Effect of Substrate Temperature on Morphology and Electrochemical

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Effect of Substrate Temperature on Morphology and Electrochemical Performance of Radio Frequency Magnetron Sputtered Lithium Nickel Vanadate Films Used as Negative Electrodes for Lithium Microbatteries M. V. Reddy,* B. Pecquenard, P. Vinatier, and A. Levasseur ICMCB-CNRS/ENSCPB, UniVersite´ de Bordeaux 1, 33607 Pessac Cedex, France ReceiVed: NoVember 13, 2005; In Final Form: January 9, 2006

Lithium nickel vanadate thin films were prepared by radio frequency magnetron sputtering at various substrate temperatures (Ts). These thin films have been investigated as anode electrode material in the use of microbatteries. Films were characterized by Rutherford backscattering spectroscopy, nuclear reaction analysis, Auger electron spectroscopy, glancing-incidence X-ray diffraction analysis, Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and high-resolution transmission electron microscopy techniques. The anodic electrochemical performances of the films have been evaluated by cyclic voltammetry at a scan rate of 1 mV/s and by galvanostatic cycling, with lithium metal as the counter and the reference electrode, and cycled in the range of 0.02-3.0 V at a current density of 75 µA/cm2. Thin films prepared at a Ts of 650 °C show a discharge capacity at the 20th cycle of 1100 ((10) mAh/g, which exhibited excellent capacity retention with a small capacity fade.

Introduction Recently, nanoscale materials have been studied extensively because of their interesting properties1-11 and wide range of applications.1-11 Present-day lithium ion batteries (LIB) are the choice of portable power sources for electronic applications and also hybrid electric vehicles as a result of their high energy density, rechargeability, and safety in operation.12 The recent technological developments on miniaturized systems are in strong demand for developing compact power sources with high efficiency and small ideal dimensions, which are suitable for portable devices. For the past several years, various thin film electrode materials for microbatteries have been studied.13-24 These batteries have many applications in the areas of complementary metal oxide semiconductor-based integrated circuits, radio frequency (rf) identification tags for inventory control and anti-theft protection, smart cards, implantable medical devices, and so on. The electrochemical performance mainly depends on the method of preparation, which will effect the morphology, structure, and composition of the materials. In this context, we fabricated the LiNiVO4 thin films, using a bulk powder target by a rf magnetron sputtering technique, which can be used as a negative (anode) electrode material for lithium microbatteries. Rf-magnetron sputtering was a widely used deposition technique in the microelectronic industry. This method offers various stoichiometric or nonstoichiometric elemental thin film compositions, with different morphologies, by varying deposition parameters.13,14,20,23,24 Prepared thin films by this process showed interesting structural and electrochemical properties.14,20,23,24 Also, to study intrinsic and electrochemical properties, thin films are of ideal geometry and clean surface, without the use of any conducting carbon or binder during fabrication. Presently, we * Corresponding author. Present address: Department of Physics, Solid State Ionics/Advanced Battery Lab, National University of Singapore (NUS), Singapore 117542. E-mail address: [email protected]. Tel.: +6568742605. Fax: +65-67776126.

report the effect of substrate temperature (Ts) during sputtering on morphology, and the structure and the electrochemical performance of lithium nickel vanadate films are discussed. Experimental Section Preparation of Target. The target was prepared from ∼18 g of LiNiVO4 powder cold pressed at a 260 MPa pressure, and then the target was sintered at 600 °C for 8 h in air. LiNiVO4 powder was obtained by mixing high purity Li2CO3, NiO (prepared from Ni(NO3)26H2O), and V2O5, which were weighed in stoichiometric amounts (all initial reactants were purchased from Aldrich). The mixture was heated in a platinum crucible at 730 °C in air for 12 h, with a heating rate of 3 °C/min. The powder X-ray diffraction pattern of the crystalline LiNiVO4 target is shown in Figure 1a. The obtained powders are yellow in color, with a structure that was similar to inverse spinel and with a characteristic cubic lattice parameter, a ) 8.218(2) Å (JCPDS 38-1395). Li and Ni atoms occupy octahedral sites (16d), V atoms are in a tetrahedral site (8a), and O ions are on the 32e site. The intensity of the (220) hkl line is much stronger than that of the (111) line as a result of the presence of a transition metal atom (V) in the tetrahedral site, whereas normal spinel, like LiMn2O4, has a (111) line with the strongest intensity and a (220) line with the weakest intensity. Thin Films Preparation. Lithium nickel vanadate films were prepared by a rf sputtering technique, where the whole sputtering unit was attached to an argon-filled glovebox to avoid contamination. Pure argon (99.99%) and oxygen (99.9%) were used as a carrier and a reactive gas during sputtering. Before the deposition of the thin films, vacuum, ∼4 × 10-5 Pa, was applied into the sputtering chamber. The sputtering conditions are as follows: Ts, 50, 500, and 650 °C ((5; substrates are heated in situ (500 and 650 °C) during deposition); distance between target substrate, 8 cm; power, 30 W; partial pressure of oxygen, 10 mPa (1%); and total working pressure, 1 Pa. Oxygen gas in the sputtering chamber was introduced to maintain the stoichiometry

