Quartz Window LiMn2O4 Gas Flow

0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.126

ITO substrate Plasma Plume

O2 Gas * Corresponding author. E-mail address: [email protected] (H. Otsuji).

Lens Heater

Rotating

Vacuum System

Fig. 1. PLD equipment.

H. Otsuji et al. / Thin Solid Films 506 – 507 (2006) 120 – 122

nm, laser fluence of 2 J/cm2, pulse width of 25 ns, and repetition rate of 10 Hz (COMPex205, LAMBDA PHYSIK). The outline figure of the PLD equipment used for thin film preparation is shown in Fig. 1 [5]. LiMn2O4 thin films were grown onto ITO coated glass substrate under various deposition conditions of the substrate temperatures (Ts) and the oxygen gas pressures ( P O2). Detailed deposition conditions are shown in Table 1. The crystallinity of thin films was characterized by X-ray diffraction (XRD) and the surface morphology of the thin films was observed by atomic force microscopy (AFM).

(400)

h:60nm

m 1u w:

(a)

700 oC

600 oC

500 oC

(b)

480.7nm

237.3nm

87.21nm

Fig. 4. AFM images of LiMn2O4 thin films prepared at the various substrate temperatures under P O2 = 100 mTorr. (a) Ts, (b) grain size.

Electrochemical properties of the LiMn2O4 thin film as the electrode were studied by the cyclic voltammetry employing a three-electrode cell. Lithium metal was used as counter and reference electrodes. Potentials are referred to lithium metal. Electrolytes used were propylene carbonate (PC) containing 1 mol/dm3 LiClO4. Electrochemical experiment was conducted under Ar atmosphere. 1um

: ITO substrate

(3 11 )

Intensity (arb. units)

(111)

: LiMn2O4

h:100nm

Shot counts Target Substrate Target – substrate distance Substrate temperature (Ts) Oxygen gas pressure ( P O2)

KrF excimer laser Wavelength: 248 nm Laser fluence: 2 J/cm2 Repetition rate: 10 Hz 30,000 shots LiMn2O4 tablet (purity 99.9%) ITO (Indium Tin Oxide) coated glass 35 mm 500 – 700 -C 10 – 300 mTorr

h:150nm

1um

Table 1 Deposition condition of LiMn2O4 thin films Laser

121

700ºC

10

20 30 40 50 Scattering Angle 2θ (deg.)

60

Fig. 2. XRD patterns of LiMn2O4 thin films prepared at the various substrate temperatures under P O2 = 100 mTorr.

: LiMn2O4

(a)

300mTorr

100mTorr

10mTorr

(b)

221.3nm

237.3nm

109.4nm

Fig. 5. AFM images of LiMn2O4 thin films prepared at the various O2 gas pressures under Ts = 600 -C. (a) P O2, (b) grain size.

: ITO substrate

Current(mA)

(400)

(311) MnxOy

(111)

Intensity (arb. units)

300mTorr

100mTorr

10mTorr 10

20 30 40 50 Scattering Angle 2θ (deg.)

60

Fig. 3. XRD patterns of LiMn2O4 thin films prepared at the various O2 gas pressures under Ts = 600 -C.

m

1u

w:

h:100nm

500ºC

h:150nm

h:100nm

600ºC

0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5

600ºC

500ºC

700ºC 3

3.5

4

4.5

5

Potential(V) Fig. 6. Cyclic voltammograms of LiMn2O4 thin films prepared at the various substrate temperatures under P O2 = 100 mTorr. Scan rate: 2.0 mV/s, electrolytes: 1.0 mol/l LiClO4/PC.

122

H. Otsuji et al. / Thin Solid Films 506 – 507 (2006) 120 – 122

Fig. 2 shows XRD patterns of LiMn2O4 thin films prepared at the various substrate temperatures. The major diffraction peaks come from the crystallized spinel LiMn2O4 phase and minor peaks from the ITO coated glass substrate. XRD pattern of the substrate temperature of 500 -C shows poorly crystallized LiMn2O4. As the substrate temperature rises from 600 to 700 -C, the quantity of (111) preferred orientation increases. Fig. 3 shows XRD patterns of LiMn2O4 thin films prepared at the various oxygen gas pressures. As seen in the figure, good crystallinity with (111) orientation was acquired at P O2 = 100 mTorr. Spinel LiMn2O4 phase was hardly crystallized at the oxygen gas pressure of 10 mTorr and the quantity of (111) orientation decreased at P O2 = 300 mTorr. Fig. 4 shows AFM images of LiMn2O4 thin films prepared at the substrate temperatures of 500, 600, and 700 -C. The particle size increases with increasing substrate temperature as expected due to grain growth. It has been known that the particle size influences directly battery performance and it has reported that decrease of particle size improves the cyclability and discharge capacity [6]. We found the crystallinity at 500 -C is poor although the small grain size of 87 nm is obtained. Fig. 5 shows AFM images of LiMn2O4 thin films prepared at the oxygen gas pressures of 10, 100, and 300 mTorr. As seen in the figure, there is no clear relation between growth of the particle size and increase of oxygen gas pressure. Fig. 6 shows cyclic voltammograms of LiMn2O4 thin films as electrodes prepared at the various substrate temperatures. All the cyclic voltammograms show two couples of redox peaks around 3.7 and 4.3 V, which are the characteristic of LiMn2O4 [3,4]. The split of the redox peaks into two couples indicates that the electrochemical reactions of the insertion (or extraction) of lithium ion proceed in two steps, which can be written as the following two reactions:

Current(mA)

3. Results and discussion

0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5

10mTorr 100mTorr

3

3.5

300mTorr 4 4.5 Potential(V)

5

Fig. 7. Cyclic voltammograms of LiMn2O4 thin films prepared at the various O2 gas pressures under Ts = 600 -C. Scan rate: 2.0 mV/s, electrolytes: 1.0 mol/l LiClO4/PC.

PO2 = 100 mTorr was 0.7 V which is smaller than at P O2 = 300 mTorr. This is probably due to change of composition of Li– Mn –O as changing the oxygen gas pressure. It is necessary to analyze composition of the deposited Li– Mn – O films in detail in the future. At oxygen gas pressure of 10 mTorr, no oxidation and reduction currents were clearly observed due to extinction of spinel LiMn2O4 phase at P O2 = 10 mTorr (XRD pattern in Fig. 3).

4. Conclusion LiMn2O4 thin film prepared at the substrate temperature of 700 -C and the oxygen pressure of 100 mTorr shows the good crystallinity of (111) preferred orientation, and its cyclic voltammogram shows two couples of redox peaks around 3.7 and 4.3 V, and the peak separation was about 0.7 V. The resulting characteristic is comparable to that of LiMn2O4 thin films deposited on Pt or Pt coated substrates. The LiMn2O4 thin films prepared on ITO coated glass substrate show good electrochemical properties, and may be able to apply to cathode materials of lithium ion batteries.

ð1=2ÞLiþ þ ð1=2Þe þ 2MnO2 SLi0:5 Mn2 O4 ð1=2ÞLiþ þ ð1=2Þe þ Li0:5 Mn2 O4 SLiMn2 O4 As the substrate temperature rises from 500 -C to 700 -C in Fig. 6, the current of redox peaks increases. This result has a close relation with increasing quantity of LiMn2O4 (111) orientation in XRD patterns in Fig. 2. Fig. 7 shows cyclic voltammograms of LiMn2O4 thin films as electrodes prepared at the various oxygen gas pressures. Good electrochemical property of LiMn2O4 thin films is obtained at P O2 = 100 mTorr. The peak separation at

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