Growth of LiMn2O4 thin films by pulsed-laser deposition and their electrochemical properties in lithium microbatteries

Materials Science and Engineering B72 (2000) 36 – 46 www.elsevier.com/locate/mseb

Growth of LiMn2O4 thin films by pulsed-laser deposition and their electrochemical properties in lithium microbatteries C. Julien a,*, E. Haro-Poniatowski b, M.A. Camacho-Lopez a,b, L. Escobar-Alarcon c, J. Jimenez-Jarquin b b

a LMDH, UMR 7603, Uni6ersite´ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris cedex 05, France Laboratorio de Optica Cuantica, Uni6ersidad Auto´noma Metropolitana Iztapalapa, Apdo. Postal 55 -534, Mexico DF 09340, Mexico c Departamento de Fı´sica, Instituto Nacional de In6estigaciones Nucleares, Apdo. Postal 18 -1027, Mexico DF 11801, Mexico

Received 2 November 1999; accepted 15 December 1999

Abstract Films of LiMn2O4 were grown by pulsed-laser deposition (PLD) onto silicon wafers using sintered targets which consisted in the mixture of LiMn2O4 and Li2O powders. The film formation has been studied as a function of the preparation conditions, i.e. composition of the target, substrate temperature, and oxygen partial pressure in the deposition chamber. Composition, morphology and structural properties of PLD films have been investigated using Rutherford backscattering spectrocopy, scanning electron microscopy, X-ray diffraction and Raman scattering spectroscopy. The films deposited from target LiMn2O4 +15% Li2O have an excellent crystallinity when deposited onto silicon substrate maintained at 300°C in an oxygen partial pressure of 100 mTorr. It is found that such a film crystallizes in the spinel structure (Fd3m symmetry) as evidenced by X-ray diffraction. Well-textured polycrystalline films exhibit crystallite size of 300 nm. Pulsed-laser deposited LiMn2O4 thin films obtained with a polycrystalline morphology were successfully used as cathode materials in lithium microbatteries. The Li//LiMn2O4 thin film cells have been tested by cyclic voltammetry and galvanostatic charge-discharge techniques in the potential range 3.0 – 4.2 V. Specific capacity as high as 120 mC/cm2 mm was measured on polycrystalline films. The chemical diffusion coefficients for the LixMn2O4 thin films appear to be in the range of 10 − 11 –10 − 12 cm2/s. Electrochemical measurements show a good cycleability of PLD films when cells are charged-discharged at current densities of 5 – 25 mA/cm2. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Lithium manganese spinel; Pulsed-laser deposition; Lithium microbatteries

1. Introduction Lithiated transition-metal oxides (LTMOs) such as LiCoO2, LiNiO2, and LiMn2O4 have received significant attention due to their industrial applications especially in rechargeable lithium-ion batteries [1]. These materials are applied on the cathode side where Li is, respectively extracted and stored during the charge-discharge cycle of the battery. It has been reported that the spinel structure LiMn2O4 exhibits a specific capacity about 120 mAh/g [2 – 4], where composite sample electrodes were used. LiMn2O4 crystallizes with a spinel structure and belongs to the Fd3m (O7h) space group * Corresponding author. Tel.: +33-144-274561; fax: + 33-144273854. E-mail address: [email protected] (C. Julien)

with cubic lattice parameter a = 8.239 A, [2]. The cubic spinel LiMn2O4 structure is described with the Mn and Li cations on the 8d and 4a sites, respectively, and the oxygen ions on the 32e sites. Half of the octahedral interstices are occupied by the Mn ions forming 3D framework of edge-sharing MnO6 octahedra. Lithium ions occupy tetrahedral sites, which share common faces with four neighboring empty octahedral sites at the 16c position. Together tetrahedral sites form a three-dimensional network of transport paths 16c-8a16c through which lithium ions diffuse [2]. The classical insertion/deinsertion reactions in a Li//LiMn2O4 cell can be written as [Mn4 + ]2O4 + Li+ + e − l Li[Mn3.5 + ]2O4,

(1)

Li[Mn3.5 + ]2O4 + Li+ + e − l Li2[Mn3 + ]2O4,

(2)