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Figure 2. RBS spectra of LiNiVO4 film on carbon substrate; thickness, 240 ((5) Å. Symbols: the open squares represent experimental RBS data, and the continuous line is the simulated data by RUMP.

Figure 1. (a) Powder X-ray diffractions of LiNiVO4 target; Miller indices (hkl) are shown. Glancing-incidence angle geometry X-ray diffraction pattern of (b) amorphous film (Ts ) 50 °C), angle of incidence is 3° and (c) Ts ) 650 °C, angle of incidence is 5°. Cu KR radiation. The thickness of the film is t > 1 µm, silicon (100) substrate.

of the LiNiVO4 films. The absence of oxygen in the vanadium composition was always e 0.75.23 Presputtering was carried out under identical conditions for 2 h to remove any contaminated atoms on the target surface. Thin films were deposited on various substrates, depending on the analysis and characterization technique used. Substrate preparation determines the surface properties, and these properties are directly or indirectly related to the film formation stages of adatoms, the interface formation, the nucleation, and the film properties, such as good adhesion, pinhole density, porosity, film microstructure, morphology, and mechanical properties. Substrates of stainless steel (SS), with the dimension 1.32 cm2, and carbon, ∼1 × 1 cm2, were mechanically cut from sheets of SS and carbon. Their surfaces were polished to get a mirror finish with silicon carbide paper in various sizes ranging from 4000 down to 500. Further polished films were cleaned with distilled water and acetone, using an ultrasonic bath, then the substrates were dried in an oven (∼100 °C) and transferred into a glovebox (Ar filled). Silicon (100; commercial) and aluminum foil are used for scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies. The preparation process for high-resolution transmission electron microscopy (HRTEM) specimens of amorphous films (Ts ∼ 50 °C) follows: LiNiVO4, with a film thickness of ∼200 Å, is directly sputtered onto a copper grid, under the above-mentioned sputtering conditions. As a result of experimental difficulties in depositing thin films with a Ts of 500 or 650 °C onto a copper grid, we have studied (TEM) only at a Ts ∼ 50 °C. Characterization. The composition and thickness of thin films were studied by the Rutherford backscattering technique (RBS; 4He+, 2 MeV) by using RUMP software,25 and the