0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 5 9 8 - X

C. Julien et al. / Materials Science and Engineering B72 (2000) 36–46

which normally lie in between the voltage 4.0 and 3.0 V, respectively [5]. In Eqs. (1) and (2) the brakets denote species in octahedral B sites, as is usual in spinel A[B2]O4 structure chemistry. The 3D network of M–O bonding is held rigid and it is accessible only by lithium ions. For x B0.9, 120 – 135 mAh/g can be extracted from the spinel oxide. Hence, the spinel LiMn2O4 is considered to be superior to its layered counterparts when its 3D channel path for lithium-ion diffusion is only taken into account. Preparation of LiMn2O4 in thin-film form may have advantages from a point of view of fundamental studies (because it is a binder-free material with a well-defined interfacial area) and of the emerging field of microbatteries as well. Thin films of LTMOs have been synthesized by a variety of techniques including sputtering, spray deposition, reaction of metals, and pulsed laser deposition [6–17]. In the fabrication of LiMn2O4 thin films, formation of the spinel structure is known to be crucial for obtaining a good rechargeability of the cells. Various aspects of LiMn2O4 thin films prepared by RF sputtering [10], electron beam deposition [7,8], pyrolitic preparation [11], and pulsed-laser deposition [15–17] have been reported. Thin films of amorphous LiMn2O4 have been prepared onto substrates maintained at lowtemperature (B 150°C) by Shokoohi et al. [7] and Bates et al. [8] with reactive electron beam evaporation and Hwang et al. [10] with RF magnetron sputtering. Pulsed-laser deposition (PLD) technique is a successful method in the growth of materials containing volatile components with complex stoichiometries. For this reason, it is well suited to films of LTMOs, where lithium loss to volatilization could occur in conventional evaporation methods. Recently, Cairns et al. reported the electrochemical behavior of LiMn2O4 films grown by PLD onto stainless steel substrates [15,16]. The authors used a LiMn2O4 target irradiated at 308 nm in vacuum and under 100 mTorr oxygen atmosphere; the substrate was heated at 800°C. The obtention of crystalline thin dense films without postdeposition annealing was claimed and the good electrochemical performance of PLD films was demon-

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strated. Recently, Morcrette et al. [17] have grown PLD LiMn2O4 films on various substrate materials with a good crystallinity when films were formed under 0.2 mbar at 500°C. In the present work, we report on the growth of thin LiMn2O4 films onto substrates maintained at low temperature (5300°C) using the pulsed-laser deposition from a sintered composite LiMn2O4 target irradiated with a Nd:YAG laser (532 nm). The structural characterizations of these films have been carried out by various techniques, i.e. X-ray diffraction (XRD), Raman scattering (RS) spectroscopy, and electron scanning microscopy (SEM). Li//LiMn2O4 microbatteries were made and tested using polycrystalline thin LiMn2O4 films as cathodes. We present the results of electrochemical measurements on these cells by investigating the cyclic voltammetry, the charge-discharge profile, the chemical diffusion coefficient of lithium in LixMn2O4, and the specific capacity as a function of the cycle number.

2. Experimental

2.1. Growth conditions PLD films were grown onto silicon substrates maintained at various temperatures in the range 100–300°C. PLD targets were high purity LiMn2O4 powder (Cerac products) crushed and pressed at 5 tons/cm2 to make tablets 2 mm thick and 13 mm diameter. Three types of target were prepared with excess of lithium, i.e., Li/ Mn\ 0.5 by adding Li2O. The typical substrates, i.e. Si (100) wafers, were previously cleaned using HF solution. A schematic of the PLD apparatus is shown in Fig. 1. Target and substrates were placed inside a vacuum chamber with a diffusion pump yielding pressures of 5×10 − 5 Torr (1 Torr= 133.3 Pa). The target was rotated between 1–10 rotations per min with an electric motor to avoid depletion of material at any given spot. The laser used in these experiments is the 532 nm line of a doubled frequency pulsed Nd:YAG using power densities close to 108 W/cm2 with 10 ns pulse width at a repetition rate of 10 Hz. The films were grown in the operating partial pressure of oxygen in the range 50–300 mTorr. The rate deposition was about 150 A, /min. Growth conditions for the PLD LiMn2O4 films are listed in Table 1.

2.2. XRD and Raman scattering measurements

Fig. 1. Schematic diagram of the pulsed-laser deposition chamber.

The X-ray diffraction measurements were carried out using a CuKa radiation source (l= 1.5406 A, ) in a Siemens D-5000 diffractometer. Raman spectroscopy measurements were performed at room temperature in air with a Jobin-Yvon U1000 double monochromator

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Table 1 Growth conditions for the PLD LiMn2O4 filmsa Sample

Substrate temperature (°C)

Oxygen pressure (mTorr)

Target composition (% Li2O)

Structure

LIMN1 LIMN2 LIMN3 LIMN4 LIMN5 LIMN6 LIMN7 LIMN8 LIMN9 LIMN10 LIMN11 LIMN12 LIMN13

250 300 100 200 300 300 200 100 300 300 200 300 100

200 50 50 50 100 50 50 50 100 50 50 100 100

15 15 15 15 15 5 5 5 5 10 10 10 10

p p a p m a a+Mn–O a+Mn–O a+Mn–O P A P A

a

m, microcystalline; p, polycrystalline; a, amorphous; Mn–O, manganese sub-oxide.

using the 514.5 nm line of an argon-ion laser (Coherent Innova 70) at a power level of 20 mW. The laser spot size at the surface of the sample was about 100 mm2. The signal was detected with a photomultiplier and a standard photoncounting system. Finally, the surface morphology of the film was observed with a scanning electron microscope (Philips XL30).

ature (Ts), partial oxygen pressure(P(O2)), and target composition (Fig. 2). A large lithium loss is observed above 400°C as shown in Fig. 2a. The influence of the oxygen partial pressure for two target compositions shows that up to an atmosphere of 300 mTorr the Li/Mn ratio does not change significantly. Thus, films deposited from LiMn2O4 + 15% Li2O target at Ts = 300°C, P(O2)= 300 mTorr are nearly stoichiometric.

2.3. Electrochemical measurements Electrochemical measurements were carried out on Li//LiMn2O4 cells with a lithium metal foil as anode and a polycrystalline film as cathode of 1.5 cm2 active area using a Teflon home-made cell hardware. The silicon substrate was mounted on Ag wire with silver paint and covered by insulating epoxy leaving only the PLD film as active area. The electrolyte consisted of 1 M LiClO4 dissolved in propylene carbonate. According to Guyomard and Tarascon, PC/LiClO4 at 25°C is fairly stable up to 4.5 V [18]. Electrochemical titration was made by charging and discharging the cells using the galvanostatic mode of a Mac-Pile system in the potential range between 2.8 and 4.2 V. Quasi open-circuit voltage profiles were recorded using current pulses of 5 mA/cm2 supplied for 1 h followed by a relaxation period of 0.5 h. Electrochemical potential spectroscopy (ECPS) was performed using 5 mV potential steps.

3. Results and discussion

3.1. Film composition Composition of the LiMn2O4 films deposited on silicon wafer was determined from the Rutherford backscattering spectroscopy analysis. The Li/Mn ratio has been studied as a function of the substrate temper-

3.2. X-ray-diffraction Pulsed-laser deposited LiMn2O4 films are pin-hole free as revealed from optical microscopy and well adherent to the substrate surface. The good film integrity is favourable for electrochemical testing. Fig. 3 shows the X-ray diffraction patterns of PLD LiMn2O4 films grown onto silicon wafers maintained at Ts =300°C in P(O2)= 100 mTorr as a function of the target composition. LiMn2O4 films grown from target LiMn2O4 +5% Li2O are poorly crystallized (Fig. 3a). As the amount of Li2O increased in the target, the XRD patterns develop features expected for the regular spinel. They are indexed using the Fd3m symmetry. When a film is grown from the target LiMn2O4 + 15% Li2O, the X-ray diagram (Fig. 3c) displays peaks at 2u= 16, 36 and 47°, which are attributed to the (111), (311), and (400) Bragg lines, respectively [2]. The X-ray diagram of the LiMn2O4 target is presented as a reference (insert in Fig. 3). The X-ray diagram of a LiMn2O4 film formed at low substrate temperature exhibits the amorphous nature of the layer. The typical peaks of the polycrystalline spinel phase in LiMn2O4 films appear upon increasing the substrate temperature (Ts = 300°C) in oxygen partial pressure P(O2)= 100 mTorr using a lithium-rich target. The manganese-oxygen framework is well defined in this structure.