lithium amount is estimated by 7Li(pR)4He, nuclear reaction analysis (NRA).26 The homogeneity of thin films was carried out by using Auger electron spectroscopy (AES; VG Micro lab 310 F). The crystal structures of the target and thin films were studied by using a X-ray diffractometer (XRD, Philips PW1730) and a glancing incidence XRD (SEIFERT XRD 30003 PTS-3 diffractometer), with Cu KR radiation, and with a step size of 0.05° and 25 s. Raman spectra of thin films were recorded by using a DILOR DIDACRAM microspectrometer, a He/Ne laser with a wavelength of 632.8 nm, and a Power 3 mW was used as the excitation source for thesemeasurements. The surface morphology of the films was analyzed by SEM (JEOL JSM5200 microscope) and AFM (Dimension 3100, Digital instruments), by the taping mode. HRTEM (JEOL 3010) was used to study the surface structure and lattice images. Electrochemical Measurements. For cyclic voltammetry (CV) and battery studies, LiNiVO4 thin films were deposited on SS substrates (good fellow), which act as working electrodes, and the lithium metal foil was used as a counter and reference electrode. A polypropylene celgard membrane was used as a separator, and 1 M LiPF6 (EC: DEC; Merck) was the electrolyte. The geometrical area of the electrode is 1.32 cm2 and ∼0.3 µm is the thickness of the films used to carryout electrochemical measurements. We note that, during the preparation of the thin film electrodes, neither conducting carbon nor binder was used. Homemade Teflon-covered SS current collector (similar to the swagelok type) test cells were used for electrochemical analysis. The cells were fabricated in an Arfilled glovebox. The CV measurements were carried out by using a computer-controlled VMP (multichannel potentiostat/ galvanostat) system (Bio-logic, France). Galvanostatic dischargecharge cycling was carried out at a constant current mode by the use of a Bath lab battery tester. Results and Discussion Characterization of Thin Films. The chemical composition and thickness of the thin films was determined by the use of ion beam techniques, such as RBS and NRA, which are the most sensitive methods of characterization. The simulated spectra (continuous line) by the RUMP software and the experimental spectra obtained from the RBS analysis of the LiNiVO4 film, deposited at ∼50 °C, are shown in Figure 2. The chemical composition of the thin film was LiNi1V1((0.03)-

Lithium Nickel Vanadate Films

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Figure 3. Cross-section and top-surface SEM photographs of films with Ts ) 500 °C (a,b) and Ts ) 650 °C (c,d). Bar scale, 2 µm. Silicon substrate (Si100).

O4((0.1). The RBS spectra clearly shows the absence of any other surface impurities in the LiNiVO4 film. The oxygen content in the thin film varies from 6-10%, depending on the film thickness and also on the amount of oxygen that comes from the carbon substrate surface. The lithium content was not determined by RBS as a result of the low scattering cross section of lithium; to overcome this, we used NRA and a lithium content of Li1 ((0.1). A deposited sputtered lithium nickel vanadate film, with a Ts of 50 °C, is shown in Figure 1b. Board peaks in the XRD pattern are characteristics of the amorphous nature of the film, with the strongest line in the XRD pattern being hkl (311), which corresponds to the orientation of the Si(100) substrate. Whereas the thin film prepared at Ts ) 650 °C, which shows a major phase of LiNiVO4, is shown in Figure 1c, with a lattice parameter of a ) 4.22 (2) Å, a 25-nm crystallite size, in addition to small impurity peaks of NiO and Li3VO4 that are also seen in the pattern. An AES analysis of the atomic (Ni, V, and O) concentrations of the thin films are homogeneous with depth (figures not shown), and the lithium depth profile at various thicknesses, by NRA, shows lithium concentrations are in good homogeneity with the film thickness. Raman spectroscopy studies of thin films are carried out to compare the stretching frequencies of the film and the bulk powder. These studies were carried out on films deposited on silicon substrate (Si100) at a Ts of 500 and 650 °C. Raman spectra of LiNiVO4 films showed a main stretching frequency of 335 ((5) cm-1. The stretching frequency of 788-818 ((3) cm-1 corresponds to the γ(VO4) tetrahedron. The vibration band at 335 cm-1 was assigned to the bending mode of the VO4 tetrahedron, which is Raman active; the γ2 (E) symmetry and the band located at