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3.3. SEM measurements Fig. 4a–b shows the typical scanning electron micrographs of pulsed-laser deposited LiMn2O4 films grown onto silicon wafers maintained at 300°C in oxygen partial pressure P(O2) = 100 mTorr (a) film LIMN12 from target LiMn2O4 +10% Li2O; and (b) film LIMN5 from target LiMn2O4 +15% Li2O. The surface of the film is relatively smooth with a few splashed particles. With a short target-substrate distance, the films produced exhibit a dense layer with grain size of the films significantly small.

Fig. 3. X-ray diffraction patterns of pulsed-laser deposited LiMn2O4 films grown at Ts =300°C and P(O2) =100 mTorr (a) LIMN9 from target LiMn2O4 +5% Li2O; (b) LIMN12 from target LiMn2O4 + 10% Li2O; and (c) LIMN5 from target LiMn2O4 +15% Li2O. The XRD diagram of the LiMn2O4 target is shown in insert.

The effect of target composition on the film morphology can be seen from the micrographs as long as the layer thicknesses are similar. The film grown from LiMn2O4 + 10% Li2O is a relatively dense layer with some particles incorporated (Fig. 4a), while the LiMn2O4 film displays well-defined particles with submicron-sized grain (Fig. 4b) with increasing the lithium content in the target. Discrete particles are formed on the top of the layer increasing the surface roughness. The main factor which determines the film morphology is the substrate temperature. It was reported that the high the substrate temperature, the less porous the layer [15]. Therefore, the reaction between LiMn2O4 and Li2O contributes to the formation of this dense morphology. These results are suitable fot the further utilization of PLD films because a fundamental role in terms of charge-transfer capability and cycle life is played by the morphology of the films used as cathodes in lithium microbatteries [7].

3.4. Raman spectroscopy

Fig. 2. RBS analysis of the Li/Mn ration as a function of (a) substrate temperature; (b) oxygen partial pressure; and (c) target composition.

Experimental Raman spectra of thin-films LiMn2O4 are shown at various stages of pulsed-laser deposition. Fig. 5(a–c) shows the Raman scattering spectra of LiMn2O4 films deposited onto silicon maintained at 300°C in oxygen partial pressure P(O2)= 100 mTorr as a function of the target composition. These spectra display the Raman-active mode of the silicon wafer. The strong peak centered at 521 cm − 1 corresponding to the Si substrate is observed. The RS peak positions

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of PLD LiMn2O4 films are in good agreement with those reported for the LiMn2O4 microcrystalline phase [19]. The experimental Raman data consist of a series of broad bands located between 300 and 700 cm − 1. The RS spectrum of the polycrystalline LiMn2O4 film grown with a lithium-rich target (curve a) is dominated by a strong and broad band at ca. 622 cm − 1 with a shoulder at 573 cm − 1. A band with a medium intensity is located at ca. 499 cm − 1, while four bands having a weak intensity are observed at ca. 437, 374, 320 and 303 cm − 1. Interpretation is complicated by the appearance of a higher number of Raman bands than that predicted by factor-group theory for the perfectly spinel LiMn2O4 microcrystalline phase. The cubic spinel possesses the prototype symmetry Fd3m with the standard notation Li8a[Mn2]16dO4, where the manganese cations reside on the octahedral 16d sites, the oxygen anions on the 32e sites, and the lithium ions occupy the tetrahedral 8a sites [20]. According to the results of the theoretical factor-group analysis, five modes are active (A1g +Eg +3F2g) in the Raman spectrum of the spinel LiMn2O4 crystal [21]. It is also convenient to analyse the RS spectrum in terms of localized vibrations, considering the spinel structure built of MnO6 octahedra and LiO4 tetrahedra [22,23]. The Raman band located at ca. 622 cm − 1 can be viewed as the symmetric Mn – O stretching vibration of MnO6 groups. This band is assigned to the A1g symmetry in the O7h spectroscopic space group. The position and the halfwidth of this band remain almost unchanged upon delithiation [22]. Its broadening could be related with the cation-anion bond lengths and polyhedral distortion occurring in LiMn2O4. The RS peak at 437 cm − 1 derives from the Eg species, whereas the peaks located at 374, 499 and 573 cm − 1 derive from the F2g species. According to Ammundsen et al. [24], the A1g and Eg vibrations involve only oxide ion displacements, while the F2g vibrations are characterized by

large oxygen motions and very small lithium displacements. The mode located at 374 cm − 1 (F2g species) derives predominantly from a vibration of the Li sublattice. It corresponds to the vibration of LiO4 tetrahedra in the local environment model [23]. Information for the structural quality of the PLD LiMn2O4 films can be given considering the Raman data using the shape and the frequency of two groups of peaks located in the low- and high-frequency region of the spectra. The effect of target composition is clearly observed in Fig. 4. When the PLD films are grown from target with Li2O5 10%, the RS spectra are less resolved and the peak at 622 cm − 1 is broadened toward the high wavenumber side. This phenomenon is due to the amorphous structure of the films and the high distortion of MnO6 octahedra. Fig. 6 shows the influence of the oxygen partial pressure in the deposition chamber on the quality of the film. The RS spectrum of the LIMN2 film grown at P(O2)= 50 mTorr corresponds to a highly disordered LiMn2O4 phase, while that of the LIMN5 film deposited at P(O2)= 100 mTorr exhibits well-resolved bands. Fig. 7 shows the RS spectra of LiMn2O4 films deposited from the LiMn2O4 + 15% Li2O target into oxygen partial pressure of P(O2)= 50 mTorr as a function of substrate temperature. The RS spectra show interesting differences between the film grown onto substrate at Ts = 300°C and at Ts = 100°C. The peaks situated in the low-wavenumber region, which are observed in the Raman spectrum of films grown on heated substrate, disappear when the film is grown on cool substrate. The RS spectrum of the film grown Ts = 100°C displays the amorphous nature of the layer. It is obvious that the high the substrate temperature, the more crystallized the layer. The band located at 640 cm − 1 can also be an indication of the presence of another manganese oxide such as Mn3O4 or Mn2O3.

Fig. 4. Scanning electron micrographs of pulsed-laser deposited LiMn2O4 films grown onto silicon wafers maintained at 300°C in oxygen partial pressure P(O2)= 100 mTorr (a) LIMN12 from target LiMn2O4 +10% Li2O; and (b) LIMN5 from target LiMn2O4 +15% Li2O.

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622 cm − 1 occurs for films grown at Ts =300°C in P(O2)= 100 mTorr from lithium-rich target. The RS spectrum of such a film is found to be well resolved, showing the polycrystalline nature of the films. As the stretching mode is sensitive to the film morphology, low FWHM values provide evidence for the spinel-like structure for samples grown at a high substrate temperature. These spectroscopic results indicate that the conjunction of target composition (lithium-rich), substrate temperature (Ts = 300°C), and partial oxygen pressure (P(O2)= 100 mTorr) promotes reconstruction of the stoichiometric LiMn2O4 spinel framework. It is important to note that the target composition plays the main role in the film stoichimetry. When a lithium-deficient target is used, manganese sub-oxides are formed, i.e. as Mn3O4 or Mn2O3, which are clearly observed by the appearance of Raman bands in the high-wavenumber region (around 650 cm − 1).

Fig. 5. Raman scattering spectra of LiMn2O4 films deposited onto silicon maintained at 300°C in oxygen partial pressure P(O2) =100 mTorr as a function of the target composition. (a) LIMN5 grown from LiMn2O4 + 15% Li2O; (b) LIMN12 grown from LiMn2O4 + 10% Li2O; and (c) LIMN9 grown from LiMn2O4 + 5% Li2O.

Fig. 8 shows the evolution of the full width at half maximum (FWHM) of the symmetric stretching Raman mode of LiMn2O4 films as a function of (a) substrate temperature (target LiMn2O4 +10% Li2O); (b) substrate temperature (target LiMn2O4 + 15% Li2O); and (c) target composition. It can be remarked that low values of the FWHM for the stretch located at

Fig. 6. Raman scattering spectra of LiMn2O4 films deposited onto silicon maintained at 300°C from the LiMn2O4 +15% Li2O target as a function of the oxygen partial pressure. (a) LIMN2, P(O2) =50 mTorr; and (b) LIMN5, P(O2) =100 mTorr.