818 cm-1 are due to the stretching mode of the VO4 with A1 symmetry. The silicon substrate shows a main band around 520 cm-1 (figures not shown), and also we recorded Raman spectra of bulk LiNiVO4 powder, prepared by solid-state synthesis. The spectra was similar to the thin film spectra. The observed Raman spectra compare well with the reported literature results.27 Thin films are nonideal materials compared to bulk materials.28 For a fundamental understanding, we studied the films surface structure property relations by using SEM and AFM complementary techniques, which provide a more complete representation of the sample surface. The 3D nature of the AFM can be used to calculate changes in roughness and surface area variations as a result of the differences in the deposition parameters. The surface morphology and cross section of thin films with Ts ) 500 and 650 °C are shown in Figure 3a-d. We compared the deposited films with those of the Thornton structural zone model (SZM)29 and at Ts of 500 and 650 °C, they resemble those of zone T and zone 2, respectively. The morphology of the films (Ts ) 500 °C) shows columnar grains, dense grain boundaries, and a smooth surface, whereas with Ts ) 650 °C, the morphology shows large equiaxed grains, an increased columnar structure, a rough surface, and soft recrystallized grains. The illustration of Ts ) 50 °C, with the effects from the working pressure of argon and various deposition parameters on the SEM morphology, are shown in our previous study.23 Figure 4a,b shows topographic AFM images of thin films; the observations of the films with Ts ) 500 and 650 °C, shows that the surface topography and crystal size distribution increase with increasing Ts. The three-dimensional (3D) plot corresponding to Ts ) 500 °C is shown in Figure 4c. The root-

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Figure 5. HRTEM images (Ts ) 50 °C) of ∼200 Å LiNiVO4 thin film sputtered on copper grid.

Figure 4. AFM images of LiNiVO4 film deposited on silicon (100) substrate with (a) Ts ) 500 °C; bar scale, 0.4 × 0.4 µm2; data scale, 20 nm. (c) 3D plot of Figure 4a. Bar scale: x-, y-axis, 0.100 µm/div and z-axis, 20 nm/div; data scale, 20 nm. (b) Ts ) 650 °C; bar scale, 1.0 × 1.0 µm2; data scale, 145 nm.

mean-square roughness shows 3 and 14 nm for Ts ) 500 and 650 °C, respectively. Also note that the surface roughness and morphology play an important role on the electrochemical performance.30,34 HRTEM photographs with the Ts ) 50 °C are shown in Figure 5. The HRTEM image clearly reflects thin films that are amorphous in nature; in addition, small nanosized crystallites of NiO are shown in the figure as symbols. Electrochemical Studies. Cyclic Voltammetry. CV is a wellsuited electroanalytical technique used to study electrode kinetics.2-6,8-12,16,18,19,35,36 The cyclic voltammograms of thin films prepared at Ts ) 50 and 650 °C are shown in Figure 6a,b. The cyclic voltammograms were recorded with a sweep rate of 1mV/s and at ambient temperature, with lithium metal as the counter and reference electrode. The thin film with a Ts ) 50

Figure 6. Cyclic voltammograms of LiNiVO4 films recorded at a scan rate of 1 mV/s, Ts ) (a) 50 °C and (b) 650 °C. (c) At various scan rates (mV/s; Ts ) 50 °C). The number refers to the cycle numbers or voltage values in a and b and the scan rate (mV/s) in c. Cycled in the 3.0-0.02 V range, 1 M LiPF6 (EC:DEC) was used as an electrolyte. CV were recorded at ambient temperature, and Li metal was the counter and reference electrode. The geometrical area of the electrode was 1.32 cm2.

°C shows during the first cathodic scan (lithium insertion) into the film a well-resolved intense peak at 1.56 V, and on further insertion of lithium, a broad peak is observed around ∼0.94 V and a more intense peak at the low voltage of 0.46 V. During the first anodic scan (lithium deintercalation), peaks around ∼1.32 and ∼2.56 V are seen (Figure 6a). In the second cycle, a cathodic peak occurs at ∼1.68 V, whereas thin films prepared with Ts ) 650 °C show a peak around ∼0.25 V during the first

Lithium Nickel Vanadate Films

Figure 7. Lithium nickel vanadate films cycled between 3 and 0.02 V, with a current density j ) 75 µA/cm2. (a) Voltage vs number of lithium inserted onto film, and (b) discharge capacity vs cycle number.