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There are two sets of well-defined current peaks observed in CV diagrams; they are located at 3.92 and 4.07 V for the LiMn2O4 film grown at Ts =300°C exhibiting a polycrystalline spinel structure (Fig. 10). These peaks are slightly shifted toward lower potentials for films grown at lower substrate temperature (Fig. 9). These CV features are associated with the redox process

Fig. 7. Raman scattering spectra of LiMn2O4 films deposited from the LiMn2O4 +15% Li2O target into oxygen partial pressure P(O2)= 50 mTorr as a function of substrate temperature. (a) LIMN3 deposited at Ts =100°C; and (b) LIMN2 deposited at Ts =300°C.

3.5. Electrochemical studies 3.5.1. Cyclic 6oltammetry LiMn2O4 films grown onto silicon wafers maintained at 100°C were used as cathode materials and tested in a lithium microbatteries with 1 M LiClO4 in propylene carbonate as electrolyte. Cyclic voltammetry (CV) measurements have been carried out at sweep rate a =1 mV/s using three types of PLD films. Fig. 9 shows the cyclic voltammogram for a Li//LiMn2O4 cell with the film LIMN3 which was grown onto substrate at Ts =100°C in P(O2)= 50 mTorr. Fig. 10 presents cyclic voltammograms for cells with films grown onto silicon substrate at Ts = 200°C in P(O2) = 50 mTorr (LIMN4) and at Ts = 300°C in P(O2) =100 mTorr (LIMN5). CV measurements have clearly shown that current peaks have a square root dependence on sweep rate (Fig. 11). This behavior is expected from material in which a diffusioncontrolled process occurs [25]. Based on a linear relationship between the peak current in the single-phase region ( : 3.9 V) and the square root of scan rates, a preliminary calculation of the diffusion coefficient of Li+ ions into the LiMn2O4 phase gave D :10 − 12 cm2/s.

Fig. 8. Evolution of the FWHM of the symmetric stretching Raman mode of LiMn2O4 films as a function of (a) substrate temperature (target LiMn2O4 +10% Li2O); (b) substrate temperature (target LiMn2O4 +15% Li2O); and (c) target composition.

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of Mn3 + to Mn4 + and vice-versa, when lithium is inserted into, and extracted from the LiMn2O4 spinel phase. According to Ohzuku et al. [3], the redox couple with a mid-peak potential of about 4.0 V is considered to be a single-phase insertion/deinsertion reaction of lithium ions in LixMn2O4, while the redox couple at higher potential, ca. 4.1 V is attributed to the cubic/cubic two-phase reaction.

Fig. 9. Cyclic voltammogram at sweep rate of 1 mV/s for Li// LiMn2O4 in cell with 1 M LiClO4/PC electrolyte. PLD film (LIMN3) was grown onto substrate at Ts = 100°C in P(O2)= 50 mTorr.

Fig. 10. Cyclic voltammograms at sweep rate of 1 mV/s for Li// LiMn2O4 in cells with 1 M LiClO4/PC electrolyte. PLD films were grown onto silicon substrate at (a) LIMN4, Ts = 200°C in P(O2) =50 mTorr;and (b) LIMN5, Ts = 300°C in P(O2)= 100 mTorr.

Fig. 11. Scan rate dependence of the first anodic (at potential (3.9 V) for the LiMn2O4 PLD film. The departure from a linear plot indicates a diffusion-controlled process of the Li+ ion extraction.

3.5.2. Charge-discharge profiles Typical charge-discharge curves of Li//LiMn2O4 cells using pulsed-laser deposited film grown at Ts =300°C are shown in Fig. 12. Electrochemical measurements was carried out at a rate C/100 in the potential range 3.2–4.2 V; as such, the voltage profile should provide a close approximation to the open-circuit voltage (OCV). From the electrochemical features, we may make some general remarks that are (i) an initial voltage about 3.4 V versus Li/Li+ was measured for LiMn2O4 thin-film cathode cells, which is similar to that recorded on the galvanic cell using crystalline cathode [18,26,27]; (ii) the cell-voltage profiles display the typical plateaus currently observed for LixMn2O4 cathodes; and (iii) voltage of each plateau is a function of the structural arrangement in the film and thus depends on the substrate deposition temperature. These potentials slightly increased for films grown at high substrate temperature. This is consistent with many literature data and ensures that at Ts = 300°C the material particles are electrochemically active. Fig. 13 gives the incremental capacity curves (−(x/ (V) versus cell voltage for the Li//LiMn2O4 thin-film cell; they are complementary to charge-discharge profiles (Fig. 12). There are two well-defined voltage regions shown by the variation of the incremental capacity, which displays two sets of peaks centered at 3.88 and 4.04 V. The electrochemical process seems to be a

Fig. 12. First discharge curve of a Li//LiMn2O4 microbattery using a LiMn2O4 film grown by laser ablation technique in 100 mTorr oxygen pressure onto substrate heated at 300°C.

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and reversible peaks shown in Fig. 13. The process of lithium extraction is accompanied by a change in coloration of the LiMn2O4 films which is due to the oxidation of Mn3 + cations. The original black color transforms in dark brownish on charge. Considering the experimental error, these results suggest that about 0.7 Li can be removed at charge potential up to 4.2 V. The PLD LiMn2O4 film grown at Ts = 300°C has interesting electrochemical properties for the microbattery application. In the high voltage region, such a cell delivers a specific capacity of 120 mC/cm2 mm. This value could be compared with the theoretical specific capacity of a LiMn2O4 film 228 mC/cm2 mm (63 mAh/ cm2 mm) assuming a density 4.28 g/cm3 and a theoretical gravimetric capacity 148 mAh/g for a total extraction of Li+ ions from the host matrix. Fig. 14 shows the discharge curves of Li//LiMn2O4 microbatteries at different current densities. As the current density is increased, cell voltage is lowered and no clear plateau is observed. This is mainly due to the increasing cell polarization, which arises from the Li+ diffusion process in the cathode with increasing current density.

Fig. 13. Incremental capacity (− (x/(V) versus cell voltage for the first charge-discharge cycle of a Li//LiMn2O4 microbattery.

3.5.3. Diffusi6ity measurements The kinetics of Li+ ions in PLD LiMn2O4 films are important factors in the battery operation since they govern the intercalation/deintercalation rate. Two techniques have been used for estimating the chemical diffusion coefficients in LixMn2O4 thin-film cathodes, i.e. the cyclic voltammetry and the galvanostatic intermittent titration technique (GITT). Using CV, the trends of current peaks show a square root dependence on a as expected for a diffusion-controlled process when recorded at sweep rate a]1 mV/s, while a linear behavior is observed at lower sweep rate (Fig. 11). Thus, the chemical diffusion coefficients can be estimated using the value of the critical sweep rate, ac, from the relation DLi =

Fig. 14. The discharge curves of Li//LiMn2O4 microbatteries at different current densities.

classical intercalation mechanism for the lithium ions into the LixMn2O4 matrix as indicated by the distinct

F 2 L ac RT

(3)

where L is the film thickness. GITT is very popular for the estimation of the chemical diffusion coefficients for lithium intercalation electrodes. This technique consists of a series of current steps separated by a relaxation period. For short times and small current pulses, the solution of the infinite diffusion problem, based on the work of Weppner and Huggins [28], leads to a simple expression for the chemical diffusivity of lithium ions if the pulse duration, t, is smaller than the characteristic time t=L 2/ DLi, DLi =

  

4 mVm pt MS

2

DES DEt

(4)

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where S is the surface area, and DEt and DEs are the voltage changes during the current pulse and after the

Fig. 15. Evolution of the chemical diffusion coefficient of Li+ as a function of degree of lithium intercalation in LiMn2O4 film. PLD films were grown from target LiMn2O4 + 15% Li2O in 100 mTorr oxygen pressure on substrate heated at (a) 100°C; and (b) 300°C. Table 2 Chemical diffusion coefficients of Li+ in LiMn2O4 samples Material

Diffusion coefficient (cm2/s)

Refs.

Powder Powder PLD film PLD film Pyrolitic film Electron beam film RF-sputtered film PLD film (Ts =100°C) PLD film (Ts =300°C)

0.2–2.2×10−8 1.0×10−9 2.5×10−11 at 4 V 1–3×10−10 1×10−10 at 4.05 V 1.3×10−14 4×10−10 1.5×10−12 at 4 V

[29] [18] [15] [16] [30] [9] [31] This work

8.0×10−12 at 4 V

This work

Fig. 16. Electrochemical behavior of a Li//LiMn2O4 film battery using a LiMn2O4 film grown by laser ablation technique. This microbattery was charged/discharged at 5 mA/cm2 current density in the voltage range from 4.2 to 3.0 V.