cathodic cycle and anodic peaks around ∼1.45 and ∼2.55 V. At the end of the third cathodic scan, peaks around 1.70 and ∼0.47 V are clearly seen in Figure 6b. Voltammograms of lithium nickel vanadate films with Ts ) 50 °C (Figure 6a) are characteristic in nature to an amorphous structure of film, and in Figure 6b, CVs show the film to be nanosized or polycrystalline in nature. A CV study clearly shows that the shapes of the voltammograms are very sensitive to annealing temperature. Similar variations in the shapes of the cycling plots and cyclic voltammograms with respect to annealing temperature were reported in the literature.32,33,35 The redox mechanism and structural studies of bulk LiNiVO4 by extended X-ray absorption fine structure and X-ray absorption near edge structure,31 show that the oxidation of the nickel atom reduces to close to metallic state, whereas the vanadium oxidation state changes from 5 to 2. Further studies to understand the redox mechanism of LiNiVO4 thin films by X-ray photoelectron spectroscopy are in progress in our group. Cyclic voltammograms were recorded at various scan rates (1-10 mV/s) and are shown in Figure 6c. As expected, with low scan rates, well-resolved peaks are observed (Figure 6a) and at higher scan rates, a change in the peak voltage to higher values occurs. A rise in the current was noticed during the anodic scan, whereas during the cathodic scan, peak potential shifted to a lower voltage and an increase of current, shown in Figure 6c. The change in the peak shape with scan rate can be correlated to the kinetics of lithium intercalation and deintercalation at the electrode/electrolyte interface and to the rate of lithium diffusion. Similar studies on the effect of scan rate on CV were reported by others.8,16,18,19,36 Galvanostatic Discharge-Charge Cycling. Dischargecharge cycling of the cells with lithium nickel vanadate films prepared at Ts ) 50, 500, and 650 °C and at a current density (j) ) 75 µA/cm2 in the 3.0-0.02 V versus Li at room temperature. The voltage versus the number of lithium inserted into LiNiVO4 film profiles are shown in Figure 7a; for clarity, only selected cycles are shown in the figure. During the initial discharge process (lithium intercalation), the voltage suddenly decreased to ∼0.75 V (0.25 Li) from the open-circuit voltage (∼3.0 V), followed by a plateau at about 1.7 Li; this corresponds to a single-phase reaction. A second plateau from 1.5 to 5.5 Li

J. Phys. Chem. B, Vol. 110, No. 9, 2006 4305 reflects a two-phase reaction, after which the cell voltage decreases continuously until it reaches the lower cutoff voltage. The electrochemical behavior (cycling plots) of the first charge profile curves and subsequent discharge-charge profiles differ from that of the first discharge profile. This material undergoes a redox reaction and changes in the structure/coordination number or electrolyte decomposition during the lithium intercatation and deintercalation process.31,33 All compounds show a Coulombic efficiency (η) of >99% and an irreversible capacity loss (ICL), due to the loss of lithium between the first discharge-charge cycle of 0.4 to 1.45 Li; these values are smaller compared to the bulk LiNiVO4 material.32,33 We also note that the ICL at Ts ) 650 °C is slightly higher than the ICL at Ts ) 500 °C, which is not clear at present. The ICL is commonly observed in several of the LIB anode bulk materials31-33,37-41 and also in the thin film electrode materials21-24 that have been studied, like pure and mixed oxides of tin (Sn) and other oxides (M ) Fe, Co, Mo, Ni, V, and Ti). The ICL occurs as a result of several factors: reduction of metal ions to the respective metal nanoparticles; formation of a lower valency metal ion embedded in a “Li-M-O” bronze-like matrix, followed by the crystal-structure destruction;12,38-41 formation of the solid electrolyte interface;12,37-41 electrolyte decomposition,31,33,37-41 followed by the formation of a “polymeric film” covering the metal nanoparticles, in the case of pure/ mixed M-oxides;12,37-41 and also as a result of the intrinsic nature of the materials due to kinetic limitations and current density. The discharge capacity versus the cycle number, up to 20 cycles, was shown in Figure 7b. Discharge capacity values at the 5th cycle of films, prepared at Ts ) 50, 500, and 650 °C, varies from 800-1150 ((10) mAh/g (Figure 7b). Lithium nickel vanadate film with Ts ) 50 °C showed a slightly higher capacity fading (after 12 cycles) compared to that of others, and at Ts ) 650 °C, it showed a better capacity for retention, with a small capacity fade as a result of the nice columnar morphology and surface structure of the films. The improved electrochemical performance of the LiNiVO4 thin films compared to that of the bulk materials31,32,33 can be ascribed to the small thickness and good contact of the film with the substrate, which facilitates easy flow of electron transfers between the active material and the substrate. Also, the morphology of the films plays a role in the cycling performance. In general, thin film electrode preparation has an advantage over conventional pellet systems. Conclusions We studied the effect of Ts on the morphology of rf-sputtered LiNiVO4 thin films. The sputtered films with various conditions were characterized by RBS, NRA, AES, XRD, SEM, AFM, and TEM. Their electrochemical properties were examined by CV, and their charge discharge cycling was examined by using Li metal as the counter and reference electrode, with a 0.023.0 V cutoff. CV shows characteristic redox voltage peaks with the morphology of the films. Charge-discharge cycling results of thin films prepared with Ts ) 650 °C and with nice columnar structures, show superior electrochemical performance of 1100 ((10) mAh/g, with a small noticeable capacity fade, which is higher compared to that of bulk LiNiVO4 and that of graphite material. Preliminary studies on sputtered thin films with different morphologies show that morphologies have a strong effect on the cycling performance of lithium nickel vanadate thin films, which can be used as an interesting anode electrode material for microbattery applications.

4306 J. Phys. Chem. B, Vol. 110, No. 9, 2006 Acknowledgment. Thanks are due to H. Fuess, T. Buhrmester, and N. Martz, Department of Material Science, Technical University of Darmstadt, Germany for help with GXRD and TEM; P. Moretto, CENBG, University of Bordeaux, for his assistance in RBS and NRA measurements; M. Lahaye, ICMCB-CNRS, for help with AES and AFM; and C. Wannek and G. Sabine, ENSCPB/ICMCB, for their help. References and Notes (1) Hae, A. J.; Zhao, J.; Zou, S.; Own, C. S.; Marks, L. D.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 2005, 109, 11158. (2) Li, X.; Cheng, F.; Guo, B.; Chen, J. J. Phys. Chem. B 2005, 109, 14017. (3) Subramanian, V.; Zhu, H.; Vajtai, R.; Ajayan, P. M.; Wei, B. J. Phys. Chem. B 2005, 109, 20207. (4) Hu, C.; Zhang, Y.; Bao, G.; Zhang, Y.; Liu, M.; Wang, Z. L. J. Phys. Chem. B 2005, 109, 20072. (5) Chen, J.; Xu, L.; Li, W.; Gou, X. AdV. Mater. 2005, 17, 582. (6) Martin, C. R. Science 1994, 266, 1961. (7) Arico, A. S.; Bruce, P. G.; Scrosati, B.; Tarascon, J.-M.; Schalkwijk, W. V. Nat. Mater. 2005, 4, 366. (8) Subramanian, V.; Hall, S. C.; Smith, P. H.; Rambabu, B. Solid State Ionics 2004, 175, 511. (9) Gnanasekar, K. I.; Jiang, X.; Jiang, J. C.; Aghasyan, M.; Tiltsworth, R.; Hormes, J.; Rambabu, B. Solid State Ionics 2004, 148, 575. (10) Subramanian, V.; Jiang, J. C.; Smith, P. H.; Rambabu, B. J. Nanosci. Nanotechnol. 2004, 4, 125. (11) Gnanasekar, K. I.; Rambabu, B.; Langry, K. C. J. Electrochem. Soc. 2002, 149, H19. (12) Nazri, G. A., Pistoia, G., Eds. Lithium Batteries: Science and Technology; Kluwer Academic Publishers: Norwell, MA, 2003. (13) Meunier, G.; Dormoy, R.; Levasseur, A. CNRS Patent. W09005387, 1988. (14) Meunier, G.; Dormoy, R.; Levasseur, A. Mater. Sci. Eng. B 1989, B3(1-2), 19. (15) Jones, S. D.; Akridge, J. R. J. Power Sources 1995, 54, 63. (16) Striebel, K. A.; Rougier, A.; Horne, C. R.; Reade, R. P.; Cairns, E. J. J. Electrochem. Soc. 1999, 146, 4339. (17) Bates, J. B.; Dudney, N. J.; Neudecker, B.; Wang, B. New Trends Electrochem. Technol. 2000, 453.

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