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current pulse, respectively. m, M, and Vm are the mass, molecular weight, and partial molar volume of the host oxide, respectively. The high insertion capability has been confirmed also by measuring the chemical diffusion coefficients of Li+ ions in LixMn2O4 film grown onto silicon substrate heated as 300°C in P(O2)= 100 mTorr atmosphere, as shown in Fig. 15. DLi has been investigated in the compositional range 0.55x5 1.0 corresponding to the single-phase region. Results show that the diffusion coefficients for the LiMn2O4 film are in the range 10 − 11 –10 − 12 cm2/s. The chemical diffusion coefficient of Li+ ions decreases continuously as lithium ions are extracted from the LixMn2O4 lattice. In this case, the intercalation process is partly controlled by the number of ion occupancies in the host lattice of the crystallite in the film. The decreasing values of DLi with lithium extraction (decreasing x) is probably due to a lattice shrinkage. By comparison, some literature data of chemical diffusion coefficients of Li+ ions in various LiMn2O4 samples are listed in Table 2. Guyomard and Tarascon [18] disclosed chemical diffusion coefficient of 1.0× 10 − 9 cm2/s with a composition independent behavior for LiMn2O4 powder. The GITT data for a LiMn2O4 PLD film [15], on the other hand, was in the order of 10 − 11 cm2/s in the 4 V region (the x value was not given), whereas Hwang and Joo [31] have shown that the chemical diffusion coefficient was 4×10 − 10 cm2/s for r.f.-sputtered LiMn2O4 films. As the diffusion path for lithium ions passes through the 16c-8a-16c cavities, modification of the short-range order of occupied sites will change the diffusion coefficient in polycrystalline films. The difference between the oxide prepared in crystalline form and the film could thus be ascribed to differences in crystal perfection, i.e. static disorder or short chains with undistorted cavities.

3.5.4. Charge-discharge cycling Fig. 16 shows the electrochemical behavior of a Li//LiMn2O4 film battery using a LiMn2O4 film grown at Ts = 300°C. This microbattery was charged/discharged at 5 mA/cm2 current density in the voltage range from 4.2 to 3.0 V. The specific capacity (in mC/cm2 mm) was plotted as a function of the cycle number. The discharge capacity after 35 cycles is about 95% of that of the first cycle. An average loss of about 0.15% per cycle is estimated. This result is due to the fact that LiMn2O4 films prepared by pulsed-laser deposition have a stable Mn2O4 framework to remain intact over repeated lithium extraction and inserton. The specific capacity of 120 mC/cm2 mm is due to the good crystallinity of the LiMn2O4 films grown by the PLD technique. One of the principal properties of the films obtained by laser ablation is the high density compared with films grown by classical techniques.

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C. Julien et al. / Materials Science and Engineering B72 (2000) 36–46

4. Conclusions LiMn2O4 thin films were grown using the pulsed-laser deposition technique, in which the control of the deposition parameters promotes the film stoichiometry. Structural properties of PLD films have been investigated as a function of deposition conditions, which play important roles in the physical and chemical characteristics of the material. As a result, LiMn2O4 thin films grown onto silicon wafer heated at Ts =300°C in oxygen partial pressure of P(O2) =100 mTorr exhibit a well-structured polycrystalline spinel phase. Films deposited at lower temperature are highly disordered. Lithiated manganese oxide films were used as cathode materials in Li//LiMn2O4 microbatteries. The electrochemical behavior of LiMn2O4 films during the lithium extraction from the host structure is linked to their structural characteristics. In the voltage range 3.2–4.2 V, the LiMn2O4 cathode-active films deliver a specific capacity of 120 mC/cm2 mm. This performance is due to the good crystallinity of the LiMn2O4 films grown by PLD technique on heated substrates. One of the principal properties of the films obtained by laser ablation is the high density compared with films grown by classical techniques. For LiMn2O4 films deposited onto Si substrate heated at 300°C, the chemical diffusion coefficients of Li+ ions are in the range 10 − 11 – 10 − 12 cm2/s. The decreasing values of DLi with lithium extraction (decreasing x) is probably due to a lattice shrinkage. Overall the results indicate that the pulsedlaser deposition method can be used to growth films with promise for application in lithium microbatteries.

Acknowledgements The present work has been partially supported by the Consejo National de Ciencia y Tecnologia of Mexico under contract 4225-E9405. The authors thank Dr Saul Ziolkiewicz for his helpful comments.

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